Advances in Agronomy, Volume 42

ADVANCES IN

AGRONOMY VOLUME 42

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ADVANCES IN

AGRONOMY Prepared in Cooperation with the AMERICAN SOCIETY OF AGRONOMK

VOLUME 42 Edited by N. C . BRADY Science and Technology Agency for International Development Department of State Washington, D . C .

ADVISORY BOARD G. H. HEICHELR. J . KOHEL G . E. HAM E. L. KLEPPER R. H. FOLLEIT D. R. BUXTON E. S. HORNERJ . J. MORTVEDT N . L. TAYLORR. J. WAGENET

R. D. HARTER

ACADEMIC PRESS, INC. Harcourl Brace Jovanovich, Publishers San Diego New York Berkeley Boston London Sydney Tokyo Toronto

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CONTENTS CONTRIBUTORS. ...................................................... PREFACE.............................................................

ix xi

BIOLOGICAL EFFICIENCIES IN MULTIPLE-CROPPINGSYSTEMS

Charles A. Francis I. 11. 111.

IV. V. VI.

Introduction .................................................... Importance of Multiple-Species Systems .......................... Efficiency of Resource Use by Multiple Species.. ................. Pest Management in Multiple-Cropping Systems. .................. Biological and Economic Stability of Cropping Systems. . . . . . . . . . . . Future Applications for Multiple-Cropping Systems ................ References .....................................................

1

4 7 17 25 35 36

SEED COATINGS AND TREATMENTS AND THEIR EFFECTS ON PLANT ESTABLISHMENT

James M. Scott I. 11. 111.

Introduction ................................ The Seed-Coating Process ....................................... Coatings to Facilitate Planting. . . . . . . . . .

........................................... IV. V . Protective Coatings. ......................... VI. Nutrient Coatings ............................................... VII. Herbicide Coatings ......................... .................................... VIII. Other Coatings.. . . . IX . Treatment Processes. ............................. ......................................... X. References ..................... .............

44 48 53 55 57 61 70 71 73 75 77

CONSERVATION TILLAGE FOR SUSTAINABLE AGRICULTURE: TROPICS VERSUS TEMPERATE ENVIRONMENTS

Rattan La1 1. 11.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conservation Tillage and Sustainable Agriculture ..................

86 89

vi

CONTENTS

I11 . Mulch and No-Till Farming for Different Ecological Environments ........................................ IV . Pros and Cons of the No-Till System: Tropics versus Temperate Zones ................................. V . Noninversion and Minimum Tillage .............................. VI . Subsoiling as Conservation Tillage ............................... VII . Conservation Tillage for Problem Soils ........................... VIII . Why Conservation Tillage? ...................................... IX . Environmental Pollution and Conservation Tillage ................. X . The Systems Approach to Conservation Tillage and Supportive Cultural Practices ..................................... XI . Soil Guide to Conservation Tillage ............................... XI1 . Research and Development Priorities ............................. XI11. Conclusions .................................................... References .....................................................

95

110 117 122 126 140 160

163 177 182 183 185

MICROBIALLY MEDIATED INCREASES IN PLANT-AVAILABLE PHOSPHORUS

R . M . N . Kucey. H . H . Janzen. and M . E . Leggett I . Introduction .................................................... I1. Sources of Plant-Available Phosphate in Soils ..................... Ill . Mycorrhizal Effects on Plant Phosphate Availability . . . . . . . . . . . . . . . IV . Phosphobacterins and Organic Phosphate Mineralization ............ V . Inorganic Phosphate-Solubilizing Microorganisms .................. VI . Sulfur Oxidation and Rock Phosphate-Sulfur Mixtures ............. VII . Future of Technologies .......................................... References .....................................................

199 200 202 207 209 220 222 223

ENZYMOLOGY OF THE RECULTIVATION OF TECHNOGENIC SOILS

S . Kiss. M . Dr5gan.Bularda. and D . PaSca I. I1 . Ill . IV . V. VI . VII . VIII . IX . X.

Introduction .................................................... Technogenic Soils from Coal Mine Spoils ......................... Technogenic Soils from Power Plant Wastes ...................... Technogenic Soils from Retorted Oil Shale........................ Technogenic Soils from Iron Mine Spoils ......................... Technogenic Soils from Manganese Mine Spoils ................... Technogenic Soils from Lead and Zinc Mine Wastes ............... Technogenic Soils from Sulfur Mine Spoils ....................... Technogenic Soils from Lime and Dolomite Mine Spoils ........... Technogenic Soils from Refractory Clay Mine Spoils ..............

230 230 252 253 257 259 260 263 264 264

vii

CONTENTS

XI. Technogenic Soils from Bentonitic Clay Mine Spoils . . . . . . . . . . . . . . XII. Technogenic Soils on Sand Opencast Mine Floor Drift and Spoils.. . XIII. Technogenic Soils from Overburdens Remaining after Pipeline Construction. ...................................... XIV . Recultivation of Soils Remaining after Topsoil "Mining". . . . . . . . . . xv. Concluding Remarks ............................................ References .....................................................

266 267 269 272 272 274

EFFECTS OF NITRIFICATION INHIBITORS ON NITROGEN TRANSFORMATIONS, OTHER THAN NITRIFICATION, IN SOILS

K . L. Sahrawat 1. 11. 111.

IV . V.

Introduction ............... ........................... Effects of Nitrification Inhibitors on Physical and Chemical Processes Relevant to Nitrogen Transformations ............. Effects of Nitrification Inhibitors on Biological Nitrogen Transformations. ........ ............... Other Effects ................................................... Perspectives ............... ........ References .....................................................

279 280 290 303 305 306

COMPACTION EFFECTS ON SOIL STRUCTURE

Satish C. Gupta, Padam P. Sharma, and Sergio A. DeFranchi I. 11. 111.

IV.

Introduction .................................................... Soil Structural Parameters ....................................... Mechanisms of Soil Structure Changes during Compaction ......... Guidelines on Water Contents and Mechanical Stresses Conducive to Lrreversible Changes in Soil Structure. ...............

V. VI . References . . . .

........................ ........................ ........................

31 I 312 328 33 I 335 337 337

TISSUE CULTURE IN RICE IMPROVEMENT:STATUS AND POTENTIAL

Satish K . Raina Introduction .................................................... Embryo Culture. ................................................ Anther, Pollen, and Ovary Culture ............................... IV . Somatic Cell Culture ............................................ I.

11. 111.

339 34 1 34 1 366

...

Vlll

CONTENTS

V . Protoplasts ..................................................... VI . Overview and Strategies for the Future ........................... References .....................................................

378 385 389

BREEDING ANNUAL Medicago SPECIES FOR SEMIARID CONDITIONS IN SOUTHERN AUSTRALIA

E . J . Crawford. A . W . H . Lake. and K . G . Boyce I . Introduction .................................................... Plant Introduction: The Basis for Development .................... 111. Plant Breeding: The Creation of New Genetic Combinations . . . . . . . IV . Preservation and Commercialization .............................. V . Scope of the Future ............................................. References ..................................................... 11.

INDEX ................................................................

399 402 416 431 433 434

439

CONTRIBUTORS Numbers in parentheses indicate the pages on which the authors' contributions begin.

K. G. BOYCE (399), Department of Agriculture, Adelaide, South Australia, Australia E. J . CRAWFORD (399), Department of Agriculture, Adelaide, South Australia, Australia SERGIO A. DEFRANCHI (3 1 l), Instituto di Agronomia, Universita Degli Studi, Della Basilicata, Italy M. DHGAN-BULARDA (229), Department of Plant Physiology, Babes-Bolyai University, 3400 Cluj-Napocu, Romania CHARLES A. FRANCIS (l), Department of Agronomy, University of Nebraska, Lincoln, Nebraska 68583 SATISH C. GUPTA (31 l), Department of Soil Science, University of Minnesota, St. Paul, Minnesota 55108 H. H . JANZEN (199), Agriculture Canada, Lethbridge Research Station, Lethbridge, Alberta TIJ 4B1, Canada S . K I S S (229), Department of Plant Physiology, Babes-Bolyai University, 3400 Cluj-Napoca, Romania R. M . N. KUCEY (199), Agriculture Canada, Lethbridge Research Station, Lethbridge, Alberta T1J 4B1, Canada A. W. H. LAKE (399), Department of Agriculture, Adelaide, South Australia, Australia RATTAN LAL (85), Department of Agronomy, Ohio State University, Columbus, Ohio 43210 M. E. LEGGETT (199), Philom Bios, Inc., Saskatoon, Saskatchewan S7N 2x8, Canada D. PASCA (229), Department of Plant Physiology, Babej-Bolyai University, 3400 Cluj-Napoca, Romania SATISH K. RAINA (339), Biotechnology Centre, Indian Agricultural Research Institute, New Delhi 110012, India K. L. SAHRAWAT (279), International Crops Research Institute for the SemiArid Tropics, ICRISAT Patancheru P. 0.. Andhra Pradesh 502 324, India JAMES M. SCOTT (43), Department of Agronomy and Soil Science, University of New England, Armidale, New South Wales 2351, Australia PADAM P. SHARMA (31 I), Department of Soil Science, University of Minnesota, St. Paul, Minnesota 55108

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PREFACE

This volume continues the broad subject focus that is typical of Advances in Agronomy. One review covers the use of a series of cell and tissue culture techniques to improve rice characteristics such as insect and disease resistance, cold tolerance, and lodging. The potential of annual Medicagos for semiarid areas such as those in West Asia and North Africa is reviewed, with special emphasis given to tolerance of low pH and low nutrient contents, especially phosphorus. The influence of soil microorganisms on the soil-plant cycling of phosphorus is the subject of one chapter. Mechanisms for phosphate solubilization by fungi and bacteria receive the most attention. The effect of nitrification inhibitors on nonnitrification activities such as movement and loss of nitrates from soils, ammonia fixation and volatilization, and denitrification is also reviewed. An excellent and comprehensive review of conservation tillage for tropical and temperate climates is the subject of another contribution. It should be of special interest to scientists in the developing countries. The subject of sustainable agriculture also receives attention in a fine review of multiple cropping opportunities in temperate regions. The effect of soil compaction on the relationship between macroscopic and microscopic soil structure parameters is reviewed, along with mechanisms by which microscopic soil structure changes with compaction. The potential of enzymatic activity to indicate the evolution of mine spoils and plant wastes into agricultural and forest soils is covered, and refers to work in countries with developed market economies and those with centrally planned economies. A final contribution provides an excellent review of the influence of coating seeds with various chemicals and adhesives, and other coating materials, on factors such as seedling establishment, legume inoculation, and protection from disease, insects, pests, and weeds. We are indebted to scientists from six countries for these reviews, a fact that again emphasizes the international significance of agronomy. N. C . BRADY

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ADVANCES IN AGRONOMY, VOL. 42

BIOLOGICAL EFFICIENCIES IN MULTIPLE-CROPPING SYSTEMS' Charles A. Francis Department of Agronomy University of Nebraska Lincoln, Nebraska 68583

1. Introduction 11. Importance of Multiple-Species Systems A. Historical Background B. Pasture Systems C. Grain and Root Crop Systems 111. Efficiency of Resource Use by Multiple Species A. Efficiencies in Time and Space B. Light Use C. Water Use D. Nutrient Use IV. Pest Management in Multiple-Cropping Systems A. Weed Management B. Insect Management C. Plant Pathogen Management V. Biological and Economic Stability of Cropping Systems A. Variations in Biological Output B. Income Stability C . Buffering and Compensation in Systems D. Statistical Analysis of Multiple-Species Systems VI. Future Applications for Multiple-Cropping Systems References

I. INTRODUCTION Biological complexities and interactions in multiple-species cropping systems present an interesting challenge to scientists who work to improve system productivity. A number of efficiencies in resource use become operative when two or more crops are present in the same field during 'This article is a contribution from the Department of Agronomy at the University of Nebraska, Lincoln, Nebraska 68583.

I Copyright 0 1YR9 by Academic Press. Inc. All rights of reproduction in any form re5erved.

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CHARLES A. FRANCIS

the same year, and these can be most complex when crops are grown simultaneously. Such interactions have been called the “integration efficiencies” of cropping systems (Hanvood, 1984). Different types of weed, insect, and plant pathogen relationships with crops may occur when the cropped field is not planted to one homogeneous species (Altieri and Liebman, 1986). There is both biological and economic buffering in systems in which there is production of more than one crop in the field (Lynam et al., 1986). Information about these biological efficiencies can lead to management options that differ from those in monoculture agriculture, and the increasing literature on multiple cropping is worthy of review (Francis, 1986). An early review by Aiyer (1949) described the principal mixed cropping systems in India. Key papers on the nature of competition and resource use were published by de Wit (1960) and Donald (1963). Another major review of cropping patterns appeared in the book “Multiple Cropping,” which included the 1975 Symposium papers from the American Society of Agronomy (Papendick e f al., 1976). Since then several symposia have been sponsored by international centers or bilateral aid organizations, for example, by the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT) (1981) in India and by the United States Agency for International Development (USAID) in Tanzania (Keswani and Ndunguru, 1982). Most of these papers and reviews are descriptive in nature, based on empirical observations from existing cropping systems or data from relatively simple experiments on component technology. The reviews by Willey (1979a,b) presented excellent descriptions of previous research and provided some of the first insight on how the information then available could be used to explain biological interactions in complex cropping systems. Since then several books have brought more of the sometimes obscure data for several regions of the developing world to light. These include the publications of Beets (19821, Steiner (1982), Gomez and Gomez (1983). and Francis (1986). There now exists a body of information on which to base some generalizations about multiple-species systems and to use for developing recommendations and management tools. Some confusion exists in the terminology used by authors to describe multiple-cropping systems. An attempt to standardize the terms and their usage was made at the 1975 Symposium, and the definitions published in the proceedings of the conference (Andrews and Kassam, 1976) are helpful. With much of the field research published in languages other than English (notably Spanish and French), there is more confusion brought into discussions of these complex systems. Table I presents a series of definitions that were published recently (Francis, 1986) and that form the common language for the discussion that follows.

3

MULTIPLE-CROPPING SYSTEMS Table I Definitions and Terminology in Common Usage with Multiple-Cropping Systems"

Miiltiple cropping: the intensification of cropping in temporal and spatial dimensions; growing two or more crops on the same field in one year Seqiienticil cropping: growing two or more crops in sequence on the same field per year; the succeeding crop is planted after the preceding crop has been harvested; crop intensification is only in the time dimension; there is no intercrop competition Double cropping: growing two crops per year in sequence Triple cropping: growing three crops per year in sequence Quadruple cropping: growing four crops per year in sequence Ratoon cropping: cultivation of crop regrowth after harvest, although not necessarily for grain Intercropping: growing two or more crops simultaneously on the same field; crop intensification is in both the temporal and spatial dimensions; there is intercrop competition during all or part of crop growth Mixed intercropping: growing two or more crops simultaneously with no distinct row arrangement Row intercropping: growing two or more crops simultaneously; one or more crops are planted in rows Strip intercropping: growing two or more crops simultaneously in different strips wide enough to permit independent cultivation but narrow enough for the crops to interact agronomically Relay intercropping: growing two or more crops simultaneously during part of the life cycle of each; a second crop is planted after the first crop has reached its reproductive stage of growth but before it is ready for harvest Cropping index: number of crops grown per year on a given area of land x 100 Cropping pcrttern: yearly sequence and spatial arrangement of crops or crops and fallow on a given area Cropping system: cropping patterns used on a farm and their interactions with farm resources. other farm enterprises. and available technology that determine their makeup Lnnd eqrrii~ulentrrrtio (LER): ratio of the area needed under sole cropping to that under intercropping at the same management level to give an equal amount of yield; LER is the sum of the fractions of the yields of the intercrops relative to their sole crop yields Monocriltitre: repetitive growing of the same sole crop on the same land Rottition: repetitive cultivation of an ordered succession of crops (or crops and fallow) on the same land; one cycle often takes several seasons or years to complete Sole cropping: one crop variety grown alone in pure stand at normal density; synonymous with solid planting; opposite of intercropping Agrisili~ic.rrltrire:growing of trees for timber but with cultivated crops grown beneath Competition effect: competition of intercropped species for light, nutrients, water, CO,. and other growth factors Complementarv effrct: effects of one component on another that enhance growth and productivity. as compared to competition Componcnt crops: individual crop species that are a part of the multiple-crop system Component technology: procedure for growing each component crop fnterplanting: all types of seeding or planting a crop into a growing stand; used especially for annual crops grown under stands of perennial crops Overyielling: production of component crops in an intercrop that is higher than the sum of appropriate monoculture crops; this is indicated by an LER greater than unity (continued)

4

CHARLES A. FRANCIS Table I (Continued)

simultaneous growth of two or more useful plants on the same plot: this includes mixed cropping, intercropping, interculture, interplanting, and relay planting Spufial urrungement: physical or spatial organization of component crops in a multiplecropping system

Simultaneous polyculture:

“From Tables 1.2, 1.3, and 1.4 in Francis (1986).

II. IMPORTANCE OF MULTIPLE-SPECIES SYSTEMS In order to understand how multiple-species systems function in both crop and pasture mixtures, it is valuable to briefly examine the history of multiple cropping. A wide range of species mixtures are currently used in both temperate and tropical regions, and they have evolved through conscious intervention of farmers to meet their goals of producing food and income. Much of this development was due to trial and error, but careful observation of what was successful undoubtedly led to many of the patterns that are seen today in mixed cropping systems (Plucknett and Smith, 1986). Both the history and prevalence of multiple-cropping systems are explored here.

A. HISTORICAL BACKGROUND Early cropping systems were certainly mixtures of desirable species used for food, fiber, and other needs in the community. Plucknett and Smith (1986) describe six stages in the evolution of crop domestication over the past 10,000 years, a process that moved at different rates and reached different stages around the globe (Table 11). Monocropping is a relatively recent innovation in agriculture. From gathering and later protection of preferred plants to gardening and subsistence farming, mixtures of crops have emulated to some degree the natural ecosystem and its diversity. Several reviews have described the evidence for plant diversity in these early systems (Baker, 1970; Hawkes, 1970; Sauer, 1947). Authors have also described the relative stability that this diversity brings to the natural ecosystem, although there is not total agreement in this area (Goodman, 1975; Hall, cited in Smith and Francis, 1986). Multispecies cropping systems probably began in the tropics and today are generally more diverse in the lowland cultivated areas in these regions.

MULTIPLE-CROPPING SYSTEMS

5

Table 11 Crop Domestication Stages from Gathering to Commercial Farming“ Stage

Characteristic

Gathering Protection of preferred plants

Wild plants in native stands Wild plants in native stands; volunteer plants around camps and along trails Transplanted seedlings, roots, cuttings of wild plants, planting of seed crops Trees, shrubs, herbs, and grasses, usually grown in polycultural assemblages under shifting agriculture conditions Polyculture common in tropics, less so in temperate areas; cash crops often grown in separate fields With tropical tree crops, polyculture still common, but trend is toward monocropping

Gardening Subsistence farming

Subsistence and cash cropping

Commercial farming “From Plucknett and Smith (1986).

Diversity of cultivated systems declines with higher altitudes in the tropics and with increasing latitude (Harris, 1976). Mixtures of species were chosen by farmers over the centuries to make use of rainfall and native soil fertility, and empirical choices of patterns were made among the bestperforming combinations observed. Plucknett and Smith (1986) describe the components of these systems in Africa, Asia, and the Americas. Many multiple-cropping systems persist today on farms on which resources are limited and the level of new technology is low. Intensive cropping systems, often with mixtures of species, have reached high yield levels through the use of pesticides, improved cultivars, and other high-input technology in countries such as China, Taiwan, the Philippines, and Thailand.

B. PASTURESYSTEMS The vast areas grazed by indigenous animals over millenia and the large expanses of rangeland used for domesticated cattle, sheep, and goats are actually important “intercropping” systems (Gomm el al., 1976). Native grasslands and mixed timber-grass ranges in the western United States are typical ecosystems that have been managed for grazing ruminant animals €or more than a century. These lands have been exploited through ruminants because they cannot be economically cropped in other ways due to low rainfall, topography, or fertility limitations. There has been serious depletion of the grazing capacity of some areas due to lack of

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CHARLES A. FRANCIS

understanding of the biology of the ecosystem and how management of animal numbers should fit the productivity of the range (USDA, 1936, 1941). Some range lands have been renovated by controlling unwanted, nonedible plant species (Herbel et al., 1973; Clary, 1974) and thus increasing the carrying capacity of the land (Cook, 1966). Judicious application of fertilizers can shift the species balance to one that is more desirable for grazing animals (Rogler and Lorenz, 1957). Interseeding cool season grasses into the range can increase productivity with good precipitation (Wight and White, 1974), and planting forbs and browse species can also improve the carrying capacity of high mountain valleys (Plummet-, 1968). These are adaptations of the natural, diverse ecosystem, which has a capacity to buffer dry matter production through a series of dissimilar seasons during which rainfall and temperature are unpredictable. Yet relatively little is known about how to fine-tune these systems due to limited knowledge about their biological interactions. Seeded pasture mixtures of grasses and legumes provide the basis for sustainable forage production for ruminants in temperate regions. Research on these mixtures has provided the majority of information on interspecific competition and complementarity (de Wit, 1960; Donald, 1%3). Both intraand interspecific competition are intense in high densities of this mixed intercropping situation. One of the best known systems combines white clover (Trifolium repens L.) with various grass species (Chestnutt and Lowe, 1970). Although initial planting densities of components influence their populations, later management including height of cutting, frequency of cutting or grazing, and amount of fertilizer applied have a greater influence on the eventual relative proportions of these perennials. Mixtures of legumes and cereals were also important for forage production in the temperate zone before the use of high levels of chemical fertilizers. Both the advantages and drawbacks of these systems have been reviewed (Brown, 1935; Haynes, 1980; Nicol, 1934, 1936; Wilson, 1940). Results from maizedat and maize-soybean mixtures for fodder showed the former to be highly productive but low in protein, and the latter to have potential for both high tonnage and protein content (Mason and Pritchard, 1987). More details on competition for resources are given in later sections.

c. GRAINANDROOTCROP SYSTEMS Multiple cropping of cereals, grain legumes, and root crops forms the basis of farming systems for many subsistence farmers in the developing world. Annual crops planted with perennials are another type of intercropping that is common in some lowland and medium-elevation farming regions. It has been estimated that high proportions of basic cereals are

MULTIPLE-CROPPING SYSTEMS

7

produced in multiple-cropping systems in many parts of the world (Francis, 1986). This includes 76% of maize, 90% of millet, 95% of peanut, and 99% of cowpea in Nigeria (Okigbo and Greenland, 1976); 84% of maize, 56% of peanut, 81% of beans, and 76% of pigeon peas in Uganda (Jameson, 1970); and 90% of beans in Colombia, 80% of beans in Brazil, and 60% of maize in all of the Latin American tropics (Francis et al., 1976). Reviews have summarized the agronomic literature on cereals (Rao, 1986) and legumes and starchy roots (Davis et al., 1986) in multiple-cropping systems. Most of the results found in published form are based on empirical field studies, and there are limited definitive results on competition and resource use. The recent books on multiple cropping focus primarily on cereals, grain legumes, and root crops in complex cropping systems (Beets, 1982; Francis, 1986; Gomez and Gomez, 1983; Steiner, 1982).

Ill. EFFICIENCY OF RESOURCE USE BY MULTIPLE SPECIES Agronomic studies over the past three decades have led to descriptions of a number of distinct relationships between and among crop species grown in mixture. Results have most often been expressed in final grain yield and on occasion total dry matter; much less often have measurements been made of light, water, or nutrient use. Yet final component crop yield is one valid indicator of the integrative success through the entire growing season of that component crop in competing for scarce growth resources in the specific environment or cropping pattern. There is a general agreement that when interspecific competition for a given limiting factor is less than intraspecific competition among plants for that same factor, there is a potential for “overyielding,” or higher total production in the intercrop pattern (Andrews, 1972; Willey, 1979a). A number of models and terms have been developed to describe this partitioning of inter- and intraspecific competition (for example, Hart, 1974; Hill and Shimamoto, 1973; Trenbath, 1975). Willey (1979a) attempted to simplify the types and final results of competition in two-crop systems by defining three broad categories of interactions using expected yields as those which would occur if inter- and intraspecific competition were equal: 1. Mutual inhibition, in which the actual yield of both species is less than expected. This is a rare occurrence in the field and only a few cases have been reported (Ahlgren and Aamodt, 1939; Donald, 1946; Harper, 1961). 2. Mutual cooperation, in which each species produces more when two crops are planted together than when planted alone; this can occur more

8

CHARLES A. FRANCIS

frequently at low levels of technology and when crop densities are relatively low and suboptimal. 3. Compensation, in which one crop produces more and the other produces less than expected; the most common situation, this relationship involves a more competitive (dominant) crop and a less competitive (dominated) crop in the mixture (Huxley and Maingu, 1978). A number of different models for compensation were presented and described by Willey (1979a), most based on the replacement series methodology, which compares a series of density combinations ranging from a pure stand of species A through various mixtures to a pure stand of species B. These models help to describe the gross effects of competition and compensation but do not shed much light on the internal interactions in the system that lead to final yields. IN TIMEAND A. EFFICIENCIES

SPACE

Conceptually, there is agreement that some complementarity between species in mixed cropping must occur if there is to be an advantage in yield, or overyielding, from the intercrop pattern. This complementarity could be in the temporal dimension or in the temporal and spatial dimensions (Francis, 1986; Trenbath, 1976, 1986; Willey, 1979a). Efficiency in the temporal dimension only is illustrated by double-cropping systems used in the Southeast United States or in tropical regions with a bimodal rainfall pattern. Lewis and Phillips (1976) reviewed the literature on double cropping in the United States, especially systems involving summer soybeans and winter small grains. China has used double cropping for centuries to increase food production from a scarce land resource (Xian and Lin, 1985). An intensive double-cropping pattern is favored in areas with a long growing season, sufficient rainfall for the two crops, favorable markets for both crops, and potential to use minimum or zero tillage to establish the second crop into the residue of the first. These practices save both time and soil moisture (Wicks, 1976). Planting maize with a subsequent forage crop has potential for the Southeast (Widstrom and Young, 1980). Growing maize as a double crop with legumes for green manure is another valuable option (Duncan, 1980; Smith and Prine, 1982),especially if there is a potential for seeding maize directly into legumes or their residues (Robertson et al., 1976). These systems are similar to both relay and ratoon cropping, two patterns which combine time- and space-related efficiencies. Relay cropping means that a significant part of the life cycle of the second crop overlaps with the cropping cycle of the first crop. Specific and intensive modifications of this scheme include seeding small grains

MULTIPLE-CROPPING SYSTEMS

9

into soybeans before the harvest of the latter (Clapp, 1974);relay planting peanuts, sweet potatos, or soybeans into corn (Akhanda er al., 1977);and the intensive three-crop relay intercropping of potatoes, maize, and climbing beans in the Central Highlands of Colombia (Francis, 1988).Another variation is ratoon cropping, in which a crop is allowed to regrow from the crown after one grain or forage harvest. Common examples include alfalfa, sorghum-Sudan hybrids for forage, and even ratoon cropping of grain sorghum (Duncan, 1983). Additional examples of overlapping crop growth cycles come from tropical areas, where the potential growing season may be the entire year, provided water is available. Norman (1974) describes a number of systems in northern Nigeria in which crops of different growth duration are planted together to take advantage of intermittent and unpredictable rainfall. On the south coast of Guatemala, farmers plant sesame, soybeans, or a second maize crop into the main crop of maize at about flowering time; this second crop takes advantage of residual moisture and late rains to produce a cash crop. Sorghum and maize are planted together in parts of Central America, and maize is harvested in 3-4 months, whereas the tall sorghum grows slowly at first and then stretches upward after the maize harvest, using residual moisture and fertility. When there is enough overlap in time to cause at least some competition in resource use or cause modifications in management procedures, these are called relay systems. The most intensive use of time and space occurs with the simultaneous or near-simultaneous plantings of two or more crops. Detailed growth analysis and measurement of resource use are being used to broaden the knowledge base on competition and productivity (for example, Clark and Francis, 1985a, b). This type of intercropping has received increased attention during the past two decades, and much of the work of Willey, Rao, Baker, Okigbo, Andrews, Gomez, Harwood, Trenbath, Francis, and others has been summarized in the recent book “Multiple Cropping Systems” (Francis, 1986). Most of the subsequent discussion centers on this form of intensive intercropping, and many examples are used to describe its efficiency and biological interactions.

B. LIGHTUSE The reviews of light use efficiency by Willey (1979a) and Trenbath (1976) are especially useful. Unlike rainfall and nutrients, solar energy cannot be captured and stored for later use in the way that other natural resources are managed; light is “instantaneously available” and needs to be “instantaneously intercepted and used” if this resource is going to be useful to produce photosynthate and plant dry matter (Donald, 1961). Competition

10

CHARLES A. FRANCIS

for light is really between leaves rather than between plants, and a leaf that receives light below the compensation point (level needed for photosynthesis) will soon perish (Etherington, 1976). Plants that are favored in the mixture are not necessarily those with the most leaves and foliage, but those with leaves in the best position to intercept solar radiation. There are both temporal and spatial ways in which multiple-cropping systems use light more efficiently than single crops. If water and nutrient requirements of crops are met, then light is most frequently the limiting resource. Both photosynthesis and plant growth of each component crop will be proportional to the amount of radiation that component intercepts (Trenbath, 1976). Double cropping, which includes two crops in the field in a sequential pattern, provides opportunity for much greater temporal interception of total radiation through the year compared to any single crop, unless that one crop has an extremely long growth cycle. Even long-term crops such as cassava or photoperiod-sensitive sorghum may initially grow slowly and establish full canopy only after several months, opening an opportunity for a short-cycle crop to be grown between rows of the longer-cycle component. Sugar cane also grows slowly in its first several weeks, and some intensive cultivation schemes include beans, cowpeas, soybeans, or maize between the rows of the plant crop (first planting of cane) or ratoon crop. One study from South Africa showed reduced tiller emergence and lower leaf area of cane grown with intercropped beans, but after two seasons there was no difference in final yield of cane or sucrose (Leclezio et a/., 1985). It is obvious that crops that cover the ground over more of the year are going to intercept more light and thus produce more total dry matter than a single crop of shorter duration. Willey and Roberts (1976) concluded that light energy was often the most important factor in overyielding by crop mixtures that exhibited temporal complementarity. How much light is intercepted over the entire growing season is primarily a function of leaf area duration (LAD) of one or more crops developing in the canopy. Willey et a/. (1983) illustrated with a sorghum-pigeonpea intercrop the relative efficiency of light interception and dry matter production compared to monocrops. With two rows of sorghum alternating with one row of pigeonpeas, all at full population in the row, sole-cropped sorghum and sorghum-pigeonpea intercepted about the same total light and produced the same dry matter (Fig. 1). At sorghum harvest, intercrop grain yield was only about 5% below that of sole crop, probably due to the high density of sorghum (Natarajan and Willey, 1981). After sorghum harvest, continued pigeonpea growth led to eventual dry matter production that was 53% of sole-crop pigeonpea, and a higher harvest index resulted in a grain yield that was 72% that of sole-crop pigeonpea. Because more of the total growing season was used and more total radiation was inter-

MULTIPLE-CROPPING SYSTEMS

1000 *

9 800' E

-

m

5 600-

!i>

400.

b

200

lo

'

,.**' ..--

20

40

60

80

100 120 140 160

Days from sowing

FIG. 1. Dry matter accumulation and light interception in sorghum and pigeonpea sown as sole crops and as a two-row sorghum: one-row pigeonpea intercrop. Values are means of 3 years. (From Willey er al., 1983.)

cepted, only 5% was lost from the sole-crop sorghum yield, and 72% of the potential sole-crop pigeonpea grain yield was obtained from the same field. Intercrops have potential to intercept light in different ways than sole crops. Changing the spatial organization of plants to achieve greater efficiency of light interception and conversion to dry matter is more complex. To achieve optimal (high) levels of light interception, it is possible to increase plant density of a monoculture crop. There is little potential for further increasing interception by mixing crop species (Willey and Roberts, 1976). Although intercrops often intercept a higher proportion of total radiation than low densities of sole crops frequently found on subsistence farms, this total interception for at least part of the season could be

12

CHARLES A. FRANCIS

achieved by seeking a more nearly optimum density of the sole crop(s) (Willey, 1979a). Willey (1979a) concluded that “better spatial use of light . . . (will have) to be achieved through more efficient use of light rather than greater light interception” and will hold the most promise for further increasing yield potential of crop mixtures, Advantages of different leaf inclination or modified leaf dispersion in the canopy have been minimal in the field studies reported to date (Willey, 1979a; Rhodes, 1970) and in simulation models (Trenbath, 1974). It would appear that fine-tuning an intercrop system with plants of similar height and growth habit will have little effect on light interception and dry matter production, since most of the light is being intercepted when densities of the component crops are adequate. This was confirmed by studies of mixtures of different-height maize hybrids, which show no advantage in yield (Pendleton and Seif, 1962), and slight advantages in mixtures of sorghum genotypes with different heights (Osiru, 1974). Most of the traditional mixtures of food crop species-maize-bean, sorghum-pigeonpea, banana-coffee, maize-cassava-involve intercrops of plants with dissimilar size and growth cycle in the field. This type of intercrop gives a better vertical distribution of leaves in the total canopy. Willey (1979a) and Trenbath (1976) described the potential advantages of modified light distribution in a canopy of distinct species, while building on the theoretical work of Kasanaga and Monsi (1954). If a tall crop, especially a cereal with C , light response, were combined with a shorter dense crop with C, response, the total use of light could be enhanced in the mixture (Crookston and Hill, 1979; Willey, 1979a). Practical examples of this reaction include the maize-bean intercrop combination, with apparent differences in total yield depending on the bean plant type (Clark and Francis, 1985b). In this study in Colombia, climbing beans outperformed bush beans by 28% due to a longer development cycle and leaf area duration; maize yield was unaffected by bean plant type. In a related study, intercropped maize-beans achieved full cover (leaf area index of 4.0) 3 weeks before monocrop maize and 1 week before monocrop beans, further explaining the potential for overyielding in an intercrop mixture of the two species (Clark and Francis, 1985b). In another crop pattern, Fawusi (1985) found greater light interception (lower transmission) in a maize-okra intercrop when the two components were planted in alternate hills, and Fawusi et al. (1982) found greater light interception due to larger leaf area index (LAI) in a more leafy cowpea cultivar in maize-cowpea intercrops. Willey (1979a) also suggests the potential for choosing crop components adapted to differences in light quality that occur due to vertical distribution of leaf area (Allen er al., 1976; Szeicz, 1975). One notable example of great vertical distribution of components and leaf area is the multistoried tropical rain forest and the analogous cropping systems such

MULTIPLE-CROPPING SYSTEMS

13

as coconut palm-papaya-pineapple mixture grown in the Philippines and Indonesia. Difficulties of converting this rainforest ecosystem to a cropping system are reviewed by La1 (1986). There can also be annual crops such as maize or rice grown under the taller crops (Nelliat et al., 1974). These systems provide the principal efficiencies that can be achieved in light interception and dry matter production through use of appropriate intercrops. A method used to quantify these concepts in light use, light use efficiency (LUE), was defined and explored by Trenbath (1986). He also presented computer simulations of light interception by different intercrop canopies. These could be useful in planning alternative cropping patterns and testing them in an efficient way before planting extensive trials in the field.

C. WATERUSE Intercropping studies in which water use was measured are limited. Water is often the most limiting factor in crop growth, and thus the ability of roots to explore a large soil volume and extract water is critical (Etherington, 1976). Trenbath (1976) describes the highly interrelated water and nutrient relationship with regard to root growth and interactions with the soil solution. Water may be depleted as far as 25 cm from a single root under experimental conditions (Klute and Peters, 1969), and in the field mobility of water to roots may be even greater (Stone et al., 1973). Intercrops may be more efficient in exploring a larger total soil volume if component crops have different rooting habits, especially depth of rooting (Willey, 1979a). For example, a deep-rooted component crop may be forced to develop even deeper roots if grown together with a shallowrooted crop (Fisher, 1976; Whittington and O’Brien, 1968). It would appear that roots of some intercropped species grow in the same general region (Lai and Lawton, 1962). This was the case in maize-bean intercrops in Colombia where lodging in maize was significantly reduced compared to sole-cropped maize; although evidence is circumstantial, the authors attribute the reduced maize root lodging to the intermingled roots of beans, which helped anchor the maize (Francis et al., 1978a). Where moisture is the most limiting resource, intercrops may offer both a temporal and spatial advantage in water use (Baker and Norman, 1975). An early component such as millet in an intercrop with maize could more efficiently use water that would be in excess for either sole crop (Kassam, 1973). Water use efficiency may be confounded by nutrient availability, as shown by the differences in nitrate leaching between sole cropping and parallel multiple cropping (alternate rows or strips) of pigeonpea-maize and sugarcane-black gram (Yadav, 1982). In this study, more nitrate was

14

CHARLES A. FRANCIS

lost below the root zone in the sole crop patterns. No differences were shown in total quantities of water taken up nor in patterns of water depletion when sole crops of cowpea and sorghum were compared with their intercrop (Shackel and Hall, 1984). Although water extraction patterns were different in an intercrop of sorghum-peanut, greater water use efficiency was found in two patterns of sole-cropped sorghum (Shinde and Umrani, 1985). In a comparison of sole crops and several intercrops in India, the wheat-mustard intercrop had greater water use efficiency than any of the other options including other intercrop combinations with wheat (Sinha et al., 1985). These results illustrate some of the complexity of water use by crops and the current lack of in-depth understanding of what to expect when crops are planted together. D. NUTRIENTUSE An excellent review of multiple-cropping fertility relationships was that of Sanchez (1976). As noted above, mobility of soluble ions such as nitrate is the same as that of water in the soil, and roots may attract nitrates from as much as 25 cm away in the soil solution (Barber, 1962; Barley, 1970; Klute and Peters, 1969; Trenbath, 1976). Nutrients such as ammonium, calcium, phosphorus, and potassium are strongly held on surfaces of soil particles and are present in low concentration in the soil solution; they move almost entirely by diffusion (Brewster and Tinker, 1970; Olsen and Kemper, 1968). The distance moved as measured by the depletion of phosphate, for example, may be as little as 0.7 cm (Bhat and Nye, 1973). The ability of an intercrop to make more efficient use than sole crops of soluble and nonsoluble nutrients will depend on the extent of root growth of component species, soil water levels, and how completely the intercrop mixture explores the entire soil mass in the rooting zone. Biological efficiency is likely to result when the intercrop either explores a larger soil mass or explores the same soil mass more completely, compared to sole plantings of the same species. There is also a possibility of differences in time of peak demand for different nutrient elements by components in the mixture (Willey, 1979a). There is much debate about the release of nutrients from one crop in a mixture for use in the same season by another. Willey (1975) cites evidence that shade trees in coffee, tea, and cacao plantations shed their leaves, and the decomposing material makes nutrients available for the lower-story crop. Sanchez et al. (1985) described the soil physical properties and fertility consequences of tree crop culture and fallow periods between tree and crop cycles. Some approaches to the study of nitrogen relations and root interactions have employed nodulating and nonnodu-

MULTIPLE-CROPPING SYSTEMS

15

lating legume variants (Nambiar et al., 1986) and plastic barriers in the ground to separate the roots of intercropped species (Willey and Reddy 1981). Root densities were affected by pearl millet-peanut intercrop as compared to sole crops (Vorasoot, 1983). Below-ground effects are less easy to visualize and to study than those of the growing crop canopy. Higher total nutrient uptake by intercrops than by sole crops has been reported by several authors: for example, nitrogen (N) (Dalal, 1974), potassium (K) (Hall, 1974a,b), and magnesium and calcium (Dalal, 1974) all show this effect. Merences in total yield by intercrops has been explained by this greater uptake, although it is difficult to know if this is the cause or the effect of greater dry matter production (Willey, 1979a). Contrasting results were reported by Baker and Blamey (1985), who found less N uptake by a sorghum-soybean intercrop compared to sole-cropped sorghum; intercropping still produced significantly higher yields than sole cropping. The greatest advantage for intercrops was found at low phosphorus (P) levels in cowpea-maize intercrops in Costa Rica (Chang and Shibles, 198513) and at low N levels in soybean-maize intercrops in Iowa (Chui and Shibles, 1984), compared to sole crops of the components. Another advantage of intercrops is the conservation of fertilizer nitrogen for a subsequent crop or mixture: pigeonpeas planted with maize in India were found to capture nitrogen and make this more available for the subsequent sugarcane crop, compared to maize alone before the cane (Yadav, 1981). Wahua (1983) presented the concept of a nutrient supplementation index (NSI) to describe the fertilizer needs of an intercrop mixture and illustrated the concept with an example from the maize-cowpea alternate row system. Competition for nutrients, especially in pasture mixtures, was reviewed by Haynes (1980). He concluded that legumes in general are poor competitors with grass species for nitrogen. Poor competitive ability of white clover for phosphorus (Jackman and Mouat, 1972a,b), for potassium (McNaught, 1958; Mouat and Walker, 1959), and for sulfur (Neller, 1960; Walker and Adams, 1958)was ascribed to different root morphology compared to the associated grasses. When pastures are fertilized with phosphorus, the response is generally an increase in total dry matter production and an increase in the proportion of legumes in the mixture (Baylor, 1974; Rabotnov, 1977). This is similar to the competition for nitrogen reported for the cowpea-maize intercrop, a relationship which changed with N application and stage of cowpea growth (Chang and Shibles, 1985a).Efficiency of nitrogen use by maize changed with intercroppinglegumes, both with legume density and nitrogen rate (Ofori and Stern, 1987). There may be short-term yield reductions in cereals due to intercropped ground cover legumes, but long-term benefits of nitrogen and reduced soil erosion make them advantageous (Leach et al., 1986). Additional complexities of nutrient use in pastures can result from other fertilizer element applications,

16

CHARLES A. FRANCIS

clipping or grazing patterns, rainfall, species and varieties included in the mixture, and method of fertilizer application (Haynes, 1980). Also important are the effects of these several treatments on symbiotic Rhizobium spp. and the resulting nodulation patterns (Bergersen, 1971). Nutrient requirements for legume nodulation and symbiosis have been reviewed by Munns (1977). Intercropping cowpeas, maize, and melons increased the rhizosphere counts of bacteria for the first two crops but not for the melons (Wahua, 1984). He also found more bacteria in the intrarow than in the interrow intercropping of these species. Yadav and Prasad (1986) observed changes in phosphorus use efficiency by sugarcane as a result of intercropping the cane with mung beans. Differences among legume species (soybean, black gram, peanut) were observed in their bacterial activity and effect on intercropped maize yields in India (Singh et al., 1986). Duncan (1980) found differences among hybrids of grain sorghum planted after a crimson clover green manure crop, but no hybrid-byplanting date interaction. Ten soybean varieties were planted in all combinations with three leguminous cover crops in Malaysia, and a significant variety-by-intercropping system interaction was observed (Mak and Pillai. 1982). Early results of studies in controlled environments seemed to prove that excreted nitrogen from legumes could be taken up by nonlegumes (early literature reviewed by Willey, 1979a). It was suggested that legumes in a lower story that were shaded would fix less nitrogen and thus provide less to grasses in the field. Since the effects of nitrogen are often confounded with water and light competition between two component crops, it is difficult in the field to sort out these interaction effects. Important factors that may influence the potential of a legume to provide nitrogen to an intercropped cereal include densities of the two crops, light intercepted by the legume and thus its ability to fix nitrogen, species of legume, and limitations of other nutrients, especially phosphorus. Willey (1979a) described the importance of both direct transfer during a given cropping season and the availability of residual nitrogen for a subsequent cereal crop. For example, Agboola and Fayemi (1972) showed that mung beans gave higher transfer to maize in the same season, but cowpeas gave a greater contribution of residual nitrogen to the next crop of maize The fertility relationships in an intercropping system are raised to a more complex level when animals are introduced into the system. Most data comes from studies of manure applications to plots, but some work has documented the relationships between rice and fish, for example, Manjappa et al. (1987). Not only is the crop-fish mixture of interest, but the authors also mixed four species of fish in careful proportions for the experiment in raising fingerlings. This is but one example of the biological efficiencies that can be used to advantage in crop-animal systems, and

MULTIPLE-CROPPING SYSTEMS

17

that could be used by farmers to increase diversity in food production and income. Much more research is needed in this complex field.

IV. PEST MANAGEMENT IN MULTIPLE-CROPPING SYSTEMS Much less information is available from the literature on weed, insect, and pathogen relationships in multiple-cropping systems, compared to data on agronomy and physiology of mixtures. One review (Litsinger and Moody, 1976) highlighted the importance of integrated pest management in these complex systems. Another review stressed the importance of using recently acquired biological information about pest species to reduce the need for active control measures in cropping systems (Altieri and Liebman, 1986). Reviews listed by Altieri and Liebman that cover literature on cover crops, agroforestry, strip cropping, and living mulches include those of Altieri and D. K. Letourneau (1982), Altieri and D. L. Letourneau (19841, and Cromartie (1981). There is general agreement that species diversity in multiple cropping reduces most insect pest problems, and the cropping intensity of carefully designed multiple-species mixtures can successfully outcompete weeds. This review presents relevant biological information on weed, insect, and disease control management, and how this can bring another dimension of biological efficiency to complex systems. A summary of how pest problems are influenced by crop and variety choice as well as cultural practices or degrees of intensity in cropping systems is presented in Fig. 2 (Litsinger and Moody, 1976). Although there are exceptions to these general relationships, Fig. 2 is useful to show how the crop itself and the temporal and spatial arrangement of crops can influence pest severity. It is important to keep in mind these complex and simultaneously occurring relationships, since change in any component of the system is likely to influence others.

A. WEEDMANAGEMENT

Interpretation of studies of weeds in traditional intercropping systems is complicated by the multiple uses of weeds by farmers: weeds are not always considered pests (Bye, 1981; Chacon and Gliessman, 1982; Kapoor and Ramakrishnan, 1975; Mishra, 1%9; Weil, 1982). In fact, weed pressure may be the most serious factor limiting food production in developing countries (Holm, 1971; Muzik, 1970); control of weeds may present the highest labor demand of the entire year and may even limit the area planted

18

-

CHARLES A. FRANCIS High P a t Potential

Low Pest Pdential

- CROP ITSELF

Large Pest Complex N d Competitiw with W W S Susceptible Variely

Crop Species

Tolerant Variety

Small Pest Complex Highly Competitive with Weeds

Resistant Pure Line

Annual

Perennial

Long-Maturins

Short-Maturing

Resistant Multigenic

--CROP ARRANGEMENT IN TIME

Monoculture

Crop Species Rotation

Continuous Planting

Discontinuous Planting

Asynchronous Planting

Synchronous Planting

Season favorable to Pest

Season UnfavorWe to Pest

CROP ARRANGEMENT IN SPACE

Sole Cropping

Low Planting Density Large Field

large Host Crop Area

Host Fields Aggregated

Rowor Strip Intercrwping

-

Mixed Intercrooping High Planting Density Small Field Small Host Crop Area

Host flelds Scattered

RG.2. Kinds of crops and their arrangement in time and space evaluated as to the potential development of pest problems. Some effects are seen to be high in pest potential, some intermediate, and some low. (From Litsinger and Moody, 1976.)

(Moody, 1977). Many factors influence weed incidence in the field, and this provides a wide range of options for management. These factors include crop species and varieties, crop densities of sole species or intercrop components, spatial organization of crops, fertility levels, cropping and weed history of the field, and integration of animals in the system. Perhaps this complexity has discouraged some researchers from working on weed control for intercrop systems; it certainly has forced attention to narrower questions that can be answered through a reductionist research approach. Although there is no other logical way to begin to study weed management, the reductionist approach may ignore important interactions that are critical in the field situation. These interactions form the basis for use of biological efficiency in controlling weeds in complex systems. There is an increasing concern about complete reliance on chemical

MULTIPLE-CROPPING SYSTEMS

19

herbicides for weed control, both in temperate and tropical countries (Akobundu, 1980; Walker and Buchanan, 1982). Some of the problems include lack of flexibility in choosing cropping options due to herbicide residues (Bender, 1987), herbicide resistance in weed species (Lebaron and Gressel, 1982), personal safety and effects on the environment (Pimentel et al., 1980), and lack of purchasing power by some farmers to gain access to this technology (Akobundu, 1980). Because many of the crops planted in developing regions are intercropped, there are far fewer options in use of chemicals (Moody and Shetty, 1981). Given the alternatives of cultivation, intensive densities to compete with weeds, allelopathy in some combinations, and other biological methods of control, these need to be emphasized in a discussion of biological efficiency of intercrops (Plucknett et al., 1977). More practical experiments today are including intercrop combinations and testing both the yield and economic consequences of alternative weed control strategies (for example, Zaffaroni et al., 1982). There are large differences both among species and among varieties within species in competitive ability with weeds (Litsinger and Moody, 1976). These differences are due to variations in plant growth habit, time of planting relative to rainfall and temperature cycles in the field, and combinations of species in an intercrop pattern. Association of different weeds with different crops is a common occurrence (Plucknett et al., 1977; Muenscher, 1980). In the temperate zone, rotations of summer crops such as maize, sorghum, or soybeans with winter cereals such as wheat or barley or with a perennial such as alfalfa will help to break the reproductive cycles of weeds. Use of different herbicides in these dissimilar crops also promotes better long-term weed control compared to continuous monoculture of one summer crop such as maize. Francis et al. (1986) stressed both the linear and cyclical nature of biological processes in the field, which can be influenced by management, including choice of crops and rotations. There are specific references to crops that have different competitive ability with weeds. In maize intercrop systems, mung bean was more competitive than peanut with weeds due to its rapid early growth, and there were differences between two mung bean cultivars in ability to suppress weeds (Bantilan er al., 1974). Other differences have been observed among crop cultivars in their ability to compete with weeds, for example, in rice (Kawano et al., 1974), squash (Stilwell and Sweet, 19741, potatoes (Yip et al., 1974), maize (Moody and Shetty, 1981), and soybeans (McWhorter and Hartwig, 1972). Study of the particular traits of those varieties or selections that show competitive ability versus weeds may reveal what traits are useful in promoting this biological type of control. One of the most easily applied management methods to reduce weed

20

CHARLES A. FRANCIS

problems is increasing density of sole crops or intercrop components. Shading of the soil and competition for water and nutrients will certainly suppress weed germination and growth (Altieri and Liebman, 1986; Staniforth and Weber, 1956). Highest crop yields and greatest weed suppression often are found with the highest densities of components in an intercrop trial, for example, in pigeonpea-sorghum combinations in India (Shetty and Rao, 1981). Use of cover crops can also help to control weeds by competing for growth factors (Altieri and Liebman, 1986). The live mulch can produce a low-growing, high-density cover that suppresses weeds between rows of taller, desirable crop species such as maize or sorghum. Legumes are often used for cover because they are less competitive with cereals and have the capacity to produce nitrogen. Melon and sweet potato were shown to replace hand weedings in yam or yam-maize-cassava systems in Nigeria (Akobundu, 1980). Control of weeds in perennial crops or perennial-annual mixtures may be a longer and more costly activity (Wycherley, 1970). Yet there is less cultivation to bring up weed seeds, greater suppression of weeds by shading, and thus more stability in a well-designed perennial system once the pattern is established (Litsinger and Moody, 1976; Rao, 1970). Use of a perennial ground cover under a perennial tree crop can provide excellent weed control over time, as shown by the kudzu-oil palm intercrop, which is practiced on the coastal plain in Ecuador. The kudzu also provides nitrogen for the associated tree crop. Another common intercrop in the medium elevations of the Andean Zone is banana-coffee, in which case judicious hand cultivation and selection toward low-growing, noncompetitive weed species provides relatively inexpensive weed control based on knowledge of the biology and competitive nature of the tree crop species. Allelopathy is an active weed control mechanism in some monocrop systems with crop species as unrelated as oats (Fay and Duke, 1977), squash (Chacon and Gliessman, 1982), and cucumbers (Putnam and Duke, 1974; Lockerman and Putnam, 1979). Since intercropping involves planting of two or more dissimilar species, it is important that allelopathy not provide interference between or among crop components. This may limit the options for designing new patterns but will provide a useful and economic mechanism for weed control. There needs to be selectivity of the allelopathic effect toward the unwanted weed species without affecting the desired crops in the mixture (Altieri and Liebman, 1986). Using extracts of squash leaf applied to different crop species under controlled conditions, Gliessman (1983) demonstrated this differential selectivity. He had observed earlier that squash was consciously planted into maize-bean intercrops in the lowlands of Central America, primarily to control weeds in the system. Lack of allelopathy or interference provides an opportunity

MULTIPLE-CROPPING SYSTEMS

21

for intercropping and control of weeds in an economical way, for example, the use of soybeans as a short-term cash crop in new plantings of eucalypts in Brazil (Couto et al., 1982). Each of these interactions adds complexity to the pattern of crops and weeds and makes it more difficult to understand, yet knowledge of the interactions can lead to economical and sustainable control methods. Altieri (1983) presented an intriguing look at the role of weeds in the total ecosystem, citing the potential consequences of complete elimination of current weeds from the farming environment (adapted from Tripathi, 1977): 1 . Herbicide-susceptible weeds are replaced with more resistant selections. 2. Decrease in overall dry matter production per unit area results from eliminating weeds. 3. Drastic reduction in total genetic resources in ecosystem occurs. 4. Insects that have attacked weeds now attack crop plants. 5. Reduction in beneficial insects that use weeds as alternate food, shelter, or breeding sites results. 6. Soil erosion increases due to lack of weeds in field after harvest. 7. Nutrients previously mined, taken up, and stored by weeds are lost. Looking at weeds as “ecological components” of the total system (Altieri, 1983) may lead to new perspectives on management of weeds as compared to their total control. New methods for simulation analysis of crop-weed competition have been proposed by Spitters (1984).

B. INSECT MANAGEMENT The agroecology approach to understanding insect population dynamics and pest management is proposed by Altieri (1983) as an alternative to current practices directed at control. This includes considering a larger biogeographic region rather than a single field, studying natural ecosystems to find sustainable models that can be emulated in cropping systems, and looking at the interactions among crops, weeds, insect pests, and their natural enemies in building biological control systems. Multiplecropping systems provide one option in the array of possible methods that help to implement this approach. In a unique review of literature on 198 insect species that attack crop plants, Risch et al. (1983) found that 53% showed lower abundance in multiple-species mixtures than in sole crops, 18% were more abundant in mixtures, 9% showed no difference, and 20% were variable in their response. Altieri and Liebman (1986) cited a number of multiple-cropping systems in which insects were less prevalent than in sole crops.

22

CHARLES A. FRANCIS

Several specific examples of cropping system effects on insect incidence and damage illustrate the concepts above. A maize-peanut intercrop had a reduced incidence of maize borer compared to sole-cropped maize in the Philippines (Raros, 1973). In Nigeria, cowpeas planted into a sorghummillet intercrop about one month after the cereals are planted have less insect damage on the grain legume (Baker and Norman, 1975), extending the range of cowpea cultivation into a region where it would otherwise not be planted. Monoculture cucumber had much higher levels of striped cucumber beetle than mixtures of cucumber with two other crop species (Bach, 1980). Altieri and Liebman (1986) presented evidence that cabbage aphids and flea beetles were less prevalent on cauliflower planted with vetch or with weeds compared to sole-cropped cauliflower. Several possible mechanisms were presented by Hasse and Litsinger (1981) that could explain why insects are less prevalent in multiple crop systems. They are grouped into mechanisms that interfere with insects finding their hosts and mechanisms that influence reproduction of the insect population and its survival. Their data are summarized in Table 111. Insects may have more difficulty finding host plants in an intercrop due to presence of other nonhost plants, to camouflage of the preferred host, to changes in the texture or color of the total background, to masking of a chemical attractant, or to presence of a repellent from a nonhost plant. Even if insects successfully find the host, there may be interference with reproduction and survival. Mechanical barriers may be present in the form of nonhost crop plants, insects may leave the field more quickly if it is not a pure crop stand, or there may be differences in either the microclimate or the natural enemy population in the intercrop compared to a sole-crop environment. Tahvanainen and Root (1972) described the complex interaction of biological, physical, and climatic conditions of the intercrop system to provide an “associational resistance” to insects, compared to sole crops of component species. Root (1973) suggested that this resistance could function as a result of two major characteristics of the intercrop environment: he proposed a “natural enemy” hypothesis and a “resource concentration” hypothesis. The first would be explained by a higher population of natural enemies of insect pests due to the diversity of the intercrop pattern in the field. Predators tend to have broad habitat adaptation and could adapt to and persist in the intercrop environment better than in a sole crop (Altieri and Liebman, 1986). Resource concentration in the form of a single type of host plant (uniform food source) would favor buildup of a pest species more than would occur in a diverse intercrop combination. Both visual and chemical stimuli from an intercrop would be less than from a sole crop, and an individual insect might have greater difficulty locating the desired host in this situation (Altieri and Liebman, 1986).

MULTIPLE-CROPPING SYSTEMS

23

Table Ill Possible Effects of Intercropping on Insect Pest Populations"

Factor

Explanation

Example

Interference with host-seeking behavior Camouflage

A host plant may be

protected from insect pests by the physical presence of other overlapping plants Certain pests prefer a crop background of a particular color and/or texture

Crop background

Masking or dilution of attractant stimuli

Repellent chemical stimuli

Presence of nonhost plants can mask or dilute the attractant stimuli of host plants leading to a breakdown of orientation, feeding, and reproduction processes Aromatic odors of certain plants can disrupt host finding behavior

Camouflage of bean seedlings by standing rice stubble for beanfly Aphids, flea beetle, and Pieris rapae are more attracted to Cole crops with a background of bare soil than to ones with a weedy background Phyllotrera cruc$erue in collards

Grass borders repel leafhoppers in beans, populations of P1u:ella xylosrella are repelled from cabbagehomato intercrops).

Interference with population development and survival Mechanical barriers

Lack of arrestant stimuli Microclimatic influences

Biotic influences

All companion crops may block the dispersal of herbivores across the polyculture; restricted dispersal may also result from mixing resistant and susceptible cultivars of one crop by settling on nonhost components The presence of different host and nonhost plants in a field may affect colonization of herbivores; if a herbivore descends on a nonhost, it may leave the plot more quickly than if it descends on a host plant In an intercropping system favorable aspects of microclimate conditions are highly fractioned, therefore insects may experience difficulty in locating and remaining in suitable microhabitats; shade derived from denser canopies may affect feeding of certain insects and/or increase relative humidity, which may favor entomophagous fungi Crop mixtures may enhance natural enemy complexes (See natural enemy hypothesis in text)

"Data from Hasse and Litsinger, 1981.

24

CHARLES

A. FRANCIS

These authors presented a series of examples to substantiate the two hypotheses in sole crop-intercrop comparisons in the field. Willey et al. (1983) presented additional examples from studies of intercrops in India. Although insect species were similar in plant crop and ratoon crop of sorghum, the severity of damage was greater on the ratoon crop in Georgia (Duncan and Gardner, 1984). More information is needed on these insect relationships to be able to generalize about systems and to predict the potential success of new combinations. Integrated pest management (IPM) is a logical approach to be applied in multiple-cropping systems. The need for basic information on the biology of insects and their hosts and natural enemies is obvious. Careful study of these interactions can lead to new management options that involve minimum cost and maximum use of cultural approaches to control. The concepts were presented by Litsinger and Moody (1976) and by Altieri and Liebman (1986). Since multiple-crop systems often extend the cropping season, there is a need to know what happens to pest populations during the entire year and how they are affected by having a crop mixture available for a longer period. Are there changes in weed species and densities in the more complex systems? The single principle that emerges is the need to consider a range in pest management strategies and specific control measures. These include cultural control, rotations, resistant varieties, biological control agents, and judicious applications of pesticides. The concept extends beyond insects to plant pathogens and weeds as well, and complex interactions in the total system need to be considered.

C. PLANTPATHOGEN MANAGEMENT Less is known about disease dynamics and plant pathogens in multiplecrop systems, compared to weeds and insects. The species diversity of natural ecosystems and thus the dispersion of individual host species apparently restricts the spread of plant pathogens (Browning, 1975). In an intercrop combination, there is a mixture of susceptible and resistant (nonhost) plants, and thus greater distance from one host plant to another (Altieri and Liebman, 1986). The more the intercrop system resembles the diversity of the natural (“resistant” or “tolerant”) ecosystem, the more success there will be in avoiding destructive levels of plant diseases (Larios and Moreno, 1977). In contrast, there may be multiple-species combinations that change the microclimate, e.g., cause higher humidity, and thus favor greater disease incidence. However, it is difficult to generalize. The cassava-maize intercrop has less incidence of cassava scab than sole-cropped cassava, but doubling over the maize to allow light to the lower crop causes an increase in the disease (Larios and Moreno,

MULTIPLE-CROPPING SYSTEMS

25

1977). To illustrate the complexity of these relations, angular leaf spot on dry beans was highest in a bean-maize intercrop and lowest in beansweet potato and bean-cassava intercrops. Altieri and Liebman (1986) presented several examples of how cropping system influences nematode populations and severity of problems. More research is needed on potential intercrop combinations before they are widely promoted for farmer acceptance.

V. BIOLOGICAL AND ECONOMIC STABILITY OF CROPPING SYSTEMS Sustaining yield and income from the total farming system may be a more important objective for farmers with limited resources than maximizing either yield or income (Francis, 1985). In addition to harvested yield and immediate income from crop sales, the family objectives include maintaining food supply and income through the year, minimizing risk of failure in every season, keeping cash costs at a minimum, and meeting other social obligations in the community. These are factors not often considered by the crop scientist involved in developing new technology. From these concerns of farmers comes the notion of yield and income stability, a new dimension or yardstick by which to measure success in a plant breeding or agronomy research and extension program. The criterion of yield stability has been used by plant breeders to evaluate small grains, maize, and other crops during the steps of genetic improvement. Regression methods of Finlay and Wilkinson (1963) and Eberhart and Russell (1966) are those most frequently used for analysis. The criteria for favorable o r stable varieties o r hybrids usually include a mean yield above the mean of all genotypes, a response to improving environments (b) that is not significantly different from unity ( b = 1 .O), and minimal deviations of yield from the regression line. This concept has been extended to evalution of bean variety yields in contrasting cropping systems (sole-cropped versus intercropped with maize) and consecutive seasons (Francis et al., 1978b,c) and t o analysis of yield components of sorghum hybrids (Heinrich et af., 1985). Although this analytical approach to stability has not been used for total system yields, income, o r risk, it would appear to be a useful methodology to quantify the results of whole farm systems. More important than the specific method of analysis is choosing an appropriate criterion by which to measure stability. As suggested above, this may be more complex than a single number such as grain yield or net income, and some of the criteria listed may be dificult to quantify.

26

CHARLES A. FRANCIS

Measures of biological stability have been reviewed by Trenbath (1974) and by Willey (1979a). New analytical tools were described by Mead (1986). There have been some reports of specific comparisons of stability in the intercrop systems as compared to sole cropping (Francis and Sanders, 1978; Rao and Willey, 1980), and these are discussed in the following section. These two articles also present an analysis of economic stability, although it is only a comparison of the three contrasting crop patterns and not extended to a whole-farm system. Biological diversity is important in yield stability, according to the majority of authors, although there is limited experimental evidence of this relationship (Willey, 1979a) and some conflicting reports. Perhaps the most interesting biological and economic aspect of intercropping is the potential for compensation among components of the system. This could be called the biological or economic “buffering” in the system (Francis, 1986) that leads to greater stability of total yield or income of intercrops. Willey (1979a) concludes that much more research is needed to assess the stability of cropping systems, especially to assure farmers that a new system will not be less stable than a traditional form of intercropping. A. VARIATIONS IN BIOLOGICAL OUTPUT

Conventional wisdom about intercropping in traditional agriculture is that relatively low-yield systems are more stable over a range of conditions and seasons (Willey, 1979a). This is attributed to greater diversity within each intercropped field and the ability of a mixture of crop species to react differently to a given set of climatic constraints in a given season, and as a total system to produce a more consistent yield. There is sustained biological production in natural ecosystems, but generally there is no harvest and most of the dry matter produced is recycled within the system. Multispecies cropping systems approximate the natural plant mixture to some degree, and there is a range of diversity within both natural ecosystems and cropping systems (see Fig. 3). Genetically more diverse systems such as the 13-crop mixture in Nigeria (Fig. 4) traditionally have been stable but lower-yielding due to the low level of added inputs and lack of available improved technology for this type of system. In contrast, single-cross maize hybrids have extremely high grain yield potential, but this can be realized and sustained only through systematic and frequent additions of fertilizer, irrigation in some regions, and pesticides to control unwanted weed and insect species. The long-term sustainability of the latter type of system and its applicability for many subsistance farmers has been challenged (Francis et al., 1986). There is little quantitative evidence to

MULTIPLE-CROPPING SYSTEMS Natural Ecosystems Tropical rain forests

Maximum Genetic Diversity

27

Cropping Systems Shifting cultivation in humid forests 12-crop mixtures in Africa

Temperate zone forests

Maize-cassava-bean Maize-bean

Natural grasslands

Bean cultivar mixture Boreal forests Multiline cereals Wheat varieties Spartina marshes

Geothermal pools

V

Double -cross maize hybrids Single-cross maize hybrids

Minimum Genetic Diversity FIG. 3. Spectrum of genetic diversity in natural ecosystems and in cropping systems. (From Smith and Francis, 1986.)

support this contention, but today there is a growing concern among development experts about the need to search for alternatives that depend more heavily on internal, renewable resources available on the farm (Francis and King, 1988). Variance in yields is one possible measure of stability. Francis and Sanders (1978) showed greater variation in sole-crop maize and sole-crop bean yields compared to intercrop maize-bean in an analysis of 20 experiments in Colombia. In a similar analysis from more than 90 trials in India, Rao and Willey (1980) found greater variation in sole-crop sorghum and sole-crop pigeonpea than in the sorghum-pigeonpea intercrop. However, Mead (1986) presented an example to show that these measures ignore some information about the structure of the data set. Pairs of observations of sole-crop and intercrop yield from a series of locations or environments need to be analyzed with that pairing in mind in order not to lose information and be misled by the results. Mead suggests some variant of the Finlay and Wilkinson (1963) method as a better alternative, depending on a rational choice of the environmental index. This index generally is the mean of all genotypes in a breeding trial, and in the case of cropping systems would logically be the mean of all systems in a given

28

CHARLES A. FRANCIS

Distance, rn

Ca - Cassava

Cu - Melon

- Peenut L - Laganaria

M - Maize Pk

D3 - 0. bulbMera

Pp . Pigeonpea

A

04 - D. cayenensis

P

- Pumpkin - Rice

D1 - Dioscorea rotundata D2 - 0. alaia

V . Voandzeie

FIG.4. Spatial distribution of 13 crop species on and between raised mounds in Nigeria. (From Okigbo and Greenland, 1976.)

site, although there are still problems with this approach. An alternative for analyzing risk is presented in the next section. Using the summary by Willey (1979a), the review of Trenbath (1974), and the data above, a list of stability comparisons of intercrops with sole crops can be constructed (if only a single crop is listed, the intercrop is two or more dissimilar components of the same species).

MULTIPLE-CROPPING SYSTEMS

29

1. There is a substantial improvement of stability in intercrop: barleyoat (Daniel (1955); maize-bean (Francis and Sanders, 1978); and sorghumpigeonpea (Rao and Willey, 1980). 2. There is a marginal improvement of stability in intercrop: soybean (Byth and Weber, 1968; Schutz and Brim, 1971); oats (Frey and Maldonado, 1967; Qualset and Granger, 1970); sorghum (Ross, 1965); and maize (Funk and Anderson, 1964). 3. There is no improvement of stability in intercrop: barley (Clay and Allard, 1969) and oats-rye (Pfahler, 1965).

Although there is little confounding of crops across groups, it would be ill-advised to conclude from these few reports that a barley-oat intercrop will always show increased stability and an oat-rye intercrop will not, for example. These are only indications of what is happening biologically in the mixtures, and no doubt the most dissimilar components in the above intercrops had the best chance of demonstrating increased stability. Differences among components could be in plant height, maturity, timing of resource use, root structure, or carbon metabolism; the greater the differences, the more likely there would be overyielding (Andrews, 1972), and perhaps greater stability. B. INCOMESTABILITY Variation in gross or net income from crop production systems is a function of yields, prices, and costs, and the information presented above provides a general guideline on the stability of income as well as biological productivity. What distinguishes intercropping systems from monoculture is the introduction of several new factors into the income equation: relative prices of two or more commodities; efficiencies of production, which result in lower production costs; and compensation between the two or more crop components. The last topic is described in another section. Some of the stability related to differences in component crop prices can be achieved by diversification on the farm and does not necessarily require intercropping. The discussion focuses on how biological efficiencies affect income stability. Lower variation among locations and seasons in intercropping systems compared to sole cropping can lead to apparent improvements in both yield and income stability. A maize-bean study in Colombia (Francis and Sanders, 1978) showed a probability of at least breaking even of 0.65 with a maize monocrop, 0.80 with a bean monocrop, and 0.92 with the maizebean intercrop. Rao and Willey (1980) reported the probability of at least breaking even as 0.91 with a pigeonpea monocrop, 0.95 with a sorghum monocrop, and 1 .OO with the sorghum-pigeonpea intercrop. The latter data set was reported in a different form by Willey et al. (1983), in which

30

CHARLES A. FRANCIS

the percentage risk of failure (probability of failure x 100) was plotted against the “disaster level income” in rupees per hectare for three cropping patterns of sorghum and pigeonpea (Fig. 5). The advantage of intercropping is clear from this presentation, which is based on 94 trialS in India. Relative prices received for component crops can also influence the economic success of an intercropping pattern. This same relationship is important in diversifcation of cropping and would cause different decisions in allocation of land to different crops if prices could be anticipated before planting. Francis and Sanders (1978) reported net returns from the intercrop and the two sole crop patterns at a range of bean:maize price ratios from I :1 to 8: 1, a range which included all the known actual price ratios in Central and South America at that time. Two additional variables were introduced into the analyses: differences in bean and maize yields and differences in production costs. The decision to plant sole-crop beans versus intercropped maize and beans depended on both bean yields and costs of materials and labor in the production of the sole-crop climbing beans, as well as on the price ratio. There was no unique decision at all levels of input and yield. At the prevailing price ratio, yield level on the farm, and production cost of subsistence farmers there was a clear advantage to intercropping, an indication of why this system persists in the middle elevations of Colombia, where the experiments were conducted. Mead (1986) described a method of evaluating stability based on risk analysis. This assumes that (1) risk is the best way to measure stability; (2) the bivariate distribution of yields can be described by a simple model; 80

1

70 60,

f .--

2

s

Sole pigeonpea

50. 40.

30 ’

10 i

250

1000

1750

2500

3250

Disaster level of income (Rslha)

FIG.5. Yield stability of sorghum and pigeonpea in sole cropping and intercropping: the probability of crop failure. (From Willey et al., 1983; data from Rao and Willey, 1980.)

MULTIPLE-CROPPING SYSTEMS

31

(3) yields for two species can be quantified on a single scale such as economic value; and (4) variation among years and among locations is similar, so that data sets can be combined for analysis. With data from sole-crop sorghum and a sorghum-pigeonpea intercrop converted to net income from the systems using a price ratio of I .8:1 (pigeonpea:sorghum), a comparison of incomes between the two systems can be plotted; this is shown in Fig. 6a. Yields (incomes) falling on the line b = 1.0 would be expected from a completely additive (nonoveryielding) situation in the intercrop, and would be the same as planting two adjacent fields in the two crops to diversify income. It is apparent in Fig. 6a that intercropping often produces higher income. If the probability of getting an intercrop income greater a lOOr

0

50 Sole-crop yield

100

b

Monocrop risk

80

FIG. 6. Bivariate plot of intercrop yield (return) against sole crop sorghum for 51 experiments in India: (a) intercrop and sole crop yields (returns); (b) relative risk graph for the fitted model. (From Mead, 1986.)

32

CHARLES A. FRANCIS

than some fixed level is plotted against the probability of a sole-crop sorghum income greater than that same fixed level, the results from these trials appear as in Fig. 6b (Mead, 1986). As shown by the figure lines, a monocrop risk of 0.5 of not reaching a fixed level of income is reduced to about 0.26 with the intercrop. Conversely, an intercrop risk of 0.5 would correspond to a risk of about 0.73 in sole-cropped sorghum. The graph clearly indicates why farmers plant a diverse mixture of species rather than sole crops, and one of the reasons is the compensation or buffering that can occur in the intercrop.

C. BUFFERING AND COMPENSATION I N SYSTEMS Compensation among yield components in agronomic crops has long been recognized and described (Grafius, 1957). Since yield is the product of heads or pods or ears per unit area, number of seeds per head or pod or ear, and weight of individual seeds or kernels, reduction of any one of these may result in an increase in another. What other component changes depends on temporal development of the plant (Castleberry, 1973: Grafus, 1957; Heinrich et al., 1983). The relationships among these yield components and their capacity to change results in “buffering” or a compensation among them and a certain stability of biological production (Heinrich et al., 1983). Compensation could be viewed at a number of different levels. Removal of some seeds from a sorghum head causes the remaining seeds on that head to increase in weight; removal of the main head causes greater tiller devolopment, depending on the stage of removal. Removal of the top ear in maize causes development of one or more lower ears. This could be called intraplant compensation. At the plant level, increasing density of seeding reduces dry matter produced per plant; some spatial organizational effects also are well known, such as row spacing changes or geometric distribution of individual plants. This could be called interplant compensation within a species. In an intercrop pattern in which two or more species are involved, higher yields of one component due to favoring that component with higher density, earlier planting, meeting specific fertility needs, or spatial organization to promote its growth often will be accompanied by lower yields of other crops in the mixture (Donald, 1963; Davis and Garcia, 1983, 1987; Davis ef al., 1987; Gliessman, 1986). That the reduction of yield in one component is not proportional to the increase in yield of another is due to complementarity in resource use, as described in previous sections. Willey and Reddy (198 1) were able to partition the aboveground and belowground interactions in a way that demonstrated the importance of intimate inter-

MULTIPLE-CROPPING SYSTEMS

33

actions in the root zone between pearl millet and peanut. Thus it is a combination of complementarity in resource use and of the ability of one component to use resources not needed by another component that lead to compensation in systems and overyielding in the right combinations of crops. Economic buffering in a production system is more difficult to document. Part of the advantage of having diversity in the system is being able to provide a range of products to the market. Before planting, the strategy could be to produce a range of crops to (1) take advantage of those with highest market value and (2) spread the risk of drastic change in price of one or more commodities. It is assumed to be less probable that prices of several crops will go down between planting and harvest than might occur with a single crop, thus the diverse mix of enterprises would provide a more stable income than any single crop. This argument relates to diversity, with or without intercropping. The other dimension of economic buffering is related to the nature of intercropping or mixtures of species in each field. The first factor is overyielding; if higher total yields result from intercropping, there will be an economic advantage if the most valuable species are well represented in the harvest. The second factor relates to the biological buffering described above. If two crops are planted throughout a field, rather than in half of a field, and if a devastating insect, disease, or drought situation selectively eliminates or strongly suppresses one component, the remaining crop is likely to take advantage of the available resources and produce a relatively high yield over the entire area. This would not occur in a diverse system in which the farm was divided into a series of sole-crop enterprises. From this biological and economic review, it could be concluded that appropriate intercropping combinations are more financially stable than sole crops. Introducing other measures of stability could expand this argument, namely, diversity and stability of food supply, distribution of food and income, and risk associated with alternative cropping strategies.

D. STATISTICAL ANALYSIS OF MULTIPLE-SPECIES SYSTEMS Statistical comparisons among intercrop alternatives and between intercrop and monocrop systems have been cited throughout the review. The simplest and most frequent type of statistical evaluation is the analysis of variance. This was discussed in detail by Mead (1986) and will appear in a forthcoming book by W. T. Federer (personal communication). The limitations of analysis of variance methods relate to assumptions about uniformity of variance among treatments and normal distribution of observations on those treatments. The researcher’s judgement is critical in

34

CHARLES A. FRANCIS

deciding whether these assumptions are met in a given trial (Mead, 1986). Mead (1986) presented valuable guidelines on design of experiments and treatments and how to present results on main effects and interactions from these trials. Researchers find it necessary to evaluate complete systems on some quantitative basis, and this is difficult when more than a single crop is involved and there are farmer criteria other than biological yield and net income by which success is measured. One simple index used in descriptive reports is the cropping index, or cropping intensity index, which is a measure of crops per year on a given field. Double-cropping or relaycropping wheat and soybeans would give an index of 2.0, while monoculture maize would give an index of 1.0. Averages may be reported for a specific region, for example, “Valley X has a cropping intensity of 1.8,” indicating that, on the average, 1.8 crops per year are harvested from each field in the area. This index tells nothing about the productivity of those crops nor about efficiency of resource use. A frequently used index is the land equivalent ratio (LER), which in fact evolved from the relative yield total (RYT) used by de Wit and van den Burgh (1965). This is the area needed under monoculture to produce the same yield as that same area would produce with intercropping; the concept has been reviewed by Willey (1979a,b). Distribution of the LER has been studied by Oyejola and Mead (1981) and reviewed by Mead (1986), with the conclusion that standard analyses can be conducted with this index. The most difficult step is choosing a valid denominator for the index, since this choice can drastically affect the results of the analysis and the conclusions for a trial. Any index or reduction of data into simpler form results in some loss of information. Mead and Willey (1980) and Mead (1986) describe an “effective LER,” which can guide experimental changes in proportions of the component crops and lead to eventual choices of the best combinations and desired products from the system. Another modification of this index is the “area-time equivalency ratio” (Harwood, 1979), which takes into account the time the component crops are in the field, an evaluation of temporal as well as spatial efficiency of intercropping. Other indexes that have been used to describe competition and resource use were reviewed by Willey (1979a). One of these is the relative crowding coefficient, which describes whether each species in a mixture produces more or less yield than expected in a replacement series (de Wit, 1960; Hall, 1974a,b). Another is the aggmsivity index or the measure of relative increase in one component compared to increase in another component (McGilchrist, 1965); if the index is equal to zero, both species are equally competitive. The competition index (Donald, 1%3) requires the calculation of equivalence factors for each species; this factor is the number of plants

MULTIPLE-CROPPING SYSTEMS

35

of one species that are equally competitive as one plant of another species, and the product of the factors gives the index. Willey (1979a) described a number of trials in which this index has been applied with some success. In spite of the number of index approaches that have been proposed, the LER remains the index most frequently used by active researchers in the area of multiple cropping. It does give a measure of biological efficiency and can be used for standard statistical analysis, subject to the limitations outlined by Mead (1986).

VI. FUTURE APPLICATIONS FOR MULTIPLE-CROPPING SYSTEMS Continued observations of natural ecosystems, traditional, and improved farming systems in the field and experiments with closely planted species will build the knowledge base of how plants influence each other. It is this information base that will provide the foundation for improvements in multiple-cropping systems and from which technicians and farmers can design new mixtures and ways to organize compatible crops spatially in the field. Understanding of critical interactions between species and among crops, pests, and environmental conditions is still in its early stages, and far less is known about intercropping than about high-technology monoculture. The future importance of multiple cropping is difficult to predict: mechanization, increases in farm size, and greater specialization all work to reduce the diversity and complexity of agroecosystems. In spite of these trends, there are some areas in which double cropping has become standard practice, such as the winter wheat-soybean pattern in the southeast United States. Short-cycle, photoperiod-insensitiverice varieties have made possible two and three crops per year in South China and the Philippines when fertilizer and other inputs are available and the price justifies intensification of production. Although it is unlikely that intensive cerealgrain legume intercrops will become commercially viable on a broad scale, they will continue to be important for subsistence farmers. Other embodiments of the multispecies concept that are likely to become increasingly important are grass-legume mixtures for pastures, overseeding legumes into cereals during or after the crop cycle in temperate zone, and more intensive rotations including relay overlapping of some species. Multiple-cropping systems observed around the world appear to be a response by farmers to scarcity of production resources and a desire to make the most efficient possible use of what is available (Francis, 1986). The many variants of these systems, based on biological efficiencies

36

CHARLES A. FRANCIS

outlined above, may lead to the lowest possible production costs and the most stable, low-risk strategies to provide food and income for the family. How much research and development occurs in the future with these complex systems depends in large part on how national research decision makers view the importance of the small farm sector of agriculture. Field research is expensive. Simulation analysis of alternative cropping patterns may provide an efficient tool for future research on complex systems (Barker and Francis, 1986; Spitters, 1983; Trenbath, 1986; Whisler et al., 1986). A number of potentials have been discovered by observations of current farmer practices and especially by controlled experiments at research stations. There are several factors that should be considered as research planning proceeds for the next century, when food needs will grow with the burgeoning population: 1. More efficient use of sunlight, water, and nutrients can result from an appropriate intercropping or double-cropping pattern. 2. Increased biological diversity can lead to more production stability and reduced risk of failure in these complex systems. 3. Improved economic stability can result from diversification of crops, especially intercropping, which leads to buffering and compensation in each field. 4. Finite supplies of fossil fuels force the consideration of alternative production systems based on biological efficiencies such as fixation of nitrogen, reduced pesticide use due to lower pest incidence in diverse systems, and nutrient cycling in rotations and mixtures of diverse species.

Further research and development of improved multiple-cropping systems could be both timely and appropriate as a viable strategy to help solve the world food challenge through improved biological efficiency.

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Altieri. M. A., and Letourneau, D. K. 1982. Crop Prof. 1, 405430. Altieri, M. A., and Letourneau, D. L. 1984. C R C Crit. Rev. Plunr Sci. 2, 131-169. Altieri, M. A., and Liebman, M. 1986. In “Multiple Cropping Systems’’ (C. A. Francis, ed.), pp. 183-218. Macmillan, New York. Andrews, D. J. 1972. Exp. Agric. 8, 139-150. Andrews, D. J., and Kassam, A. H. 1976. I n “Multiple Cropping” (R. 1. Papendick, P. A. Sanchez, and G. B. Triplett, eds.), pp. 1-10. Amer. SOC.Agron. Spec. Publ. 27, Madison, Wisconsin. Bach. C. E. 1980. Ecology 61, 1515-1530. Baker, C. M.. and Blarney, F. P. C. 1985. Field Crops Res. 12, 233-240. Baker. E. F. I.. and Norman, D. W. 1975. Proc. Crop. Sysr. Workshop I . R . R . I . , Los Bano.s, Philipp. pp. 334-361. Baker, H . G . 1970. “Plants and Civilization.” Wadsworth. Belmont. California. Bantilan, R. T.. Palada. M. C., and Harwood, R. R. 1974. Philipp. Weed Sci. Bull. I, 1436. Barber. S. A. 1962. Soil Sci. 93, 3 9 4 9 . Barker. T. C., and Francis, C. A. 1986. I n ”Multiple Cropping Systems” (C. A. Francis, ed.), pp. 161-182. Macmillan, New York. Barley. K. P. 1970. A h . Agron. 22, 159-201. Baylor. J . E. 1974. In “Forage Fertilization” (D. A. Mays, ed.), pp. 171-188. Amer. SOC. Agron.. Madison, Wisconsin. Beets, W. C. 1982. “Multiple Cropping and Tropical Farming Systems.” Westview, Boulder, Colorado. Bender. J. 1987. Crop Prod. News Univ. Nehr.. Lincoln. Coop. E x / . Serv. 7, Oct. 23. Bergersen, F. J. 1971. Annrr. Rev. Plonr Phvsiol. 22, 121-140. Bhat, K. K . S.. and Nye. P. H. 1973. Plont Soil 38, 161-175. Brewster. J. L., and Tinker. P. B. 1970. Soil Sci. Soc. A m . Proc. 34, 421426. Brown, H. B. 1935. Effect of soybeans on corn yields. Buton Rorrge, L o . Agric. Exp. Sfu. Bull. 265. Browning. J. A. 1975. Proc. A m . Phvroputhol. SOC. 1, 191-194. Bye. R. A,. Jr. 1981. J. Ethnohiol. 1, 109-123. Byth. D. E. and Weber. C. R. 1968. Crop Sci. 8, 44-47. Castleberry, R. M. 1973. Effects of thinning at different growth stages on morphology and yield of grain sorghum ( S o r g h m hicolor (L.) Moench). Ph.D. thesis, Univ. of Nebraska, Lincoln. Chacon. J. C.. and Gliessman, S. R. 1982. Agro-Ecosyslems 8, 1-11. Chang, J . F., and Shibles. R. M. 1985a. Field Crops Res. 12, 133-143. Chang. J. F.. and Shibles. R. M. 1985b. Field Crops Res. 12, 14.5-152. Chestnutt. D. M. B.. and Lowe. J. 1970. Occus. Symp. 6th. Br. Gross/. Soc. pp. 191-213. Chui. J. A. N., and Shibles, R. 1984. Field Crops Res. 8, 187-198. Clapp. J . G.. Jr. 1974. Agron. J . 66, 463465. Clark. E. A,. and Francis. C. A. 1985a. FirldCrups Rrs. 11, 151-166. Clark. E. A,. and Francis. C. A. 1985b. Field Crops Res. 11, 37-53. Clary. W. P. 1974. J. Ronge Munoge. 27, 387-389. Clay. R. E.. and Allard. R. W. 1969. Crop Sci. 9, 407-412. Cook, C. W. 1966. Development and use of foothill ranges in Utah. P r o w Utuh Agric. Exp. Sto. Bull. 461. Couto. L.. de Barros, N. F.. and Rezende, G. C. 1982. Aust. For. Res. 12, 329-332. Cromartie. W. J. 1981. In “CRC Handbook of Pest Management in Agriculture” (D. Pimentel, ed.). Vol. I. pp. 223-251. CRC Press, Boca Raton. Florida. Crookston. R. K., and Hill, D. S. 1979. Agron. J. 71, 4 1 4 4 .

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Dalal, R. C. 1974. Exp. Agric. 10, 219-224. Daniel, G. H. 1955. J . Natl. Inst. Agric. Bot. 7, 309-317. Davis, J. H. C., and Garcia, S. 1983. Field Crops Res. 6, 59-75. Davis, J . H. C., and Garcis, S. 1987. Field Crops Res. 16, 105-115. Davis, J. H. C., Woolley, J. N., and Moreno, R. A. 1986. In “Multiple Cropping Systems” (C. A. Francis, ed.), pp. 133-160. Macmillan, New York. Davis, J. H. C., Roman, A., and Garcia, S. 1987. Field Crops Res. 16, 117-128. de Wit, C. T. 1960. Versl. Landbouwkd. Onderz. 66, 1-82. de Wit, C. T., and van den Bergh, J. P. 1965. Neth. J. Agric. Sci. 13, 212-221. Donald, C. M. 1946. J. Counc. Sci. Ind. Res. Aust. 19, 32-37. Donald, C. M. 1961. Syrnp. SOC.Exp. Biol., 15th, Mech. Biol. Compet. Proc. pp. 282-313. Donald, C. M. 1963. Adv. Agron. 15, 1-118. Duncan, R. R. 1980. Cereal Res. Commun. 8, 539-544. Duncan, R. R. 1983. Crops Soils 35, 10-1 I . Duncan, R. R.. and Gardner, W. A. 1984. Can. J . Plant Sci. 64, 261-273. Eberhart, S. A.. and Russell, W. A. 1966. Crop Sci. 6, 36-40. Etherington, J . E. 1976. “Environment and Plant Ecology.” Wiley, New York. Fawusi, M. 0 . A. 1985. Field Crops Reg. 11, 345-352. Fawusi, M. 0 . A., Wanki, S. B. C., and Nangju, D. 1982. J . Agric. Sci. 99, 19-23. Fay. P. K . , and Duke, W. B. 1977. Weed Sci. 25, 224-228. Finlay. K . W . . and Wilkinson, G. M. 1963. Aust. J. Agric. Res. 14 742-754. Fisher, N . M. 1976. Symp. Intercrop. Semi-Arid Areas Proc.. Morogoro. Tanzania. Francis. C. A. 1985. Agric. Hum. Values 2, 54-59. Francis, C. A., ed. 1986. “Multiple Cropping Systems.” Macmillan, New York. Francis, C. A. 1988. In “Agroecology and Small Farm Development” (M. A. Altieri, ed.), CRC Press, Boca Raton, Florida (in press). 27, 67-75. Francis, C. A., and King, J. W. 1988. Agric. Sysr. (U.K.) Francis. C. A., and Sanders, J. H. 1978. Field Crops Res. 1, 319-35. Francis, C. A., Flor, C. A.. and Temple, S. R. 1976. In “Multiple Cropping” (R. 1. Papendick, P. A. Sanchez, and G. B. Triplett, eds.), pp. 235-253. Amer. SOC.Agron. Special Publ. 27, Madison. Wisconsin. Francis, C. A., Flor, C. A., and Prager, M. 1978a. Crop Sci. 18, 760-764. Francis. C. A., Prager, M.. and Laing, D. R. 1978b. Crop Sci. 18, 242-246. Francis. C. A., Prager. M.. Laing. D. R., and Flor. C. A. 1978~.Crop Sci. 18, 237-242. Francis, C. A., Harwood, R. R., and Parr, J. F. 1986. Am. J. Alternative Agric. 1, 65-74. Frey. K. J.. and Maldonado, U . 1967. Crop Sci. 7, 532-535. Funk, C. R.. and Anderson. J. C. 1964. Crop Sci. 4, 353-356. Gliessman. S. R. 1983. J . Chem. Ecol. 9, 991-999. Gliessman. S. R. 1986. In “Multiple Cropping Systems” (C. A. Francis, ed.), pp. 82-95. Macmillan. New York. Gomez, A. A., and Gomez, K. A. 1983. “Multiple Cropping in the Humid Tropics of Asia.” International Development Research Centre (1DRC)- 176e. Ottawa. Gomm. F. B., Sneva, F. A.. and Lorenz. R. J. 1976. In “Multiple Cropping” (R. I. Papendick, P. A. Sanchez, and G. B. Triplett, eds.), pp. 103-115. Amer. SOC.Agron. Spec. Publ. 27, Madison, Wisconsin. Goodman, D. 1975. Q . Rev. Biol. 50, 237-266. Grafius, J. E. 1957. Agron. J. 49, 419423. Hall, R. L. 1974a. Aust. J . Agric. Res. 25, 739-747. Hall, R. L. 1974b. Aust. J . Agric. Res. 25, 749-756. Harper, J. L. 1961. Symp. SOC.Exp. Biol., ISth, Mech. Biol. Compet. Proc. pp. 1-39.

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ADVANCES IN AGRONOMY, VOL. 42

SEED COATINGS AND TREATMENTS AND THEIR EFFECTS ON PLANT ESTABLISHMENT James M. Scott Department of Agronomy and Soil Science University of New England Armidale, New South Wales 2351, Australia

I.

11.

111.

IV.

V.

VI.

VIl. VIII.

1x. X.

Introduction A. Why Seeds Are Coated B. Evolution of Seed Coatings C. Definitions The Seed-Coating Process A. Coating Equipment B. Adhesives C. Coating Materials Coatings to Facilitate Planting A. Precision Sowing B. Improved Ballistics lnoculant Coatings A. Rhizobia B. Vesicular-Arbuscular Mycorrhizal Fungi C. Other Organisms Protective Coatings A. Diseases B. Insects. Pests, and Other Fauna C. Protection against Herbicides Nutrient Coatings A. Need for Early Seedling Nutrition B. Macronutrients C. Micronutrients D. Efficacy of Nutrient Seed Coatings E. Injury Caused by Fertilizers Herbicide Coatings Other Coatings A. Hydrophilic Coatings B. Hydrophobic Coatings C. Oxygen Supply Treatment Processes Conclusions References

43 Copyright 0 1989 by Academic Press, inc. All nghfs of reproduction in any form reserved.

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I. INTRODUCTION A. WHY SEEDSARE COATED The successful establishment of crop and pasture species from seed depends on a broad array of factors including the species sown, the inherent vigor of the seeds, the soil type and its fertility, the climatic conditions, the time of year, sowing depth, soil tilth, method of soil cultivation and sowing, and the presence or absence of antagonistic or beneficial organisms such as weeds, insects, diseases, rhizobia, or mycorrhizas. Farmers have an opportunity to control only some of these factors; many factors remain uncontrolled and can, either singly or in combination, cause a delay or reduction in establishment. Commonly, farmers attempt to overcome some of these adverse conditions by applying materials such as herbicides and fertilizers to the whole area of land to be planted. Such broad-acre applications can be expensive and there is a risk of considerable financial loss if establishment is inadequate or fails altogether. An alternative approach is to apply materials either in “bands” adjacent to the seed or on the seeds themselves in seed “coatings” in an effort to increase the effectiveness of the treatments. Seed coating is a mechanism of applying needed materials in such a way that they affect the seed or soil at the seed-soil interface. Thus, seed coating provides an opportunity to package effective quantities of materials such that they can influence the microenvironment of each seed. By not having to treat the remaining bulk of their soil, farmers may be able to save on the inputs required and the associated costs of applying them. Because seed coatings offer such opportunities for cost saving and increasing effectiveness, they have been studied widely for many years and yet, with some exceptions (for example, precision coating of sugarbeet and some vegetable seeds, fungicide and insecticide seed treatment of grain crops, and inoculant coatings on legume seeds), much of the world’s crop and pasture seed is still sown without any coating. In this review, an attempt has been made to draw not only upon the scientific literature, but also the patent literature, which contains much of the current expertise associated with seed coatings. The review places particular emphasis on areas that have not been dealt with in detail before, namely, the seed coating process and nutrient and herbicide seed coatings.

B. EVOLUTION OF SEED COATINGS Seed coatings have evolved from those which protect the seed from fungal and insect attack to a diverse range of coatings, the objectives of

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which include the protection of rhizobia, supply of micro- and macronutrients, protection from birds and rodents, supply of growth regulators, attraction of moisture, supply of oxygen, germination stimulation, germination delay, increase in seed weight or size, and the supply of selective herbicides or herbicide antidotes. In spite of a considerable amount of research, reliable and effective seed coatings are not currently available for many crop and pasture situations. Even in the case of legume inoculant coatings, which have been intensively studied, there is still no universally accepted practice of inoculation, particularly for coatings applied well in advance of planting (as in preinoculation). The treatment of seeds with fungicides and/or insecticides is a relatively common practice compared to other coatings and, provided the materials are not phytotoxic, few problems occur. Problems with seed coatings become much more apparent when relatively large quantities of coating materials are applied to the seeds. Reports of ineffectiveness of coatings or lower seedling establishment due to coatings are relatively common in the literature. Successful results with coatings are also reported, however and it is these success stories that indicate that seed coatings do have the potential to overcome some of the problems of plant establishment. Much of the literature related to seed coatings consists of reports of ad hoc testing of various chemicals, coating materials, additives, etc. applied to seeds in ways that are often ill-defined; these diverse reports indicate that, so far, there has been little concerted effort to view seed coating as a branch of science requiring many basic principles to be understood before substantial progress can be made. This view is supported by Heydecker and Coolbear (1977), who stated in their review of seed treatments that “progress in the technology of pelleting is not facilitated by the fact that manufacturers keep their materials and processes a closely guarded secret.” I n fact, a significant proportion of the literature consists of work validating or evaluating the efficacy of proprietary products that are applied to seeds. Factors that may be crucial to the success or failure of coated seeds (such as the particle size distribution of the coating material, the exact specifications of the adhesive used, or the porosity of the coating) have largely been ignored by researchers reporting on seed coatings. By not specifying precisely how coatings have been prepared, researchers have made it virtually impossible for their work to be validated by others, and hence, there are many reports of workers finding different results from supposedly similar coatings. Because of the limitations mentioned above, the potential of seed coatings has not yet been explored thoroughly. When coating materials and techniques are identified more precisely, producing repeatable coatings will become relatively straightforward and an understanding of how each coating affects seeds under various conditions can be developed.

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Figure I shows how some of the factors that affect the performance of seed coatings can be conveniently grouped into seed, coating, soil, and aerial environments and then further partitioned into physical, chemical, and biological sectors. Much of the research done to date has been conducted in such a way that general principles have been difficult to transfer from one area of seed coating research to another. Knott and Lorenz (1950) reviewed much of the early work on seed coatings, which largely concerned the development of relatively inert coatings that permitted the rounding and enlarging of small seeds (particularly vegetable and sugarbeet seeds) sufficiently to facilitate precision mechanical planting. More recently, reviews

I

AERlAL ENVIRONMENT

AERIAL ENVIRONMENT

BIOLOGICAL FIG. 1. Schematic diagram showing the relationships among factors affecting the performance of coated seeds. Factors that are italicized are those on which some research has been reported.

SEED COATINGS AND TREATMENTS

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have covered seed treatments and coatings with an emphasis on osmoconditioning (Heydecker and Coolbear, 1977),seed treatments, especially those containing fungicides and insecticides (Jeffs, 1986), and pelleting and other pre-sowing seed treatments (Tonkin, 1979, 1984).

C. DEFINITIONS The literature contains many inconsistencies in its terminology regarding seed coating; for example, pelleting has been defined as a process of inoculation followed by lime or clay (Johnson, 1971), a process whereby the number of seeds per pellet is not accurately controlled (Roos and Moore, 1975), a process primarily aimed at creating single seeds to aid precision planting (Johnson, 19751, and a process of creating more or less spherical units for precision sowing, usually incorporating a single seed with the size and shape of the seed no longer readily evident (International Seed Testing Association rules, cited by Tonkin, 1984). In view of such inconsistencies, I would first like to define some of the terms commonly found in the literature before reviewing aspects of the production of coated seeds and of the many types of coated seeds which have been described. 1. Seed coating. A general term for the application of finely ground solids or liquids containing dissolved or suspended solids to form a more or less continuous layer covering the natural seed coat: includes pelleting and many other seed treatments. 2. Seed treatment. A broad term that does not specify the application method but merely indicates that seeds are subjected to a compound (chemical, nutrient, hormone, etc.), process (such as wetting and drying), or to various energy forms (e.g., radiation, heat, magnetism, electricity). This also includes the less commonly used term seed dressing which refers to the application of finely ground solids (usually a fungicide or insecticide) dusted onto the surface of seeds in small quantities to protect seeds from disease and/or pests. 3. Seed pelleting. The application of solid materials to seeds in sufficient quantity to make the pelleted seed substantially larger and/or heavier and approach a spherical or elliptical shape. 4. Seed soaking. A process by which seeds can be led to absorb nutrients, protectants, growth regulators, etc. by immersing them in appropriate solutions for extended periods. 5 . Seed tablet. A composite of seed and solid materials formed by compression in a tablet press such as is used in the pharmaceutical industry.

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6. Seed inoculation. The application of microorganisms (e.g., rhizobia, bacteria, mycorrhizas) to seeds.

II. THE SEED-COATING PROCESS Specific information on the chemical and mechanical engineering aspects of seed coating is rare in the scientific literature. Most information resides in the patent literature and as “art” or “skill” in commercial organizations involved in the production of coated seeds and hence is not widely available. Much can be learned, however, from the literature from other fields in which the binding of particles in granules or tablets is important, such as in the iron ore, fertilizer, and pharmaceutical industries. Rumpf (1962), for example, has reviewed the literature concerning the bonding mechanisms between particles with particular emphasis on the development of strong granules that are able to withstand mechanical handling. A broad review of the literature concerning the pelletizing of iron ore is provided by Goldstick (1%2), and Newitt and Conway-Jones (1958) describe in detail the processes of granule formation. Coating small quantities of seeds with a uniform and consistent quantity of material is a difficult task for research workers in agronomy. Gilbert and Shaw (l979), for example, noted the difficulty of preparing relatively large seed pellets containing sulfur and overcame the problem by placing the sulfur close to the seeds (within 2 mm) to simulate the effect of coating. Any physical effects of seed coatings are thus ignored by this simulation and the results cannot be interpreted as being the same as if the seeds were actually coated.

A. COATINGEQUIPMENT The equipment used in the granulation industries has been described by Lyne and Johnston (1981) and Kapur (1978);the methods and equipment used for the coating of tablets in the pharmaceutical industry have also been well described (Lachman et al., 1970). The most commonly applied seed coatings are those in which a trace quantity of fungicide and/or insecticide is applied to seeds in such a way that this small quantity is evenly distributed among the seeds. Excellent reviews of the many types of equipment used to apply such materials are provided by Purdy (1967), Harris (1975), and Jeffs and Tuppen (1986). When relatively large quantities of materials are applied to seeds, rapid continuous flow equipment (such as can be used for fungicide application)

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is inappropriate, because larger coatings require equipment that allows some “residence” time so that the coating can accumulate on the seeds’ surfaces. This is most commonly achieved in equipment similar in principle to a cement mixer, such as an open, inclined drum or pan that rotates at constant speed (5-35 rpm, depending on diameter) while the load of seed, adhesive, and coating material tumble in the drum, with the adhesive and coating material usually being added sequentially. Such equipment has been used by commercial seed coating companies as well as by research workers (e.g., Fraser, 1966). The process involves the gradual accumulation of successive layers of adhesive and coating material on the seed and the operation of such equipment requires considerable skill (Kirk, 1972). Some of the best descriptions of the pan coating process for coating seeds are contained in patents (e.g., Funsten and Burgesser, 1951; Ostier, 1953). The quantities of seed which can be processed in coating pans are quite limited: pharmaceutical coating pans most commonly permit coating of up to 100 kg of tablets in a few hours. However, some continuous flow operations (in which only a relatively small quantity of coating material is being applied) can allow processing rates of up to 7 metric tons (Mg) of product per hour (F. E. Porter, personal communication). One of the problems of seed coating is that usually only singulated coated seeds are required and the production of any oversize material is quite undesirable. In contrast, most of the agglomerating equipment used for the production of granules in the mineral industries is designed for cpntinuous operation, relying on the screening out of agglomerates too large or small for the required product and their subsequent return to the processing chain. Such machines can generate four times as much return material as that being taken off as acceptable product; this type of agglomeration is usually carried out in an inclined horizontal drum or in a pelletizing pan (inclined disk). These machines can produce tens of metric tons per hour of operating time (Lyne and Johnston, 1981). The equipment’s diameter, rotational speed, angle of inclination, and the characteristics of the coating material (e.g., particle size distribution, moisture content) can all have a profound effect on the speed of agglomeration and the size and quality of the agglomerates produced. Among the more novel processes that have been used for coating seeds are extrusion, compression, and fluid-bed methods. The extrusion of large pellets containing several seeds per pellet has been described by Hall et al. (1974). A range of smaller pellets, containing from one to several seeds per pellet, have been described in patents of processes in which the extrusion process is followed by rolling of the pellets (Coated Seed Ltd., 1975a,b). The production of seed tablets by compression has been described by

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Hirota (1972b). Further descriptions are contained in the patents of Brink (1975) and Adams (1971). Other researchers have patented methods by which damage to the seed during compression can be avoided (Brink, 1971; Knapp, 1973). Clifford (1971) patented a method of producing a tablet which allows for adequate “ventilation” of the seed within the tablet and thus allows the free entry of air and water into the seed. Fluid-bed processing of seed has been utilized for the application of inoculants (Nack and Porter, 1965; Mullett et al., 1974), protectant coatings (Dannelly, 1981a), and germination delay coatings (Schreiber and La Croix, 1970). A further novel method of coating “seeds” is that reported by Rogers (1983) whereby somatic embryos produced by tissue culture can be encapsulated within gelatinous capsules, which are then coated with a biodegradable polymer to improve handling characteristics. B. ADHESIVES The process of seed coating usually involves the use of adhesives (also known as glues, binders, or stickers) to bind materials to the surface of seeds; coating without them using, for example, water alone will usually lead to fragile coats that are extremely prone to dusting and cracking and subsequently to the loss of the active ingredient. Such dusting and breakage can also lead to severe problems in handling the seed due to hazards to operators and to problems in mechanical planting. The physical integrity of coated seeds is of great importance in any handling, transport, and planting operations. In spite of this, no literature was found on methods available for evaluating the physical quality of seed coatings; in the pharmaceutical literature, however, there are published methods available for testing the strength and integrity of tablets and coatings (Lachman et al., 1970). When fine particles are agitated, as in a coating drum, they tend to aggregate naturally even without adhesives (due to cohesive mechanical, van der Waals, and electrostatic forces) and hence the need for an adhesive is for one that assists these natural forces of aggregation while allowing the packing of particles and “densification” to continue while the particles are tumbling. Thus, the adhesive required need not be extremely strong but must be an appropriate adhesive, one that has affinity for both the natural seed coat and for the coating material. Current adhesive technology permits adhesives to be selected with the appropriate affinity for selected substrates, the required degree of water solubility (or insolubility), the required strength and plasticity to prevent

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breakage, dusting, etc., and the most appropriate viscosity for ease of application. However, studies of the binding qualities of adhesives used for seed coating are relatively rare. Again, this is in contrast to other industries (e.g., the pharmaceutical, building, and manufacturing industries) in which there has been intensive research carried out into bonding mechanisms, bond strengths, failure to bonds, etc. The tablet making and coating processes of the pharmaceutical industry are quite closely akin to those of seed coating, and the processes and adhesives used have been well reviewed by Lachman et al. (1970). Millier and Bensin (1974) have shown that the degree of attraction or repulsion of moisture by the seed coating can have large effects on the germination of coated seeds. In terms of their binding ability and their propensity to cause undesirable agglomeration of seeds, there are large differences between the performance of adhesives. Hirota (l972a) examined a range of adhesives for binding diatomaceous earth to seed of Vicia villosa and found that a mixture of methyl cellulose and gum arabic performed the best. The coating of seeds with activated carbon has been successfully achieved with gum arabic plus a plasticizer (Sharples, 198I), methyl cellulose (Vogelsang, 1954), or polyvinyl acetate (Nagju, 1973). Tests of the performance of adhesives for adhering lime to grass seeds were conducted by Hathcock et al. (1984a), who showed methyl cellulose to be the most effective; when the coated seeds were sieved for 1 min, methyl cellulose resulted in the best retention of lime, but this was still only 71% of the lime applied. Scott (1975b) also evaluated a range of adhesives and coating materials for seed coating, but the number of interactions involved make it difficult to draw general conclusions about the efficacy of particular adhesives. By far the most studied aspect of adhesives used in seed coating is not their binding ability but rather their effect on the survival of rhizobia following seed inoculation. Those recommended by various authors include gum arabic (Brockwell, 1962; Norris, 1972), methyl cellulose (Faizah et al., 1980; Norris, 1972), gelatin and casein (Thompson, 1961),and caseinate salts (Lloyd, 1979). In practice, however, methyl cellulose is most widely used due to its ease of use, availability, low cost, and low rate (3% w/v solutions) compared to gum arabic (up to 45% w/v). Many claims regarding the efficacy of adhesives are contained within the patent literature. There are patents covering the use of mineral oil (Rushing, 1982),plastic resins (Eversole and Roholt, 1963),polyvinyl acetate (Barke and Luebke, 1981), and insoluble polyelectrolyte complexes (Dannelly, 1981a)to bind pesticides to seeds, polyethylene oxides to prevent erosion of surface-sown seed (Porter and Kaenver, 1976), polyurethanes to bind lime in a way that resists coat abrasion (Porter and Kaerwer,

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1974), blends of polyvinyl alcohol and polyvinyl acetate to bind vermiculite (Kirk, 1972), and polyelectrolytes (Hedrick and Mowry, 1953) or dextran (Peake, 1956) to aggregate soil around the seed, thereby improving the aeration of the sown seeds. Kitamura et al. (1981) also patented a process for the surface coating of fine powders with a water-soluble binder to aid subsequent adhesion of the powder particles during the coating process. C. COATINGMATERIALS The literature concerning the various solid materials used in seed coatings is concentrated, as with adhesives, on the effects of the materials on the survival of rhizobia (e.g., Brockwell, 1962; Lowther, 1975). The most common materials used as protectants for rhizobia include lime, gypsum, dolomite, or rock phosphate. Other materials mentioned in the literature include clay minerals such as montmorillonite (Bergersen et al., 1958; Hirota, 1972a; Burba, 1981) and vermiculite (Sharples and Gentry, 1980), which have been used principally as carriers for chemicals or as pelleting materials. Several other mineral materials that have been found to be useful for different purposes (e.g., pelleting, inoculation, or simply as diluents) are bauxite (Norris, 1973), diatomaceous earth (Hirota, 1972a), pumice, sand (Burba, 19811, and talc (Brinkerhoff et al., 1954; Vartha and Clifford, 1973a). Many organic materials have been used in coatings, usually as protectants or sources of nutrition for rhizobia or the developing plant. These include blood (Brockwell, 1962), bonemeal (Faizah et al., 1980), peat, poultry manure, moss (Hirota, 1972a),and mucilage (Harper and Benton, 1966). There is little information in the literature regarding the specifications of coating materials used by workers studying seeds coatings. The chemicals analysis, pH, purity, and particle size distribution are rarely noted. Thus, the many reports of coatings made with lime, for example, have been produced using materials from different sources and perhaps with different physical and chemical attributes. The effect of particle size of solid materials, in particular, is crucial to any coating or agglomeration process; such effects have been well researched in the iron ore-pelletizing industry. Urich and Han (1962), for example, report that iron ore pellets made with smaller particles (73% < 15 pm) had a porosity one-third and a strength three times that of pellets made from larger particles (6% < I S pm). Not only is the mean particle size important, but the particle size distribution also affects the quality of granulation (Newitt and Conway-Jones, 1958). The addition of small

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particles to a material containing relatively large particles, will result in more rapid granulation, increased granule strength, and decreased porosity. Thus, this is one of the dilemmas in seed coating; although high integrity and strength of the coating are desirable, low porosity is undesirable because it may restrict the movement of air to the seed. The question of how the particle size of seed coating materials can affect the performance of seed coatings has been addressed by few workers. Loperfido (1975) noted in a patent that many commercial coatings for precision sowing contain particles that are sufficiently fine that they can decrease germination due to the limitation of free gas and water exchange between the seed and the soil. In his patent, he claims the use of relatively large, hydrophobic, polypropylene beads (15G750 pm in diameter), which can be coated on lettuce seed to permit precision sowing without loss of germination ability. In this way, he suggests the coating can have a porosity of 15-25% made up of voids 2-100 pm in diameter. Sharples (1981) also investigated coatings to facilitate precision sowing of lettuce seed and suggested a model of seed coatings in which relatively large particles (minimum diameter of 180 pm are coated in a layer adjacent to the seed to facilitate oxygen diffision to the seed. Outside this inner layer, he claims that smaller particles (such as diatomaceous earth or clay), can be applied without detriment to the germination of the seed.

Ill. COATINGS TO FACILITATE PLANTING The mechanical planting of seeds is facilitated by having seeds that are of uniform size and shape, have sufficient size and weight to be easily separated mechanically, and flow readily without clumping together; seed coatings have been employed to achieve all of these features. A. PRECISION SOWING Increasing the size and weight of seeds is particularly useful for very small seeds (such as some vegetable and flower seeds), thus permitting precision planting, which results in uniform plant populations and can eliminate the need for crop thinning (Johnson, 1975; Robinson and Mayberry, 1976; Robinson et af., 1983). Such coatings can increase the weight of small seeds by 4-500 times the raw seed weight (Burgesser, 1951~). Coatings have also been used to change the shape of seed (e.g., from flat to round) and to add powdered lubricants to aid in the planting operation.

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Roos and Moore (1975) cite a survey of vegetable growers in the United States which found that 43% of growers used precision planting techniques for their crops and that coated seed played a significant role in such plantings. Roos and Moore also found that commercially coated lettuce seed produced by seven different companies did not differ in performance from raw seed except for causing a slight delay of 1-2 days in emergence. Seed coatings have also been widely used to aid the sowing of sugarbeet seeds, which have an irregular shape. Funsten and Burgesser (1951) describe a patent of a process in which the use of a nonswelling sub-bentonite (i.e., dominated by Ca and Mg ions) permits the coating of sugarbeet without using any binder. Another method of coating seeds for precision sowing, proposed in a patent by Hamrin (1973), suggests the encapsulation of carrot seeds within a gel coating. Several patents concerning coatings for precision sowing suggest that emergence can be a problem under high moisture conditions and claim that if this occurs, the aeration of the seed can be improved by blending of vermiculite with bentonite in the coating (Burgesser, 1951b), by using acid-activated bentonite (Burgesser, 195la), or by coating the seed with relatively large (150-750 pm in diameter) polypropylene beads (Loperfido, 1975). Sachs et al. (1981) demonstrated that delays in emergence observed with clay-coated sweet pepper seed were due to a reduction in oxygen supply; they reported that improved commercial formulations of coated sweet pepper seed have recently become available but, as with much of the information concerning commercial preparations, there was no disclosure of how these formulations achieved their results. B. IMPROVED BALLISTICS Increasing the weight of seeds to aid in aerial sowings has been described by several workers. For pasture seeds, Scott (1975a) found that doubling the weight of grass seeds through coating increased their terminal velocity in air, but similar coatings had little effect on the velocity of the more dense legume seeds. Coating grass seeds to a weight of 2-10 mgheed, he found that when aerially sown they behaved similarly to legume seeds and small fertilizer granules, thus reducing the separation of seed and fertilizer that can occur in such sowings. Hay (1973) found that seed coatings increased the capacity of aerially sown seed to penetrate into standing vegetation compared to raw seed but that this advantage was negated if any wind disturbed the vegetation. Similar coatings are also claimed to have improved the ballistics of rice seed aerially sown into flooded rice paddies (Mickus and Munson, 1978).

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IV. INOCULANT COATINGS A. RHIZOBIA I . Inoculation Processes Methods of inoculating seed of temperate species have been reviewed by Burton (1967) and Brockwell(1977), and many aspects of the inoculation of tropical legumes have been summarized by Date (1976). The process of inoculation is principally concerned with choosing a culture of viable rhizobia of the appropriate strain contained in a suitable carrier or medium and applying it to the seed of the legume host. By far the most common method of inoculation is to apply the rhizobia within a coating on the surface of the seed, usually employing an adhesive that improves the binding of the inoculum to the seed and aids the survival of the rhizobia until planting (e.g., Burton, 1961). Other techniques have been employed, such as impregnating the seed using vacuum (Porter, 1960) or by pressure impregnation (Brockwell and Hely, 1962), but it is generally agreed that the seed coat itself is a relatively hostile environment for rhizobia. One of the adverse conditions faced by rhizobia following inoculation is rapid desiccation, the effect of which may be lessened by growing the organism in a complex organic substrate, such as peat, to which the organisms can become tightly adsorbed and thereby gain protection. Thus, peat is usually the preferred camer for rhizobia inoculated on seed. Adsorption onto other substrates such as montmorillonite clay can also aid in the protection of rhizobia from desiccation (Marshall and Roberts, 1%3). Rhizobia may also require protection from microbial antagonisms (Bergersen et al., 1958) and from toxic materials released from the natural seed coat (Thompson, 1961; Hale, 1976; Hale and Mathers, 1977). The work of Hale and Mathers has been further developed and included within a patent for a commercial process that assists the survival of large numbers of rhizobia on seed such as white clover (which otherwise would be toxic to the rhizobia) by the inclusion of an adsorbent in the inoculum (Coated Seed Ltd., 1983).

2 . Lime Coating Early work in Australia (Loneragan et al., 1955) and New Zealand (Lobb, 1958) established that on acid soils the inoculation of legume seeds

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M. SCOTT

could be made much more effective if the inoculated seed was enclosed within a lime coat. Today, lime coating of legumes following inoculation is widely practiced in both Australia and in New Zealand, particularly on acid soils. A practical guide to the operations involved in the lime coating of small batches of legume seeds was given by Plucknett (1971). In Canada, Kunelius and Gupta (1975) found lime coating increased alfalfa establishment on soils with a pH of 5.0-5.6. However, in the United States, some workers have shown that lime coating is not nearly as beneficial as has been shown in Australia and New Zealand (e.g., Olsen and Elkins, 1977). This experience may be associated with somewhat higher soil pH, as was found by Lowther (1974) in New Zealand; he demonstrated that lime coating could even depress plant growth at or above a pH of 6.2, supposedly due to a lime-induced nutrient deficiency. With tropical and subtropical legume species, the benefits of lime coating are less clear. Although positive responses to lime coating have been obtained by some workers (e.g., Cook, 1978), others have claimed that lime coating can be ineffective (e.g., Norris, 1967). A useful review of this controversial area is given by Snyder and Kretschmer (1981). B. VESICULAR-ARBUSCULAR MYCORRHIZAL FUNGI The effect of vesicular-arbuscular (VA) mycorrhizas on the growth of plants has received considerable attention from researchers. Seed inoculation of white clover with VA mycorrhizas was found to be ineffective by Boatman et al. (1980), due perhaps to loss of viability of the mycorrhizas or to the soil already being colonized by effective mycorrhizas. Powell (1979) found that the pelleting of either ryegrass or white clover with soil that was heavily infested with VA mycorrhizas increased yields by 4080%. Nevertheless, large-scale inoculation is not yet feasible because a means of culturing VA mycorrhizas in vitro has yet to be developed (Gianinazzi-Pearson and Diem, 1982).

C. OTHERORGANISMS Most work on the application of organisms other than rhizobia or VA mycorrhizas to seeds concerns disease-controlling organisms. The inoculation of cotton seeds with spores of Trichoderma spp. has been found to control Rhizoctonia solani (Elad et al., 1982) while the application of some biotypes of Trichoderma viride suppressed damping-off diseases of peas and beans (Papavizas and Lewis, 1983). Similarly, Rhizoctonia bataticola of gram was controlled with coatings containing certain Bacillus

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and Streptomyces species (Singh and Mehrotra, 1980). A reduction in the severity of take-all of wheat has also been shown to be possible using Pseudomonas spp., and methods of wheat seed inoculation are being investigated (Wong and Baker, 1984). Antibiotics have also been used successfully in seed treatments to reduce disease.

V. PROTECTIVE COATINGS A. DISEASES The research work cited above concerning the biological control of disease is dwarfed by the literature concerning the use of seed-applied fungicides to control plant diseases. Such seed coatings or treatments are commonly used to combat diseases that can cause enormous losses to plant populations and productivity. Today, fungicidal coatings are routinely applied to seeds of a wide range of crops to protect against fungal pathogens resident in soils and also carried in the seed coat itself. A useful historical account of the development of fungicide and insecticide seed treatments was given by Callan (1975) and a detailed text on all aspects of seed treatments has been compiled by Jeffs (1986). Some examples of the many successful fungicide seed treatments which have been developed include the control of Septoriu nodorum on wheat (Cunfer, 1978), bunt and smut diseases of cereals (Alcock, 19781, powdery mildew of barley, Rhizoctonia in cotton (Cole and Cavill, 1977), and sugarcane downy mildew of maize (La1 et a f . , 1979). Newer fungicides have been developed that not only provide some protective action but can also have a curative effect on some diseases (Nesmith, 1984). An example of such a fungicide is metalaxyl, which is more effective in controlling Pythium and Phytophthora diseases when applied to seeds than foliar sprays (La1 et a f . , 1979). Pasture species also benefit greatly from fungicide seed treatment. Damping-off diseases of alfalfa and ryegrass, for example, can be effectively controlled by fungicide seed treatments (Falloon, 1980). Seed-applied fungicides have been found to not only improve emergence but to increase yield, presumably due to reducing the level of subclinical disease (Falloon and Fletcher, 1983). The effects of fungicides are often more noticeable when conditions suit the development of fungi, such as when low temperatures prevail (Yen and Carter, 1972). As sowing conditions become less favorable, the benefits of seed treatment with fungicide become more apparent. The compatibility of fungicide seed treatments with other pesticides has

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been examined by several workers. For example, Chatrath et al. (1977) found that either of two fungicides, carboxin or benomyl, could be used on wheat seed together with either of two insecticides, BHC or malathion, without any deleterious effect on germination or effectiveness. Some fungicides have been found to be incompatible with some insecticide seed treatments, however, the combinations adversely affecting germination and plant populations. Interactions have also been found between fungicide-insecticide seed treatments and soil-applied herbicides. Improvements in fungicide formulations have resulted in much better adhesion of the fungicide to seeds and less dusting, with the result that hazards to operators are reduced and efficacy is increased. Other developments in fungicide seed treatments that may lead to more effective control of diseases include the use of solvent delivery systems, which can improve the control of fungi within the seed (Vidhyasekaran, 1980), the development of controlled release pesticides (Anderson and McGuffog, 1983), and the improvement of integrated control practices (Nesmith, 1984).

B.

INSECTS, PESTS, AND OTHER

FAUNA

Seed coatings containing insecticides or acaricides have been used widely on many plant species, often in combination with fungicides. Examples of some of the pests controlled with seed treatments include leafhoppers on rice, barley, and soybeans; onion fly on onions; soldier fly larvae in ryegrass pastures, and shootfly in maize. Examples of the use of acaricide seed treatments include the control of spider mites in cotton and red-legged earth mites in pastures. The use of insecticides applied to the seed is a practice which, for many pests, is more amendable to integrated pest control than is overall spraying, as less chemical is used and it is localized in the area where it is needed. As with fungicides, insecticides used on seeds need to be compatible with any other pesticides being used. The theft of pasture seeds by ants can be a serious problem, particularly for surface sowings. The degree of ant theft has been reduced by seed treatments with permethrin or bendiocarb (Campbell and Gilmour, 1979) and to some degree by commercial seed coatings without insecticide, which the ants tend not to recognize as food (Johns and Greenup, 1976). The control of molluscs using seed coatings has been attempted using rnethiocarb on clover. Although some increase in establishment was observed for surface sowings in spring, no such increase was observed for autumn sowings. There appears to be a need for either a better seed coating

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formulaton of methiocarb or, alternatively, for a more effective active ingredient (Charlton, 1978; Welty et al., 1981). Some coatings designed for other purposes may also require the inclusion of an effective arthropod repellent; Dunning and Baker (1977) found that coatings for the precision sowing of sugarbeet seeds resulted in increased damage caused by millipedes. A number of workers have studied ways of using protectant seed coatings to protect seeds from birds or rodents. In some cases, pelleting of seeds with nutrients, clays, lime, etc. is sufficient because the pests may not recognize the seed as food. However, this is often not the case with birds and rodents and chemical repellents can be required. Some of the coatings used to repel birds include those containing methiocarb, endrin, or thiram (Naquin, 1978). Rodents have been successfully repelled with coatings containing mestranol on seed of Douglas fir and resorcinol on wheat seed (Fuchsman, 1972). AGAINST HERBICIDES C. PROTECTION

Materials which can aid in protecting seeds or seedlings from herbicides can be divided into those that act either chemically (i.e., antidotes) or as adsorbents.

1 . Antidotes The development of herbicide antidotes is a relatively new branch of science: the first patent was taken out by Hoffman (1964), who described “antagonistic agents” such as oximes, which reduced plant injury due to thiocarbamate herbicides. Reviews of herbicide antidotes have been written by Blair e? al. (1976) and by Pallos and Casida (1978), the latter authors concentrating on the chemistry and mode of action of the antidotes. Subsequent reviews summarizing this active area of development over recent years have been published by Hatzios (1983) and Parker (1983). The first commercially produced herbicide antidote was 1,g-naphthalic anhydride (NA). When used as a seed treatment to protect maize from S-ethyl dipropylthiocarbonate (EPTC) damage, NA permits good weed control while maintaining selectivity (Burnside et al., 1971; Hoffman, 1971). The use of NA as a seed treatment to protect maize from EPTC damage has now largely been replaced by a more effective antidote, R-25788, which is marketed as a mixture with the herbicide EPTC and renders seed treatment unnecessary (Chang et al., 1973). R-25788 has also

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been found to be effective for use on other crops such as barley (against the herbicides vernolate and EPTC) and wheat (against the herbicide triallate) (Miller and Nalewaja, 1980). Some antidotes have been shown to cause injury to the sown species in the absence of herbicide [e.g., N A on sorghum (Eastin, 1972) and allidochlor on corn (Chang et al., 1973)l. An active area of antidote development has been the protection of sorghum from acetanilide herbicides (Brinker et al,, 1981;Chang and Merkle, 1982; Ellis et al., 1980; Moshier and Russ, 1980; Schafer et al., 1980). The most effective method of antidote application has been shown to be quite different for various antidotes: MON-4606 and CGA-43089 are more effective as seed treatments than as tank mixes with herbicides, while N A is effective only as a seed treatment. Although R-25788 is effective as a tank mix, it may be more effective as a seed treatment (Spotanski and Burnside, 1972; Miller et al., 1973).

2. Adsorbents

The use of adsorbents as a means of inactivating herbicides has been reviewed by Blair et al. (1976)and Gupta (1976). Reports of the successful inactivation of herbicides using seed coatings of activated carbon include those on maize against a range of herbicides, on cotton against alachlor, on pregerminated rice seed against several herbicides (Nagju, 1973), on kikuyu seed against atrazine (Cook and O’Grady, 1978), and on Australian native grasses against diuron and chlorthal dimethyl (Hagon, 1977). Several patents also claim that activated carbon seed coatings can provide good protection against herbicides such as (2,4-dichlorophenoxy)acetic acid (2,4-D) (Vogelsang, 1954) and methylurea and triazines (Johnson et al., 1972). Hahn and Merkle (1972) found that although an activated carbon seed coating provided some protection for sorghum against the herbicide alachor, it nevertheless was far less effective than NA. Activated carbon seed coatings were also found to be relatively ineffective in protecting grasses (Bertges, 1977) and lettuce (Richardson and Jones, 1983) from several herbicides. Gupta (1976) reports that activated carbon and NA seed coatings were equally effective in protecting maize from low rates of EPTC, but at higher herbicide rates NA was more effective. The application of activated carbon in bands along the row of seed can be as effective as antidotes in protecting crops from herbicide injury, but as noted by Gupta (1976), such band applications may require high rates (up to 336 kg/ha) and also may suffer the disadvantage of protecting weeds located within the band. Activated carbon has also been examined as a minor component of

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coatings for the precision sowing of vegetables; when it was included in a vermiculite coating (at 2.5% w/w), the germination of coated lettuce seed was vastly improved due to the adsorption of germination inhibitors released by the natural seed coat (Sharples and Gentry, 1980). The use of seed coatings containing another adsorbent, polyvinyl pyrrolidone, has been proposed as a means of enhancing the establishment of direct-drilled seeds by the adsorption of phytotoxic phenolic compounds released by herbicide-killed swards (Habeshaw, 1980).

VI. NUTRIENT COATINGS The incorporation of nutrients in seed coatings provides a unique opportunity to supply each sown seedling with an accurately controlled quantity of nutrient that may be preferentially available to the sown species and less available to any neighboring weed species. This area of seed coating development has received relatively little attention in previous reviews. In addition to nutrient coatings, the soaking of seeds in nutrient solutions will also be dealt with in this section. A. NEEDFOR EARLYSEEDLING NUTRITION The need for young seedlings to utilize sources of nutrients external to the seed early in life has been clearly shown by Krigel (1967) with subterranean clover (Trifolium subterraneum). Even though subterranean clover has a relatively large seed compared to many other pasture species, Krigel observed responses to external sources of nutrients as early as 7 days after sowing for calcium, 10 days for phosphorus, 14 days for nitrogen and magnesium, and 21 days for potassium. Responses of pasture species to the early supply of external nutrients (particularly of phosphorus and nitrogen) has also been shown by McWilliam et al. (1969), Lazenby and Schiller (1%9), and Blair et al. (1974). Some species, of course, have much smaller seeds than subterranean clover and the extent of their nutrient reserves is quite limited. Ozanne and Asher (1965) suggested that the low potassium content of some pasture seeds may limit their establishment, particularly when sown at depth, and proposed that a seed coating containing a form of potassium safe for the seed may overcome such limitations. In view of the large influence which available sources of phosphorus (P) can have on early seedling growth, more information is needed concerning the P nutrition of seedlings (Silcock, 1980); the critical P concentration in the soil solution required for maximum growth is far greater

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during seedling growth than later in a plant’s life (Fox et al., 1974). Crops such as wheat require much higher rates of P up to the end of tillering than during the grain filling period (Sutton et al., 1983). With wheat growing in solution culture experiments, Ningping and Barber (1985)found a maximum concentration of P in the shoots at 25 days after sowing.

B. MACRONUTRIENTS 1 . Coatings

Although many workers have reported that nutrient seed coatings can cause damage during germination or that they supply little nutrition to seedlings, the literature nevertheless contains numerous reports of cases in which the supply of macronutrients by seed coatings has been substantial. Reports of P coatings that have been used successfully include a positive response of ryegrass to a P coating 2 years after sowing (Vartha and Clifford, 1973a), a two- to fourfold increase in the establishment of ryegrass coated with P compared to ryegrass with no coating (Vartha and Clifford, 1973b),an increase in the survival of direct-seeded lettuce grown from tablets incorporating a small quantity of P (Sharples and Gentry, 1980), and an increase in corn yield brought about by a seed coating containing as little as 0.2 kg P/ha (Guttay et al., 1957). Sulfur ( S ) has also been incorporated successfully in a number of seed coatings: gypsum-coated clover seed showed twice the establishment of lime-coated clover in a sulfur-responsive site (Lowther and Johnstone, 1979); Stylosanthes guianensis coated with elemental S or gypsum, produced significantly more yield than with the equivalent rate of broadcast S (Gilbert and Shaw, 1979);the addition of S and molybdenum to P coatings increased the establishment of clovers (Scott and Hay, 1974);and a coating containing elemental S approximately doubled the establishment of oversown legumes (Scott and Archie, 1978). Successful coatings containing nitrogen (N) appear to be those in which the N is present only in small quantities. When small amounts of an NP-K fertilizer have been applied to corn seed, for example, early growth and nutrient uptake have been enhanced (Miller et al., 1971), and the efficiency of uptake of P in particular was increased three to four times (Smid and Bates, 1971) compared to the effect of band application of the fertilizer. Other workers have found some low rates of macronutrient additions to be safe for germinating seeds, such as a coating on red clover seed containing 5% of either superphophate or an N-P-K fertilizer (Hirota,

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1972a) or a seed tablet including a small proportion of an N-P-K fertilizer and I% algin (Hirota, 1972b). There are few reports of the benefits of coatings containing two or more nutrients. One example is the increased dry matter production and nutrient uptake which resulted from the inclusion of P and zinc (Zn) in seed coatings for wheat (Gawade and Somawanshi, 1979). N and P have been applied singly and together in various combinations with lime on seeds of tall fescue and Kentucky bluegrass with the result that some N and P formulations produced better growth than coatings containing either nutrient alone (Hathcock et al., 1984b). Combinations of N and P compounds can produce large responses in the early growth of forages and crops, but the chemical form of the N and P compounds must be chosen carefully to ensure maximum effectiveness (Sheard, 1980; Costigan, 1984).

2. Soaking The soaking of seeds in macronutrient solutions, as opposed to coating seeds, has received relatively little attention. Soaking seeds in such solutions has been claimed to increase the seedling growth of sugarbeets (Miyamoto and Dexter, 1960) and grain yield and nutrient uptake of cereals. However, the mechanisms responsible forthese claimed increases in growth and yield have not been adequately explained. In contrast, negative results were obtained by Guttay et al. (1957) from corn seed soaked in a phosphate solution. Of course, compared to nutrient seed coatings, the soaking of seed can only supplement the seed’s mineral reserves to a small extent. C. MICRONUTRIENTS

1 . Coatings A range of micronutrients has been successfully coated on seeds of various species. Molybdenum has been used in many seed coatings, particularly those applied to legumes, where molybdenum is an essential element for nitrogen fixation. Such coatings have increased clover establishment (Scott and Hay, 1974) and increased the growth and yield of cowpeas equivalent to liming the soil with basic slag at 3 Mg/ha (Rhodes and Nangju, 1979). Kerridge et al. (1973) found that the inclusion of molybdenum trioxide in rock phosphate coatings was as effective as a soil application in alleviating molybdenum deficiencies of a number of tropical legume species. Gault and Brockwell (1980) studied the various forms of

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molybdenum and their compatibility with rhizobia and found sodium molybdate to be the only one which depressed nodulation. The application of zinc in seed coatings on rice has brought about yield increases (Thompson and Kasireddy, 1975) and generally been more effective than foliar sprays. Manganese applied to sugarbeet seed partially alleviated a manganese deficiency but required an additional foliar spray for complete correction of the deficiency (Farley, 1980). However, other micronutrients that have been coated on seeds have not been so successful, including magnesium (as dolomite) on legume seeds, in which case the beneficial effects were more likely due to the increased survival of rhizobia rather than to any direct nutritional effect on the plants (Brockwell, 1962).

2. Soaking The soaking of seeds in micronutrient solutions has been claimed to produce large responses in some situations, although the reasons for these responses are often not clear. Soaking seeds in solutions of magnesium salts increased the germination and fodder yield of pearl millet (Chhipa and Lal, 1976), whereas soaking oat seeds in manganous chloride alleviated a manganese deficiency (Drennan et al., 1961). Wilson and Notley (1959) overcame a molybdenum deficiency of tomatoes by soaking seed in a molybdenum solution, although the results of soaking lettuce were more variable. In other cases, negative effects of seed soaking in micronutrient solutions have been reported (Kereszteny, 1973). D. EFFICACY OF NUTRIENT SEEDCOATINGS The relative efficiency of fertilizer placement has been studied widely, particularly for comparisons between drilled and broadcast applications (e.g., Bates, 1971). Comparisons between nutrient seed coatings and alternative application methods have also been conducted. Smid and Bates (1971), for example, found that small additions of fertilizer in seed coatings were three or four times as effective in providing an early supply of P to corn seedlings as was band placement. The relative efficiency of fertilizer placed near the seed compared to broadcast applications has been shown to be higher under conditions where the available soil P is low (Fiedler et al., 1983). The early growth of buffel grass (Cenchrus ciliaris) seedlings was maximized by placing P such that it was available within 4 days of sowing (Silcock and Smith, 1982). Close placement, and therefore the availability, of nutrients to establishing seedlings appears to be most im-

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portant for immobile elements such as P, especially under cool conditions, which restrict P uptake (Klepper et al., 1983). Scott and Blair (1988a) showed that the efficacy of different calcium phosphate seed coatings [mono- (MCP), di- DCP), and tricalcium phosphate (TCP)]in promoting seedling growth of phalaris (Phalaris aquatica) and alfalfa (Medicago saliva) increased with the solubility of the P source (i.e., from TCP to DCP to MCP). Each of the P sources studied markedly increased leaf number and increased yield (MCP had a greater effect than DCP, which in turn had a greater effect than TCP), but the biggest effect was on the P content of the seedlings, which increased greatly with increases in P rate and the solubility of the P source. In a related experiment, P seed coatings applied to phalaris seed containing the equivalent of 5 kg P/ha produced plants (at 27 days after sowing) as tall as those supplied with 20 kg P/ha by drill or broadcast methods (Scott and Blair, 1988b). The heights of individual plants were also more uniform in the coated treatment condition, suggesting that the seedlings obtained more uniform access to P from the coatings. At harvest, the differences between the coated and drilled application methods were greater for the dry matter yield and P content of roots than for shoots, suggesting that the coated seeds may have an increased chance of survival compared to drilled seeds if competition from weeds were for the belowground resources of either soil moisture or nutrients. Phalaris seed coatings with DCP containing 10 kg P/ha produced significantly more growth at 27 days than those supplied with 40 kg P/ha as drill or broadcast applications; nevertheless, DCP was less effective per unit of P than was the more water-soluble MCP. When MCP seed coatings were applied to phalaris seed oversown with rattail fescue (Vulpia myuros), the coat P treatment increased the P content of phalaris more than drill P; they were not, however, different without rattail fescue, thus suggesting that the P fertilizer in the seed coating was preferentially available to the phalaris and less available to the rattail fescue. Conversely, the yield of the oversown rattail fescue was significantly increased by the drill P treatment, but not by the coat P treatment, indicating the greater availability of the drilled P granules to the weeds, with a corresponding decrease in its availability to the phalaris seedlings (Scott, 1986). No other evidence for the supposed preferential supply of nutrients by seed coatings has been found in the literature to date. Other workers have reported that relatively insoluble nutrient coatings have had little or no effect on nutrient supply. These reports include studies of the surface sowing of pasture species in Australia, in which case Dowling (1978) observed no difference between the effect of inert- and nutrientcoated seed, and the surface sowing of legumes in New Zealand, in which

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case the principal requirement was for nodulation rather than for the supply of nutrients (Lowther and McDonald, 1973; Lowther, 1974). Lime-reverted superphosphate (which contains phosphorus in the form of dicalcium phosphate) was investigated by Hay (1973) as a coating material for large pasture pellets, each containing several seeds of ryegrass, but negligible P was supplied to the seedlings within 9 weeks of sowing. Also using ryegrass, Scott et af. (1985) found reverted superphosphate coatings to be relatively ineffective in supplying P; additionally, a slow-release form of N (isobutylidene diurea) included in the coating was found to be ineffective in supplying N. Silcock and Smith (1982) found that dicalcium phosphate provided little phosphorus to buffel grass seedlings, but they did observe an increase in early emergence due to the coating. These results contrast with those of Terman et af. (1958), who found that DCP was more available to ryegrass and sudan grass than MCP in acid soils. Their results, however, were found in trials that were conducted over several months and in which the fertilizers were finely ground (to 4 0 pm) and either mixed into the soil or spread evenly at a depth of 35 mm. The effectiveness of nutrient coatings is very much dependent on several factors including the species sown (Watkin and Winch, 1974; Dowling, 1978),the time of sowing (Vartha and Clifford, 1973b; Watkin and Winch, 1974; Dowling, 1978), the soil type (Watkin and Winch, 1974), the type of coating, and the soil fertility and texture (Scott, 1975b). Just as there is no universally applicable fertilizer recommendation for all soils, regions, etc., the interactions listed above indicate that there is little likelihood of the development of any one effective nutrient coating having universal application.

E. INJURY CAUSEDBY FERTILIZERS The use of concentrated soluble fertilizer salts as seed coating materials has generally been deleterious to germination and early growth (e.g., Scott, 1975b). The differences between the various fertilizers in potential damage to plants has been evaluated by Rader et al. (1943) using a “salt index” that compares the osmotic effect of fertilizers relative to a standard of sodium nitrite (salt index 100). Thus, Rader identifies “safe” fertilizers, or ones with little osmotic effect, as dolomite (salt index 0.8). dimagnesium phosphate (4.3), calcium carbonate (4.7), superphosphate (7.8),and gypsum (8.1). The “osmotic” effects of fertilizers on the germination of seeds have been observed by many researchers as early as late last century. Much of this early work has been reviewed by Uhvits (1946), who also reported the toxic as well as the osmotic effects of NaCl on germination. The strictly osmotic effects of fertilizers or the “physiological drought”

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that they impose on seeds has been questioned by Philip (1958), who suggested that toxic effects may be the real cause of damage, particularly for those materials with diffusible solutes. This may help to explain why a fertilizer such as superphosphate, which according to Rader has a slightly lower osmotic effect than gypsum, has nevertheless been found to be quite damaging to the germination of seeds when applied as a seed coating, whereas no such reports exist for gypsum coatings. Guttay (1957) showed superphosphate to be one of the most damaging fertilizers in causing depressed emergence when sown with wheat at rates greater than 100 kg/ha. When the separate components of superphosphate (i.e., Ca(H,PO,),-H,O; CaHPO4.2H,O;Ca3(P0,),; CaF,; and CaSO,) were studied separately by Guttay, none could be identified as the primary source of injury to the germinating seedling. He suggested that the small amount of residual free acid present in superphosphate (0.6%) may result in an increase in the availability of HF, which is quite toxic to germinating seeds. 1 . Injury Caused by Nutrient Coatings When used in seed coatings, both single and double superphosphate have been found to be damaging to the emergence of buffel grass whereas other soluble sources of P (e.g., monosodium phosphate) caused little damage at moderate concentrations (Silcock and Smith, 1982). In these experiments, Silcock and Smith evaluated many phosphorus sources consisting of salts of calcium, sodium, nitrogen, and potassium. Hence, the effects of the phosphorus source alone, without the possible interference of other elements, are difficult to identify. Research work in our laboratory has shown that injury due to seed coatings containing soluble phosphorus sources such as monocalcium phosphate varies greatly between species, with legumes being much less tolerant than gramineous species (Scott, 1986; Ascher et a f . , 1987; Scott and Blair, 1988a). This is in agreement with the work of Carter (1967), who noted that species differ widely in their tolerance of fertilizers, with crucifers being more susceptible than legumes, which in turn are generally more susceptible than grasses. In an effort to prevent any reduction in germination, which can be caused by soluble nutrients, many workers have concluded that relatively insoluble materials are most appropriate for use in seed coatings. Some slowrelease nutrient materials are the subject of a patent (Coated Seed Ltd., 1975a) that claims that isobutylidene diurea (IBDU), fritted potash, dolomite, gypsum, and lime-reverted superphosphate, when used to coat turfgrass seed by extrusion, are safe forms for the supply of N , K, Mg

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plus Ca, Ca plus S, and P plus S, respectively. In evaluating slow- release fertilizer coatings, Scott and Hay (1974) reported that the main effect of the coatings is physical rather than an effect on nutrient supply. In contrast, Scott et al. (1985) showed that with nutrient-coated ryegrass, the composition of the coatings can have marked effects, with the coating containing the most water-soluble nutrients causing the longest delay and greatest depression of emergence and yet the greatest improvement in seedling growth. Other researchers too (e.g., Younger and Gilmore, 1978; Boatman et al., 1980) have noted that, in spite of reduced germination, the surviving plants still produced increased yields compared to uncoated controls.

2 . Effect of Seed Structure on Tolerance to Fertilizer Injury The mechanisms of tolerance shown by various species to contact with fertilizers at sowing has been discussed by several authors. Hadas (1982) noted that most salts affect germination “not directly through their osmotic effects-but rather through ion toxicity which depends on species susceptibility.” Guttay (1957) observed that oats showed less susceptibility to fertilizer injury than wheat, which he attributed to the oats’ caryopsis being enclosed within a lemma and palea. The seed structure of buffel grass (which has its caryopsis enclosed within glumes, lemma, and palea in a fascicle) has also been suggested as the reason for its tolerance of soluble P sources (Silcock and Smith, 1982). In the case of phalaris, McWilliam and Phillips (1971) noted that the structure of the seed resulted in a significant barrier to liquid moisture movement from the outer lemma and palea to the caryopsis. Scott (1986) found the lemma and palea of phalaris to be responsible for the protection of the caryopsis from injury due to MCP and postulated that the air space between the lemma-palea and the caryopsis may act as a semipermeable membrane, as do the air spaces in moderately dry soil (Philip, 1958), thus restricting the movement of toxic quantities of solutes into the caryopsis. Garrote et al. (1987) confirmed that the presence of the lemma and palea around seeds of a range of grass species is responsible for the protection of caryopses from injury due to soluble P sources. 3. Effect of p H Scott and Hay (1974) noted that, in general, the germination of nutrientcoated seeds was positively correlated with the pH of the coating. Scott

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and Blair (1988a) observed a similar relationship between injury and the pH of various calcium phosphate seed coatings, but when different combinations of P and N sources were compared, there was no consistent relationship with pH nor with the partial salt index of the fertilizers used (Scott et al., 1987), suggesting that neither osmotic effects nor pH alone is responsible for injury during germination.

4 . Effect of Soil Moisture

Fertilizer effects have generally been claimed to be more damaging at low soil moisture contents (Guttay, 1957; Carter, 1967). For soluble N coatings applied to cereal seeds, this same trend has been demonstrated (Scott et al., 1987). However, examining the effects of relatively insoluble nutrient seed coatings applied to seeds of ryegrass, Scott et al. (1985) observed that injury during emergence was greatest at high soil moisture contents; this suggested that at nonlimiting moisture tensions aeration of the seeds may have been inhibited.

5 . Protection of Seeds from Fertilizer tnjury Protecting seeds from damage by coatings may be possible, because certain species (e.g., buffel grass) emerge with little damage from soluble nutrient coatings because the caryopsis is enclosed within a fascicle (Silcock and Smith, 1982);similar protection may be possible for other seeds if a suitable barrier could be applied during the coating process between the natural seed coat and the nutrient coating. The protection of seeds from soluble nutrient sources has been attempted by Smid and Bates (1971), who found that coating corn with either sucrose or polyvinyl acetate resulted in a slight reduction in the toxic effects of fertilizers, but the reduction was insufficient to make nutrient coating practical. Scott et al. (1987) investigated the effect of various forms and combinations of N and P applied to cereal seed in seed coatings. They showed that, even with otherwise very damaging materials such as urea, the injury caused during the emergence of wheat can be lessened somewhat by reducing the pH of the urea seed coating and more substantially by including the urease inhibitor phenyl phosphorodiamidate in the urea seed coating. Far more work is needed to find ways of overcoming injury to seeds caused by soluble fertilizers.

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VII. HERBICIDE COATINGS Seed coatings incorporating herbicides to control weeds are a recent innovation with relatively little research yet being reported in this area. The first mention of the possibility of using the seed as a carrier for herbicides was in a patent on the use of (S)-benzyl-N,N-disec-butylthiocarbamate as a means of stimulating the growth of rice and providing a measure of weed control (Pellegrini et d ,1976). The most complete work done so far is that of Dawson (1978), who applied liquid EPTC [(S)-ethyl dipropylthiocarbamate)] to alfalfa seed already coated with lime. This herbicide-coated seed has been shown to retain its germinability after 1 year’s storage (Dawson, 1981a) and when planted, produces a band of weed control approximately 5 cm wide, with the alfalfa seedlings emerging relatively unharmed in the center of the band. Even though EPTC controls a broad range of weeds, it is highly selective, causing little damage to legumes such as alfalfa and beans. According to Dawson (1980), EPTC is effective as a herbicide applied in seed coatings because of its volatility, which permits it to move away from the seed coating and control weeds in the zone of soil around each seed. Alfalfa has, however, been found to be slightly susceptible to EPTC damage if exposed to high doses following emergence (Dawson, 1983). Successful weed control with EPTC-coated alfalfa seed has also been observed by Kapusta and Strieker (1982), who obtained higher yields from herbicide-treated seed than from conventional applications of granular EPTC, and by Krecker and Foy (1982), who found fewer weeds after sowing herbicide-treated seed compared to tests in which EPTC was applied conventionally followed by soil incorporation. Dawson (1981b) suggested even greater scope for seed-applied herbicides in a patent in which he claimed that a range of thiocarbamates (vernolate, cycloate, pebulate, and EPTC) can provide good weed control not only in alfalfa, but also in crops such as beans, soybeans, turnips, sunflowers, flax, tomatoes, and sugarbeets. The only herbicides other than thiocarbamates which have been reported as successful in seed coatings are those described by Dale (1983), who found that fluazifop-butyl, when applied in tung oil as a coating on soybean seeds, provided good control of grass weeds while causing little damage to the soybeans. A similar coating applied to cotton seeds resulted in significant damage to the cotton, perhaps due to the greater absorption of the herbicide by the cotton seeds compared to soybean seeds. In a greenhouse trial, Scott and Blair (1987) established the effectiveness of EPTC applied to alfalfa seeds in controlling germinating rattail fescue seed and markedly increasing alfalfa yield. None of eight other herbicides

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tested (representing a wide range of herbicide classes) caused both significant control of the weed and increased growth of the alfalfa. In this experiment, significant weed control was obtained with seed-applied EPTC at rates as low as 0.1 kg/ha (soil-incorporated applications of EPTC are commonly 4 kgha), indicating that using the seed as a carrier for herbicides can be an effective means for their distribution. The most effective application rates for EPTC applied to alfalfa seed were 0.4-1.2 kg/ha. The behavior of seed-applied herbicides in soil, like that of conventionally applied herbicides, can vary considerably depending on conditions. Factors such as the soil's organic matter, clay and moisture contents, its exchange capacity, pH, and microflora, as well as temperature, all affect the movement, effectiveness, and degradation of herbicides (Helling et al., 1971). Thus, future work may need to take into account some of these variables as they affect seed-applied herbicides. Confirmation of the effectiveness of seed-applied EPTC under directdrilled conditions is still needed to assess the practicability of incorporating a volatile herbicide into soil that is not substantially disturbed. Also, the compatibility of EPTC seed treatment with Rhizobium inoculation of the alfalfa still remains to be studied (Anonymous, 1981). Future experiments concerning the development of herbicide seed coatings may need to study not only the most appropriate herbicides and rates, but also the formulation of the coating. More fundamental work on the movement and adsorption of EPTC from seed coatings is needed, particularly as it is affected by sowing method, cultivation, sowing depth, soil compaction, and soil moisture content. Recent developments in controlled release technology (reviewed by Kydonieus, 1980) offer opportunities to tailor the release rate of herbicides to within desired limits. The porosity of seed coatings may also be an important factor influencing the release of herbicides, as has been found to be the case for granular formulations of thiocarbamate herbicides (Schreiber and White, 1980).

VIII. OTHER COATINGS A. HYDROPHILIC COATINGS

Hydrophilic seed coatings have been seen by many to offer exciting opportunities to promote the rapid and complete germination of seeds. The use of hydrophilic polymers as coating materials was claimed in a patent on the use of water-soluble polyelectrolytes (with molecular weights greater than l0,OOO) to enhance germination by aggregating soil clay

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particles adjacent to the seed, thus improving the movement of air and water to the seeds (Hedrick and Mowry, 1953). The use of starch graft polymers (which can absorb up to 1000 times their own weight of water) has been patented (as a seed coating procedure) by Weaver et al. (1976), with further improvements being claimed by Hall (1979), who suggested mixing fatty alcohols with the starch graft polymer to reduce the evaporation of absorbed water. The use of such materials in seed tablets containing lettuce seeds or in seed coatings on pasture seeds has shown no effect on germination (Sharples and Gentry, 1980; Campbell, 1985), whereas when used as coatings on rangeland grasses, they resulted in delayed and reduced germination and emergence (Yarris, 1982). This negative effect could be related to the depression of emergence reported for some seeds with a mucilaginous natural seed coat, which can enhance water uptake but can also result in impaired oxygen diffusion to the seed if conditions are too wet (Hadas, 1982). With crops, the use of hydrophilic polymer seed coatings on cowpea seeds resulted in a depression of emergence, whereas similar coatings on corn seed produced better emergence in only one cultivar out of four (Baxter and Waters, 1986). Other patented materials claimed to enhance water uptake by seeds include hygroscopic materials such as magnesium carbonate (Reams, 1972) and small, insoluble polymer particles (<3 pm in diameter), which can be wetted with less water than water-soluble coatings and hence are claimed to eliminate delays in germination (Dannelly, 1981b). Fine particulate coatings (such as lime) have been shown to enhance greatly the water uptake and germination of pasture seeds under some conditions, particularly when the seed is surface-sown (McWilliam and Dowling, 1970). Such effects, however, have been demonstrated only over a narrow range of limiting moisture tensions (Hay, 1973; Scott and Hay, 19741, and the effects on subsequent establishment are often inconclusive (Vartha and Clifford, 1973a). This increase in imbibition may be due to the fine lime particles in the coating acting as a “wick” or moisture-attracting materials or, perhaps, to improved seed-soil contact. Alternatively, the wetting and drying processes that the seeds are subjected to during coating may precondition the seeds for prompt germination. Such a condition, according to Lush et al. (19811, is achieved after controlled wetting and drying of ryegrass and results in an increased rate of germination, which is partly attributable to an increased rate of imbibition. B. HYDROPHOBIC COATINGS Several patents describe the use of hydrophobic seed coatings to delay seed germination for specific purposes. A seed coating process to enable

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synchronous flowering of male and female inbred parent lines in hybrid seed production by creating controlled delays in germination has been described by Porter and Scott (1980). The autumn sowing of some crops in Canada is claimed to be made possible by hydrophobic coatings which delay the germination of rape (Watts, 1976) and wheat (Schreiber and La Croix, 1970) until temperatures rise in spring. The use of a hydrophobic adhesive in seed coatings resulted in improved emergence of lettuce seed, particularly from wet soil (Millier and Bensin, 1974). C. OXYGENSUPPLY Attempts to supply oxygen to germinating seeds have been made using seed coatings containing peroxides (principally zinc and calcium), but not all of these coatings have been successful; such coatings had no effect on the emergence of carrots, decreased the emergence of onions (Brocklehurst and Dearman, 1983), but increased the emergence of ryegrass (Ollerenshaw, 1985). The chemical and physical requirements of such coatings have been described by Hatton and Baker (1987). The germination of wheat sown in cold, wet soil can be increased by seed coatings of calcium peroxide (Sladdin and Lynch, 1983); although this effect was attributed principally to the supply of oxygen, the authors suggested that the coating may also have had antimicrobial action or may have neutralized phytotoxic substances. A possible relationship between peroxide coatings and fungicide-insecticide seed treatments on wheat seed was also suggested by Thomson et al. (1983).

IX. TREATMENT PROCESSES Numerous seed treatments can be found in the literature that offer possible methods of stimulation of seed germination andor growth of young seedlings. Although some of these are not strictly seed coatings, many of the treatments employ chemicals or processes that could be incorporated into coatings or used during the coating process and hence should not be ignored. Heydecker and Coolbear (1977) and Tonkin (1979, 1984) have reviewed an extensive range of ways in which a seed’s performance can be enhanced by many types of treatments applied before sowing, including the use of soaking and hormone treatments as well as physical treatments in which seed is exposed to various forms of energy. One of the areas studied is that of wetting and drying seeds prior to sowing. This process, and the entire area of the water relations of seeds,

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has been the subject of a review by Hegarty (1978). Hadas (1982) described three stages of water uptake by seeds: the first is a rapid initial phase of passive imbibition, which is followed by a transition stage during which relatively little water uptake occurs. If dried at this transition stage, the seed is then “primed” for rapid germination upon subsequent wetting. The third and final stage is the growth phase, which only occurs in viable, nondormant seeds and which coincides with radicle emergence and thus signals the end of germination. Heydecker et al. (1975) “primed” seeds of many plant species by soaking the seeds in osmotically controlled solutions of polyethylene glycol (PEG), thus dramatically reducing the time from sowing to emergence of seedlings. Watt (1978) stimulated the germination of Queensland bluegrass (Dicanthiurn sericeum) by a controlled wetting and drying of seeds and termed this priming of seeds hydropedesis. Lush et al. (1981) found that controlled hydration and dehydration reduced the lag phase of germination by stimulating the development of the seed’s embryo without commencing the breakdown of the endosperm. They suggested that this reduction in the lag phase could assist establishment when conditions for germination were suitable for only a limited period. Although wetting and drying can be viewed separately from seed coating, many coatings are nevertheless applied with water-based adhesives and the seeds do in fact undergo wettingdrying cycles during the coating process; thus, it is possible that some coating processes result in a reduced lag phase during germination. Several researchers have examined ways of using inorganic chemicals to stimulate seed germination. Hendricks and Taylorson (1974) were able to accelerate the germination of seeds by treating the seeds with nitrate, nitrite, and amine and ammonium salts. Others have achieved similar results by soaking seeds in magnesium, copper, boron, and zinc solutions but the reasons for these effects have not yet been adequately explained. Growth regulators and hormones have also been used as seed-treating agents by a number of researchers who have reported stimulation of germination and growth. Puls and Lambeth (1974) soaked tomato seeds in kinetin, gibberellic acid, or potassium nitrate solutions and found an increased rate of germination; they postulated that this was due to these growth regulators correcting enzyme and substrate deficiencies at the initial stage of germination. Nelson and Sharples (1980) also found that soaking seeds of tomatoes, peppers, and sugarbeets in growth regulators resulted in a stimulation of germination, particularly at low temperatures. Similar treatments have also been shown to be effective in improving the emergence of sugarbeet seedlings under conditions of combined moisture, temperature, and impedance stress (Akeson et al., 1981). Some undisclosed seed treatments, presumably using hormones, can break thermodormancy of lettuce (Robinson et al., 1983). It may also be possible to alter apical

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dominance with seed-applied growth retardants such as those shown to affect barley by Woodward and Marshall (1987).

X. CONCLUSIONS Establishment is markedly improved when sown species possess good “establishment ability” and can compete aggressively with weeds (Cook, 1980). This statement applies to pasture establishment, as well as crop establishment. Establishment ability, however, has a relatively low heritability and is therefore difficult to select for in plants (Donald, 1963). Donald stated that the success of a competitor depends on “its capacity to make rapid use of its immediate supplies and then, by growth of its roots and foliage, extend its exploitation into a greater spatial part of the environment.” In competitive situations, even a slight early advantage of one species over another can ultimately translate into a strong competitive advantage resulting in its dominance (Cocks and Donald, 1973). Successful plants in mixed sowings are usually those that can exploit a different source of a limiting resource, such as those with deeper roots to reach moisture or ability to fix atmospheric nitrogen (Hall, 1978). Seed coatings are seen as one way in which seedlings may have an improved “competitive ability” conferred upon them. Strategies that encourage the rapid growth of the desired species whilst having minimal (or even negative) effects on the growth of its neighbors should enhance the development of seedlings, enable them to attain a large size before they encounter a period of stress, and lead t o improved establishment in the longer term. In crop establishment, early seedling growth is of great importance in obtaining optimum plant stands and maximizing yields. Even large-seeded crop species can respond to nutrient supplies external to the seed as early as 1 week after sowing (I. M. Wood, personal communication). In spite of the potential benefits that coatings offer, many of the world’s crop and pasture seeds are today sown without any coating. This fact is probably due to some of the problems in the use of coatings such as incompatibility between chemicals or processes, little or no positive effect observed, or depressions in germination or emergence. Such problems may not result in an establishment failure but they do tend to limit the potential most people see for seed coatings and they underline the fact that much of the science behind seed coatings is poorly understood. Seedling plants are often faced with more than one limiting factor governing their early growth. The chief limitations on establishment (nutrient deficiencies, competition from weeds, diseases, pests, climatic limitations,

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etc.) all need to be overcome in order for establishment to be successful. There is no literature available on the use of combinations of fungicide, herbicide, antidote, and multielement nutrient seed coatings to aid plant establishment. It is clear from much of the literature reviewed that the problems faced by plants during the establishment phase are not going to be solved by one or two simple seed coatings acting as a panacea for all limitations to early seedling growth. As shown in Fig. 1, the physical, chemical, and biological properties of seed coatings interact strongly with a myriad of other factors including the plant species, seedling vigor, soil type and fertility, climatic conditions, time of sowing, method of cultivation and sowing, weeds, pests, and diseases. Research in the area of seed coatings has, to date, been notable for the occurrence of confounded treatment factors, which occur partly because of the problems of producing coatings differing in only one factor at a time. Such confounding has been recognized by others as one reason for the difficulty of extracting general principles from such research findings (Vartha and Clifford, 1973a; Scott, 1975b). Additionally, some of the information contained in the patent literature consists of claims that are not well substantiated. The scientific literature contains relatively little information on the mechanisms responsible for the observed effects of treatments; an understanding of such mechanisms is necessary if large gains in knowledge are to be made. In the future, a more deliberate attempt needs to be made to resolve the complexity involved in seed coating research by developing sound principles on which to build a model of seed coatings and their interactions. A multidisciplinary effort from chemical and mechanical engineers, chemists, agronomists, soil scientists, and microbiologists could combine to reduce some of the biological complexity referred to above to its essential parts. A first step towards such a model could perhaps be based on the model proposed by Sharples (1981), who has suggested a seed coating with sufficient porosity (due to the relatively large size of particles adjacent to the seed) to eliminate any delays in emergence. Further fundamental work is needed concerning the interactions between soil moisture content and the hydrophilic-hydrophobic properties of seed coatings, such as the research reported by Millier and Bensin (1974). Far more research will be needed before an understanding is possible of the uptake of soil moisture and oxygen by coated seeds and the interactions with soil texture and seed coating parameters such as particle size distribution or the adhesive used. In addition to any physical factors, the chemistry of seed coatings will markedly influence their effectiveness. The susceptibility of various species to soluble nutrients during germination needs better understanding. More fundamental work could be carried out on the diffusion and uptake of

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solutes from seed coatings (Philip, 19581, the duration of susceptibility of seeds to soluble salts during imbibition, the development of cell membrane integrity during germination (Hegarty, 1978), and the further development of means of protecting seeds from injury. It appears that large increases in early plant growth and in the efficiency of fertilizer usage are possible with the use of soluble phosphorus sources. The solution appears to be to overcome the problems of damage from soluble fertilizers to emerging seedlings rather than to use relatively insoluble nutrient sources, which many workers have found to be of comparatively Little benefit. The marked influence that phosphorus can have on early seedling growth makes this element a prime target for research into nutrient seed coatings. The close placement of P is especially important under cool conditions (Robinson et al., 1959), in low P soils (Fiedler et a / . , 1983), and in soils in which P is applied at low rates (Anghinoni and Barber, 1980). It also appears that young seedlings require higher concentrations of P in the soil solution for maximum growth than are required by more mature plants (Fox et al., 1974). The fact that phosphorus fertilizers are generally less damaging to seeds during germination and emergence than nitrogen fertilizers (Olson and Dreier, 1956; Klepper et al., 1983) makes phosphorus the better initial candidate for the development of macronutrient seed coatings that are “safe” for germinating seeds. Eventually, reliable multielement nutrient coatings should be capable of development. If sufficient basic information can be accumulated to describe adequately the behavior and performance of the many materials that can be included in seed coatings, then ultimately, quite complex coatings that will be effective over a wide range of conditions should be capable of development. Such seed coatings offer a unique opportunity to use lesser amounts of fertilizers, pesticides, etc. in a way that may be more effective, safer ecologically, and more economical.

ACKNOWLEDGMENTS The financial support for some of the research by the author reported in the review was provided by the Australian Meat and Livestock Research and Development Corporation, the Rural Credits Development Fund of the Reserve Bank of Australia, and the Australian Wool Corporation and is gratefully acknowledged. Thanks are due to Dr. Alan Andrews and especially to Dr. Graeme Blair, who provided constructive criticisms of drafts of this review.

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Tonkin, J. H. B. 1979. In “Advances in Research and Technology of Seeds” (J. R. Thomson, ed.), Part 4, pp. 84-105. Pudoc, Wageningen. Tonkin, J. H. B. 1984. In “Advances in Research and Technology of Seeds” (J. R. Thomson, ed.) Part 9, pp. 94-127. Pudoc, Wageningen. Uhvits, R. 1946. Am J. Eot. 33, 278-285. Urich, D. M., and Han, T. M. 1%2. In “Agglomeration” (W. A. Knepper. ed.), pp. 669713. Wiley (Interscience), New York. Vartha, E. W., and Clifford, P. T. P. 1973a. N . Z . J . Exp. Agric. 1, 3 9 4 3 . Vartha, E. W., and Clifford, P. T. P. 1973b. N.Z. J . Exp. Agric. 1, 181-186. Vidhyasekaran, P. 1980. Seed Sci. Techno/. 8, 357-362. Vogelsang, P. 1954. U.S. Pat. No. 2,671,985 (16 Mar). Watkin, E. M., and Winch, J. E. 1974. An assessment of shallow soil pastures in Ontario. Rep. of A.R.D.A., Ontario. Canada, Proj. No. 85045. Watt, L . A. 1978. Ausr. J . Agric. Res. 29, 1147-1 155. Watts, H. 1976. U.S. Pat. No. 3,947,996 (6 Apr). Bagley, E. B.. Fanta, G. F., and Doane, W. M. 1976. U.S. Pat. No. 3,981,100 Weaver, M. 0.. (21 Sep). Welty. L. E., Anderson, R. L., Delaney, R. H. and Hensleigh, P. F. 1981. Agron. J . 73, 813-816.

Wilson, G. L., and Notley, L. F. 1959. Aust. J . Agric. Res. 10, 621-627. Wong, P. T. W., and Baker, R. 1984. Soil Eiol. Biochem. 16, 397404. Woodward, E. J., and Marshall, C. 1987. Ann. Appl. Biol. 110, 629-638. Yams, L. 1982. Agric. Res. (Washington D.C.) 31, 13. Yen, S.-T., and Carter, 0. G. 1972. Ausr. J . Exp. Agric. Anim. Husb. 12, 653-661. Younger, D. R., and Gilmore, J. M. 1978. Trop. Grass/. 12, 163-169.

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ADVANCES IN AGRONOMY, VOL. 42

CONSERVATION TILLAGE FOR SUSTAINABLE AGRICULTURE: TROPICS VERSUS TEMPERATE ENVlRONMENTS Rattan La1 Department of Agronomy Ohio State University Columbus, Ohio 43210

I.

Introduction

11. Conservation Tillage and Sustainable Agriculture

111.

IV.

V.

VI. VII.

VIII.

IX.

A. Tillage B. Mulch C. Crop Rotations Mulch and No-Till Farming for Different Ecological Environments A. Mulch Farming B. No-Till Systems Pros and Cons of the No-Till System: Tropics versus Temperate Zones A. Soil Temperature B. Soil Moisture C. Hard-setting D. Compaction E. Erosion F. Soil Fertility Noninversion and Minimum Tillage A. Semiarid and Arid West Africa B. Southern and Eastern Africa C. South and West Asia Subsoiling as Conservation Tillage Conservation Tillage for Problem Soils A. Low Plant-Available Water Reserves B. Poorly Drained Soils C. Crusting and Compaction D. Leaching Why Conservation Tillage? A. Improvements in Soil Structure B. Soil and Water Conservation C. Favorable Soil Moisture and Soil Temperature Regimes D. Soil Chemical and Nutritional Properties and Fertilizer Response E. Root Growth F. Energy Conservation G. Preventing Soil Degradation and Maintaining Soil Fertility Environmental Pollution and Conservation Tillage 85 Copyright Q 1989 by Academic Press. Inc. All rights of reproduction in any form reserved.

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X. The Systems Approach to Conservation Tillage and Supportive Cultural Practices A. Agroforestry and Alley Cropping B. Cover Crops C . Live Mulch D. Rotations and Multiple Cropping E. Summer Fallowing XI. Soil Guide to Conservation Tillage XI1. Research and Development Priorities XIII. Conclusions References

I. INTRODUCTION Soil degradation is a serious global problem. It is estimated that the total historical loss of arable lands due to soil degradation is more than the whole area now under cultivation (FAO, 1983; UNEP, 1986). The current annual rate of loss of arable land to soil degradation is estimated at 5-7 million hectares (UNEP, 1986), and it is feared that the rate may increase to 10 million hectares (ha) annually by the year 2000 (Dudal, 1982). With an ever-increasing demand for prime agricultural land for nonagricultural uses, however, the per capita arable land is shrinking rapidly. Even if the total arable land area of the world is maintained at 14.39 x 10' ha, the per capita arable land is projected to decline to 0.23 ha, 0.15 ha, and 0.14 ha by the years 2000, 2055, and 2100, respectively, due to the expected increase in population. The provision of the minimum dietary requirements on the basis of a meager 0.14 ha of per capita land, therefore, can only be met by technological innovations that bring about drastic increases in productivity. With existing proven technologies the best agriculturist have been able to do to date, is to maintain constant per capita production of livestock, edible plants, and commercially harvested wood for the decade ending in 1982 (Hortenstein, 1986). The planners must, however, consider the options of increasing per capita production or at least maintaining at this level until the world population stabilizes by the year 2100! Soil degredation, the decline in soil quality, affects about 35 % of the earth's land surface (Mabbutt, 1984). The problem is, however, more severe in the tropics than in the temperate zone. Although the processes involved in soil degradation are the same in all ecological regions, the relative importance of different degradative processes is, nonetheless, different in temperate and tropical regions. Processes leading to soil degradation include accelerated soil erosion by wind (Fig. 1) and water, salinization and alkalization, leaching, physical degradation including

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FIG. 1. Wind erosion is severe in the West African Sahel and in other and and semiarid regions of the world.

compaction and hard setting, laterization (Fig. 2), and biological degradation (Lal, 1988). The predominant degradative processes in the tropics are erosion and desertification, leaching, physical degradation and soil compaction, laterization, and biological degradation. More than half of the estimated 11 million hectares of forest cleared annually (Lal, 1986b; FAO, 1983), is believed to be brought under cultivation to replace degraded agricultural soil (Pimentel et al., 1986, 1987).

FIG.2. About 300 million ha are covered by hardened laterite in the West African region.

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An inevitable consequence of intensive land use is the pollution of natural waters and environments. An intensive use of chemical fertilizers, herbicides, and pesticides increases both crop production and risks of pollution. The recovery of nitrogenous fertilizers by crops is hardly 5060% and often as low as 12% (Gilliam et al., 1985). The unrecovered fertilizers and pesticides are washed away in surface runoff or leached out in the seepage water. The annual discharge of pollutants to waterways from agricultural lands in the United States is estimated at 1079.2 million tons (t) of total suspended solids, 477.3 tons of total dissolved solids, 1.16 million tons of total P, and 4.65 million tons of total N (Duttweiler and Nicholson, 1983). The energy inputs of modern intensive agriculture are becoming debatable even for progressive farmers of western Europe and North America (Pimentel, 1984; Shahbazi and Goswami, 1986). Agricultural efficiency, measured as caloric outputfinput, progressively declines to an uneconomic level with an increasing degree of soil degradation, requiring increasing inputs of tillage, fertilizers, irrigation, and pesticides (Pimentel, 1984). Even if some farmers can live with these intensive inputs for a time, neither the world as a whole nor the farmers of the tropics and subtropics can afford such wasteful luxuries, paid for heavily in both environmental and economic prices. Soil and environmental degradation, environmental pollution, and expensive inputs are all related to soil mismanagement and to inappropriate agricultural practices. The latter include deforestation and expansion of agricultural activities to marginal and steep lands, excessive plowing to produce clean seebed and fine tilth, monoculture, reduction in the frequency and duration of fallowing, indiscriminate use of chemicals, etc. These agricultural practices, although they no doubt yield high short-term gains, are destructive in the long run. Low-input sustainable agriculture is a resource management strategy aimed at reducing dependence on energy-based inputs. It is based on the use of innovative soil and crop management techniques and the use of renewable inputs to attain satisfactory returns, optimize resource use, and preserve a healthy balance of soil, food, people and environment. Sustainable alternative agricultural systems involve the use of new crops and cultivars that are adapted to specific soil- and environmental-related constraints, multiple and rotational cropping systems based on legumes and agroforestry techniques, integrated pest management, and conservation tillage. These practices based on principles of conservation farming are not always high-yielding, especially on a short-term basis. Adoption of these practices, however, reduces the risks of soil degradation, preserves the soil’s productive potential, decreases the level of inputs required, and sustains productivity over the long term.

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Conservation tillage is an integral component of sustainable agriculture. It is aimed at preserving the resource base. The objectives of this review are to define the relation between conservation tillage and sustainable agriculture; identify appropriate conservation tillage systems for different soils, crops, and agroecological regions; define their potentials and constraints; and outline research and development priorities. Appropriate examples are chosen from the tropics and are compared with a few relevant examples from the temperate regions. The discussion highlights tillage needs for major ecological regions of contrasting soils and climates, with a view to sustaining satisfactory returns and reducing risks of environmental degradation.

II. CONSERVATION TILLAGE AND SUSTAINABLE AGRICULTURE Given that land is a finite resource and that it is being depleted by degradation and nonagricultural uses, it is essential that agricultural practices be adopted that preserve its productive potential. Conservation tillage is one such practice. Conservation tillage is a generic term encompassing many different soil management practices. It is generally defined as “any tillage system that reduces loss of soil or water relative to conventional tillage; often a form of non-inversion tillage that retains protective amounts of residue mulch on the surface” (Mannering and Fenster, 1983). This definition may be rather narrow if the concept is to be applied to a broad range of ecological environments. Important criteria for a tillage system to be classified as conservation tillage include

(i) presence of crop residue mulch; (ii) effective conservation of soil and water; (iii) maintenance of a favorable level of or improvement of soil structure and organic matter contents; (iv) maintenance of a high and economic level of productivity; (v) minimal needs for chemical amendments and pesticides; (vi) preservation of ecological stability; and (vii) minimal pollution of natural waters and environments. These criteria can be satisfied by a range of cultural practices, such as

(i) using crop residue as mulch (ii) adopting noninversion or no-tillage systems; (iii) using crop rotations based on cover crops, buffer strips, and/ or agroforestry;

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(iv) improving infiltration capacity of soil through rotation with deeprooted perennials and modification of the root zone; (v) increasing surface detention capacity of the soil through using rough and cloddy seedbeds and ridge-furrow systems; (vi) enhancing the biological activity of soil fauna through soil surface management; and (vii) reducing cropping intensity to conserve soil and water resources and improve soil fertility. Conservation tillage is, in fact, an umbrella term that encompasses a wide range of cultural practices. It is an ecological approach to seedbed preparation and soil surface management. Therefore, suitable practices differ among different ecological regions. The criteria and objectives listed above involve three principal practices of soil and crop management: tillage, mulch, and rotations; these will be discussed in greater detail.

A. TILLAGE The usefulness of the moldboard plow as the means of weed control and of seedbed preparation has long been brought into question by the rapid technological advances in herbicides and in our understanding of the processes governing accelerated soil erosion. The monopoly of the plow as a soil management tool has, therefore, been replaced by other technological innovations (Unger, 1980; Phillips et al., 1980a). Methods of seedbed preparation significantly influence soil structure, soil and water conservation, weed infestation and pest incidence, the decomposition rate of soil organic matter content, the activity and population of soil fauna, the soil temperature regime, seed germination and seedling emergence, nutrient uptake and fertilizer use efficiency, and crop growth and yield. Mechanical seedbed preparation also represents a major investment in crop production, both in terms of fixed and variable costs. Consequently, a range of tillage systems have been developed to alleviate the soil-related constraints to crop production. These tillage systems include (i) complete elimination of preplanting mechanical seedbed preparation, as in a no-till system (Fig. 3); (ii) cultivation in the row zone only, as in strip tillage or zonal tillage; (iii) subsoiling in the row zone to loosen the compacted subsoil, as in chiseling or paraplowing;

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FIG.3. No-tillage: crop residue mulch covering the interrow zone.

(iv) using noninversion and minimum tillage systems, as in disk planting systems, wheel-track planting, plow planting systems, or ridge planting; (v) conventional tillage, including a complete range of primary and secondary tillage operations; and (vi) using a ridge-furrow system to increase the surface detention capacity of the soil and/or to provide surface drainage (Sprague and Triplett, 1986). Timing of tillage operations can also be adjusted to facilitate operations during the periods of peak labor demand. The latter includes practices such as plowing at the end of rains in the tropics, and fall plowing followed by spring disking in temperate zone. Tillage is an energy-intensive activity and uses as much as 11% of the energy used on an average farm in North America. Energy requirements also vary widely among different types of tillage systems used. The diesel fuel requirements are estimated at 17.2, 10.5, 5.9, 3.9, 2.9, and 1.21/ha for moldboard plowing, chisel plowing, disk plowing, cultivating, planting, and spraying, respectively (Shahbazi and Goswami, 1986). Land clearing, seedbed preparation, planting, and weeding also form a major component (70-90%) of labor needs for traditional farming in tropical Africa (IITA, 1981).The choice of an appropriate and an energy-efficient tillage method is, therefore, an important consideration in farm economy.

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B. MULCH Crop residue mulch is an important agricultural tool with which to conserve soil, maintain the quantity and quality of water running off the agricultural land, regulate soil temperature and moisture regimes, improve soil physical conditions by enhancing biological activity of soil fauna, and increase soil fertility. Crop residues are also used for nonagricultural purposes; they can be energy sources, industrial materials, and environmental protection tools (Larson, 1979). In addition, crop residues are a major component of livestock feed. In the tropics, crop residues are used for fencing, roofing, and as a source of household fuel. The residues that can be used for mulching are, therefore, always in short supply. There are four ways of procuring crop residue mulch at the scale of practical farming.

(i) The residue from the previous crop can be used as mulch through some form of no-till, minimum tillage, or no- inversion tillage system. For the system to be successful, however, it is necessary that an appropriate rotation is followed so that at least one crop leaves enough residue that can be used as mulch. In general, cereals produce considerable amounts of a slow-to-decompose residue that can be used as mulch for the succeeding crop. Important cereals are maize (Zea mays), sorghum (Sorghum bicolor), rice (Oryza sativa), and millet (Pennisetum and Panicum sp.) in the tropics, and wheat (Triticum aestivum), barley (Hordeum vulgare), and oats (Avena sativa) in the temperate climate. Leguminous crops, such as soybeans (Glycine max) and cowpeas (Vigna unguiculeta), either do not produce enough residue or it decomposes more readily than those of cereals (De Vleeschauwer et al., 1978, La1 et al., 1980). (ii) Specific crops can be grown to produce biomass that can be used as a source of mulch for the succeeding one or two crops. These crops are commonly referred to as cover crops and may be either legumes or grasses. Some important leguminous crops are kudzu (Pueraria phaseoloides), stylo (Stylosanthes guianesis), centro (Centrosome pubescens), mucuna (Mucuna utilis), etc. for the tropics, and alfalfa (Medicago sativa L.), tall fescue (Festuca arundinacea), hairy vetch (Vicia villosa), and crimson clover (Trifolium incarnatum) for the temperate zone. Grasses often have more beneficial effects on soil structure than legumes (Pereira et al., 1958). However, some grasses are eradicated with difficulty (La1 et al., 1978). Some useful grasses for the tropics include guinea grass (Panicum maximum), elephant grass (Pennisetum purpureum), molasses grass (Setaria sp.), and bahia grass (Paspalum notatum). Some useful grasses for the temperate zone are rye (Secale cereale), rye grass (Lolium multiflorum), and bluegrass (Poa pratensis).

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Food crops are grown through the mulch produced by the cover crop. Some cover crops die naturally and produce a nicely mulched seedbed ready for seeding through with a no-till system, e.g., mucuna grown on Alfisols in subhumid regions (Fig. 4). Seasonal crops are also grown through the live mulch of a cover crop that has been only partially suppressed by mechanical mowing or slashing or by herbicide.

(iii) A form of mixed cropping specifically to produce mulch can be adopted. The mixed cropping may be in the form of alternate strips planted to a food and a mulch-producing crop (Fig. 5). The latter strip may be used to grow annual grass and/or legume covers, or perennial shrubs and trees. The vegetation in the mulch strip is pruned regularly and prunings are used as mulch for the food crop. Rather than a strip, a narrow but contoured hedge of grass and woody shrubs can also be used to control runoff and erosion and provide a reliable source of mulch. (iv) The mulch material may be imported. For some high-value cash crops, such as horticultural crops, it may be economical to bring in the mulch material from another field. It is also possible to use some synthetic materials (polythene sheets or emulsified soil conditioners) as mulch. Polythene sheets are used as mulch for pineapple (Ananas comosus) production

FIG.4. Mucuna utilis is widely recommended as a cover crop for the subhumid regions of West Africa.

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FIG.5. Mixed cropping of cereals with legumes provides the needed diversity to create a stable production system.

in Hawaii (Ekern, 1%7); tomatoes (Lycopersicon esculentum) in the United States (Geraldson et al., 1966), sisal (Agave sisalana) in Tanzania (Hopkinson, 1969), and seed yam (Dioscorea sp.) in Nigeria (IITA, 1984). Petroleum products, such as bitumen, are also used as mulch under special circumstances (Blore, 1964; Collis-George et al., , 1963; De Vleeschauwer et al., 1978). Bringing in large quantities of mulch for Large-scale food crop productions, however, can be economically prohibitive.

c. CROP ROTATIONS Cropping sequences (rotations) and crop combinations (mixed and relay cropping) are an important consideration for conservation tillage. Oversimplification of an ecosystem by continuous monocropping is bound to cause ecological imbalance and create problems in soil, hydrology, and biotic environments. Diversification is, therefore, an important component of a successful conservation tillage. Successful and ecologically compatible crop rotations are those that involve growing crops in temporal or spatial arrangements and sequences such that the soil surface is continuously protected by a canopy cover, nutrients and water reserves are efficiently utilized by growing deep-rooted

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crops in rotation with surface feeders, pests are kept under control by growing cereals in combination with legumes, soil fertility is maintained by growing open row, soil-depleting crops in association with closed canopy, soil-conserving crops, the soil surface is covered by growing crops that produce less residue in sequence with those that produce a large amount of biomass, and continuity and stability of macropores is improved or maintained by rotating shallow-rooted crops with those characterized by a deep taproot system. In addition to choosing appropriate crop species, selection of suitable varieties is equally important for conservation tillage to be successful. Plant breeders must develop cultivars that are able to germinate and establish a stand through an unplowed and trashy seedbed, are relatively tolerant to suboptimal conditions of soil temperature and moisture regimes and unfavorable microclimates in the seed zone, and can withstand possible high pest incidence in a mulched rather than a cleanly tilled seedbed.

Ill. MULCH AND NO-TILL FARMING FOR DIFFERENT ECOLOGICAL ENVlRONMENTS

N o single conservation tillage system has a universal application. Different types of conservation tillage are needed to alleviate specific soilrelated constraints. The specific soil-related problems include accelerated erosion, too much or too little water, supraoptimal or suboptimal soil temperature, surface sealing and soil compaction, low water infiltration rate, high losses of plant nutrients by leaching or water runoff, groundwater pollution, heavy weed infestation, and the need to manage crop residue. There is more than one solution to each of these problems. One common cultural technique, or subsystem, for solving these problems is the judicious management of crop residue. A. MULCHFARMING There are four possible options for dealing with crop residue: carting it away, burning, plowing under, or leaving it on the surface as mulch. If socioeconomic and biophysical conditions permit, ecologically the most desirable option is that of leaving the residue on the surface as mulch. Among the important reasons for choosing this option is the fact that proper use of crop residue can be the best means of controlling wind and water erosion in both tropical and temperate climates (Lal, 1984a; Phillips e t a / . , 1980a; Larson, 1979; Skidmore et af., 1979; Gupta ef af., 1979);lowering

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the maximum soil temperature in the tropics, where supraoptimal temperatures adversely affect crop growth and yield (Harrison-Murray and Lal, 1979; Maurya and Lal, 1981; Gupta and Gupta, 1986); improving water infiltration in soils that are prone to crusting and compaction (Lawes, 1962; La1 et al., 1980); increasing soil-water storage in the root zone (Lal, 1975; Tisdall and Adem, 1986); and increasing crop growth and yield, especially in those tropical soils that have low nutrient reserves and are prone to frequent drought stress (Lal, 1975; ICAR, 1982; Sanchez and Salinas, 1981; Schoningh and Alkamper, 1985). In addition to improving production in grain crops (annuals), mulches are also profitably used for managing perennial crops, e.g., coffee (Coffeaarabica, and tea (Camellia rhea) in Kenya (Jones er al., 1961; Northmore, 1963; Mehlich, 1965; Mitchell, 1967, 1968), sisal (Agave sisalana) in Tanzania (Northmore, 1963; Lock, 1969), and plantains (Musa sp.) in Nigeria (IITA, 1982). The most widely used conservation tillage techniques, therefore, are based on using the crop residue mulch. A relevant example of the beneficial effects of mulch on the yield of beans (Phaseolus vulgaris) is shown by the data from a study conducted in Western Samoa (Table I). Some mulch materials, such as coconut (Cocos nucifera) fronds, increased bean yield by almost 400%. Any mulch (even stones) was better than no mulch. Mulches decreased the incidence of Southern Blight and increased yield. Mulches caused delay in the germination of the fungus Sclerotium rolfsii and reduced the degree of infection. Similar beneficial effects of a range of mulch materials on yields of cassava (Manihoc esculenta), maize, cowpeas, and soybeans were shown in field experiments conducted on Alfisols and Ultisols in Nigeria

Table I Effects of Mulch Materials on Grain Yield of Dwarf Beans in Western Samoa" Treatment

Total yield (9)

Increase over control (%)

No mulch (control) Black polythene White polythene Wood shavings Aluminum foil Stones Grass cuttings Coconut fronds

998.0 3972.8 3877.1 3260.9 3741. I 3744. I 3817.7 4958.3

0 297 288 226 274 275 282 396

"From Reynolds (1970).

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by Okigbo and La1 (1982) (Table 11) and Maduakor et al. (1986). Even the sawdust- and stone-mulched plots produced greater yields than the unmulched control in some cases. In Nigeria, mulching was as effective in increasing crop yields as fertilizer (ICRISAT, 1985). Experiments conducted at different locations in India showed approximately a 25% increase in the yield of wheat, 40% increase in the yield of tobacco (Nicotiana tabacum), 80% increase in the yield of sorghum, and 10% increase in the yield of barley (ICAR, 1982). In addition to surface applications, slot mulching also increased crop yields. Increases in yields are due to favorable soil temperature and moisture regimes (Minhas et al., 1986). In northern Thailand Ratchdawong et al. (1984) reported that mulching increased yields by 78% in upland rice by 33% in peanuts (Arachis hypogaea). Mulching also resulted in significant yield increases of maize and

Table I1

Effects of Mulch Materials on Yields of Cassava, Maize, Cowpeas, and Soybeans" Yields Mulch

Cassava

Maize

Cowpeas

Soybeans

Unmulched control Maize stover Maize cobs Oil palm leaves Rice straw Rice husks Pennisetum sp. Elephant grass Guinea grass Andropogan straw TYPha Cassava stem (chipped) Pigeon pea tops Pigeon pea stem (chipped) Legume husk Soybean tops Eupatorium Mixed twigs (chipped) Sawdust Black plastic Translucent plastic Fine gravels

16.4 def 16.4 def 17.8 cdef 17.1 def 17.9 cdef 28.3 a 14.2 ef 16.6 def 15.5 f 18.5 cdef 16.7 def 20.9 cd 22.9 bc 19.9 cde 26.4 ab 22.9 bc 18.8 cdef 18.5 cdef 20.5 cde 30.5 ab 27.7 ab 22.9 bc

3.0 c 3.3 cd 3.3 cd 3.2 cd 3.5 bcd 3.7 abc 3.3 cd 3.3 cd 3.6 bcd 3.5 bcd 3.1 cd 3.8 abc 3.7 abc 3.5 bcd 4.4 a 4.2 ab 3.6 abc 3.4 bcd 3.7 abc 3.0 cd 2.7 d 3.1 cd

0.6 a 1.1 a 1.1 a 1.2 a 1.0 a 1.1 a 1.2 a 0.9 a 2.1 b 1.0 a 1.0 a 0.8 a 1.1 a 1.0 a 1.0 a 1.0 a 1.0 a 1.0 a 0.9 a 0.9 a 1.0 a 1.0 a

0.6 de 1.5 abc 1.4 abcd 0.9 bcde 1.5 abc 0.8 de 1.4 abcd 1.3 bcd 1.5 a b I .2 bcde I . 1 bcde I .4 abcd 0.9 cde I .3 bcd 1.5 abc I .2 bcde 1.2 bcde 1.2 bcde 1.9 a 1. I bcde I . I bcde I .O bcde

"From Okigbo and La1 (1982). 'Letters following the yield figures refer to Duncan's New Multiple Range test; figures followed by the same letter are not significantly different at the 5% level.

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cowpeas in an eastern Amazon Oxisol (Schoningh and Alkamper, 1985). In the United States, Unger (1978a,b) observed significant benefits of mulch on water conservation and on grain yield of sorghum in Texas. The effects of mulch on crop response are greatly influenced by the amount of rainfall and its distribution; by microclimate; and by the soil moisture storage. In general, mulch has less beneficial effects in regions with greater than with lesser rainfall. Mulch farming may under some conditions, in fact, have adverse effects on crop growth. These include suboptimal soil temperatures and slow warming in the spring in northern latitudes and high risks of pest incidence. Seed germination and seedling establishment are often more severe problems in mulched than in unmulched seedbeds. In spite of its overwhelming advantages, therefore, mulch farming cannot be practiced for all soils, climates, and crops. For some arid regions, such as the West African Sahel, mulch material is not always available in the quantity needed. There is a minimum quantity of mulch needed for conserving soil and water (Lal, 1976a), regulating soil moisture and temperature regimes, improving soil structure (La1 et al., 1980), and increasing crop yields. Unger et al. (1986) observed that grain yield increased by 0.016 t/ha/mm of rain when the mulch amount was 0-0.4 t/ha, and 0.27 t/ hdmm of rain when the residue mulch was more than 3.2 tha. Considering the short supply of mulch and its alternate uses, it is important to quantify mulch requirements for soil resource management for different crops and ecological regions and to define which conservation tillage system is applicable for what environments. B. NO-TILLSYSTEMS No-tillage systems involve complete elimination of mechanical seedbed preparation and reliance on herbicides and cover crops to kill or suppress weed growth. The crop is seeded in an undisturbed soil and residue mulch is firmly anchored to the ground. Severe problems of accelerated soil erosion and high costs of energy inputs with plow-based methods of seedbed preparation have led to a wide adoption of no-tillage systems for production of row crops. The system is, however, applicable to only some soils, crops, and ecological regions. A no-tillage system is suited for those problem soils that are highly susceptible to erosion, are well drained and have a low water- holding capacity, are prone to surface sealing and compaction, and have limitations of supra rather than suboptimal soil temperatures during the seedling stage of crop establishment (Lal, 1985b; Allmaras and Dowdy, 1985; Cannell, 1985). Successful crop establishment with a notill system also depends on the antecedent soil conditions and the land

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use history. The following criteria must be met in a successful no-tillage system:

(i) Erosion is reduced to a level below the limit of tolerable soil loss. (ii) Soil-water conservation significantly decreases the frequency and intensity of short (10-15 days) midseason drought effects. (iii) Soil structure, infiltration rate, and soil organic matter content is maintained at or improved to a level favorable for crop production. (iv) Satisfactory seed germination and seedling emergence for an adequate crop stand and an acceptable weed control are possible. (v) Acceptable economic returns and satisfactory yields are sustained over a long time period. The latter point needs further clarification. Even a lower level of absolute yields with a no-tillage system is acceptable provided that it is economic and that it is sustained without causing severe degradation to soils and environments. Some examples of the successful use of the no-tillage system are discussed below.

1 . Tropics Because of a higher rate of residue decomposition in the tropics than in the temperate regions, an adequate amount of crop residue mulch rather than its excess is an important determinant of crop performance. Another important determinant is the high susceptibility of soils in the tropics to compaction and hard setting. Considering these two factors, a no-till system is likely to succeed better in the upland soils of the humid and subhumid than in the semiarid or arid tropics (Lal, 1985a). Humid and subhumid tropics. There are many examples of the successful use of no-tillage systems for production of row crops in regions where rainfall exceeds the potential evapotranspirationfor at least 7 month a year. These regions are characterized by one or two long growing seasons, which permit production of sufficient biomass that can be used as mulch. The no-till system has been successful in West and central Africa for cultivation of maize, cowpeas, soybeans, yams, and cassava (Lal, 1982). Maize is grown in regions with an annual rainfall of 800-1500 mm. Experiments conducted in Nigeria have shown that satisfactory yields are obtainable even on a long-term basis provided that maize is grown in rotation with legumes, weeds are adequately controlled, crop residue mulch is available, and soil compaction is curtailed by avoiding excessive vehicular traffic (Rockwood and Lal, 1974; Curfs, 1976; Wijewardene, 1980; Lal, 1982; Osuji, 1984: Lal, 1986a-c). A long-term experiment with maize

100

RATTAN LAL

on an Alfisol at Ibadan, Nigeria, showed that if vehicular traffic is eliminated, satisfactory maize yields are obtainable even with continuous cropping of maize for two crops a year for 17 consecutive years (Lal, 1982, 1984a). Maize grain yields could have been even better if suitable crop rotations were observed. In another experiment on similar soils, La1 (1986d) reported satisfactory maize yields for no-till with residue mulch for 8 consecutive years. In comparison, however, lower yields were obtained for crops on plowed or ridged seedbeds (Fig. 6). In the same ecological region, equivalent or greater yields of cowpeas and soybeans were obtained with no-till than with a plow-based system (Nangju, 1979)(Table 111). In the coastal savanna zone of Ghana, Leyenaar and Hunter (1976) observed yield reduction in ridged versus flat- planted maize. Soil compaction is a severe yield-limiting factor for the mechanized notill systems, especially on soils with predominantly low-activity clays (Lal, 1985a; Kayombo et al., 1986a,b). Drastic declines in yields of maize are observed with a no-till system that renders a soil prone to severe compaction (Table IV). It is important, therefore, that soils intended for grain

II

4.5

U

4

U

-- 3.5

i3 j

2.5

2

9 1.5

1

0.5 0 I

1981

1982

I

I1 I 11 1983 1984 Consecutlvecrops 1

1980

1

I

1985

I1

1 ' 1 1 ' 1

1986

1987

FIG. 6. Maize grain yield for 10 consecutive crops of maize grown on an Alfisol at Ibadan, Nigeria, with no-till or plowed methods of seedbed preparation, with and without crop residue mulch.

101

CONSERVATION TILLAGE Table I11

Effects of Tillage Methods on Grain Yields of Soybeans and Cowpeas Grown on a Tropical Alfiil at Ibadan, Nigeria"

1973"

1974'

Tillage methods

I

I1

Cowpea yield (kg/ha) Ridges Conventional plowing Strip tillage No tillage

1211 b 1499 a b 1496 a b 1609 a

1252 a 1318 a 1377 a 1579 a

Soybean yield (kg/ha) Ridges Conventional plowing Strip tillage No tillage

1916 b 1813 b 2228 a 2017 ab

1979 I1

1

I1

1185 b 1274 b 1538 a 1649 a

1039a 1003 a 861 b 651 b

I545 a I505 a 1238 b 1173 b

-

0c 2490a 2569 a

-

1

2107 ab 2387 a 1984 bc 1656 c

1818 b

"From Nangju (1979). "I, First season (March-July); 11, second season (August-December). 'Letters following the yield figures refer t o Duncan's New Multiple Range test: figures followed by the same letter are not significantly different at the 5% level.

production with a no-till system are cleared, developed, and managed to minimize risks of soil compaction. Soil compaction is less with manual than with mechanized land clearing methods (La1 and Cummings, 1979; Lal, 1981; Hulugalle et a f . , 1984), and less with manual than with motorized farm operations. Soil compaction can be alleviated by growing deep-rooted legumes such as mucuna (Hulugalle et al., 19861, pigeon pea (Cajanus Table IV Reduction in Maize Grain Yield Due to Traffic-Induced Soil Compaction on a Tropical Altis~l'''~

Observations Grain yield (t/ha)

Yield decline after 1978 (%) ~

Years

Tillage method

1975

1976

1977

1978

1979

1980

No-till Plowed

2.8 2.7

4.5 4.0

4.8 3.9

5.0 4.0

3.8 2.9

3.0

No-till Plowed

-

-

-

-

-

11

21

1.0

30 73

~~

"From La1 (1984a). "Mechanized harvesting was introduced in 1980. Yield reduction is calculated considering the yields of 1975-1978 at 100.

102

RATTAN LAL

cajan) (Hulgugalle and Lal, 1986), or deep-rooted perennials such as Leucaena leucocephala (Juo and Lal, 1977). Soil compaction can also be alleviated by mechanical means. The paraplow has proven effective in loosening a compacted Alfisol without soil inversion. This equipment, however, is an energy- intensive device, is prohibitively expensive for small landholders of the tropics, and requires a powerful tractor to pull it. Tropical root crops, such as yams and cassava, are grown in humid regions with mean annual rainfall of 15W3000 mm. The soils of this region have a pH of 4.5-5.5 and have low plant-available nutrient reserves. Many experiments have shown that the most important factor for obtaining a high tuber yield in these environments is the mulch rather than tillage (Vine, 1981; Ohiri, 1983; Anazodo and Onwualu, 1984; Anazudo, 1986; Opara-Nadi and Lal, 1987a, b; Hulugalle and Lal, 1986). Germination and emergence of yams is often faster and greater on mulched than on unmulched ridges and mounds (Hahn et al., 1979). Mulching increases tuber yield of yams through its favorable effects on soil temperature and moisture regimes. The data in Tables V, VI, and VII show a significant effect of mulch on fresh tuber yield. The tuber yield, however, can be drastically limited if the effective rooting depth is less than 50 cm. If the rooting depth is restricted either by the presence of a hardpan layer or by a high water table, tuber yield is increased by growing yams on raised seedbeds. Among cereals, upland rice is known to have a poor germination, low stand, and reduced grain yields when grown under no-tillage rather than Table V Effects of Mulching on Fresh Tuber Yield of Yam"

Treatment

Tuber yieldh.' (t/ha)

Complete surface mulch Mulch incorporated Row zone mulch Unmulched control Mean

13.6 a 10.9 b 12.8 ab 13.4 ab 12.7

"From Opara-Nadi and La1 (1987a). "The least significant difference between means at the 5% level of significance is 2.6. 'Means in a column followed by the same letter are not significantly different at the 5% level by Duncan's New Multiple Range test.

CONSERVATION TILLAGE

I03

Table VI

Effectsof Tillage Methods and Mulching on Fresh Tuber Yield of Yam"

Tillage methods

Mulch application

Tuber yield'.' Wha)

No-till No-till Conventional plowing Conventional plowing

Without With Without With

9.8 a 11.9 ab 10.9 a 13.9 b

"From Opara-Nadi and La1 (1987b). "The least significant difference between means at the 5% level of significance is 2. I. 'Means in a column followed by the same letter are not significantly different at the 5% level by Duncan's New Multiple Range test.

a plow-based system (Ogunremi and Lal, 1986). Poor seed-soil contact, damage to young seedlings by birds, and nutrient imbalances in the soil are major problems. Transplanting of rice seedlings through the killed mulch of a cover crop, although labor-intensive, may be an agronomically satisfactory method of obtaining a good crop stand. In western Cameroon, however, Ambassa-Kiki er al. (1984) reported no differences in rice grain yield among no-tillage, minimum tillage, and conventional tillage methods when rice was grown on vertisols with a high moisture retention capacity. Table VII Fresh Tuber Yield of Cassava as Influenced by Methods of Seedbed Preparation and Mulching"

Mulch application

Tillage treatment ~~

~~

~

No-till No-till Conventional plowing Conventional plowing

____

Without With Without With

Cassava tuber yieldh (t/ha) ~ _ _ _ _

~

12.7 a 16.8 b 13.1 ab 14.5 ab

"From Opara-Nadi and La1 (1987b). "Means in a column followed by the same letter are not significantly different at the 5% level by Duncan's New Multiple Range test.

104

RATTAN LAL

The mean grain yields and soil bulk density (of the 6-1 1-cm layer) were 6240 kg/ha and 1.46 g/cm3for no-tillage, 6332 kg/ha and 1.61 gkm3 for minimum tillage, and 5934 kgha and 1.56 g/cm3for the conventional tillage, respectively. Also, for hydromorphic soils with low infiltration rates, satisfactory yields of lowland irrigated rice have been reported with an unpuddled or no-till system (Rodriguez and Lal, 1985; Lal, 1986d) (Fig. 7). No-till farming has also proven successful for production of row crops in the tropics of Central and South America. In Costa Rica Shenk and Saunders (198 1) observed that no-till and reduced tillage systems produced 8150 kg ha-' N

4-

-

I

6-

0 kg ho" N 54-

3-

2-

0

I

I 1978

I

I

X 1979

I

P 1980 CROP

I

I 1981

I

L 1982

I

X 683

SEQUENCE

FIG.7. Effects of tillage methods (NT, no tillage: P, plowed) on yields of lowland rice on a coarse-textured hydromorphic soil in Nigeria. (From Lal, 1986d.)

CONSERVATION TILLAGE

105

more maize grain yields than plowed treatments (Table VIII). No-till plots also suffered significantly less damage by insects and pathogens. Despite the high yields, however, systems with high chemical input may not be the most economical or efficient practices (Zaffaroni and Locatelli, 1980). Maize, soybeans, beans, wheat, and sunflowers (Helianthus annuus) are successfully grown with a no-till system on a wide range of soils in Brazil. Yields obtained with a no-till system are often equivalent to or greater than those obtained with the conventional tillage system. In Parana, for example, Derpsch et al. (1985, 1986) reported that the mean grain yields for 6 consecutive years for wheat were 1597, 1418, and 1342 kg/ ha, and yields for soybeans were 23 14, 1856, and 1730 kg/hafor no-tillage, chisel plowing, and conventional tillage methods, respectively. Similar results in favor of no tillage are reported for other soils and ecological regions (Sidiras, 1984; Batz, 1985; Derpsch et al., 1986). Effects of tillage methods on yields, however, depended on seasons, location, and crops (Table IX). Whereas in 1979 wheat yield was greater in no- till than in plowed land for Londrina, the reverse was the case for Rolandia. In 1978, no-till crops outyielded those under conventional tillage for both sites. Soybean yield was less with conventional plowing than in a no-till or minimum tillage system for both sites. Wheat yields were increased and soil physical and nutritional properties improved when a no-till system was adopted in conjunction with appropriate cover crops (Kemper and Derpsch, 1981). The use of a no-till system also brought about significant improvements in soil temperature and moisture regimes and in water infiltration (Sidiras et al., 1985a). In Sao Paulo, Brazil, Silveira ef al. (1981) observed that mechanical weeding of coffee drastically altered soil structure. In Minas Gerais, Brazil, the grain yield of beans was 959 kg/ha with the no-till system and 712 kg/ha with conventional tillage.

Table VIII Effects of Tillage Systems and Weed Management on Growth and Grain Yield of Maize in Costa Rica"

Treatment

Grain yieldh (kdha)

Plant height" (cm)

Plowed, pre-emergence herbicides Plowed, postemergence directed paraquat Slashed at planting, postemergence directed Preplant glyphosate ( I .3 kg adha)

2397 b 2959 a 2819 a 3034 a

233 a 223 b 241 ab 249 a

"From Shenk and Saunders (1981). "Meansfollowed by the same letter do not differ significantly at the 5% level of probability using Duncan's list.

106

RATTAN LAL Table IX Effects of Tillage Methods on Grain Yield of Crops at Londrina and Rolandia, Parana, Braziln

~~

Site

No tillage (%) (kg/ha)

Londrina Rolandia

1713 1319

123 161

Londrina Rolandia

1844 2650

115

Londrina K o1and ia

1987 3 I52

139 I05

86

Minimum tillage (kg/ha) (%) Wheat yield (1975) 101 1404 120 98 I

Conventional tillage (kglha) (%)

=

0.05)

818

100 100

160 70

I607 3090

100 100

I40 120

Soybean yield (1978-1979) 105 1434 1505 106 3010 3204

100 100

151

Wheat yield (1979) 1676 104 31 17 101

1387

LSD (p

86

"Adapted from Kemper and Derpsch (1987).

A no-till system has proved effective for soil and water conservation and for production of pastures and grain crops on Alfisols prone to crusting and accelerated erosion in Northern Territory, Australia. McCown et al. (1980, 1985) and McCown (1984) have developed a system involving production of maize and soybeans in association with pastures using a notill system. Cultivation of sugarcane (Saccharurn officianale) in northern and central Queensland, Australia, is also widely practiced with a no-till system. In addition to producing equivalent yields, the no-till system has proven highly effective for erosion control on undulating terrains (Bureau of Sugar Exp. Station, 1984; Freebairn et al., 1986). Many crops are being grown on an experimental scale with a no- till system in the humid regions of South and Southeast Asia. Blevins (1984) described the potential for minimum tillage for growing many crops of the region in Bangladesh. The potential of no-till farming has been assessed and the system has been found promising for many regions in India. Thamburaj (1980) reported that cassava tuber yield in Madras was satisfactory when no-till was combined with the use of residue mulch. Wijewardene (1982) reported that a range of upland crops can be grown in Sri Lanka with a no-till system to produce economic returns. Usefulness of the conservation tillage systems has also been assessed for northern Thailand, where Ratchdawong et al. (1984) reported satisfactory crop yield and less runoff and erosion with no-till cropping compared to twice-plowed plots (Table X). Furthermore, higher yields were obtained with than without mulch. A considerable amount of research conducted in Indonesia

107

CONSERVATION TILLAGE Table X Mean Yields of Upland Rice and Peanut as Affected by Different Tillage Systems in Northern Thailand"

Treatments Yield (t/ha)

Cultivation ( I )

Cultivation (2)

Rice grain Peanut

0.9 I .5

0.4

I .3

Cultivation (I) plus mulch

No tillage plus mulch

1.6 2.0

0.8 I .4

"From Ratchdawong et a / . (1984).

has also proved the usefulness of no-till farming for erosion control and for producing satisfactory crop yields. Suwardjo et al. (1984) observed that in Indonesia mechanical tillage including plowing reduced structural stability. These researchers, therefore, recommended the use of minimum tillage with mulching for soils prone to erosion. The data in Table XI show that satisfactory yields are obtained with no-till systems for maize, peanuts, mung beans (Phaseolus aureus), and upland rice. Based on this other data, practical and extension-type recommendations can be made to the fanners of the region. Lowland rice is an important crop in Southeast Asia. Puddling, cultivating the soil when soil moisture content is near the saturation point in order to destroy soil structure and decrease percolation, is the conventional method of seedbed preparation. The puddling system has many disadvantages including high energy and labor costs, additional time needed in Table XI Effects of Tillage Methods and of Mulching on Yield of Crops Grown in Indonesia"

Yieldh (t/ha) Treatments

Maize

Peanuts

Mung beans

Upland rice

Deep tillage plus mulch Deep tillage Plowed, mulch W no-till Plowed No-till with mulch

4.0 a 3.5 a 4.1 a 3.8 a 3.9 a

2.8 a 2.8 a 2.1 a 2.1 a 2.6 a

0.99 a 0.65 b 0.92 a 0.62 b 0.74 b

3.6 a 3.0 b 3.9 a 3.0 b 3.2 c

"From Suwardjo et ul. (1984). "Means in a column followed by the same letter are not significantly different at the 5% level. 'fb, Followed by.

108

RATTAN LAL

seedbed preparation when the turnaround time is limited for double cropping, and low yields of the succession planting of upland crops due to poor soil structure. Consequently, the potential of the no-till system has been evaluated for the cultivation of lowland rice. Shad and De Datta (1986) reported satisfactory yields in the Phillipines with no tillage in some seasons but not in others. Other workers have also reported satisfactory yields of lowland rice in the Phillipines with a no-till system (Mabbayad and Buencase, 1967; De Datta and Karim 1974; Sharma and De Datta, 1986). 2 . Temperate Regions

No-till farming is now being widely practiced for cultivation of row crops in Europe, North America, and the temperate regions of Japan, Australia, and Chile (Triplett et al., 1968; Triplett, 1986; Langdale et al., 1978; Cannell, 1985; Unger, 1980; Unger and McCalla 1980; 1984; Phillips et al., 1980b; Sprague and Triplett, 1986). Rapid technological advances in the development and use of herbicides and appropriate seeding equipment, accelerated soil erosion with the conventional methods of seedbed preparation, and high energy costs have been responsible for this transformation. The no-till system is satisfactory for well- drained soils, and longterm yields are sustainable with appropriate levels of inputs. The data in Table XI1 on maize grain yields from Wooster, Ohio, in the United States for three cropping systems show high yields for 7-12 years of continuous

Table XI1 Maize Grain Yields (kg/ha) for 7-12 Years of Continuous Cropping with No-Till and Plowed Systems in Ohio"

Soil Wooster silt loam

Hoytville clay loam

Rotation

No-till

Plow

No-till

Plow

Continuous maize Maize-soybean Maize-oats-meadow

9400 9480 10450

8420** 8720* 9720**

6820 7920 8180

8000** 8260 8390

"From Van Doren er a/. (1976). *Significant at 5% level of probability. **Significant at 1% level of probability.

109

CONSERVATION TILLAGE

no-till farming. The no-till fields produced higher maize grain yields than did plowed land with the well-drained Wooster silt loam but lesser yields than that of plowed land with the slow-draining Hoytville clay loam. In a subsequent report Dick e? al. (1986a) observed that over a 22-year period, maize grown by no-tillage practices on poorly drained soils averaged 0.45 t/ha lower yield than if plowed treatments were applied. Similar results were obtained for yields of soybean and oats. On a well-drained soil, however, the average maize grain yield over a 23-year period was 0.84 t/ha more than from fields where plowed treatments were used (Dick e? al., 1986b). Draining, therefore, is an important determinant for the success of no-till farming. Satisfactory yields of wheat and summer cereals were also reported for a no-till system used on well-drained soils in Canada (Johnson, 1977; Ketcheson, 1977) (Table XIII). Satisfactory yields of wheat and barley are also obtained in the United Kingdom and Europe with notill systems adopted on well-drained soils (Cannell, 1985). No-till farming is also widely adapted in the Australian Wheat Zone, i.e, New South Wales and Victoria (Newman, 1978). The temperate zone climate of western Europe poses some s.evere problems for the successful adoption of a no-till system. The soil temperature in spring is sub-optimal and soil moisture is near saturation, decomposition of crop residue is slow, and anaerobic conditions prevail. Consequently, yield depressions with no-till systems are common (Baeumer, 1970). Low crop yields are caused by poor germination, slow and inhibited initial seedling growth, and anaerobiosis-caused nutritional imbalance.

Table XI11 Effect of Tillage and Residue Management Treatments on 15-year Average Wheat Grain Yields at Melfort, Saskatchewan, Canada"

Treatment

Plow in fall Heavy duty cultivator in fall Disk in fall Chop straw in fall No fall treatment Bum in spring "From Johnson (1977).

Yield (kdha) 1810

2030 1980 1960

2090 1960

110

RATTAN LAL

IV. PROS AND CONS OF THE NO-TILL SYSTEM: TROPICS VERSUS TEMPERATE ZONES

For soils and environments in which the no-till system is ecologically suitable, it has the following advantages over the conventional tillage systems: erosion control, moisture conservation, savings in labor and fuel, reduction in machinery cost, timeliness of operations, possibility of double cropping, and lessened risks of environmental pollution. It may also have some disadvantages such as high weed infestation, additional herbicide costs, anaerobic conditions, and possible yield depressions. The no-till system is apparently suited to tropical uplands, where its advantages outweigh the disadvantages. There are subtle differences in tropical vis-a-vis temperate regions that must be considered when assessing the applicability of the no-till system; they are discussed in the following sections. A. SOILTEMPERATURE The no-till system with crop residue mulching causes slow warming of soils during spring in the temperate regions. Delayed sowing and slow initial crop growth are due to suboptimal soil temperature regimes in notill systems in the Corn Belt of the United States (Amemiya, 1977; G f l i t h , 1977). The data in Fig. 8 show that soil temperature was below optimal (<25"C) almost 8 weeks after sowing (Cruz, 1982). Soil temperatures were even lower in a treatment utilizing maize, which produces high residue amounts, than when soybean which produces less residue, was used. In contrast, soil temperatures at Ibadan, Nigeria, in the subhumid tropics are supraoptimal (>30"C) for at least 2 months after sowing (Fig. 9). In the tropics, germination and crop establishment can be adversely affected by high soil temperatures. A no-till system with crop residue mulching lowers the maximum soil temperature and improves germination, seedling establishment, and crop growth and yields (Lal, 1982).

B. SOILMOISTURE In accordance with the soil temperature regime, soils in temperate regions can be too wet and often poorly drained during spring. Consequently, anaerobic conditions prevail in a no-till system during the seedling stage of crop growth. The data in Fig. 10 show high soil moisture content during early spring on a no- till plot in loess-derived soils in West Germany. The volumetric moisture content in May-June may be as high as 40%. In con-

111

CONSERVATION TILLAGE l

I

'

I

'

l

'

I

!

'

-.

1

'

,

MIN MORRIS. MINNESOTA

15.1

-

COSl4OCTON.OHIO

12.9 10.7 -

8.4 .

I

'

MlN

SOIL

6.2. CLEMSON. SOUTH CAROLINA

I

b

~

I

~

~

'

~

'

trast, however, low water-holding capacity and erratic and unreliable rainfall are responsible for more droughty conditions in conventional plots compared with those under the no-till system in soils of the tropics. Occurrence of frequent drought stress is one of the major yield-limiting constraints in upland soils of the tropics. This is not to say that drought stress is not a problem in northern

'

.

112

RATTAN LAL MAY 2,1973

AUG 29, 1973

a

-5cm

48-

NO TILLAGE

46 -

1 rz,'b

PLOWED

40-

3632-

28-

241

jj 2 4 1 Ln

1

6

8

10

12

14

16

18 20 TIME OF

6

8

10

12

14

16

18

20

DAY

FIG. 9. Soil temperature in no-till and plowed systems in a tropical Alfisol. a, maize; b, soybeans; c , cowpeas. (From Lal, 1976a.)

latitudes. Mid-season drought can also be a yield-limiting factor in the temperate zone. In that event, no-till farming may be a yield-stabilizing technology.

C. HARD-SETTING Some tropical soils containing low levels of organic matter contents,

113

CONSERVATION TILLAGE

0-lOcm

20* Tllled

10-

0,

I

I

I

1

intervol I

I

I

I

I

I

20

FeFoyT5 50- 60 cm

110-!2Ocm

0

MAY

JUNE

JULY

FIG. 10. Effects of tillage methods on soil moisture during summer in a loess-derived soil in Germany. (From Ehlers er a / . , 1980.)

predominantly low-activity clays, set hard on drying (Mullins et al., 1987). Such soils are widely prevalent in the lower latitudes. This hard-setting inhibits seedling establishment and root growth and lowers crop yield. The no-till system is difficult to adopt in hard-setting soils, which are widely prevalent in arid and semiarid regions. Such hard-setting soils have extremely low infiltration rates and the surface soil is so hard that it is difficult to sow crops without resorting to some form of mechanical loosening. Hard-setting soils are relatively less predominant in temperate than in tropical climates.

D. COMPACTION Whereas hard-setting is a soil characteristic, compaction is caused by interaction betwen soil properties and its management or by anthropogenic factors. Soil compaction is a more severe problem in soils subjected to excessive vehicular traffic than in soils with less traffic. Compactability is also related to the nature and amount of clay, quantity and distribution of crop residue mulch, and antecedent soil moisture content. All other factors remaining the same, upland soils in the tropics are more

114

RATTAN LAL

-

-

NT NO TILLAGE

a

-3

P - P L O W E D...........

1976

1

1

I

I

I

I

1

20

30

40

50

60

t

70

4

2i . .......... ...........

%,,),,

978

1

1

.........................

I

t

I

I

I

I

I

20

30

40

50

60

........... 20

t

70

................................ 30

40

50

60

70

TIME (min) Fw. 11. (a) Decline in iditration rate of a tropical Alfsol in Nigeria caused by motorized farm operations for no-till and plowed systems. (Recalculated from Lal, 1984b.) (b) Effects of no-till (NT), conventional plowing (CP), and plowing with motorized farm operations (CT) on water infiltration rate. (From Derpsch et al., 1985.) (c) Effects of long-term tillage on infiltration in a Brazilian Latosol. (From Da Siiva et a / . , 1981.)

1 I5

CONSERVATION TILLAGE

lb

Intensity of rainfall 60 mmhr-I

X

-

CP (y = 95.12~ -O-;

r = 0.98)

CT ( y = 129.65~''? r = 0.99)

I

I

20

30

I 40

TIME OF RAINFALL

I

I

50

60

(min)

C

0. a Conventional tillage for more than 20 years One year of conventional tillage after cleaned with bulldozer Native forest

0

20

40

60

80

loo

INFILTRATION TIME (min)

FIG. 11. (confinurd).

120

116

RATTAN LAL

compaction-pronethan equivalent soils in the temperate zone. Low levels of soil organic matter content, absence of the natural ameliorative effects of freezing and thawing, and predominance of low-activity clays are some factors responsible for high compactability of soils in the tropics. La1 (1984a) observed that soil compaction can set in even 2-3 years after adopting mechanized no-till farming on a tropical Alfisol. The data in Fig. Ila show a drastic decline in infiltration rate of an Alfisol used for continuous cropping to maize with motorized farm operations. Similar drastic effects on decline in infiltration rates were reported for Brazilian Latosols (Fig. l l b and c). Vehicular traffic also causes severe compaction in the soils of temperate regions (Lindstrom et al., 1981; Soane, 1983; Gupta and Allmaras, 1987) (Fig. 12). The formation of plow-pan is a severe yield-limiting factor in many temperate zone soils. Root penetration is severely restricted by the plow- pan layer, which normally occurs at the 20-30-cm depth (Ehlers et al., 1980). Loosening the plow-pan requires subsoiling and deep tillage. Although compaction is serious in both temperate and tropical regions, attainment of the same degree of compaction may occur sooner in the tropics than in the temperate climates.

E. EROSION The on-site adverse effects of accelerated soil erosion are generally more severe in the shallow soils of the tropics than in the temperate regions for the following reasons (Lal, 1987a):

i. The effective rooting depth of most residual soils is shallow due to common occurrence of the root-restrictive layers at shallow depth. The root restriction in subsoil may be due to either adverse physical or nutritional propertie s. ii. A high proportion of plant-available nutrients and soil organic matter reserves are confined to the top few centimeters of the soil profile. iii. Chemical amendments and other inputs of improved technologies are expensive and not readily available. As a consequence of the above factors, the range of tolerable soil loss for most uplands is 1-2 t/ha/yr. In addition, harsh climate and poor structural stability render soils of the tropics highly susceptible to accelerated erosion. Therefore, the need for adoption of a conservation tillage for erosion control is more critical in the tropics than for the temperate zone soils.

117

CONSERVATION TILLAGE

0-0 conventional plowlng

A 4conservallon llllage

i

10

20

o -0

no till

0 A

wheel tracks no wheel tracks

0

1

30

40

SO

60

TIME (min)

FIG. 12.

Effects of wheel traffic on compaction of a soil in Minnesota. (From Lindstrom

e r a / . , 1981.)

F. SOILFERTILITY As a general rule, residual upland soils of the tropics are old, highly weathered, and often excessively leached. The nutrient reserves are low and concentrated in the top few centimeters of the soil profile (Obeng, 1978; Sanchez and Buol, 1975; Lal, 1987b). In addition to the low nutrient capital, the essential nutrients are not present in the right balance. The potential productivity of soils is, therefore, low.

V. NONINVERSION AND MINIMUM TILLAGE Other forms of conservation tillage are used whenever the disadvantages of a no-till system outweigh its advantages. The aims of conservation tillage

118

RATTAN LAL

are to provide a seedbed suitable for optimum crop growth at minimum cost and to conserve the soil resource and its productivity. These objectives are also achievable by minimum tillage or noninversion tillage systems. The noninversion or minimum tillage systems are especially applicable in soils that are prone to severe compaction and crusting and have low water infiltration capacity. These soil conditions are major yield determinants in semiarid and arid regions of Africa, Asia, and South America. Many researchers have documented that in arid and semiarid tropics the no-till system causes severe yield depressions of sorghum, maize, peanuts, cotton (Gossypium hirsutum), and upland rice and is not a suitable tillage method for these environments (Nye and Greenland, 1964; Charreau and Nicou, 1971; Buanec, 1972 1974; Nicou, 1974). The data from Chile in Table XIV, for example, show that the no-till system decreased maize yields by about 18%. On a Norfolk loamy sand in central Alabama, Elkins et al. (1983) reported a 20-30% increase in soybean yield following chiselling and slit planting to ameliorate the plow-pan. For hard-setting soils and those with a root-restricting layer, therefore, some form of minimum tillage is necessary to conserve soil and water and facilitate crop growth. A. SEMIARID AND ARID WEST AFRICA

Shallow and compacted soils of subhumid and arid regions are low in organic matter content and prone to crusting (Obeng, 1978). These soils respond to some form of mechanical loosening. The most important con-

Table XIV Effect of Tillage Systems on Grain Yields of Maize in Chile"

Treatmentsh A: B: C: D: E:

Plow, harrow; harrow; plant, roll Plow; in tandem: harrow, plant, roll Plow; in tandem: harrow, plant In tandem: harrow, plant No-till

Grain yield (kg/ha) 11,110 1 1,230 10,580 10,810 9420

"Adapted from Luchsinger et a / . (1979). qreatment A is conventional tillage; B, C, and D are minimum tillage; and E is no tillage (with atrazine for weed control).

I19

CONSERVATION TILLAGE

sideration is to break the surface crust or seal and to improve water infiltration into the soil by some form of minimum or reduced tillage. The needed tillage may be performed manually (on small farms), by animal traction, or by motorized equipment. In Ghana, for example, Nye and Greenland (1964) and Ofori and Nandy (1969) observed significant yield improvements in maize in plowed soil compared with no-till or minimum tillage methods of seedbed preparation. The maximum yield was, however, obtained with plowing to about a 20-cm depth (Table XV). On average, shallow plowing increased the maize yield by about 15%. In northern Cameroon the yields of peanuts and rice were also improved by chisel plowing or minimum tillage system (Vaille, 1970; Ambassa-Kiki et al., 1984). The most extensive research on minimum tillage systems for semiarid West Africa was conducted by the Institut Recherche Agronomie Tropicale (IRAT) of France. The results of these experiments, conducted in the early 1960s, have indicated that for those soils prone to crusting and compaction some form of mechanical tillage is necessary (Charreau and Nicou, 1971; Nicou, 1977; Chopart 1978, 1981, 1983, 1984; Chopart et al., 1979). In Bambey, Senegal, and at 3 sites in Togo, Chopart et al. (1981) found that the highest yields of peanuts, millet, maize, and rice were obtained when soil was mechanically loosened. The lowest yields were obtained with a no-till system using a crop residue mulch. Significant yield improvements were observed when crop residues or farmyard manure were incorporated into the topsoil (Table XVI). In Togo, the maximum yield of maize was obtained by a treatment involving plowing under a green manure crop. Chopart and Kone (1985) observed that plowing improved yields of maize and cotton in central Ivory Coast. The usefulness of loosening the surface soil has also been demonstrated for the West African Sahel (Hoogmoed and Stroon- Snijder, 1978; Boer and Hoogmoed, 1979).

Table XV

Effects of Tillage Methods on Maize Grain Yield (th in ) a Subhumid Region of Ghana”.’

First season

Second season

Treatment

1%5

1966

1965

1966

No-till Plowing to 22-cm depth Plowing to 37.5-cm depth

1.9 2.0 2.0

2.3 2.8 2.6

1.1 1.2 1.3

0.8 1 .O 1 .O

“Adapted from Ofori and Nandy (1969). bThe least significant difference at the 5% level of significance is 0.3 for the first season and 0.1 for the second.

120

RATTAN LAL Table XVI Effects of Tillage Systems on Soil-Water Use and Crop Yields at Bambey, Senegala Water use (mm)

Crop yields (kg/ha) Treatments

Peanuts Millet Maize Rice Total Use by millet Total stored

No-till, no mulch Chiselling, mulch Chiselling, double mulch Plowing, residue incorporated Plowing, dung incorporated

1469 1420 1651 2073 1985

1462 1689 1435 1779 1781

-

-

1515

1765

3041 2687

3417 3660

-

-

89 96 171 187 -

99 97 184 137 -

171 161 227 188 -

“Adapted from Chopart er a / . (1981).

Plowing with animal traction, a form of noninversion minimum tillage, is a practical and economical approach for these environments (Pagel, 1975; Munzinger, 1982; Pingali et al., 1986). Not all soils in semiarid West Africa respond as positively to mechanical loosening as do those of Senegal, Togo, or Burkina Faso. Soils of northern Nigeria, although developed in a similar rainfall regime, respond less favorably to mechanical tillage than those of Senegal. For example, Dunham and Aremu (1979) observed that maize yields with no tillage could be as high as 95% of those obtained with conventional tillage. This was particularly true when recommended doses of fertilizer were used (Dunham, 1982, 1984). Dunham (1983) also observ.ed that rather than plowing the entire field using inversion or noninversion tillage, making the traditional hill-in-furrow by hand hoe is a useful technique, well adapted to the erratic and low rainfall regime of semiarid West Africa. AND EASTERN AFRICA B. SOUTHERN

Soils of the semiarid environments in southern Africa also respond to noninversion minimum tillage. A relevant example is that from Sebele, Botswana. The data in Table XVII show that minimum tillage by chiseling produced yields equivalent to those produced by moldboard plowing. Chiseling or strip tillage in the seed zone is a satisfactory form of conversation tillage for these soils (Gibbon et al., 1974). In Kenya, improvements in crop growth were observed when minimum tillage rather than a no-till system was used (Muchiri and Gichuki, 1982). Similarly, MaCartney et al. (1971) reported that at Kongwa, Tanzania, ripping to loosen the compacted soil before the onset of rains is necessary to break the surface seal, improve water infiltration, and facilitate deep root penetration. Also

121

CONSERVATION TILLAGE Table XVII Effects of Tillage Methods on Yields (t/ha) of Four Crops at Sebele, Botswana during 1976-1977"*b

Tillage Moldboard plow Chisel Sweep Plowed strip

Autumn Spring Depth (cm) Sorghum Cowpeas Maize Sunflowers Maize Sunflowers 20 20 10 30

0.92 I .20 1.05 1.35

0.89 0.80 0.78 0.71

2.40 2.23 2.31 2.31

0.43 0.40 0.58 0.56

2.19 2.29 1.60 1.52

0.68 0.57 0.60 0.81

*

NS

NS

NS

*

NS

"From the Botswana Dryland Farming Research Scheme (1982). '*, Signifcant differences at 5% level: NS, not significant by the least significant difference method.

at Kongwa, site of the ill-fated Peanut Scheme, Northwood and MaCartney (1971) observed that failure of the no-tillage system was due to mechanical impedance of roots. Once the soil has been loosened, requirements of an appropriate seedbed could be met by the cultivation of a narrow strip sufficient for germination and seedling establishment. The data in Table XVIII show that the maximum maize grain yield was obtained for a 20cm-wide strip cultivated to a 9-cm depth. For West Kilimanjaro and Table XVIII Effects of Depth and Width of Cultivation Strip on Maize Grain Yield (kg/ha) at Two Sites and Plant Height at One Site in Tanzania"

Width of cultivation (cm) Depth of cultivation (cm)

2.5

10 745

9

526 656

5 9

1516 1475

5

5 9

SE

30

45

1075 1242

1081 1231

56 (Width) 39 (Depth)

West Kilimanjaro 1317 1581 1424 1592 I568 I547

1419 I343

NS" NSh

909

20

Kongwa I120 I247

Sambwa (plant height at 4 weeks after sowing, cm) 18.1 19.8 21.4 18.2 19. I 18.9 22.0 23.7 20.7 20.4

"From Northwood and MaCartney (1971). "NS. not significant.

0.9 (Width) 0.6 (Depth)

122

RATTAN LAL

Sambwa, however, even a 2.5-cm-wide strip produced satisfactory growth and yield. Satisfactory yields of maize and cowpeas were also reported with a minimum tillage system at Morogoro, Tanzania (Huxley, 1982). Minimum tillage systems found to be suitable for these soils require more energy inputs than traditional systems (Ellen, 1984) but can be performed by animal traction. The use of animal traction in these regions also facilitates the timeliness of seedbed preparation. Early sowing is critical in these regions of short rainy seasons because the crop should be sown soon after the rains begin. Consequently, animal traction is an economical proposition (Oluoch-Kosura, 1983).

C. SOUTHAND WEST ASIA Noninversion soil tillage, such as disking to break the crust and to incorporate the crop residue in the topsoil layer, is the form of conservation tillage widely practiced in the Indian subcontinent and in arid West Asia. Shallow tillage is often performed with animal-powered implements. Over and above the beneficial effects of shallow tillage, crop residue mulch, if available, also increases crop yield (Gupta and Gupta, 1986). Bhatnagar et al. (1983) reported that favorable water conservation and the best yields of wheat and peanuts were obtained by a combination of minimum tillage and mulching on irrigated and coarse-textured soils in hnjab, India. The most desirable combination involved one disk plowing to a depth of about 15 cm followed by one disk harrowing; residue mulch at a rate of 5 t/ha was surface-applied about 1 week before sowing (Table XIX). Hakimi and Kachru (1976) reported that the maximum yields of barley in Iran were obtained by shallow tillage to about a 5-cm depth. Both inversion tillage with the moldboard plow and no-tillage produced less yield than the minimum tillage (Table XX). For soils prone to crusting and hard-setting, therefore, shallow tillage without inversion and with the crop residue retained on the surface as mulch is the most appropriate conservation tillage.

VI. SUBSOILING AS CONSERVATION TILLAGE Some residual soils with a clayey B horizon and a low level of organic matter, have low infiltration rates, low plant-available water reserves, poor root growth, and low yields. Periodic soil inversion involving both primary and secondary tillage operations can relieve these restrictions and improve crop yields. Conservation tillage for these soils is that which creates a favorable porosity and maintains it at the desired level. There

123

CONSERVATION TILLAGE Table XIX

Effects of Tillage Methods and Residue Management on Yield of Wheat and Peanuts for Two Coarse-Textured Soils from Punjab, India" Sandy loam soil Sandy soil Treatmen t No tillage, residue removed (TI) One disk plowing and disk harrowing, residue removed (T2) Tz plus residue mulch at 5 tonnedha, applied 1 wk before sowing (T,) T, plus residue mulch applied 2 wk after sowing (T.) TZplus 2.5 t/ha residue incorporated and 2.5 tlha applied as mulch LSD"

Wheat

Peanuts

Wheat

Peanuts

Nm'

N,,/

I .6 I .7

I .7 1.8

1.7 2.0

3.9 4.4

4.3 4.7

2.3

2.6

2.3

4.7

5.0

1.9

2. I

2.2

4.4

4.6

1.7

2.2

2.3

4.6

4.8

0.21

0.24

0.2

0.2

0.2

"From Bhatnagar et a/. (1983). bLSD. least significant difference at the 5% level of significance. 'NW, 80 kg N/Ha dNlzo.120 kg N/Ha

are also other soils that have developed massive structure due to past mismanagement; their productivity can be restored by deep ripping, subsoiling, and soil inversion. These soil-loosening treatments, however, are not required every year. Once loosened, these restored soils may produce satisfactory yields for 3-5 years. Table XX

Effects of Tillage Methods and Depth of Cultivation on Barley Grain Yield (kglha) in Iran, 1973-1975" Tillage depth (cm) Tillage type

5

Moldboard plow plus disk Disk Field cultivator Field cultivator plus disk No tillage Mean

2780 2490 3200 3180 2910 a"

I5

25

2490 2160 2750 3100

2440 2320 2620 2780

2630 ab

2540 b

-

-

Mean 2570 bch 2320 c 2860 a b 3020 a 1830'

"Adapted from Hakimi and Kachru (1976). b R o or ~ column means followed by the same letter or letters are not significantly different at the 5% level of probability by Duncan's New Multiple Range test. 'Not included in statistical analysis because depth effect was absent.

124

RATTAN LAL

A relevant example is that from Israel. For these arid-region soils, it is necessary to loosen the compacted soil layer to facilitate root growth into the subsoil and to obtain high crop yields (Table XXI). Stibbe and Ariel (1970) observed that this loosening or soil-inversion treatment is not necessary at every growing season. Once macroporosity is improved, a notill system can be practiced. In Israel, for example, summer crops can be grown with a no-till system in the winter-fallowed wheat stubble. Khan (1984) reported that the maximum yield of peanuts in Kharagpur, in northern India, was obtained with the inversion tillage treatment, involving moldboard plowing and disk harrowing. The increase in yield following plowing in comparison to the no-tillage treatments was 67-78% (Table XXII). The yield increase caused by plowing was attributed to high infiltration rate and to a low soil bulk density. Vittal et al. (1983) reported that deep tillage in Hyderabad, Central India, improved depth of penetration of the wetting front, root growth, and infiltration rate. These beneficial effects of deep tillage were attributed to the lowering of bulk density and increasing of macroporosity (Table XXIII). Crop yields were also increased by deep compared with shallow tillage (Table XXIV). The yield increase is partly due to low runoff and reduced soil erosion. Moldboard plowing is known to improve infiltration rate (Table XXII), increase depth of penetration of rains (Table XXIII), and increase crop yields (Table XXIV). For Alfisols at Hyderabad, India, Klaij (1983) measured 60.5 mm of water runoff and 232 kdha of soil loss from shallow-tilled watershed in comparison with 48.7 mm runoff and 336 kg/ha of soil loss for a deeptilled soil. The soil bulk density for the two tillage treatments was 1.48 and I .55 g/cm3, respectively. Similar beneficial effects of plowing have been reported for crusted and compacted soils in temperate regions. The Table XXI Effects of Depth of Plowing and Tillage Methods on the Relative Dry Root Weight Distribution (in %) in Depth from Mature Sorghum"

Depth of soil layer (cm)

Plowed at 40 cm

Plowed at 25 cm

Subsoiled at 40 cm

No tillage

0-30 30-60 60-90 90-120 120-150 150-180 0-180

20.1 23.4 24.6 20.6 10.8 0.5 100.0

29.7 17.9 11.9 13.8 8.8 5.7 87.8

18.9 16.4 13.1 12.0 15.3 4.2 79.9

15.9 8.3 9.7 6.6 7.1 2.9 50.5

"From Stibbe and Ariel (1970).

125

CONSERVATION TILLAGE Table XXII

Effects of Tillage Methods on Water Infiltration Rate and Yield of Peanuts on a Lateritic Sandy Loam Soil at Kharagpur, India in 1978-1979"

Infiltration rate (cdhr) Treatment

Pod yield (t/ha)

Initial

Final

4.5

1.6 2.0 1.9 2.0 2.3 0.02

~

No tillage Wedge plow. disk harrow (ox-drawn) Rotary tiller (tractor-driven) Cultivator (tractor-driven) Moldboard plow (tractor-driven) LSD*

2.16 2.82 2.33 3.09 3.22 0.19

5.5 5.2

5.3 6.1 0.02

"From Khan (1984). "LSD, least significant difference at the 5% level of significance.

increase in random roughness created by moldboard plowing increases infiltration rate in soil. The beneficial effects of mechanical loosening are often short-lived, however, for soils that are subsequently subjected to excessive traffic and other mismanagement. For example, Adeoye (1982) reported that although Table XXlII Effect of Tillage Methods on Characteristics of an Alfisol in Central India"

Measurements Depth of penetration

Units cm

Depth of wetting front 24 hr after 18.2 mm rainfall

cm

Depth of wetting front 24 hr after flooding

cm

Bulk density during crop growth period 0-15 cm 15-30 cm 30-45 cm

g/cm' g/cm' gkm'

Infiltration rate Initial Final

cdhr cdhr

~~

"From Vittal er a / . (1983). "Numbers in parentheses are standard errors.

Shallow tillage"

Deep tillage"

15.5

23. I

(3.2)

(1.7)

15.0

(2.0)

19.2 (1.7)

42.0

63.0

I .80 1.83 I .85 26.3 4.1

I .73 I .72 1.75

45.9 8.2

I26

RATTAN LAL Table XXIV

Effects of Tillage Methods on the Yields of Crops in Central India"

Crop

Shallow

Deep

Amount of rainfall during crop growth period (mm)

Sorghum Finger millet Castor Castor Sunflowers Pigeon peas Sorghum Pearl millet Castor Sorghum Pearl millet Castor

17.4 I .o

18.1 1.5 4.3 2.8 8.1 2.9 29.5 22.3 12.7 26.5 16.3 12.7

264 124 264 269 149 149 259 399 62 1 609 529 883

Grain yieldb (qh) Type of season Subnormal

Normal Above normal

3.8 2.8 8.1 2.5 26.6 19.4 7.8 26.1 16.0 9.6

~

Rainy days

(no.)

Calendar year

21 15 27 37 11 11 34 19 49 48 40 36

I979 1980 1979 1980 1972 1972 I980 I973 1978 I978 1978 1973

~~

"From Vittal et al. (1983). "Quintal or 100 kg.

plowing to a 15- or 30-cm depth increased water shortage and rate of wetting in northern Nigeria, these beneficial effects from increased porosity disappeared by the end of the first cropping period. Mechanical loosening, in such cases, is a vicious cycle difficult to escape.

VII. CONSERVATION TILLAGE FOR PROBLEM SOILS The objectives of conservation tillage are to alleviate the soil-related constraints to crop production, sustain satisfactory yields, and preserve the natural soil resource and its productive potential. Major soil-related constraints that can be partly alleviated through one or another form of conservation tillage include:

(i) low plant-available water reserves and erratic rainfall; (ii) poor trafficability on swelling soils; (iii) low infiltration rates; (iv) massive soil structure and poor drainage; (v) salt accumulation in the root zone; (vi) acidity of the subsoil horizons; (vii) excessive leaching; (viii) excessive runoff and resultant loss of water, nutrients, and soil;

CONSERVATION TILLAGE

127

(ix) the presence of hardened plinthite on or near the surface; and (x) the problem of cultivating steep croplands. Relevant examples of applications of conservation tillage alleviating these problems are discussed below. A. Low PLANT-AVAILABLE WATERRESERVES

One of the major constraints of the arid and semiarid regions is the low and erratic rainfall, low water infiltration rate, high temperatures, high evaporative demand, and frequent drought stress. Crop yields are limited by the low amount of plant-available water (Lawes, 1%5; Lal, 1979). Most of the rains received are high-intensity thunderstorms and are concentrated in a short rainy season. Because of low infiltration capacity, a high proportion of rainfall is lost as runoff. Most uplands in the tropics and subtropics have low plant-available water reserves (Lawes, 1965; Lal, 1979), and crops suffer from frequent drought stress. The strategy for water conservation in the root zone, therefore, is to allow more time for the water to infiltrate. This is done by using a conservation tillage system called tied ridges. Traditionally, peasants in Africa grow their crops on ridges but tieing is rarely practiced. Ridging with addition of crossties in the furrow creates a series of individual basins that increase the surface detention capacity to hold surplus water (Fig. 13). This system allows more time for water to infilitrate into the soil and increases crop growth and yield. The tied-ridge system has been widely used in East Africa (Rentice, 1946), semiarid West Africa (Rodriguez, 1981), and in the southeastern United States (Unger, 1984). Tied ridging is known by many names including basin listing, furrow blocking, furrow damming, furrow diking, etc. Whereas planting on simple contour ridges may cause yield depressions due to high soil temperatures, lodging (Lal, 1973; Leyenaar and Hunter, 1976), and increased risks of soil erosion (Kowal, 1970), planting on tied ridges decreases the risks of drought stress and increases crop yield. Laws (1966) observed that in northern Nigeria the highest yield of sorghum was obtained when alternate furrows were left open, and the lowest yields where all furrows were left open. The increase in grain yield ranged from 0.29 to 0.44 h a . Yield increases were also obtained in peanuts by tieing alternate furrows. The practice of tied ridging has also been found beneficial for water conservation in semiarid and arid regions of Burkina Faso. Rodriguez (1981) observed significant increases in yields of maize planted on tied ridges in comparison with that planted on the flat or on simple ridges. Hulugalle (1986) observed that the grain yields of cowpea were 1529 and 2315 kg/ha (LSD,,, = 649) in 1985, and 1981 and

128

RATTAN LAL

FIG. 13. The tied-ridge system is also called basin listing, furrow damming, or furrow blocking.

2373 kg/ha (LSD.05= 1607) in 1986 on simple and tied ridges, respectively, for similar soils and environments. Hulugalle (1986, 1987) observed that tied ridges increased profile water content by an average of 30.5 mm and 24.6 mm per week. Consequently, the grain yield of cowpea was increased by 500-800 kg/ha. The increase in profile water content is apparently due to an increase in infiltration capacity. In another study in Burkina Faso Hulugalle et al. (1987a) observed that tied ridges in combination with rockbunds (Fig. 14) produced the maximum grain yield of sorghum. With the recommended rate of fertilizers, the sorghum grain yields for planting on tied ridges and on the flat were 620 and 260 kg/ha with rockbunds and 490 and 170 kg/ha without rockbunds, respectively. The tied-ridge system has been widely researched in eastern and southern Africa. Prentice (1946) reported significant increases in yields of cotton, sorghum, and maize planted with the tied-ridge system in comparison with those grown on the flat (Table XXV). As a result of this research information, Le Mare (1954) and Brown (1963) observed that the practice of tie ridging was encouraged in the Lake Victoria region and that crop yields were greater on tied ridges than on open or tied ridges. Encouraged by these findings, engineers developed appropriate farm tools to prepare tied ridges by motorized equipment (Dagg and MaCartney, 1968). Also in Tanzania, Northwood and MaCartney (1971) observed significant

129

CONSERVATION TILLAGE

FIG. 14. Rockbunds are used to retard runoff velocity and encourage infiltration.

Table XXV

Effects of Tied-Ridge System on Crop Yield in Tanzania" Yield (kg/ha) Year

Rainfall (mm)

Crop

Flat

Tied ridge

Difference

1939 1939 1940 1942 1943 1943 I944 1944 1944 1945 1945 1945

610 610 787 1245 584 584 660 660 660 787 787 787

Cotton Sorghum Sorghum Cotton Maize Cotton Cotton Sorghum Sorghum Cotton Sorghum" Sorghum

323 202 808 1049 172 617 101 853 343 684 1467 976

542 734 1122 854 825 853 393 869 798 1234 3747 892

"Modified from Prentice (1946). 'Straw yield.

(%)

+67.8

+ 263.3 + 38.9 -

18.0

+ 380.0 + 38.0 + 290.0 + 2.0 + 133.0 + 80.0 + 139.0 - 9.0

130

RATTAN LAL

increases in soil moisture conservation and in grain yield in maize sowed on tied ridges (Table XXVI). Beneficial effects on tied ridges have also been demonstrated by research experiments conducted in Botswana (Dryland Farming Research Scheme, 1984). Grain yields of sorghum and cowpeas were drastically increased when every furrow was tied (Table XXVII). The yield increases were apparently due to the greater availability of water in the root zone produced by tied ridges than by the open furrow method of seedbed preparation. The depth of penetration of wetting front after a 34-mmrainfall was 11.1, 17.3, and 24.0 cm for untilled soil, untied ridges, and tied ridges, respectively. Similar results are reported from the Shair Haneger region of Israel. The data of Monn et al. (1984) in Table XXVIII shows a 44% increase in the yield of wheat sown on 1.6-m-widebeds of tied ridges. In arid regions of Israel, Rawitz et al. (1983) observed that erosion loss from disked and plowed plots were approximately 10 times greater than that from ridged plots (up and down the slope), and 25 times higher than that from plots under basin tillage or the tied ridge system. Supplemental irrigation is another possibility for reducing the adverse effects of drought stress in arid and semiarid regions. In addition to conserving rainwater, the bed-furrow system is also a useful technique for improving irrigation efficiency. This technique saves water and increases irrigation efficiency. The semipermanent bed-furrow system combined with no tillage or minimum tillage is an appropriate conservation tillage for improving water storage in the root zone. The system of irrigation using tied ridges has been successfully used in the semiarid climate of Texas (Musick et al., 1977; Stewart et al., 1981; Stewart and Musick, 1982).

Table XXVI Effects of Tillage Methods on Moisture Conservation and on Grain Yield of Maize in 1966-1%7 in Tanzania"

Available moisture in top 1.83 m (mm) Treatment

Flowering

Harvest

Disk plow and harrow Rip and disk harrow Rip and ridge Rip and direct drill Rip and tied ridge

29 39

61 82

-

110 98

35 42

"From Northwood and MaCartney (1971).

Maize grain yield (kdha) 1180 1014 1196

-

1285

131

CONSERVATION TILLAGE Table XXVII

Effects of Tied and Untied Furrows on (a) Yields of Sorghum and Cowpeas and on (b) Soil Moisture Conservation at Sebele, Botswana" ~

Yield (t/ha) 1972-1973

1973-1974

Treatment

Sorghum

Cowpeas

Sorghum

Cowpeas

Open furrow Alternate furrows tied Every furrow tied LSDb

0.34 0.63 0.57 0.15

0.90 0.75 0.77 NS

3.17 4.31 4.58 0.29

0.83 0.91 1-23 0.39

mm Waterlm soil in 1973-1974

Treatment

25 Oct

20 Dec

Harvest ~~~

Open furrow Alternate furrows tied Every furrow tied

149 162 178

156 181 195

127 133 I37

"From Botswana Dryland Farming Research Scheme (1984). bLSD, least signifcant difference at the 5% level of significance.

Irrigation often causes alterations in soil structure, probably due to the nature and amount of salts present in water and hence due to poor water quality (Moreno et al., 1986). With deteriorating soil structure, use of mulching and no-till systems can restore soil structure and improve crop yields, as has been observed in Victoria, Australia (Adem et al., 1984; Tisdall and Adem, 1986). Chaudhary et al. (1985) also reported an increase

Table XXVIII

Effects of Tied-Ridge System on Soil and Water Conservation and on Grain Yield of Wheat on a Calcic Haploxeralf in the Shaar Haneger Region of Israel" ~~

Treatment

Runoff (mm)

Soil erosion (kdha)

Yieldb (kdha)

Flat sowing Tied ridges (1.6-m wide bed) Tied ridges (0.6-m wide bed)

17.7 7.9 7.4

293 I56 72

975 1403** 964

"Modified from Morin er a/. (1984). '**, significant at 1% level of probability.

132

RATTAN LAL

in maize grain yield for the coarse-textured sandy soils at Ludhiana, Punjab, India, by supplemental irrigation. The maximum yield was, however, obtained by a combination of subsoiling and irrigation. B. POORLYDRAINED SOILS Severe yield depressions are observed when upland crops are grown on lands with slow or poor internal drainage. In Trinidad Lindsay et al., (1983) observed that on a tropical Inceptisol with impeded drainage, maize grain yields were 5.6, 1.9, and 1.3 t/ha for conventional tillage, no tillage, and minimum tillage, respectively. Most notable among soils in the tropics with low infiltration rate and impeded internal drainage are the Vertisols. These are poorly drained, fine- textured soils, generally developed on level lands and in depressions. The infiltration rate of these soils is often as low as 0.2 cm/hr (ICAR, 1984). Vertisols are formed in climates ranging from subhumid temperature and Mediterranean to semiarid and subhumid tropical with marked dry and wet seasons. There are about 260 million ha of Vertisols in the world including 105 million ha in Africa, 58 million ha in Asia and the Far East, 48 million ha in Australia, 27 million ha in Latin America, 10 million ha in North America, 6 million ha in the Near and Middle East, and 5 million ha in Europe (Beek et al., 1980). These soils predominantly contain swelling clay minerals and develop wide and deep cracks during the prolonged dry season. When dry, these soils have a hard consistency. They are plastic and sticky when wet, however. The optimum soil moisture range for friable consistency and tillage is narrow. Just as the tied-ridge system improves water conservation in soils with crusted surface and low infiltration rate, graded and open ridge-furrow systems provide surface drainage to remove surplus water from these poorly drained Vertisols. A relevant example is the use of this system for growing cotton in Tanzania. A system of raised and cambered beds, 7-8 m apart, produced good yields over a number of years in the western cotton-growing areas of Sukumaland in Tanzania (Spence and Smithson, 1966). At the International Crops Research Institute for the Semi-Arid Tropics (ICRISAT), Hyderabad, India, a broad-bed-and-furrow (BBF) system has been developed to reduce runoff and erosion and to permit cropping during the monsoon season (Kampen, 1982). The package of cultural practices for management of Vertisols using the BBF system include the following: (i) The BBF system is installed on a semipermanent basis, and maintenance is done only during the dry season. If any tillage is necessary, it is done on already existing broad beds.

I33

CONSERVATION TILLAGE

(ii) To avoid traffic on wet soil, dry seeding is recommended just before the first rains occur. The seed is sown deep enough to avoid germination following a small amount of rainfall. The optimum sowing depth is 5-7 cm for sorghum, pigeonpea, maize, and millet. Small grains, e.g. Setaria, are sown at a shallower depth. (iii) The rainy-season crop is harvested soon after it attains physiological maturity, so that the post-rainy-seasoncrop is established when soil moisture is favorable. (iv) The post-rainy-season crop is intercropped or relay cropped through the standing previous-season crop by a no-till system. Also in India, Klaij (1983) developed a tillage concept that involves the BBF and a ridge-furrow system. By adopting a semipermanent ridgefurrow system, Klaij visualized the seedbed as comprising three separate zones: a traffic zone, water management zone, and the seedling environment zone (Fig. IS). Within the framework of this zonal tillage, it was observed that the grain yield of pearl millet decreased with increase in depth or cross-sectional area of the soil disturbed (Fig. 16). The runoff and erosion from shallow-tilled soil were less than those from deep-tilled soil. Also in Bundelkhand, Central India, yield of black gram (Cicer arietinurn) was more than doubled by the raised- bed technique (ICAR, 1984). The mean grain yields was 479, 1063, and 1 123 kg/ha for control, raised

f-

50c-rlmcm-

I

I I

f

I

S = Seedling Environment

I

t

WATER

Zone

I

-=-

I

I

I I

I

MANAGEMENT

I

1

t

I

ZONES

FIG.15. The Role of Tillage. A zonal tillage concept based on three zones: traffic zone, water management zone, and the seedling environment zone. (From Klaij, 1983.)

I34

RATTAN LAL

0

c

a s 9 w

>-

I 0

T2 T3 T4 T2 T3T4

IIDCENTER ROW FIG.16. Effects of the depth and cross-sectional area of the soil disturbed on the grain yield of pearl millet in central India. (From Klaij, 1983.) T2, T3, and T4 refer to different tillage treatments corresponding to different depths and cross-sectional areas of plowing.

beds made across the slope, and raised beds made along the slope, respectively. There was no difference in grain yield among the three spacings used, implying that adequate surface drainage conditions were provided for by the bed spacing, which was between 3 and 9 m (Table XXIX). Making raised beds along a slope, however, increases the risk of soil erosion, unless the land has already been graded, as was the case in this study. An important consideration is to provide a well- aerated root zone. A satisfactory level of aeration was apparently not achieved in the ICAR experiment by merely constructing drainage ditches in the control treatment. Table XXIX Effects of Raised Beds and Spacing on Grain Yield of Black Gram (Cicer sp.) on a Vertisol in Central India"

Grain yield (kg/ha) Raised bed spacing (rn)

Along slope

Across slope

Control"

3 6 9

1183 1113 1075 1 I23

I107 I174 907 1063

558 693 185 479

Mean

"From [CAR (1984). 'The control treatment had drainage ditches without raised beds.

135

CONSERVATION TILLAGE

Poor drainage is also caused by impeded surface and subsurface flow due to the flow-inhibiting terrain. Valley bottom soils are often poorly drained, and farmers overcome the drainage-induced constraint by constructing mounds and ridges (Kowal and Stockinger, 1973). At Abet in northern Nigeria (9"40' N, 8" 10' E) Tarawali and Mohamed-Saleem (1987) produced drastic increases in grain and stover yields of sorghum by increasing the ridge height to 45 cm (Table XXX). Tied ridges on poorly drained soil are predictably less useful than open ridges. The latter provide the much-needed free-drainageflow. It is for the reasons of poor drainage that soil inversion and plowing improve crop yields on slowly permeable soils, as was also reported for Trinidad by Lindsay et al. (1983). The flat seedbed and open ridge-furrow system are, however, prone to accelerated soil erosion, especially at the onset of rains. In that event, use of ridges in combination with crop residue mulch may decrease runoff losses and reduce sediment concentration in the overland flow (Loch et al., 1987). In temperate regions, the adverse effects of poor drainage on crop growth are confounded with those due to the suboptimal soil temperatures that occur during the seedling stage in early spring. Suboptimal soil temperature regimes are one reason for low yields of wheat, oats, and barley observed with no-till system on poorly drained soils in the United Kingdom (Cannell et al., 1986) and elsewhere in northern latitudes. On clayey soils of low permeability at Saskatoon, Canada, Greaves and Bomke (1986) observed more grain yield and more N uptake by barley in seedbeds prepared by moldboard plowing than in those prepared by reduced or minimum tillage systems. Stunted seedling growth on poorly drained soils is

Table XXX

Effects of Ridge Height on Grain and Stover Yield of Sorghum on a Poorly Drained Soil at Abet in Northern Nigeria" Sorghum yieldb (kg/ha) Tillage system

Grain

Stover

Flat, undisturbed Flat, disturbed Ridge (I5 cm) Open ridge (30 cm) Tied ridge (45 cm) Open ridge (45 cm)

1724 1827 2023 2338 2190 2246

3666 4301 5243 5294 5317 5453

"From Tarawali and Mohamed-Saleern (1987). bThe least significant difference ( p = 0.05) is 400 kg/ ha for grain and I163 kg/ha for stover.

I36

RATTAN LAL

also caused by low availability and inhibited uptake of nitrogen and other essential nutrients (La1 and Taylor, 1970; Cannell, 1979). That is why the ridge drainage (or ridge tillage) and raised-bed systems of seedbed preparation produce more maize grain yields in Ohio than no-till or plowed methods (Table XXXI) (Eckert, 1987; Fausey, 1984). Because of a favorable response to ridge tillage, no-tillage or minimum tillage techniques are being adapted to develop permanent ridge-furrow systems. C. CRUSTING AND COMPACTION Regardless of the ecological region, compaction and crusting are the major production constraints to intensive row- crop agriculture (Awadhwal and Thierstein, 1985; Voorhees and Lindstrom, 1983, 1984). Unplowed soils often have more bulk density than plowed soils (Kay et al., 1985). Soils prone to the formation of surface seal become susceptible to accelerated runoff and erosion and produce low yields because of poor crop stand and severe drought stress. In addition to crusting, some soils of the arid and semiarid regions are naturally compacted and restrict root growth and development. In Zambia, for example, Lenvain and Panwelyn (1986) observed high soil bulk density of 1.50-1.76 g/cm3 at 10-50-cm depths. Shallow, eroded, and compacted soils also occur extensively in West Africa (Charreau, 1970) and in semiarid regions of India (Vittal et al., 1983). Mechanical loosening is an ameliorative conservation tillage to improve crop growth on such problem soils. Soil compaction is also caused by wheel traffic on the soil surface and by the formation of plow-pan in the subsurface horizons (Lal, 1985a; Voorhees and Lindstrom, 1983, 1984; Kay et al., 1985). Compaction Table XXXI Effects of Tillage Method and Drainage on Maize Grain Yield on a Poorly Drained Soil in Ohio"

Grain yield (t/ha) Tillage

Tile

No tile

Moldboard plowing Ridge tillage Slot plant Till plant

10.4

8.3

11.2 11.4

9.9 9.7

LSD (.05)

NS

0.8

~

~

"Modified from Eckert (1987).

I37

CONSERVATION TILLAGE

caused by wheel traffic decreases root growth (Ehlers et al, 1980; Negi e t a / . , 1981), decreases water uptake, and reduces crop yield (Fausey and Dylla, 1984). In Nigeria, for example, Kayombo and La1 (1986) and Oni and Adeoti (1986) observed than an increasing number of tractor passes prior to sowing decreased germination and yields of maize, cowpeas, soybeans, and cassava. In southern Brazil Da Silva et al. (1981) and Klamt et al. (1986) found decreases in the proportion of water-stable macroaggregates and reductions in the equilibrium infiltration rate due to the repeated passages of motorized tillage equipment. One method of reducing the traffic-induced compaction is adoption of no-till or reduced tillage methods so that the number of passes involved in seedbed preparation is reduced. Traffic elimination is an important step in reducing risks of soil compaction (Pidgeon, 1981). Guided traffic is another useful innovation. Farm operations are done in such a way that the tractor wheels always follow the same tracks. Another beneficial approach is the use of mechanical loosening devices, e.g., the paraplow. The latter is a slant-legged soil-loosening machine that loosens the subsoil without inverting it. This process can temporarily increase yield, as has been observed on some compacted Alfisols in western Nigeria (Table XXXII) and elsewhere (Brain et al., 1984). It is important to realize, however, that a moderate level of compaction in the seed zone by wheel tracks may improve seed germination and seedling establishment (Sidiras and Vieira, 1984).

Crust formation limits water infiltration, inhibits gaseous exchange, and Table XXXII

Effectsof Compaction Alleviation by Paraplow and Chiseling on Maize Grain Yield on a Tropical Alfisol"

Treatment

Maize grain yield (kglha)b

Chisel planter Strip tillage Paraplow, once a year No tillage, hand Paraplow, once every 2 years No tillage, tractor

2309 b 2261 b 2734 a 1998 c 2661 a 21 10 bc

~~

~

~

"Modified from Garman and Juo (1983). "Means followed by the same letter are not significantly different at the 5% level by Duncan's Multiple Range test.

138

RATTAN LAL

decreases seed germination and seedling emergence. In subhumid and humid regions, seedling establishment on crust- prone soils can be improved by using strip tillage and no-tillage systems. Experiments conducted in western Nigeria by Nangju et al. (1975) showed that seedling emergence of soybeans was 1, 33,51, and 54% with ridges, plowed flat, strip tillage, and no-tillage methods of seedbed preparation (Table XXXIII). For soils of arid and semiarid regions, however, it is necessary to break the crust mechanically. In the West African Sahel Hoogmoed and Stroosnijder (1984) observed that beneficial effects on tillage toward breaking the crust are short-lived. Frequent cultivation is needed to break the crust mechanically. New crust is, however, quickly formed on freshly tilled soils. Attempts have been made to develop special farm equipment to break the crust without damaging the seedlings.

D. LEACHING Adequate leaching is necessary to maintain proper salt balance in the root zone. Whereas prolonged periods of excessive leaching may lead to Table XXXIII

Effects of Tillage Methods on Seedling Emergence, Seedling Fresh Weight, Soil Temperature, and Soil Moisture Regimes in Cowpeas and Soybeans“

a

Tillage method

Maximum soil temperatureb (“C)

Soil moistureh

Conventional tillage, ridges Conventional tillage, flat Strip tillage Zero tillage

43 41 39 36

8.3 11.2 15.8 14.4

Conventional tillage, ridges Conventional tillage, flat Strip tillage Zero tillage

43 41 39 36

Soybean cv. bossief 8.5 0.9 e 11.6 33.4 d 16.9 50.7 c 14.3 53.9 c

Emergence (%)

Cowpea cv. vitad.e 83.2 b 89.4 b 96.7 a 97.8 a

Days to emergence 12 e

Seedling fresh weight‘ (€9

3a 3a

0.49 1.32 I .55 I .60

12 e 6d 5e 5e

0.24 0.53 0.46 0.43

4b

~~

“From Nan& er al. (1975). bMean of the first 10 days after planting. ‘Determined at 20 days after planting. “Rainfall 6.4 mm one day after planting, 33.3 mm at 9 days after planting. ‘Numbers followed by the same letter are not significantly different at the 5% level by Duncan’ Multiple Range test.

CONSERVATION TILLAGE

139

depletion of plant nutrients and acidification, restricted leaching causes salt accumulation and salinization. An appropriate conservation tillage for salt- affected soils improves drainage, increases leaching, and improves porosity of a soil with massive structure. The latter is caused by a high proportion of Na' in the exchange complex. Deep plowing to 80-cm depths and extensive soil loosening are often necessary to reclaim sodic soils (Cockroft and Tisdall, 1978). Deep tillage and use of crop residue mulchs can also influence redistribution of salt within the soil profile. Some acid soils require application of lime amendments for good crop growth. Lime acts both as a nutrient and as a neutralizer of active acidity. Unsurprisingly, lime must be incorporated into the surface horizon to neutralize acidity effectively (Moschler et al., 1973). The adoption of a no-till system, however, precludes the possibility of mixing lime within the root zone. Although a slow process, continuous application of lime to the surface in no-till, however, may cause the lime to be gradually translocated to the subsoil horizons. This implies that crop growth is adversely affected in a no-till system until the lime gets properly mixed into the root zone. Because of a high soil acidity, effectiveness of lime can be greater in a no-till compared with a plowed seedbed. In Virginia, for example, Moschler et al. (1973) reported that lime increased maize yields more than twice as much in no-till culture as in conventional tillage. In coarse-textured permeable soils, however, surface-applied lime is readily translocated into the root zone (Rodriguez and Lal, 1985; Maurya and Lal, 1979a,b). In humid tropical regions of southeast Nigeria, Friesen et al. (1982) noted a rapid movement of surface-applied lime and observed high leaching losses even when lime was broadcast on the surface. Some soils, especially uplands in the humid and subhumid tropics, are likely to suffer considerable leaching losses of essential plant nutrients. In southwest Nigeria, Ghuman and Lal(1984) observed that leaching losses of chloride and nitrates were high in no-till plots, especially because worm channels serve as preferential pathways for translocation of these chemicals in saturated or nearly saturated flow. On a kaolinitic Ultisol in southeastern Nigeria, Arora and Juo (1982) observed that nitrate leached readily in both the fallow and cropped plots. The recovery of applied nitrate ranged from 22 to 60% depending on the number of split applications made. Similar observations are made in the temperate zone climate. In Kentucky, for example, Thomas et al. (1973) observed that a large fraction of the nitrate was lost from the top 90 cm of soil under killed-sod plots. No nitrate was, however, lost from the conventionally tilled plots. It is feared, therefore, that risks of translocation of pollutants to groundwater and surface waters are greater with no-till and conservation tillage than with conventional tillage. This topic is discussed at length in Section IX. Whereas preferential movement of water and dissolved solids in

140

RATTAN LAL

biochannels can cause high leaching losses of nutrients and other chemicals in no-till compared with plowed soil, comparatively higher concentrations of organic matter in the surface layer of no-till seedbeds causes chemicals to be sorbed and retained in the surface horizon. There are, therefore, many compensatory mechanisms, and it is difficult to draw generalized conclusions.

VIII. WHY CONSERVATION TILLAGE? Apart from alleviating specific soil-related constraints, conservation tillage methods are used for growing crops in diverse soils and ecological environments. There are many reasons why a conservation tillage system in which crop residue is used as mulch is preferred over the conventional tillage system based on soil inversion and residue incorporation. A widely used conservation tillage method is the no-till system. Some of the proven benefits, regardless of the ecological regions, of the no- till system are listed below. A. IMPROVEMENTS I N SOILSTRUCTURE

Properly implemented, no-till and conservation tillage systems improve soil structure, increase the relative proportion of biochannels and macropores, increase water infiltration, and decrease susceptibility to crusting. In some soils no-till also increases homogeneity of soil structure (Boone and Kuipers, 1970), and decreases bulk density. In Brazil Machado (1976) observed lower bulk density and more total porosity in the 0-15-cm layer of no-till plots than in plowed soil (Table XXXIV). In northern Nigeria Table XXXIV Effects of Tillage Methods on Soil Physical Properties of the 0-15 cm Layer of a Tropical Soil in Brazila Treatment

Organic matter (%)

Bulk density (dcm’)

Total porosity (%)

Forest soil Pasture No tillage Conventional tillage

4.4 3.4 3.4 I .5

1.20 1.24 I .21 1.35

57.0 55.6 54.7 49.4

“From Machado (1976).

141

CONSERVATION TILLAGE

Ike (1986) observed more water-stable aggregates in the surface (0-20cm) layer of no-till and manually tilled plots than in mechanically tilled soil (Table XXXV). The surface (0-5-cm) layer of soil tilled by manual or mechanical methods had less bulk density than the no-till soil. Similar results were obtained by Aina (1979) for an Alfisol in southwest Nigeria (Fig. 17). Adem and Tisdall(l984) oberved in southeastern Australia, that the volume of water-stable aggregates increased by 48% when a cultivated red-brown earth was left untilled for 6 months (Fig. 18). In southwest Nigeria, La1 (1982) observed significantly higher infiltration rates in notill compared with plowed plots. In Parana, Brazil, Sidiras and Roth (1985) found significantly more infiltration in no-till compared with plowed soil. Depending on the antecedent soil conditions, however, mechanical soil disturbance may have a beneficial effect on the structure of some soils. For example, in Adelaide, South Australia, Dexter et af. (1982) observed that mechanical weed control measures had a significant positive effect on soil macrostructure as compared to that of plots treated with the chemical weed control. Improvements in soil structure by no tillage and conservation tillage are attributed to high biotic activity, especially of earthworms. Earthworms thrive better in no-till than in plowed soil (Lal, 1987a; Lavelle, 1984). Ehlers (1975) attributed the higher infitration rate of loess soil in Germany to greater numbers of worm channels and to their continuity and stability, which was better in no-till than in plowed soil (Table XXXVI). Haynes (1981) reported that the maximum number and weight of earthworms was in plots planted to grass in New Zealand. Suppression of grass by application of herbicides decreased the activity of earthworms. The most severe suppression of earthworm activity, however, was observed in cultivated plots (Table XXXVII). House and Parmelee (1985) reported that the

Table XXXV Effect of Tillage Treatments on the Percentage of WaterStable Aggregates of a Typic Haplustult in Northern Nigeria"

Tillage treatmentb Depth (cm)

No-till

Manual tillage

Mechanical tillage

0-5 5-20

46.3 81.1

55.9 66.9

30.5 28.0

"From Ike (1986). 'The least significant difference ( p ments is 8.5%.

=

0.05) between treat-

142

RATTAN LAL WATER-STABLE

AGGREGATES GREATER THAN 2.36 mm (%I

+

IWO SOIL

281

OBA SOIL

-I

Frc. 17. Effects of tillage methods on water-stable aggregates of an Alfisol in southern Nigeria. (From Aha, 1979.)

earthworm number in plowed and no-till soil, respectively, was 637 and 2202/m2for a sorghum-rye rotation, and 191 and 1210/m2for a sorghumclover rotation in plots in Georgia (in the United States). As a result of intense biotic activity, the soil under continuous no-till develops a structural profile distinctly different than that of the plowed soil (Fig. 19). The number of visible biochannels or earthworm channels (Fig. 36) is often used as an index of the status of soil structure. In fact, the suitability of a soil for no-till culture is related to the number of biochannels visible during the field examination. Gowman et al. (1977) suggested that a high number of visible pores (>lo0 pm in diameter) is the

t

50

1

1

I

I

1

1

Oct. Feb. Oct. Feb. Oct. Feb. I980 I981 1981 1982 I982 1983 Spring Summer Sprlng Surmer Spring Sumner DATE

FIG. 18. Improvements in aggregation of a cultivated red-brown earth in Australia following adoption of a no-till method. (From Adem and Tisdall, 1984.)

143

CONSERVATION TILLAGE Table XXXVI Number of Earthworm Channels per Mete$ in Plowed and No-Tillage Lwss Soil in Germanf Channel diameter (mm) Plowed

Depth (cm)

2-5

2 20 30

21 60 124 174

60

5-8 5 18 58

I65

No-till

8-11

Total

2-5

5-8

8-11

Total

1 1

29 79 187 348

75 99 209 183

40 41 91 I72

2

117 141 305 363

5 9

1

5 8

"From Ehlers (1975).

most important soil requirement for successful application of a no-till system in the United Kingdom. As reported in Sections IV and VII, the infiltration rate in an untilled soil can be lower than in plowed soil due to the traffic-caused soil compaction. In addition to the effects of vehicular traffic, however, the presence of residue cover may also in some soils decrease infiltration rate and moisture retention. In some cases, plant residue may cause hydrophobicity. In loess-derived soils in northern Nigeria, for example, Maurya (1986) observed a lower infiltration rate with mulch than without the crop residue mulch (Fig. 20).

B. SOILAND WATER CONSERVATION Soil erosion is effectively controlled by no-till and conservation tillage systems. The beneficial effects of the no- till system on soil and water

Table XXXVII Effects of Herbicides and Cultivation on Number and Total Weight of Earthworms in a Soil from New Zealand" Total weight

Mean weight

Treatment

Number (per mZ)

Wm2)

(g)

Grass Herbicide Cultivation

831 2 31 141 ? I I 6?4

341 k 14 36 f 4 221

0.41 0.26 0.33

"From Haynes (1981).

144

RATTAN LAL

CONTINUOUS

CONVENTIONAL TILLAGE

NO TILLAGE @

cecomposition and Mineralization of

MIXED SOIL

FIG.19. Structural profile of a no-till soil. (From House and Parmelee, 1985.)

conservation have been extensively documented for North America (Harrold et al., 1972; Harrold and Edwards, 1972; SCSA, 1973; McGregor et al., 1975; Phillips et al., 1980a; Griffith et al., 1986).The degree of erosion control achieved depends on the quantity of crop residue mulch (Loch et al., 1987).The conservation effectiveness of no-till systems has also been demonstrated for a wide range of soils in the tropics and subtropics (Lal, 1984a). In southwestern Nigera La1 (1976a) observed that a no-till system drastically reduces soil erosion on slopes of up to 15%. In Parana, Brazil, Sidiras and Roth (1985) observed, using a rainfall simulator test, greater infiltration in no-till and minimum till fields compared with plowed land.

* HTILLED

L

.s

RESIDUE

0

I

I

120

d

NO TILL TILLED

NO RESIDUE

I

180 0 60 120 180 240 TIME (min) Re. 20. Effects of crop residue mulch on the infiltration rate of a loess-derived soil in northern Nigeria. (From Maurya, 1986.) 0

60

I45

CONSERVATION TILLAGE

In northern Thailand Ryan (1986) observed that water runoff and soil erosion increased with increase in frequency and intensity of mechanical tillage (Table XXXVIII). In upland rice culture, the mean annual soil erosion decreased from 12.2 t/ha for conventional tillage to 0.8 and 1.0 t/ha for no-till and minimum tillage treatments, respectively. In Queensland, Australia, Mullins et af. (1984) observed that high soil erosion losses of 200400 t/ha/yr measured on lands with burnt trash are reduced to tolerable levels by adopting a no-till or a minimum till system. There are several mechanisms involved in erosion control by conservation tillage. Conservation tillage reduces both the sediment origin and its transport. Some conservation tillage effects are sediment-limited, whereas others are transport-limited. The principal mechanisms involved in erosion control are

(i) Reduced soil erosion due to improved aggregation and a high proportion of water-stable aggregates. Aggregation is improved by a high soil organic matter content, high biomass carbon, and high biotic activity of soil fauna. Soil detachability is also reduced by a high proportion of roots concentrated in the top soil horizons (Maurya and Lal, 1979a). (ii) Reduced inter-rill erosion and soil splash due to the prevention of raindrop impact on the soil surface. (iii) Reduced rill erosion due to decreases in runoff rate, amount, and velocity. The high infiltration rate in conservation tillage land is due to the lack of a surface crust and the presence of stable and continuous biochannels. Crop residue mulch reduces the velocity of water runoff. Table XXXVIII Effects of Method and Intensity of Tillage on Runoff and Erosion from Upland Soils in Northern Thailand“ Treatment

Runoff (m3/ha/yr)

Erosion (tlhalyr)

Effects of tillage intensity (average for the period 1983-1985) I763 3.0 Two cultivations Two cultivations plus mulch 1290 2. I I051 I .8 Scarifier 1.5 1021 N o tillage Effects of tillage methods (average for the period 1981-1985) Bare fallow 3788 49.1 Conventional tillage 1777 12.2 Minimum tillage 563 I .o N o tillage 457 0.8 “From Ryan (1986).

146

RATTAN LAL

-

MAIZE

5

No till

MPlowed

"1

* 0 to

O'I 01

1 APR

2 MAV

3

1

2 3 JUNE

1

2 3 JULY

1

T I ME

FIG. 21. Effects of no-till and conservation tillage on soil moisture reserve in (a) tropical and (b) temperate zones (9 = soil moisture content) (Maury soil, Kentucky). [(a) From Lal, 1976; (b) from Thomas er a / . , 1973.1

CONSERVATION TILLAGE

e

147

b IQ

FIG.21. (continued).

c. FAVORABLE SOIL MOISTUREAND SOIL TEMPERATURE REGIMES Through effects on soil structure, aggregation, total porosity, and poresize distribution, tillage methods influence wettability, moisture retention characteristics, water transmission, the depth of the wetting or drying front, water extraction patterns, and transport of water and solutes through the profile (Ehlers, 1976a; Allmaras er af., 1982; Ghuman and Lal, 1984). Conservation tillage has a moderating effect on soil temperature and moisture regimes, prevents extremes, and regulates the rate of evaporation. Tillage methods also affect soil moisture through altering root distribution and morphology (Barber, 1971). Consequently, all other factors being the same, plant-available water reserves in a soil managed by conservation tillage are likely to be greater than in plowed soil. This is especially true when the soil moisture content corresponds to the first and second stages of evaporation. High moisture reserves of no-till and conservation tillage soils have been demonstrated by data from both tropics and the temperate regions (Lal, 1982; Ojeniyi, 1986; Blevins et al., 1971; Van Doren and Allmaras, 1978; Grifith et al., 1986). Examples of higher soil moisture reserve in no-till soil than in plowed soil are shown in Fig. 21a and b, summarizing data from tropical and temperate regions, respectively. The high soil moisture content is due both to improved soil structure and to the decrease in evaporation due to the crop residue mulch. The improvements in soil structure, however, take a long time. Furthermore, structural improvement is minor, it occurs, in single-grained or sandy soils. In these

148

RATTAN LAL

cases, the high soil moisture content is mostly due to the presence of a crop residue mulch. Gupta and Gupta (1986) reported that for a sandy soil from Rajasthan, in western India, moisture conservation in the coarsetextured soil was mostly due to the reduction in evaporation by crop residue mulch (Fig. 22). Soil temperature is influenced by the quantity and properties of crop residue mulch (color, durability, reflectance, composition, contact, etc.), and by soil properties. The latter include soil texture, clay mineralogy, bulk density, and moisture content. These soil properties influence a soil's thermal conductivity and heat capacity (Allmaras et al., 1977).The effects of conservation tillage on soil temperature are also confounded by the proportion of surface area covered by crop residue mulch and by the prevalent climate (winter versus summer). Consequently, trends in soil temperature for conservation versus conventional tillage are different in summer or tropical regions than in winter or temperate climates. In the tropics, the maximum soil temperature is generally less and the minimum more in no-till than in plowed soil (see Fig. 9). In the temperate zone or in winter, although the maximum temperature is lower the minimum is not drastically higher in no-till than in plowed soils. The lower maximum soil temperature and slow warming in the spring are responsible for poor seedling growth in no-till plots in temperate climates (Moody et al., 1963; Van Wijk et al., 1959) (Fig. 23). The undesirable microenvironments in seed zone (Chaudhary et al., 1985) are a major obstacle to successful adaptation of the no-till form of conservation tillage in temperate climates. In summer, however, as in tropical climates, the decrease in maximum

-I JULY-AUGUST FIG.22. Moisture conservation in a sandy soil in Rajasthan, India, by the crop residue mulch. (From Gupta and Gupta, 1986.)

149

CONSERVATION TILLAGE 25

$

21

3

s

a W

a 5

17

13 2

0

4

6

10

8

WEEK

FIG.23. Soil temperature regime in no-till and conventionally plowed plots in the temperate United States. (From Cruz. 1982.)

soil temperature in no-till soil is proportional to the amount of surface area covered by mulch (Van Doren and Allmaras, 1978) (Fig. 24). Similar effects are observed in the tropical climates (Lal, 1975). In arid regions of northwestern India the use of crop residue mulch lowered the maximum soil temperature by 5-10°C regardless of the intensity of mechanical tillage (Fig. 25a) (Gupta and Gupta, 1986).

AT = -.W + 2106 (0.01060,0010) a MFRAC I (.30-.13);EON 15

W

a

3 I-

a

II:

w

2.0 AT 10 cm I h

b I

0

I

.2

I

I

.4

I

I

.6

I

I

.8

I

I 10 .

FRACTIONAL RESIDUE COVER ( MFRAG 1 FIG. 24. The effects of surface area covered by mulch on soil temperature in the Corn Belt of the United States. (Van Doren and Allmaras, 1978.)

RATTAN LAL

150

NO DlSKlNG

NO MULCH

40

MULCH

4eL

-

44 -

- -No

40 -

-

b

I

6.' w

IT 3

w

4 c

i

32-

-

28 -

-

24 -

-

20 -

plant cover

Tlllage H Chisel plough 0 4 Conventional

-

36-

w

-Dense

3-cm soil depth

-

6-cm soil depth

FIG.25. Effects of residue mulch and of tillage intensity on the temperature regime of (a) sandy soil in western India and (b) an Oxisol in central Brazil. [(a) From Gupta and Gupta, 1986; (b) from Derpsch er al., 1985.1

CONSERVATION TILLAGE

151

Plastic sheets used as mulch influence soil temperature differently than does straw mulch. The transparent plastic mulch may create a greenhouse effect at the soil-atmosphere interphase. In northern India, for example, Tripathi and Katiyar (1984) observed a paddy straw mulch lowered the maximum soil temperature by 6-73°C and raised the minimum by about 3°C. In contrast, a polythene asphalt emulsion raised the maximum soil temperature by 4 4 ° C . Similar results were obtained for Alfisols in the subhumid regions of western Nigeria (Harrison-Murray and Lal, 1979; Maurya and Lal, 1981) (Fig. 26).

1F’lc. 26. Soil temperature regime of an Alfisol as influenced by the nature and properties of mulch material used. (From Maurya and Lal, 1981.)

152

RATTAN LAL

D. SOIL CHEMICAL AND NUTRITIONAL PROPERTIES AND FERTILIZER RESPONSE Effects of tillage methods on soil chemical properties differ among soils, climatic regimes, crop rotations, and the period of time for which the tillage systems have been in operation. Soil chemical properties are predictably different due to tillage-induced alterations in soil temperature and moisture regimes, biotic activity of soil fauna, and the return of crop residue to the soil surface in conservation tillage and its mixing and incorporation in the plowed layer in conventional tillage. In general, the surface layer of a soil managed with conservation tillage contains more organic matter and possesses a relatively higher fertility status than soil managed by conventional tillage. The chemical properties of the subsoil, however, may be more favorable in soil managed by conventional than conservation tillage. The differences in organic matter content among tillage treatments are also influenced by the climate. The data from soils of the tropics show that continuous use of conservation tillage for 5-10 years causes the top soil layer to have a higher organic matter content, more cation exchange capacity, and more basic cations than the plowed soil (Aina, 1979; Lal, 1986a). These conclusions are supported by the data on an Alfisol from western Nigeria (Fig. 27) (Aina, 1979). La1 and De Vleerchauwer (1982) reported that favorable chemical properties of the surface layer of no-till soil in Nigeria may partly be due to a high proportion of earthworm casts. Machado (1976) reported more available phosphorus and higher levels of exchangeable cations in the 0-15-cm layer of no-till soil than of plowed soil in Brazil.

w

ORGANIC MATTER (70)

LL 0

FIG. 27. Organic matter content of a tropical Alfisol as influenced by tillage methods. (From A h a , 1979.)

CONSERVATION TILLAGE

I53

Favorable soil chemical properties are also observed in conservation tillage systems in temperate zone soils. In Kentucky Blevins et al. (1977, 1983) observed that in the 0-2-cm layer, organic carbon and nitrogen were approximately twice as high in surface soil of the no-tillage soil as of the plowed soil. Similar results, indicating an increase in the concentration of plant-available nutrients in the surface layer of no-till soil, have been reported by others (Coutts et al., 1977; Ellis and Howse, 1980; Stinner et al., 1983). Modifications in physical, chemical, and nutritional properties by conservation tillage alter soil’s response to fertilizers and chemical amendments. The differential fertilizer response may also be due to the mode of fertilizer application. Whereas fertilizers are usually broadcast on the surface layer in no-till and conservation tillage, they are incorporated into the plow- layer in conventional tillage. In addition, important determinants of fertilizer response are root growth, soil fauna, and mulch. The latter influences fertilizer response both directly and indirectly. Indirectly, crop residues influence nutrient availability through altering temperature and moisture regimes and influencing losses in seepage and surface runoff. The effects of residue mulch in preventing nutrient losses due to runoff and eroded soil are important factors in improved fertilizer use efficiency on steep lands (Lal, 1976a; McDowell and McGregor, 1984). Directly, crop residue may contribute or immobilize plant-available nutrients. Crop residues with a low C:N ratio (from leguminous plant materials) contribute readily available nitrogen, whereas those with a high C:N ratio (cereals) may immobilize available soil nitrogen into an unavailable form through microbial activity. The fertilizer response, therefore, depends on the antecedent soil properties, drainage conditions, the quality and quantity of crop residue mulch, and the prevalent climatic conditions, The response to applied nitrogen may follow either of the two patterns shown in Fig. 28, depending on internal drainage, the antecedent soil conditions, and the quality and quantity of crop residue mulch. Nitrogen is a major nutrient whose uptake is influenced by tillage methods. No-till or conservation tillage may require, under some soils and moisture regimes, additional nitrogen to produce yields equivalent to those produced by the conventional tillage. In Nigeria, for example, Kang et al. (1980) observed that yields of no-till maize were less than yields of plowed maize with no or low rates of N application, but equal with greater rates. White et al. (1985) also reported, using data from studies in Queensland, Australia, that the slope of the curve relating wheat yield to N rate was maximal when the residue of the previous sorghum crop was incorporated into the soil. Sharma (1985) observed that production of irrigated forage in northern India was thwarted by the low availability of N in notill plots. The nitrogen requirements of conservation tillage, however, can

RATTAN LAL

154

T

PLOWED

-.-.-. NO TILL

n

A

ez K

0

0 W

fU A

W

K

N RATE-

FIG.28. Crop response to nitrogen with conservation and conventional tillage systems as affected by soil properties: (a) eroded soil or poorly drained soil or soil with residue mulch of high C:N ratio; (b) uneroded soil of good structure or with residue mulch of low C:N ratio.

be reduced by adopting soil and crop management systems that involve suitable rotations, growing leguminous cover crops, and growing nitrophilic crops in association with leguminous shrubs and woody perennials. The nitrogen requirement of conservation tillage eventually decreases in comparison with plowed soil because of the reduced losses due to erosion and the equilibrium attained between the amount of nitrogen immobilized and released. On soils similar to those studied by Kang et al. (1980), for example, La1 (1982) observed equivalent or better responses by maize to nitrogen on no-till compared to plowed soil about 10 years after these

155

CONSERVATION TILLAGE

treatments were imposed. Similar results are reported from Canada by Greaver et at. (1986). Soil drainage is an important determinant of the nitrogen response to tillage methods. Crop response to N on poorly drained soil is similar to that on eroded soils (see Fig. 28a). In Canada, Greaver and Bomke (1986) observed for a northern clay soil that the nitrogen response of barley varied among tillage methods depending on soil wetness. On a Typic Paleudult along the southeastern coastal plain in the United States, Campbell et al. (1984a) oberved that conservation tillage does not work well on poorly drained soils where fragipans exist. The nitrogen response was lower on conservation tillage soil than on conventional tillage plots. Response to P in relation to tillage methods depends more on soil chemical and mineralogical composition than on soil physical properties. For soils with a low capacity for P fixation, tillage methods have little effect on P uptake. For Alfisols in western Nigeria, for example, Kang and Yunusa (1977) observed that broadcast and hill methods of P application were equally effective in supplying adequate P to the maize crop at P application rates exceeding 20 kg/ha. Juo and La1 (1979) observed a satisfactory rate of P movement for Alfisols containing predominantly lowactivity clays (Fig. 29). For soils with a high capacity to fix P and with low plant-available reserves, however, incorporating fertilizer in the soil makes it more readily available to plants than when it is broadcast on the surface. The response to P is, therefore, lower with a no-till system, in which the fertilizer is broadcast, than with a plow system, in which it is incorporated into the soil.

70 T

-

NOTILLAGE CONVENTIONAL TILIAGE

10

n -. 2.5

7.5

12.5

17.5

22.5

27.5

32.5

37.5

42.5

47.5

DEPTH Rc. 29. P profile of no-till and conventionally plowed plots of a tropical Alfisol. (From Juo and Lal, 1979.)

156

RATTAN LAL

E. ROOT GROWTH Fertilizer use efficiency is also influenced by proliferation of the root system. Tillage methods influence the root-depth distribution and the relative proliferation in the surface versus subsoil horizons. Root growth in relation to tillage is influenced by factors that determine pore size distribution, stability and continuity of pores, and soil moisture and temperature regimes. In general, conservation and reduced tillage systems favor more root growth in the surface layer immediately beneath the residue mulch. The roots are'thickened, and the quantity (weight and number) or the total weight of root system is reduced in conservation tillage compared to that seen in a plow-based system. A much higher percentage of roots is found in the surface than in the subsoil horizons. Some isolated roots also grow actively in the deeper horizons of the conservation tillage soil compared with plowed land. Deep root penetration is facilitated by worm holes and biochannels. As a consequence of the differences in root system, water extraction patterns also differ among tillage systems. Generalized rooting depth distribution patterns for no-till and conventional tillage systems are shown in Fig. 30. The pattern is, however, modified by the type of conservation tillage used (minimum tillage, ridge tillage), land use history, antecedent soil properties, and crop and soil management practices adopted. In Nigeria Maurya and La1 (1979a) observed that in an uncompacted soil managed with manual farm operations, there was a greater root density in the surface layer of the no-till compared with the plowed plots. Furthermore, there were a few roots that penetrated beyond the I-m depth

ROOT DENSITY ---+

T

I I=

-NO

w

_ _ - _ _PLOWED

n

n

TILL

FIG.30. A generalized root profile in relation to tillage methods.

CONSERVATION TILLAGE

157

through biochannels that existed only in the undisturbed soil of the notill plot. Similar results have been reported for Nigeria by Osuji (1984). In Cameroon Ambassa-Kiki et al. (1984) observed that root distribution was restricted in the no-till soil (Fig. 31). Restricted root growth in an untilled soil was also observed in the West African Sahel by Chopart (1984) (Fig. 32). Similar trends in root developments were observed for a loessderived soil in Germany by Ehlers et al. (1980) (Fig. 33). Ehlers et a/. (1983) observed significant positive influence of worm channels on water conductance and on root growth of oats in a no-till soil.

F. ENERGY CONSERVATION Through its emphasis on reducing inputs, successful use of conservation tillage causes significant savings in energy costs without jeopardizing productivity. Tillage and petrobased chemicals such as fertilizers and pesticides are energy-intensive inputs. The data in Table XXXIX show that fuel costs represent as much as 13.6 and 20.0%of the total costs for maize and soybean production, respectively. Field machinery consumes as much as 469 trillion BTUs of energy used in agricultural production in the United States (Ritchie, 1983). The total energy use includes 764 trillion BTUs for fertilizers and pesticides, 370 for transportation, 263 for irrigation, 75 for crop drying, and 8 1 trillion BTUs for miscellaneous uses. Conservation tillage saves energy through reducing the frequency and intensity of tillage and decreasing fertilizer and irrigation needs by conserving soil and water.

ZERO MINIMUM CONVENTIONAL FIG.31. Effects of tillage methods on root system development of upland rice in Cameroon. (From Ambassa-Kiki er a / . , 1985.)

158

RATTAN LAL

50 I

(v

E

MNO TILL @=4 PLOWED

MYS AFTER SEEDING

FIG.32. Effects of no tillage and plowing on root system development in semiarid West Africa. (From Chopart, 1984.)

Energy conservation is an important reason for adopting conservation tillage. Whereas subsistence farmers of the tropics use few energy-based inputs, North American and western European agriculture is based on somewhat excessive use of commercial energy (Table XL). An important reason for adopting conservation tillage, therefore, is the conservation of nonrenewable energy.

SOIL DEPTH (cm)

FIG. 33. Rooting density in 10-cm soil layers and total root length on plowed and no-till loess soil in Germany. (From Ehlers et af., 1980.)

159

CONSERVATION TILLAGE Table XXXIX Cost of Production ($/acre) of Maize and Soybean in the United States” ~

~~~~

Variable

Maize

Soybean

Fuel Fertilizer Chemicals Drying Subtotal Other variable costs Total variable costs

16.00 40.50 15.00 4.40 75.90 41.60 117.50

13.60 7.50 13.50

-

34.60 33.50 68.10

“From Ritchie (1983).

G. PREVENTING SOILDEGRADATION AND MAINTAINING SOIL FERTILITY Ecologically compatible agriculture should be aimed at preventing soil degradation, maintaining soil’s productive potential, and reducing environmental pollution. Non-point source pollution is a major environmental hazard of modern agriculture. The problem is particularly severe in the United States and other economies geared to commercial surplus production. When soil degradation and desertification are severe problems, the question of crop yields is of secondary importance. In that event, the important question is not whether conservation tillage works but how to make it work. Table XL Commercial Energy (lO’*J) Used for Inputs to Agricultural

Fertilizer

Machinery

Irrigation

Pesticides

Percentage of world total

Region

A

B

A

B

A

B

A

B

A

B

North America Latin America Near East Africa

750 153 86 38

1429 468 351

1299 148 50 30

1427 349 167 73

36.6 6.1 30.8 1.2

42.0 13.7 54.7 3.1

55.3 5.3 1.4 1.2

64.5

28.2 4.1 2.2 0.9

22.0 6.3 4.3 1.4

111

“From Shahbazi and Goswami (1986). ’A, 1972-1973; B. 1985-1986.

13.8 8.3 8.3

160

RATTAN LAL

IX. ENVIRONMENTAL POLLUTION AND CONSERVATION TILLAGE The material presented in the previous sections indicates that low crop yields with conservation tillage are associated with the following soil and environmental factors: soil compaction, crusting and hard-setting soil, poor drainage, low soil temperatures, high P fixation capacity of the soil, use of crop residues with high C:N ratios, and damp and humid climates, which cause anaerobic decomposition of crop residue mulches. Low crop yields are caused by poor stand establishment, damage to seedlings by rodents and birds, and possibly by a high incidence of weeds, insects, and pathogens. A considerable literature exists relating the incidence of pests to the use of crop residue mulch. There are also research reports showing lesser pest incidence with conservation or mulch tillage than with conventional tillage (Shenk and Saunders, 1981). A serious environmental issue of modem times is the pollution of natural waters. This is particularly true in countries such as the United States, where the land area under conservation tillage has drastically increased over the two decades ending in 1985. For example, the land area in conservation tillage in the United States has been increasing at an average rate of 4.2% per year during the quarter of a century ending in 1985 (Mannering et al., 1987). Consequently, the use of pesticides, especially herbicides, has also increased. Herbicide use is especially heavy for maize and soybean production, because these crops are grown in the United States with heavy dependence on herbicides regardless of the tillage methods used. It is estimated that the annual discharge of pollutants to rivers in the United States amounts to more than a billion t of suspended solids, about one- half billion t of dissolved solids, one million t of P, and about 5 million t of N. Conservation tillage is also a concern in relation to the groundwater quality (Logan ef at., 1987). The use of no-till and conservation tillage systems is also expanding rapidly in some countries of the tropics and subtropics, such as South and Central America (especially Brazil), Australia, southern Africa, and southeastern Asia. In contrast to the United States and western Europe, however, little is known about the retention, biodegradation, and movement of these chemicals in surface and subsurface waters in tropical environments. The movement of agricultural chemicals is related to that of water in both surface and subsurface flows. Although conservation tillage decreases surface runoff and erosion, it is also known to increase the drainage and subsurface tile flow. High infiltration and subsurface drainage in conservation tillage are facilitated by the presence of large numbers of earthworm

CONSERVATION TILLAGE

161

channels. These channels serve as preferrential waterways that conduct water (Phillips, 1981; Gold and Loudon, 1982) and dissolved chemicals in the aqueous phase to the subsurface horizons. Because of a high infiltration rate, it is likely that the infiltrating water in no-till plots carries greater amounts of nitrate and the added nitrogeneous fertilizers than that in plowed plots (Tyler and Thomas, 1977). However, the leaching losses of total nitrogen are highly variable due to the many compensatory mechanisms involved (Wild, 1974; Kanwar et al., 1985). Similar to the loss of added fertilizers, infiltrating water in conservation tillage is also likely to transport more pesticides than water from plowed plots because the total amount of water infiltrating through the soil column in no-till plots is more than that from the plowed land. The effects of tillage systems on transport of fertilizers and pesticides are, however, confounded by other factors, including the time of application in relation to the time of plowing, the amount and distribution of rainfall, and the prevalent climate. The latter influences the amount and rate of volatilization and degradation. Conservation tillage, through its moderating effect on climate, alters the rate of uptake and degradation of pesticides (Glotfelty, 1987). A high proportion of pesticides is transported in surface runoff. Some pesticides (such as trifluralin, endrin, and toxaphone) are extremely insoluble and are transported along with solids only. Pesticides absorbed on clay and organic matter are washed away in surface runoff and eroded sediments. Because conservation tillage reduces the rate and amount of water runoff and sediment loss, it also decreases the losses of absorbed pesticides. It has now been proved that the quantity of pesticides that volatilizes is usually much larger than that which moves with runoff or leaching (Taylor, 1978; Glotfelty, 1987). Soil incorporation of pesticides and low soil moisture contents decrease volatility. This implies that volatilization losses of pesticides are likely to be greater in conservation than in conventional tillage systems. There are, however, other factors that decrease volatility of pesticides in conservation tillage soils. Important among these are diffusion and convective flow. The net effects of all these compensatory factors are, therefore, difficult to estimate. In addition to volatilization, major pesticides losses occur in water runoff and seepage flow. Most pesticides move in water runoff as soluble compounds (Fig. 34). Therefore, crop and soil management practices that reduce soil losses but not runoff volumes may have little effect on pesticide losses (Wagenet, 1987). Leaching loss is the third major source of pesticide loss into natural wzters. Some pesticides, such as paraquat, atrazin, and metolachlor, are relatively immobile in soils of average organic matter content. Because surface soil in conservation tillage contains relatively

162

RATTAN LAL W

-3

-2

-1

0

1

2

3

4

5

6

LOG (solubility, ppmw)

FIG. 34. Partitioning of pesticides between sediments and water in runoff samples, with the range of reported literature values indicated by the solid bars. (From Wanchope et a / . , 1985; Wagenet, 1987.)

more organic matter than that in conventional tillage, the retention of these pesticides in the top layer is likely to be greater with conservation than with conventional tillage. However, the presence of large and relatively continuous macropores in conservation tillage soil is another complicating factor. The presence of well-established macropores may influence pesticide leaching in two ways (Wagenet, 1987):

(i) If rainfall exceeds the infiltration rate and surface ponding occurs, the macropores serve as conduits to transmit water and dissolved pesticides to less biologically active subsoil horizons. The pesticides in this horizon of low organic matter are neither easily absorbed nor biodegraded and are transmitted en masse to the ground water. (ii) If heavy rains do not occur immediately after the pesticide application, then pesticides solubilize and diffuse into relatively small pores. Whenever saturated flow does occur through the large pores, it will transmit relatively clean water to the subsoil horizons. At present, there is little if any field data from the tropics or temperate zone to verify either of these scenarios and their relative importance in pesticide movement in relation to tillage methods.

CONSERVATION TILLAGE

163

X. THE SYSTEMS APPROACH TO CONSERVATION TILLAGE AND SUPPORTIVE CULTURAL PRACTICES Is sustainable agriculture necessarily based on low inputs? The word input is a relative term. At present, African agriculture is based on low or no commercial inputs. In contrast, agriculture in North America is heavily dependent on commercial inputs. Ecologically, no-input agriculture can be as harmful to the African environment as are excessive inputs and intensive agriculture in North America to that environment. Whereas farmers in North America must make an earnest effort to reduce inputs, subsistence farmers in Africa can achieve substantial yield improvements by even marginal increases in added inputs. Sustainable agriculture emphasizes reduction in chemical and energyintensive industrial inputs. The objective is to optimize the use of energyrelated inputs. High crop yields are, however, possible if other nonindustrial inputs are increased. These inputs may include improved cultivars, new crops, efficient cropping systems, improved tools, increased fertilizer use efficiency, and systems of integrated pest management. These inputs are in accord with principles of good farming and land stewardship. Good farming, by this definition, is that which is ecologically and environmentally compatible. The effectiveness of conservation tillage in soil and water conservation and resource management is greatly enhanced by adopting the systems approach. Conservation tillage requires a special set of cultural practices that may be different than those needed for conventional tillage. There may be some crops and varieties that are more suited to conservation tillage than others. The rate, time, mode, and type of application of fertilizers and other amendments are also likely to be different, as would be the measures for pest control. Conservation tillage also requires different types of seeding equipment and farm machinery to manage the uneven and trashy soil surface. Some crop rotations and farming systems are apparently better suited to conservation tillage than others. Mulch being an integral component of conservation tillage, cultural practices that ensure the production and availability of a large quantity of residue mulch are compatible with conservation tillage. Regulating vehicular traffic is an important consideration for reducing the risks of soil compaction. Method, time, and type of harvesting equipment used have important effects on soil compaction. Conservation tillage, therefore, is not just a single concept but a package of cultural practices that are specifically developed and adopted to conserve soil and water resources, sustain high and satisfactory returns, minimize degradation of soil and environments, and preserve the

164

RATTAN LAL

soil resource. The interrelationship between conservation tillage and supportive cultural practices is shown in Fig. 35. Some of the cultural practices specifically developed to enhance the effectiveness of conservation tillage are briefly described below.

A. AGROFORESTRY AND ALLEYCROPPING Agroforestry refers to a technique of growing food crop annuals in association with woody perennials to optimize the use of natural resources, minimize the need for inputs derived from nonrenewable resources, and reduce the risks of environmental degradation. The practice is also referred to as agrisilviculture, Taungya, and by many other names drawn from different cultures and languages (Roche, 1973). King (1968) lists 79 woody species and genera and 42 agricultural crops grown in one or another form of agroforestry used in the tropics. The most common tree species used in the tropics are Nauclea diderrichii, Lovoea trichilioides, Khaya ivorensis, and Tectona grandis. Lately, the emphasis has been shifted to some woody perennials (e.g., Leucaena, Gliricidia, and Flemingia).

FIG.35. The system approach to conservation tillage.

CONSERVATION TILLAGE

I65

Alley cropping is a form of agroforestry in which food crop annuals are grown between two adjacent hedgerows of leguminous shrubs and woody perennials (Kang et al., 1981, 1985). The woody perennials are regularly pruned to minimize shading and to procure nitrogen-rich mulch for food crop annuals. Satisfactory crop yields are obtained provided that compatible species are chosen and that the plant-available reserves of soil water are sufficient to meet the evapotranspiration needs of both species. The system is normally suited for humid and subhumid regions in which precipitation exceeds evapotranspiration during the crop-growing season. At present the system is labor-intensive and is suited more for resourcepoor farmers of the tropics than for large-scale commercial farming. MaizeLeucaena alley cropping can be economically promising if hired labor is available at low cost (Verinumbe et al., 1984). Field experiments conducted in the subhumid tropics have shown that when properly established maize grown in association with contour hedges of Leucaena leucocephala and Gliricidia sepium produces satisfactory yields (Fig. 36). The data in Table XLI show that despite the reduction in cropped area, maize grain yield with alley cropping was equivalent to that of no-till treatments. The yields of cowpeas, however, was drastically reduced by alley cropping. In the case of cowpeas, the yield reduction was due to poor stand establishment and reduced germination. An alleleopathic effect is a likely reason for the poor germination. In semiarid

FIG.36. Alley cropping of maize with Leucaena leucocephala.

166

RATTAN LAL Table XLI

Effects of Methods of Seedbed Preparation and of Hedgerow Spacing of Leucaena and Gliricidiu on Grain Yield of Maize and Cowpeas"

Grain yield (t/ha) Treatment

Maize

Cowpeas

Plowed No-ti11 Leucaena: 4 rn Leucaena: 2 m Gliricidia: 4 rn Gliricidia: 2 m

3.6 4.0 3.7 3.8 3.6 3.3

447 1193 58 I 503 670 678

"Unpublished data of La1 (1984).

and arid climates, however, growth suppression and yield reduction in food crop annuals are caused by excessive competition for soil moisture (Singh and Van Den Beldt, 1986; Nair, 1984). Contour hedges decrease runoff velocity and reduce its sediment transport capacity. Sediments trapped by the contour hedges facilitate the formation of natural terraces. Experiments conducted on relatively steep lands in the Philippines have shown that compared with croplands contour hedges of Leucaena reduce sediment transport by several orders of magnitude (Loch, 1985; Pacardo and Montecillo, 1983). The effectiveness of contour hedges in trapping sediments has also been demonstrated in Indonesia (Sukmana et al., 1985). Closely spaced narrow strips of shrubs or woody perennials are likely to be more effective in soil and water conservation than widely spaced single-row hedges. There is, however, an optimum spacing for erosion control and for satisfactory growth and yield of food crop annuals. The optimum spacing depends on slope gradient, soil type and its susceptibility to erosion, rainfall, crop species, and the soil and crop management system. Loch (1983) observed that erosion control by contour hedges depends on the sediment-carrying capacity of the water runoff. Hedges trap sediments as long as the sediment transport capacity of the overland flow is not yet fulfilled. The data in Table XLII show that 2-m-apart contour hedges of Leucaena are more effective in reducing runoff and soil erosion than are the 4-m- apart hedges. In comparison with conventional plowing, hedges of Leucaena and Gliricidia also reduced losses of cations and plant nutrients. The data in Fig. 37 show that growing contour hedges of perennial shrubs drastically influenced the accumulative infiltration. The ac-

167

CONSERVATION TILLAGE Table XLII Effects of Contour Hedges of Leucaem and Gliricidin on Runoff, Soil Erosion, and Total Nutrient Loss for Maize Grown in the First Season (April-August 1984) and Cowpeas Grown in the Second Growing Season (September-December 1984)

Treatment

Runoff (% of rainfall)

Erosion (t/ha)

Plowed No-till Leucaena: 4 m Leucaena: 2 m Gliricidiu: 4 rn Gliricidiu: 2 m

Maize (Rainfall 727 mm) 29.9 14. I6 0.8 0.026 I .2 0. I7 I .3 0.07 4.9 1.62 2.2 2.05

Plowed No-till Leucaena: 4 m Leucaena: 2 rn Gliricidia: 4 m Gliricidia: 2 m

Cowpeas (Rainfall 631 mrn) 2.4 0.18 0.08 0.57 0.32 0.71

0.74 0.006 0.02 0.04 0.05 0.27

Total nutrient loss (kglha)

101.2 12.9 4.8 2.1 12.3 2.2

1.13 0.29 0.10 0.61 I .69 0.53

cumulative infiltration was 83, 82, 70, 55, 54 and 40 cm per 2-hr period for Gliricidia at 4 m spacings, Leucaena at 2 m spacings, Leucaena at 4 m spacings, plowed soil, Gliricidia at 2 m spacings, and no-till treatments, respectively. There were also notable differences in runoff hydrographs among methods of seedbed preparation and hedge-row spacing treatments (Fig. 38). Despite its apparent advantages, a considerable amount of local-specific research is needed to develop appropriate alley cropping or agroforestry systems for different soils, crops, and ecological environments. Research is needed in choosing appropriate crop and tree species, suitable spacing, management of trees, and soil and crop management practices for food crop annuals. Trees are extensively grown as woodlots, on field boundaries, and along fence posts in temperate zone climates. An important research consideration would be to determine the proportion of land area needed to be allocated to trees so that ecological stability is maintained with intensive use of the remaining land as an arable area. A strong data base is needed to validate the nutrient recycling effects presumably attributed to growing deep-rooted perennials.

168

RATTAN LAL

-

-x

Leucaena - 4 m Gliricidia .4 m Notill

c-. Plowed 80

70

-E C

60

0 CI

E

=

c'

-

50

.-

m

5

8 a

40

30

20

10

0

20

40

60

80

100

120

4 4140 0

Time (min)

FIG.37. Accumulative infiltration as influenced by methods of seedbed preparation and spacing of perennial hedges. (Unpublished data of Lal, 1984.)

B. COVERCROPS Diversifying the cropping system is a necessary strategy to create ecological stability and reduce the incidence of disease and pests. Growing grass or leguminous cover crops at frequent intervals, once every 2-4 years in temperate zone and once every 1-2 years in the tropics, is necessary for successful adaptation of a conservation tillage system. Cover

I 69

CONSERVATION TILLAGE

-

Total RunoW Leucaena-4rn 176

Ghnc1dm-4m

0

10

148

205

41

53

19

Notill

Glmcidia - 2 rn

(mln) Eroslon (ke/ha)

12 0

Plowed

Rainfall = 67 3 mrn

20

30

40

50

60

70

80

90

Time attar Runoff initiation (min)

FIG.38. Effects of hedgerow spacing and methods of seedbed preparation on the hydrographs generated by a rainstorm with a total rainfall of 67.3 mm. (Unpublished data of Lal, 1986.)

crops have many advantages for conservation tillage systems (e.g., they restore fertility, control weeds, avoid repeated seeding and cultivation traffic, conserve rainwater, and reduce energy costs). In addition to controlling pests, cover crops improve soil physical properties and soil tilth and reduce soil erosion. Cover crops are beneficial regardless of the ecological region, although their advantages are relatively greater in the tropics than in the temperate zone. Adams rt al. (1970) successfully grew maize with conservation tillage in atrazin- treated sod species. Elkins et al. (1979) investigated the effects of growth retardants on many sod species and on the yield of maize grown as a succession crop. Satisfactory maize yields were obtained when sod species were adequately suppressed. Thomas et a1 (1973) and Ebelhar and Frye (198I ) reported that legumes boosted nitrogen for no-till maize production in Kentucky. They observed that growing a winter annual legume cover crop for no-till maize involves relay-sowing the cover crop in the fall before harvesting maize and killing it with herbicides in the spring just prior to seeding the next maize crop. The nitrogen contribution that could be produced by growing suitable cover crops in rotation with vegetables

100

170

RATTAN LAL

was shown by Mascianica et al. (1982). In the southeastern United States, Touchton et al. (1984) observed that winter legumes such as crimson clover (Trifolium incarnatum) and common vetch (Vicia saliva) are good nitrogen sources for succession-planted no-till cotton. The usefulness of different cover crops and their management for conservation tillage in the southern United States were discussed at length by Hargrove (1982), Unger et al. (1986), and Triplett (1986). Cover crops have long been used in the tropics for soil and water conservation in plantation crops (Okigbo and Lal, 1977). The importance of cover crops in conservation tillage for the management of some uplands in Ghana, West Africa, was demonstrated by the work of Kannegieter (1967a). Under average conditions, some grasses and legumes produce large quantities of biomass (Table XLIII). The biomass produced is useful as forage, mulch, and for other domestic uses. Kannegieter (1967b, 1969) developed a technique involving the combination of short-term pueraria fallowing and zero cultivation to reduce the requirement of fertilizer nitrogen of a maize succession crop and to abate soil erosion. Table XLIII Dry Matter Yield of Various Grasses and Legumes, Africa"

Dry matter yield (t/ha) Cover crop

West Africa

Cenchrus ciliaris (buffel grass) Seiaria sphacelaia Tripsacum laxum Stylosanrhes plus C . ciliaris Andropogan gayanus Botriochloa inscupia B. inscupf a plus Centrosema pubescens Panicum maximum plus Centrosema pubescens Pennisetum purpureum PIUS Centrosema pubescens S . sphacelaia plus Centrosema pubescens Psaphocarpus palustris Penniseium purpureum (elephant grass) Panicum maximum (guinea grass) Chlorea gayana (Rhodes grass) Pueraria phaseolodes (kudzu) Cenirosema pubescens Glycine wighiii

35.3 28.7 30.1 28.2 30.5 26.7 26.1 25.9

East Africa

23.0 25.2 11.0

-

10.0 13.0 6.5

11.3 10.0 8.3 3.7 2.3

"Based on data from Nateh and Anderson, 1962; Kannegreter. 1967a; Okigbo and Lal, 1977.

CONSERVATION TILLAGE

171

In addition to augmenting soil fertility, cover crops also improve soil structure and increase macroporosity . In northern Nigeria, Wilkinson (1975) observed significant benefits of grass fallow rotations on the infiltration of water into the savanna zone soil. The infiltration rate increased with increasing length of the fallow period. Similar observations were reported for soils of western Nigeria (La1 et al., 1979; Wilson et al., 1982) Mucuna utilis is now widely recommended as a cover crop in western Nigeria (see Fig. 4). The benefits of cover crops on soil structure and tilth improvement have also been demonstrated for East Africa (Pereira et al., 1954, 1958; Pereira, 1956; Wallis, 1960; Peers, 1%2; Stephens, 1%7). Cover crops are also found to be useful for erosion control with conservation tillage in soils of tropical America (Kemper and Derpsch, 1981). In Parana, Brazil Sidiras et al. (1985b) observed that cover crops (such as Avena strigosa, Raphanus sativus, and Lupinus albus) grown in winter have beneficial effects in controlling water erosion and on the yield of the following summer crops of beans, soybeans, and maize. Yield increases of 93% in beans, 73% in soybeans, and 81% in maize were observed when they were grown after an appropriate legume cover. The choice of an appropriate cover crop for different soils and ecological regions depends on many considerations. Some of the important considerations are (i) Ease and economics of establishment including availability of seed, (ii) Quick ground cover and growth rate during the off- season, (iii) N-fixing rather than N-consuming, (iv) Deep root system and consumptive water use, (v) Feed value for livestock, (vi) Alternate hosts for pests and cover for wildlife, (vii) Canopy height, (viii) Ability to suppress weeds, (ix) Growth duration (i.e., permanent versus annual), (x) shade tolerance, and (xi) Ease of management for growing a food crop with conservation tillage

There is considerable scope for selecting appropriate species and cultivars for suitable cover crops. Cover crops are an important tool in sustainable agriculture. C. LIVEMULCH Management of cover crops is a necessary step in conservation tillage. A difficult-to-suppresscover crop can be expensive and an energy-intensive activity. Consequently, the concept of live mulch or a green seedbed

172

RATTAN LAL

was proposed in the early 1940s (Spivack, 1942, 1984). A live mulch system is based on the principles of mixed cropping. A fast-growing perennial legume is established with the objectives of smothering weeds and growing a seasonal grain crop through it without severely suppressing the growth and yield of the food crop. A small strip is opened, with or without herbicides, to seed a seasonal food crop through an established live cover crop. The system works if the live mulch is a low-growing nonclimber and is not competitive for light, moisture, or nutrients. Like alley cropping, the live mulch system is also likely to be more successful in humid and subhumid regions with little or no water deficit than in semiarid or arid regions. The concept has been tried in West Africa with but modest success (Voelkner, 1979; Ogborn, 1980; Akobundu, 1980a; Wilson et al., 1982). Akobundu (1980a) reported satisfactory yields of maize using a live mulch system of Arachis, Centrosema, and Psophocarpus. Drastic yield suppression of food crops can occur, however, due to alleleopathic effects, smothering, and competition for moisture during periods of drought stress. D. ROTATIONS AND MULTIPLE CROPPING Crop rotations are an integral component of successful conservation tillage. Benefits of crop rotations in conservation tillage are widely recognized (Van Doren et al., 1976; Triplett, 1976; Triplett and Mannering, 1978). An ideal rotation should involve sequential cropping of a cereal followed by a legume, shallow-rooted by deep-rooted crops, fertility-depleting by fertility-conserving crops, soil-degrading by soil- regenerating crops, and crops demanding heavy inputs by those that can survive on low inputs. The objective is to create a desired level of crop diversification. Mixed and multiple cropping are the rule rather than the exception in the tropics. Although double cropping, growing two crops in a year, is practiced in the frost-free belt in the United States, mixed cropping is unusual in North America. The most commonly observed rotations in West Africa are maize-cowpea, millet-cowpea, sorghum-cowpea, and sorghum-yam as sequential crops; and cassava plus maize, cassava plus cowpeas, and maize plus yams as mixed crops (Okigbo, 1978). For tropical Alfisols, La1 (1976a,b) observed that maize-cowpea and maize-soybean rotations were compatible with no-till and conservation tillage systems. Aina et al. (1979) reported significant reduction in water runoff and soil erosion in maize plus cassava mixed cropping compared to either maize or cassava monocultures. Mixed cropping of compatible crops has been reported to maximize water use efficiency in Australia (Hulugalle and Willatt, 1985) and Nigeria (Hulugalle and Lal, 1986).

I73

CONSERVATION TILLAGE

Exploitative and intensive monocropping has more severe soil erosion risks than rotational farming. A relevant example depicting conservation effectiveness of different rotations in northern Thailand is shown by the data in Table XLIV. Runoff and soil erosion are more severe with intensive cultivation than when the land use intensity is low. Fallowing, natural or with planted cover crops, improves soil's physical conditions and reduces the risks of accelerated soil erosion.

E. SUMMER FALLOWING Fallowing, leaving the land uncropped and weed free, is commonly practiced in arid and semiarid regions to improve soil- water reserves for the succeeding crop. Increasing soil-water storage is the primary objective of the practice. It is defined as a cultural practice wherein no crop is grown and all plant growth is controlled by cultivation or chemicals during a season when a crop might normally be grown. Thus, production for one season is forfeited in anticipation that there will be at least partial compensation by increased crop production the next season. (Haas et al., 1974)

It is difficult to trace the origin of this practice, because it is widely used in many regions around the world characterized by low and erratic rainfall and marginal soil conditions. The practice has been widely used in the western United States since the latter part of the 19th century. The acreage managed under this practice increased from about 2 million hectares in 1900-1914 to about 15 million hectares in the 1960s and 1970s. The technique is considered useful in regions with uncertain rainfall, usually totalling less than 400 mdper year. The moisture-conservingefficiency of the practice has become a debatable issue, however. The net gains in soil-water conservation and in crop yields vary widely depending on soils,

Table XLIV

Effects of Crop Rotations on Runoff and Soil Erosion in Northern Thailand"

Treatment

Runoff (mm)

Erosion (t/ha)

Shifting cultivation Exploitative intensive cultivation Rice Peanut-mung bean

9 32 131 83

0.2 15.1 2.3 I .9

"From Ryan (1986).

174

RATTAN LAL

rainfall during the fallow and crop growth periods, and soil and crop management systems adopted. In the Great Plans of the United States, Greb et al. (1967) related net gains in soil-water storage during fallowing to the quantity of crop residue mulch. For the same ecological region, Smika and Wicks (1968) reported that soil-water storage was greater when herbicides rather than conventional tillage practices were used to control weeds. The storage efficiency was 35.4% for conventional tillage and 42.4% for herbicide treatment. Smika (1970) also reported some excellent yields with summer fallowing. In addition to conserving water, residue mulch also controls wind erosion, a serious problem in the Great Plains region (Black et al. 1974). Residue mulch is an important input for managing soil water in dryland crop production (Unger and Wiese, 1979). In contrast to the results reported above, however, some researchers have questioned the feasibility of bare (cropless) fallowing. It is argued thatplanted fallows, though they conserve less water, may be more suitable in terms of providing ground cover, creating residue, and improving the soil’s nutrient capital. Touchton et al. (1984) reported that water infiltration was more rapid in plots planted to legumes than in fallowed soil. In the fallowed soil, 34 kg/ha of additional N fertilizer was required to attain . near-maximum yields. In comparison, cotton yields in the plots planted to clover (Trifolium) were greater than those from the fallowed plot even without any application of nitrogenous fertilizer. Also in the southeastern coastal plains of the United States, Campbell et al. (1984b) found more satisfactory yields of soybeans seeded through a grazed cover crop of winter rye (Secale cereale) with a conservation tillage system than from fallowed land. The practice of fallowing has also been found useful elsewhere in semiarid and arid climates. On a Xeralfic AItisol in Western Australia, Hamblin (1984) conducted an experiment to compare soil properties and crop performance in plots cropped every year with those cropped every other year. Soil properties and yield progressively deteriorated in continuously cropped treatments. In arid regions of western India, fallowing was found to conserve more water in the soil profile than plowing to different depths and with different intensities (Table XLV). In Botswana, in southeastern Africa, Whiteman (1975) reported significant improvements in the grain yield of sorghum by fallowing. The data in Tables XLVI and XLVII show significant differences in grain yield when sorghum was seeded in fallowed land rather than in land previously growing a cover crop, maize, or weeds. The beneficial effects of fallowing were greater in dry rather than in normal rainfall years, and greater when weeds were effectively controlled than when plots were left weed-infested. Weed control during fallowing is also an important factor (Botswana, 1977). Experiments conducted in semiarid central Tanzania also proved that weed-free bare fallow land conserved

175

CONSERVATION TILLAGE Table XLV

Comparative Effects of Depth of Plowing and of Fallowing on Soil Moisture Storage in the Profile and on Maize Yield’ Soil moisture content (%) Depth (cm)

Plowing to 45-cm depth

Loosening to 20-cm depth

Plowing to 10-cm depth

Disking to 10-cm depth

Fallowing

6.7 9.5 10.4 11.2 11.5

5.1 7.6 8.8 8.8 8.9

3.4 7.7 7.6 8.0 8.9

2.2 4.5 6.8 7.3 8.0

9.0 9.7 9.3 11.2 13.2

2.5 5.3

-

0-7.5 7.5-15 15-22.5 22.5-30 30-37.5

Maize yield (t/ha) Grain Biomass

3.6 7.8

3.2 6.6

2.6 5.8

“Data from CAZRI (1975) and Kovda (1980).

Table XLVI Yield of Sorghum as Influenced by Previous Crop and the Weed Control System during Fall~wing‘~ Sorghum grain yield (kgha) Treatments

1970-197 I

Cropped Cover crop Maize Weeds

1103 1603 1375

Fallow Cultivated Ridged Herbicides

1907

Mean SE Rainfall (mm)

}

(1369)

1646 071

1972-1973

; :3!

I296

(1933)**

2 258

I97 1- 1972

1688 I700 1783 1540 %I17 614

353

I

}

(365)

Mean

1028

(1717)*** 2269 1368 2 194

289

“From Whiteman (1975). ?he figures in parenthesis are means; ** and *** indicate statistically signifcant differences at 1 and 0.1% probability levels.

176

RATTAN LAL Table XLVII Effects of Cultural Practices on Sorghum Production" Cultural practice Weeding Free of weeds throughout growth period Without weeding Plant population High plant population (80,OOO /ha) Low plant population (20,000 /ha) Water conservation Following a bare summer fallow Following a crop Time of planting Early-planted (early November) Late-planted (late December) Fertilizer application With fertilizer (100 N, 60 P, 40 K kg/ha) Without fertilizer

Number of years' data

Yield (kdha)

I

1802 487

4

2873 1836

3

2007 1028

3

1737 1029

3

3047 1647

"From Botswana Agricultural Research (1977).

more water and produced increased subsequent yields of peanuts, compared to continuously cropped land (Pereira et al., 1958). However, severe soil erosion occurred in the unprotected bare soil. Protection of the fallow by sowing shallow-rooting teff grass (Eragrostis abysinica) provided efficient soil conservation, controlled weeds, and enabled enough subsoil water to be stored to produce a satisfactory crop of peanuts. Erosion control, therefore, is a major consideration during fallowing. Similar observations were made in Senegal, West Africa, by Charreau (1970) (Table XLVIII). The conservation effectiveness of fallowing is drastically improved by the presence of crop residue mulch. Data from northern Kazakhstan, USSR, (Table XLIX) indicate importance of residue mulch in moisture conservation. Under the semiarid steppe conditions of northern Kazakhstan, mulching of the calcareous silty clay loam southern Chernozem soil conserved more plant- available moisture for the following wheat crop than did unmulched treatments. The beneficial effects of residue mulch in moisture conservation were observed for the entire growing season from sowing to harvest and were due to an increase in water infiltration rate. In comparison with the unmulched control, the infiltration rate increased by 78 and 85% for mulch rates of 2 and 4 t/ha, respectively. Mulching also increased the yield of spring wheat by 0.25-0.50 t/ha.

177

CONSERVATION TILLAGE Table XLVIII

Effects of Soil Surface Management during Fallowing on Runoff and Soil Erosion in Senegal"

Runoff Treatment

(mm)

(% of rainfall)

Erosion (t/ha)

Vegetation fallow Cultivated Bare soil

200 264 456

16.6 21.2 39.5

4.9 7.3 21.3

"From Charreau (1970).

Considering all pros and cons, the necessity of maintaining residue mulch at a rate of 2-4 t/ha, and the need to enhance the nitrogen status of the soil, it is logical that a suitable cover crop be sown during the fallow period. The cover crop must, however, be judiciously managed to conserve soil, water, and nutrient reserves for the grain crop to follow.

XI. SOIL GUIDE TO CONSERVATION TILLAGE The choice of the most appropriate type of conservation tillage depends on many factors. The most notable physical factors include soil properties,

Table XLIX Plant-Available Water (mm) in the 0-100-qn Soil Layer as Affected by Straw Mulch Rate during Fallowing in Northern Kazakhstan, USSR"

Growth stage Straw rate (t/ha)

0 1

2 4 LSDh

Sowing

Heading

Harvest

107.3 120.9 120.6 127.2 12.2

56.0 69.3 71.2 10.3

26.1 28.2 27.6 30.0 10.2

"From Bakaijev et a / . (1981). "LSD, least significant difference at the 5% level of significance.

178

RATTAN LAL

rainfall regime, climate, drainage conditions, rooting depth, soil compaction and erosion hazards, cropping systems, etc. In addition, there are socioeconomic considerations including farm size, availability of inputs, and marketing and credit facilities. The socioeconomic factors are as important as or more important than the biophysical factors. In addition to biophysical environments, high aspirations of members of modem society and increases in the cost of living are important factors that have altered farming systems and the type of tillage operations used. Impressive progress has been made in measuring the nutrient status of a soil using laboratory and field tests and in recommending precisely the fertilizer required to procure the desired yield levels. Soil scientists have developed reliable tests for diagnosing soil acidity, aluminum toxicity, and nutrient deficiencies and for prescribing corrective measures. However, soil scientists do not possess reliable and routinely measurable soil tests to determine the type of conservation tillage needed. We do not have a reliable and proven soil guide to assess specific tillage needs for alleviating soil- and environment-related constraints to crop production. Tillage operations are energy-intensive,form a major proportion of production costs, and have long-term effects on soil productivity and environmental quality. Developing diagnostic techniques to assess the curative tillage requirements deserves to be a high priority. Attempts have, therefore, been made to develop soil evaluation techniques to assess tillage needs. The most important factors considered in evaluating tillage needs are drainage, erosion, rooting depth, soil temperature regime, susceptibility to droughtiness, compactability, and susceptibility to crust formation. The applicability of conservation tillage can be drastically improved by developing cultural practices to alleviate production disadvantages and lower yields. Some relevant examples of soil guides to tillage needs are briefly described below. Triplett et al. (1973) developed a guide to assess application of a notill system to Ohio soils. They classified soils into 5 tillage groups primarily on the basis of surface and internal drainage. (i) Tillage group 1: These soils are perfectly suited to no-till conservation tillage systems. Soils in this category are well-drained and have a silt loam, loam, sandy loam, or loamy fine sand texture. These soils respond positively to mulch cover and require 70430% coverage of the soil surface. (ii) Tillage group 2: Soils in this group are somewhat poorly drained, have hydraulic conductivity of less than 0.5 cm/hr, and require additional management inputs to respond satisfactorily to conservation tillage. Additional inputs needed are generally in terms of providing surface and subsurface drainage. The predominant textural classes of this group are silt

CONSERVATION TILLAGE

179

loam, loam, sandy loam, or loamy fine sand. Mulch cover is also important for satisfactory crop performance. (iii) Tillage group 3: Soils in group 3 are very poorly drained and have extremely low hydraulic conductivity. These soils have loam, silt, loamy silty clay, or loam texture. In general, crop yields are better with conventional than with no-till or conservation tillage systems. (iv) Tillage group 4: Soils of this group are extremely poorly drained. Predominant textural classes in this group are silty clay loam, silty clay, or clay. These soils do not respond to mulch. (v) Tillage group 5: These soils do not respond to no-till and conservation tillage systems regardless of the management and additional inputs (e.g., peat soils, etc.). A similar guide was developed for soils of Indiana to assess the tillage needs for maize and soybean production (Cooperative Extension Service 1977, 1982). Galloway and Griffith (1978) described the most appropriate form of conservation tillage for each soil type. Within the Corn Belt of the United States, Galloway and Grifiith suggested that conservation tillage systems with proven erosion control potential can be widely adopted provided that weeds and other pests are controlled. Allmaras and Dowdy (1985) outlined nine tillage management regions (TMR) in the United States. These regions are based on climate, adapted crops, and cropping systems. Adoption of conservation tillage planting systems ranged from 2245% of the cropland in a TMR. Considerable progress has also been made in Europe towards relating the most suitable form of conservation tillage to soil properties and agronomic constraints. Two relatively independent research groups developed no-till or direct drilling systems for soils in the United Kingdom. Soane and Pidgeon ( 1975) related tillage requirements to soil physical properties in Scotland such as soil strength, aeration, water status, soil temperature, and the field situation. Pidgeon and Ragg (1979) observed that an important factor determining soil suitability for direct drilling is the inherent ability of some soils to resist or recover from compaction while maintaining a satisfactory pore-size distribution and drainage status. Similar to the soil groups proposed by Triplett and VanDoren for Ohio, Pidgeon and Ragg proposed the following groups for soils of the United Kingdom: (i) Soils suitable for no-till: These soils include well-drained, loamy soils and coarse sandy soils in which levels of organic matter are adequate (>2%). These soils have enough bearing strength to support the loads imposed without compacting to a damaging extent.

I80

RATTAN LAL

(ii) Less suitable soils: Soils relatively less suitable to no-till and conservation tillage comprise either better-structured, moderately well-drained or imperfectly drained clay soils and weakly structured, imperfectly drained loamy soils in which field drainage can control the water table. An important consideration is the interaction between climate and soil for impeded drainage. (iii) Least suitable soils: There occurs a drastic yield reduction when these soils are managed by no-till or a conservation tillage system. These soils include poorly drained and weakly structured clay loam and clay soils. In Britain Cannell et al. (1978) proposed a soil classification system based on the experimental results. There were four criteria considered in developing this classification system. These were (a) changes in soil conditions with repeated use of no-till; (b) limiting soil factors such as lack of tilth, topsoil compaction, drainage, texture, levels of organic matter, free lime, wetness caused by slow subsoil drainage, self-mulching, and presence of stone; (c) site factors such as slope, spring lines, and field variability; and (d) climate. These factors were further refined by Stengel et al. (1982, 1984) on the basis of crop performance. Three soil-related indices were prepared: structural stability, shrinkage, and compactability. A similar soil management guide was prepared for soils of New South Wales Australia, by Cooke (1982). He proposed a soil classification framework that groups together land of similar type for land management planning. A soil suitability guide for conservation tillage for soils of the tropics was proposed by La1 (1985a). A rating system was developed to assess the suitability of the type of conservation tillage for different soils. Soil and climatic properties included in developing the rating system are erosivity, erodibility, soil loss tolerance, compaction, soil temperature regime, available water-holding capacity, cation exchange capacity, and soil organic matter content. Also included is the quantity of crop residue mulch on the soil surface at seeding. Each of these factors were rated from 1 to 5 . The value of I corresponded with the characteristics desirable for notill and mulch farming and that of 5 for soil-inversion conventional tillage. The accumulative rating index in relation to the most desirable type of conservation tillage is shown in Table L. Based on the available information, general guidelines for the choice of conservation tillage in relation to soil type and climate are shown in Fig. 39. In the humid and subhumid tropics, with soils of coarse texture in the surface horizon no-tillage can be successfully applied for upland row crops. In the semiarid region, with fine textured soils some form of mechanical loosening of crusted and compacted soil is necessary. The

181

CONSERVATION TILLAGE Table L Accumulative Tillage Rating Index and the Appropriate Conservation Tillage System in the Tropics" Accumulative rating index of soil properties

Suitable conservation tillage system No-till farming, with cover crops and alley cropping Chiselling in the row zone Minimum tillage-permanent ridge furrow system Plowing at the end of the rainy season Both primary and secondary tillage

<30

30-35 3540 40-45 >45

"From La1 (1985a).

Water Erosion

0 El

Water Erosion- Crusting Water Logging - Water Eroston Water and Wind Eroston Wind Erosion Drought SIress

-

X~'/RIDGE F U R R O ~ A IWATER w

a 2 c X

w

c

PER

HUMID

H~MID

SUBHUMID

MOISTURE

SiMIARID

+ARID

REGIME

FIG.39. Tillage systems (no-tillage and surface-tillage) and conservation objectives for the tropics depending on soil texture, climatic moisture regime, and major soil conservation problems. (From Lal, 1985a.)

I82

RATTAN LAL

frequency and type of mechanical operation desired depends on soil characteristics and the crops to be grown.

XII. RESEARCH AND DEVELOPMENT PRIORITIES A systems approach is essential for the wide adaptation of conservation tillage. For the conservation tillage system to be to be successfully adopted in a wide range of soils and environments, it must fit into the overall scheme of the present and future trends in the farming systems of the region and must meet the rising social and economic aspirations of the farming community. Conservation tillage cannot be adopted in isolation. It is a basic management tool for which the supporting packages of cultural practices must be developed and researched specifically for each benchmark soil and agroecological region. These cultural practices must be designed to render the system flexible for fine-tuning by the farmer concerned. Non-point source pollution is a major environmental hazard. Conservation tillage has become a source of controversy regarding its effects on transport of herbicides and nutrients in surface and subsurface waters (Crosson, 1981; Hinkle, 1983; Baker, 1985). Long-term and large-scale ecosystem studies are necessary to assess the effectiveness of conservation tillage systems in reducing the transport of sediments, nutrients, and pesticides to natural waters. Impressive progress has been made by soil scientists in assessing soil’s nutritional status and recommending the rates of fertilizers for meeting crop requirements. There are no such tests available for routinely measurable soil properties to diagnose soil tilth-related constraints to crop production and recommend the appropriate conservation tillage systems to achieve and maintain the desired seedbed. Soil structure and tilth continue to be elusive properties difficult to diagnose and assess. Minimizing energy-related inputs of tillage, fertilizers, and pesticides is a major objective that deserves to be a high priority. The objective is to sustain satisfactory and optimum levels of economic returns while minimizing dependence on those inputs that are either not available or based on nonrenewable resources. In the tropics and subtropics herbicides and farm chemicals are not easily and economically available and high rates of application are not ecologically and environmentally compatible. The dependence on herbicides as the preferred mode of weed control should be minimized. Alleleopathic effects of cover crops and biological methods of weed control should be evaluated. Regenerative cropping systems

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must, therefore, be researched to reduce dependence on potentially hazardous chemicals. Seedling establishment is a major factor responsible for a low crop stand and low yields with conservation tillage. Poor seedling establishment may be due to an unfavorable microclimate in the seed zone or to high levels of pest incidence. There are many important factors that are need to be researched including time of sowing, seed rate, seed placement and seedsoil contact, row orientation, and integrated pest management. The role of appropriate cultivars and of seed characteristics (such as size, hardiness) cannot be overemphasized. Also important are alleleopathic effects related to anaerobic decomposition of crop residues in damp and cool environments. Suitable cultivars should be screened for the specific environments of the conservation tillage. Diversification is a key to ecological stability. Appropriate conservaton tillage should be developed for systems of row crop production integrated with those for raising livestock and growing perennial crops. Techniques of management of pastures or of woody plants and shrubs should be compatible with the specific requirements of the proposed conservation tillage systems for row crop production. The question of sustainability and of environmental quality remains a major challenge to agriculture for generations to come. What are the system’s performance indicators that assess sustainability and its ability to preserve the resource base? Should the agronomic returns be assessed in terms of production (i) per unit area; (ii) per unit time; (iii) per unit loss of an important soil property that plays a vital role in maintaining soil’s life-support processes such as pH or organic matter content; (iv) per unit loss of soil’s effective rooting depth (e.g, kilograms of grain produced per kilogram of soil eroded); (v) 0utput:input ratio evaluated in terms of calories; (vi) or per unit increment of major pollutants to first-order streams or groundwater, e.g., increase in N03-N, phosphorus, herbicides or insecticides in streams? Assessing suitability of conservation tillage in terms of the economics of crop production on a seasonal or annual basis alone is not enough. We do not possess appropriate system performance indicators.

XIII. CONCLUSIONS Conservation tillage is a systems approach to farming whereby longterm sustainability and preservation of environmental quality and resource base are given priority over the short-term economic returns. However,

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conservation tillage may not always yield the highest economic returns on a short-term basis. Conservation tillage is also soil- and site-specific and no single blueprint of cultural practices can be universally applicable. It includes no-till, minimum tillage, ridge tillage, chisel plowing, zonal tillage, and a range of other cultural practices developed to overcome specific constraints. The appropriate type of conservation tillage depends on both biophysical and socioeconomicfactors and on interactions between them. The most beneficial tillage practice is the one that creates or maintains a favorable porosity for water and air movement and root growth and development. Although basic principles governing sound management of soil and environments are the same for the tropics and temperate regions, there are subtle differences in the package of cultural practices needed to optimize the use of limited resources and to alleviate soil- and environment-related constraints to crop production. Variations in packages and cultural practices are due to differences in climate, soil, erosion risks, availability of commercial inputs, infrastructure, farm size, and socioeconomicfactors. The basic components of successful conservation tillage are based on use of crop residue mulch, reducing the intensity and/or frequency of mechanical tillage, and adoption of appropriate cropping sequences and combinations to provide needed diversity, minimize inputs, and preserve the soil resource. Major reasons for adopting conservation tillage are preventing soil erosion, providing favorable soil and microclimate environments, reducing risks of environmental pollution, minimizing the commercial inputs needed, and preserving the soil resource base. Conservation tillage is a risk-avoiding and a problem-solving approach, geared to providing satisfactory yield under the worst conditions rather than the highest yield under the best conditions. It is also aimed at alleviating specific constraints, e.g., accelerated erosion, drought stress, surface sealing and crusting, subsoil compaction, unfavorable soil temperature regimes, anaerobic conditions in the root zone, and other factors responsible for low soil fertility. Effectiveness of conservation tillage can be vastly improved by adopting other supportive practices based on principles of good farming. These include crop rotations, cover crops, mixed farming, agroforestry, and summer fallowing. The slow adoption of conservation tillage is due to the lack of suitable supporting practices that would enhance its effectiveness. Conservation tillage has become a controversial issue because its high dependence on herbicides. Herbicide transport from row crop agricultural lands is a major source of pollutants of natural waters. We do not understand the pathways, biodegradation, and transport processes in different systems of soil and crop management. Myths and suspicion should be replaced by facts through long-term and ecologically oriented field experiments.

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Conservation tillage is an important component of low-input sustainable agriculture systems. It is aimed at preserving the productive potential of soil and maintaining environmental quality. It is an approach that emphasizes the use and improvement of the natural resource rather than its exploitation and mining its productivity for quick economic gains. Conservation tillage is synonymous with good farming.

ACKNOWLEDGMENTS Help received from Karla Gutheil and Shirley Hall in typing this manuscript is gratefully acknowledged.

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Unger, P. W., Steiner, J. L.. and Jones, 0. R. 1986. Soil Till. Res. 7 , 291-301. Vaille, J. 1970. Agron. Trop. (Paris) 25, 472490. Van Doren, D. M., Jr., Triplett, G. B.. Jr., and Henry, J. E. 1976. Soil Sci. Soc. A m . J . 40, 100-105. Van Doren, D. M., Jr., and Allmaras, R. R. 1978. I n “Crop Residue Management Systems,” pp. 49-83. ASA, Madison, Wisconsin. Van Wijk, W., R., Larson, W. E., and Burrows, W. C. 1959. Soil Sci. Soc. Am. Proc. 23, 428434. Verinumbe, I., Knipscheer, H. C., and Enabor, E. E. 1984. The economic potential of leguminous tree crops in zero-tillage cropping in Nigeria: A linear programming model. Agrofor. Syst. 2, 129-138. Vine, P. N. 1981. Cassava growth in the dry season. Proc. Annu. Conf. Agric. Soc. Nigeria, Meiduguri, Bomo State, 171h, July 26-31. Vittal, K. P. R., Vijayalakshmi, K., and Rao, U. M. B. 1983. Soil Till. Res. 3, 377-384. Voelkner, H. 1979. Urgently needed: An ideal green mulch crop for the tropics. World Crops MarcWApril, 76-78. Voorhees, W. B., and Lindstrom, M. J. 1983. J. Soil Water Conserv. 38, 307-31 I . Voorhees, W. B., and Lindstrom, M. J. 1984. Soil Sci. Soc. Am. J . 48, 152-156. Wagenet, R. J. 1987. I n “Effects of Conservation Tillage on Groundwater Quality” (T. J . Logan e f a / . , eds.), pp. 189-204. Lewis, Chelsa, Michigan. Wallis, J. A. N. 1960. Notes on grasses for mulching coffee. Kenya Coflee Sept. Wanchope, R. D., McDowell, L. L., and Hagen, L. J. 1985. Environmental effects of limited tillage. I n “Weed Control in Limited-Tillage Systems” (A. F. Wiese, ed.), pp. 266281. Weed Sci. SOC.Am. Monograph Series No. 2. White, P. J., Saffigna, P. G., and Vallis, I. 1984-85. Crop stubble management and nitrogen availability. In “Soil Conservation Research Branch,” pp. 4 0 4 2 . Biennial Report, Qld., Dept. of Primary Industries. Whiteman, P. T. S. 1975. Exp. Agric. 11, 305-314. Wijewardene, R. 1980. World Crops 32. Wijewardene, R. 1982. Conservation farming for small farmers in the humid tropics. IITASrilanka Grogram, Colombu, Srilanka. Wild, A. 1974. In “Shifting Cultivation and Soil Conservation in Africa,” pp. 167-168. F A 0 Soils Bulletin 24, FAO, Rome, Italy. Wilkinson, G. E . 1975. Trop. Agric. (Trin.) 52, 97-103. Wilson, G. F. 1978. Acta Hortic. 34, 33-41. Wilson, G. F., Lal, R., and Okigbo, B. N. 1982. Soil Till. Res. 2, 233-250. Zaffaroni, E., and Locatelli, E. 1980. Energy efficiency of corn in Costa Rica, CATIE, Turrialbe. Costa Rica.

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ADVANCES IN AGRONOMY, VOL. 42

MICROBIALLY MEDIATED INCREASES IN PLANT-AVAILABLE PHOSPHORUS R. M. N. Kucey,’ H. H. Janzen,’ and M. E. Leggett2 ‘AgricultureCanada Lethbridge Research Station Lethbridge, Alberta T i J 481, Canada ‘Philorn Bios, Inc. Saskatoon. Saskatchewan S7N 2x8. Canada

1. Introduction 11. Sources of Plant-Available Phosphate in Soils A. Forms and Transformations of Soil P B. Added Phosphate Fertilizers C. Rock Phosphate as a Fertilizer Source 111. Mycorrhizal Effects on Plant Phosphate Availability A. Absorption of Unavailable Phosphate B. Alteration of Plant Growth or Enzyme Activity C. Extension of Phosphate Depletion Zone IV. Phosphobacterins and Organic Phosphate Mineralization V. Inorganic Phosphate-Solubilizing Microorganisms A. Solubilization of Phosphate in Pure Culture B. Occurrence and Numbers of PS Organisms in Soil C. Effect of Inoculation in Soils D. Mechanism of Action of PS Microorganisms VI. Sulfur Oxidation and Rock Phosphate-Sulfur Mixtures VII. Future of Technologies References

I. INTRODUCTION The importance of microorganisms in soil nutrient cycling and their role in plant nutrition has been realized for a long time. Their active part in the decomposition and mineralization of organic matter and release of nutrients is crucial to sustaining the plant productivity upon which we depend for survival. Only relatively recently have attempts been made to quantify other roles that specialized groups of soil microorganisms mediate. One group of microorganisms that is of importance to plant nutrition includes those organisms that allow plants to absorb phosphorus (P) from sources that are otherwise less available. Organisms that cause increases

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in plant-available P in the soil system belong to a diversified group including bacteria, actinomycetes, and several groups of fungi. Research has been conducted in many aspects of the study of microbially mediated increases in plant-available P. These have included research in the area of mycorrhizal fungi, P-mineralizing or -solubilizing microorganisms, and dissolution of P by the oxidation products of sulfur. The opinions regarding the feasibility of such systems for agricultural purposes depend greatly upon familiarity with the literature and the nature of related research results. The results produced in the study of these organisms are variable because of the differences in the organisms used as well as differences in soils and climatic parameters. Until now, no comprehensive review of this topic has been available. We have attempted to summarize the wide array of information on microbially induced release and increased availability of P in soils.

II. SOURCES OF PLANT-AVAILABLE PHOSPHATE IN SOILS A. FORMSAND TRANSFORMATIONS OF SOIL P

The concentration of total P in soils ranges from 0.02 to 0.5% and averages approximately 0.05% (Barber, 1984), the variation being largely due to differences in weathering intensity and parent material composition (Stevenson, 1986). Relatively high concentrations are often observed in calcareous soils of arid regions, whereas relatively low P concentrations are often observed in soils subjected to high weathering intensity. Vertical distribution of total P within the soil profile is usually quite uniform (Stevenson, 1986), although plant residue deposition may result in some accumulation of total P in the surface horizon (Barber, 1984). Soil P exists chiefly as orthophosphate, although phosphine and phosphonates have been detected under some conditions (Stewart and McKercher, 1982). The diverse soil P forms can be generally categorized as soil solution P, insoluble inorganic P, or insoluble organic P. Only a very small fraction of soil P exists in the soil solution because of its extreme reactivity. The concentration of P in the soil solution is commonly approximately 0.05 mg/liter and seldom exceeds 0.3 mg/liter in unfertilized soil (Ozanne, 1980). Inorganic P in solution exists mainly as primary or secondary orthophosphate, depending on soil pH. Organic P may also make up a large fraction of soluble P, as much as 50% in soils with high organic matter content (Barber, 1984). Inorganic P associated with the solid phase can be sorbed to the surfaces of soil constituents (Sample et al., 1980) or can occur in calcium, iron,

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or aluminum minerals (Tiessen and Stewart, 1983; Barber, 1984). Organic P, occurring in humified organic matter, organic residues, and microbial biomass, can account for 2-80% of the P content of surface soils (Dalal, 1977). Most of the organic P occurs as ester P (Stewart and McKercher, 1982). Only the P dissolved in the soil solution is directly accessible to plants. Since the concentration of P in the soil solution is normally insufficient to support plant growth, crop growth depends on continual replenishment of soluble P from inorganic and organic sources (Chauhan et al., 1979; Anderson, 1980; Barrow, 1980; Ozanne, 1980; McGill and Cole, 1981; Tisdale et al., 1985; Stevenson, 1986). While most soils contain substantial reserves of total P, most of it remains relatively inert. According to Ozanne (1980), less than 10% of soil P enters the plant-animal cycle. Consequently, P deficiency is a widespread problem and P fertilizers are almost universally required to maintain crop production.

B. ADDEDPHOSPHATE FERTILIZERS In many agricultural systems, P fertilizers are routinely applied to promote crop yields. Most of these fertilizers contain P in water-soluble forms as salts of ammonium, calcium, and potassium (Tisdale et al., 1985). Although the P in these fertilizers is initially plant-available, it rapidly reacts with soil and becomes progressively less available for plant uptake. These forms become subject to the same forces as native soil P. As a result of the various retention mechanisms, most of the fertilizer P applied (often as much as 90%) is rendered unavailable for crop uptake but is retained in insoluble form (Stevenson, 1986). Although this P may have some residual benefit, further annual applications are often necessary to maintain adequate labile P. Because P applications usually substantially exceed crop uptake, the total P concentration of many soils has increased markedly over time (Barber, 1979). Thus, soils commonly have large reserves of “fixed” P that could support long-term crop requirements if it could be economically exploited.

c. ROCK PHOSPHATE AS A FERTILIZER SOURCE The direct application of rock phosphate as a fertilizer source has received renewed interest in recent years. Upon application to the soil, rock phosphate gradually releases soluble P by various solubilization reactions. As time progresses, the unsolubilized rock phosphate becomes progressively more recalcitrant. The rate of plant-available P release from

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rock phosphate has been shown to be quite variable owing to differences in rock phosphate source, particle size of the rock phosphate, soil pH, and other soil chemical properties (Barnes and Kamprath, 1975; Kucey and Bole, 1984; Hammond et al., 1986). In general, rock phosphate is not a reliable source of plant-available P in soils with pH greater than 5.56.0 (Engelstad and Terman, 1980) and even in acidic soils, the rock phosphate must be extremely finely divided to ensure an adequate rate of soluble P release (Tisdale et d., 1985). In practice, addition rates for rock phosphate are generally 10 times that recommended for manufactured P fertilizers (Kucey and Bole, 1984). A recent comprehensive review on the benefits of direct application of rock phosphates as fertilizers has been published by Hammond et al. (1986). The uptake of P from relatively insoluble sources can be affected by the type of plant growing in the soil. It has been determined that plants vary in the cation exchange capacity (CEC) of their root systems, and plants with high CEC levels, such as ragweed (Ambrosia artemisiifolia L.) or smartweed (Polygonum coccineum Muhl), are more effective at obtaining P from rock or soil P sources than those with low CEC, such as wheat (Triticum aestivum L.) and oats (Avena sativa L.) (Drake and Steckel, 1955). Furthermore, it was found that growing a plant with low CEC root systems beside a plant with a high root CEC resulted in increased P uptake by the former. It was hypothesized that the roots caused P release by binding calcium, iron, and aluminum with organic anions (Nye and Kirk, 1987). Buckwheat (Fayopyrom esculentum L.) was able to acidify its rhizosphere and cause dissolution of rock phosphates, but maize (Zea mays L.) was unable to do so (Bekele et al., 1983). Rapeseed (Brassica nupa oleiferu) plants in that study were able to utilize rock phosphates by absorbing high amounts of calcium, which shifted the mass equilibrium in favor of soluble ions. Van Ray and Van Diest (1979) also observed a relation between the rhizosphere pH of various plant species and their utilization of rock phosphate, superphosphate, or calcined aluminum phosphate. A variety of plants has been found to use one or more of these mechanisms in the solubilization of P in their rhizospheres (Johnston and Olsen, 1972).

Ill. MYCORRHIZAL EFFECTS ON PLANT PHOSPHATE AVAILABILITY The effect of mycorrhizal fungi on plant growth and nutrient uptake has been extensively studied and reviewed (Gianinazzi-Pearson and Gianinazzi, 1981, 1985; Barea and Azcon-Aguilar, 1983; Tinker, 1984). It is

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not the purpose of this chapter to review extensively the myriad effects of mycorrhizal fungi on plant growth. Rather, our discussion will concentrate on the effect of mycorrhizae on P uptake by plants. The mycorrhizal fungi are an important part of the soil microbial system because the prevalence of these associations on plants is so common under natural soil conditions that a nonmycorrhizal plant is the exception rather than the rule. Only a few groups of plants do not normally form mycorrhizal associations (Marx and Krupa, 1978). Endomycorrhizae, which include vesicular-arbuscular (VA) mycorrhizae, and ectomycorrhizae are noteworthy for their potential economic importance (Gerdemann, 1968). The effect of mycorrhizae on plant P uptake and the effect of soil P on mycorrhizae were among the first aspects of these symbioses studied. The influence of mycorrhizae on plant P uptake has become obvious and well known (Gianinazzi-Pearsonand Gianinazzi, 1985). The relative benefits of mycorrhizal infection decrease as P availability increases (Ross, 1971),partially due to the negative effect of P on the levels of mycorrhizal infection (Hayman et al., 1975; Kucey and Paul, 1983). Infected roots and high numbers of spores are found most commonly in soils of low to moderate P-availability status, whereas soils high in P have been found to contain few spores or infected roots (Azcon et al., 1978). Split-root techniques with sudangrass (Sorghum vulgare L.) colonized by GIomus fasciculatus have shown that the concentration of P within the plant, not P levels in the soil, reduces root infection and spore production levels (Menge et al., 1978b). Studies concerning fungi-assisted uptake of P should consider that plants growing in more fertile soil are associated with low fungal biomass and so derive less benefit than a plant associated with high levels of fungi. Because phosphate ions in the soil are relatively immobile, plant roots must expend considerable energy producing enough root material for adequate P absorption in soils with low P levels (Bieleski, 1973). Mycorrhizae appear to be able to assist the plant in absorbing the P it needs. There are several possible mechanisms by which the fungus could assist host uptake of P. Research has concentrated on three areas: (i) absorption of P from sources unavailable to the uninfected plant, (ii) alteration of plant growth such that the plant produces a larger root system or alters its enzymes for absorbing P, and (iii) extension of the P-depletion zone away from the root (Gerdemann, 1975; Gianinazzi- Pearson and Gianinazzi, 1981, 1985).

A. ABSORFWONOF UNAVAILABLE PHOSPHATE The possibility that mycorrhizal fungi may be able to use forms of P unavailable to plants is most interesting from an economic point of view.

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Many forms of P, although plentiful, are essentially unavailable for plant uptake. The possibility that mycorrhizal fungi may have a mechanism by which they can assimilate these forms and pass the P onto the host has been studied by adding various inorganic P compounds to P-deficient soils and measuring symbiotic and nonsymbiotic plant P uptake. For instance, mycorrhizal soybean (Glycine max L. Merr.) shoot weights were increased by up to 56% when monocalcium phosphate was added (Ross and Gilliam, 1973). Ali (1976) obtained positive plant responses from the addition of calcium or iron phosphates. Jackson et al. (1972) found no response of mycorrhizal and nonmycorrhizal corn roots to rock phosphate addition; however, increased P uptake from rock phosphates by mycorrhizal plants has been observed by others (Waidyanatha et al., 1979; Powell et al., 1980; Cabala-Rosand and Wild, 1982). Although initially it appears that plants colonized by mycorrhizal fungi can use these “unavailable” sources of P, it must be remembered that the precipitated forms of P are in chemical equilibrium with P in solution (Russell, 1973). If solution P is removed, precipitated P can replenish the solution P. In this way, unavailable forms of P may contribute to the solution P pool, albeit to a small extent, and become available for plant uptake (Gianinazzi-Pearson and Gianinazzi, 1985). Phosphorus uptake studies using ”P-labeled phosphate have shown that the specific activities of P in VA mycorrhizal and nonmycorrhizal plants are the same, indicating that both plants utilize the same sources of P (Mosse et ul., 1973; Gianinazzi-Pearson et al., 1981; Raj et al., 1981; Asea et al., 1988). Similar findings were observed for ectomycorrhizal and nonmycorrhizal pines (Thomas et al., 1982). The conclusion, therefore, is that the mycorrhizae do not utilize unavailable P sources, they utilize the available solution forms more efficiently (Gerdemann, 1975). Long-term experiments indicate that both mycorrhizal and nonmycorrhizal plants are less able to extract P from the soil as the levels of solution P become increasingly more depleted (Powell, 1977). Certain species of host benefit greatly from mycorrhizal infection because their uninfected roots are unable to take up P present in very low concentrations (Mosse, 1973; Plenchette et al., 1983). Hosts of this type depend heavily upon the mycorrhizal fungi to absorb the solution P and transport it to the host.

B. ALTERATION OF PLANT GROWTH OR ENZYME ACTIVITY Mycorrhizae may alter the physical or physiological activity of a host root system, which in turn may allow the host itself to take up more P. The possibility that mycorrhizae alter host shoot-root ratios (S:R) has

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been studied in some detail, although the results are conflicting. Smaller S:R of VAM-colonized plants have been reported in some cases (Ali, 1976), but larger S:R have also been obtained for infected plants (Crush, 1974; Daft and El Giahmi, 1976). The results of one experiment are generally inconsistent with those obtained in other experiments, so no general conclusion can be made on the effect of VA mycorrhizae on host shoot-root ratios. It appears that the results of a study involving mycorrhizal fungi may vary with the fungus and host used, as well as with the soil they are growing in. The fungi may affect plant roots by means other than increasing their physical size. Some groups have found that mycorrhizal onion roots (Allium cepa L.) removed a greater amount of 32Psolution from areas close to the root (Owusu-Bennoah and Wild, 1979). This has prompted several groups to explore the possibility of more efficient enzymes for hydrolyzing and/or absorbing P. A soluble alkaline phosphatase that is lacking in nonmycorrhizal roots has been found in onion roots infected with Glomus mosseae. The maximum levels of this enzyme’s activity appeared at the same time that the positive growth response to mycorrhizal infection appeared in the host, at which time the enzyme accounted for 32% of the total root phosphatase activity (Gianinazzi-Pearson and Gianinazzi, 1976, 1978). Allen et al. (1981) also observed increased alkaline phosphatase activity of mycorrhizal Bouteloua gracilis. The presence and activity of many enzymes in the fungi appear to change with the age of the fungus and its state of development (MacDonald and Lewis, 1978) and may increase the availability of certain organic P forms to host plants. Other theories assume that certain mycorrhiza-specificenzymes do not play roles in plant P uptake, but rather are involved in P assimilation by the fungus, either at the level of P absorption, in active transport by the fungus, or in active transport into the host plant (Gianinazzi-Pearson and Gianinazzi, 1978). The role of endomycorrhizae in organic P hydrolysis is still unclear; however, ectomycorrhizae have been shown to directly break down phytates in soils (Gianinazzi-Pearson and Gianinazzi, 1985).

c. EXTENSION OF PHOSPHATE DEPLETION ZONE Whether or not the host itself becomes more efficient at absorbing P, a major part of the increased uptake is directly due to fungal activity (Sanders and Tinker, 1971, 1973). Experimental evidence indicates that, as well as increasing P uptake from areas close to the root, mycorrhizal roots obtain P from areas far from the root (Gray and Gerdemann, 1969).

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It has been found that mycorrhizal onion roots removed 32Pfrom areas 8.0 cm from the root surface (Rhodes and Gerdemann, 1978), whereas nonmycorrhizal roots failed to absorb any label from sources placed 1.O cm from the root surface. Diffusion in this experiment accounted for movement of the isotope of less than 0.75 cm. Addition of a fungal toxicant to the soil restricted uptake of labeled P by mycorrhizal roots to that of control plants. Thus, it appears that the fungi extend the zone of P depletion from the root surface. The uptake of P by mycorrhizal fungi is not a passive action. The fungi appear to be specific in their uptake and translocation of nutrients. When equal amounts of 32P,35S,and 65Znwere injected into a soil, the molar ratios of the nutrients taken up and translocated to the host were 35:5:1 for P:S:Zn (Cooper and Tinker, 1978). Other workers found the hyphae to be more efficient at taking up and moving P than at moving calcium (Rhodes and Gerdemann, 1978). Thus it appears that the major action of mycorrhizal fungi in facilitating plant P uptake is to increase the absorptive surface area of the mycorrhizal root system and to extend the P depletion zone away from the root surface. To date, there is no evidence that mycorrhizal roots are able to absorb P from any sources of soil P not available to the nonmycorrhizal root systems. Nonetheless, mycorrhizal fungi play a very important role in aiding plant P uptake. Because mycorrhizal fungi increase plant uptake of P, it should be possible to substitute selected mycorrhizal fungi for part of the P fertilizer now added. Glomus fasciculatus was able to substitute for up to 56 ppm P in the greenhouse cultivation of Troyer citrange (Poncirus trifoliata L.) and 278 ppm in the cultivation of Brazilian sour orange (Citrus aurantium L.) (Menge et al., 1978a). These two cultivars depend heavily on fungi for adequate P uptake. Generally, the majority of work on the use of mycorrhiza has been conducted under controlled conditions. As most agricultural soils contain native VA mycorrhizae, research on the role of specific strains in increasing P uptake has, of necessity, been done in greenhouse or field soils treated to remove the indigenous VA mycorrhizae. Most of the conclusions about the ability of selected VA mycorrhizae to increase P uptake are therefore based on tests done under altered conditions. Vesicular-arbuscular mycorrhizal isolates vary in their ability to supply certain hosts with P (Abbott and Robson, 1978) and to compete with the indigenous VA mycorrhizae (Abbott and Robson, 1982). The use of these organisms to supplement commercial fertilizers will therefore depend on our ability to select strains that have the ability to increase P uptake and to compete with native microflora. The detailed experiments that have been conducted to deter-

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mine the mechanisms VA mycorrhizae use to increase P uptake should enable us to identify the characteristics we should look for to find effective strains. Experiments conducted in sterilized soils have relevance to practical situations in which the host species are routinely grown in soils sterilized to remove disease organisms, for example, in the case of containergrown seedlings. However, in order to select species or strains which are also highly competitive, experiments must be conducted under nonsterile field conditions.

IV. PHOSPHOBACTERINS AND ORGANIC PHOSPHATE MINERALIZATION During the 1950s, farmers in the USSR and several eastern European countries inoculated a large proportion of their agricultural soils with a fertilizer consisting of kaolin impregnated with spores of the bacterium Megatherium viphosphateum (USSR Min. Agric., 1953; Rubenchik, 1956). This bacterium was later renamed Bacillus megatherium var. phosphaticum and the fertilizer was termed phosphobacterin (Menkina, 1956; Cooper, 1979). The bacteria added were reputed to increase the rate of organic P mineralization in the soil, resulting in the release of plant-available P (Menkina, 1950, 1963; Yung, 1954; Kudzin and Yaroshevich, 1962; Kvaratskheliya, 1962). The mechanism of action was, however, not fully determined by these workers. Consequently, questions remained regarding the mode of action of B. megatherium in increasing plant growth and, indeed, whether the early experiments with phosphobacterin were properly analyzed (Mishustin and Naumova, 1962). Yield increases resulting from the addition of B. megatherium to Soviet soils were reported to range from 0-70%, with 10-20% yield increases being obtained from over half of the crops inoculated (Smith et a / . , 1961). Vegetable crops responded best to inoculation; however, grains and potatoes (Solanum tuberosum L.) were also found to respond (Smith et d., 1961). Soils that gave the best results with phosphobacterins were neutral to alkaline and high in organic matter (Yung, 1954). Lime and/or organic materials such as manure were supposed to be added to alleviate acid conditions or to enrich soils low in organic matter. Experiments on phosphobacterin effectiveness conducted in the United States did not show the positive results obtained in studies from the USSR. Wheat and tomatoes (Lycopersicon esculentum) grown in greenhouse tests using six Chernozemic or chernozem-like soils did not respond positively to inoculation with B. megatherium (Smith et al., 1961). Lack of plant

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response was also observed with wheat, oats, and sorghum (Sorghum vulgare) in field tests conducted in Alaska, Minnesota, Montana, North Dakota, and Texas (Smith and Allison, 1962). It was concluded that phosphobacterin was of no practical use for farming systems in the United States and, further, that B. megafherium inoculation should not be used on a practical scale because the degradation of soil organic matter would be detrimental to the soil system. Tests conducted in India, however, did show positive responses to phosphobacterin addition (Sundara Rao and Sinha, 1963; Sundara Rao at af., 1963; Kavimandan and Gaur, 1971). The positive effects of phosphobacterins, which contained Pseudomonas jlorescens strains as well as B. megatherium strains, increased when farmyard manure and mineral fertilizers were added in conjunction with bacterial inoculants (Sundara Rao et af., 1963). These inclusions, however, confound the experimental data since the amounts of mineral and organic fertilizers added were sufficient to give optimum yields in the absence of the bacteria (Brown, 1974). If mineralization did occur, the amounts of P released would have been hidden and insignificant in the presence of the manure and fertilizers added. Bacillus megatherium was shown to cause the mineralization of nucleic acid P (Menkina, 1963) and myoinositol phosphate (Greaves and Webley, 1969) in sand culture but was not shown to cause the release of myoinositol phosphate in soil (Greaves and Webley, 1969). Although phosphobacterin has been reported to release inorganic P from organic sources in soil (Molla et al., 1984), other studies have concluded that B. megatherium was unable to do so (Martin, 1973). Although other instances of plant growth increases resulting from the addition of B. megatherium have been reported, the mode of action was determined not to be due to organic P mineralization, but rather to the solubilization of inorganic P forms, and will be discussed in a later section. The literature on the use of bacteria to increase plant-available P through organic P mineralization is somewhat contradictory, partially owing to poorly designed and improperly analyzed experiments in some cases. Because of this, it is diffcult to determine the validity of the conclusions based on some trials. In many cases, insufficient attention was paid to designing experiments to determine the mode of action of the bacteria. Conclusions about the mode of action were based on empirical data, e.g., higher yields in treated versus untreated plots. As it is now known that bacteria can increase plant growth by other mechanisms, such as the production of plant growth regulators, these conclusions may not be valid. It thus appears as if the research into phosphobacterins may have been misdirected. Plant growth responses to B. megatherium addition undoubtedly have been obtained; however, the mechanism of action is un-

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likely to be via organic P mineralization. This does not reduce the potential use of phosphobacterins for agriculture, but it does shift the area of research into other directions.

V. INORGANIC PHOSPHATE-SOLUBILIZING MICROORGANISMS A. SOLUBILIZATION OF PHOSPHATE I N PURECULTURE Microbial solubilization of inorganic P under pure culture conditions has been shown many times. Indeed, solubilization of a precipitated calcium phosphate in agar medium has been used as the initial criterion for isolation and enumeration of P-solubilizing (PS) microorganisms (Sperber, 1958a; Katznelson and Bose, 1959). Organisms growing on such media and able to solubilize P produce a clear zone around themselves due to the solubilization of the fine particles of calcium phosphate. Rates of P solubilization vary with the source of inorganic P. Louw and Webley ( 1959) observed equal levels of solubilization between tricalcium phosphate and hydroxyapatite in liquid media by PS organisms isolated from oat plants. Solubilization of both compounds was less than from dicalcium phosphate and roughly equal to the solubilization of P from basic slag. Iron and aluminum phosphates also have been shown to be solubilized in liquid media (Banik and Dey, 1982),but levels of P released were less than that released from calcium phosphate. Solubilization of P from defined P sources is a convenient method for use in comparing isolates. For practical purposes, however, the organisms must be able to cause significant solubilization of rock phosphates or increased availability of phosphatic fertilizers. Louw and Webley (1959) found that most of the 26 PS isolates they tested solubilized Gafsa rock phosphate in liquid medium, but none of the isolates solubilized rock phosphate in agar medium or variscite, strengite, or taranikite in either medium. Duff et ul. (1963) also observed a lack of solubilizing effect of PS isolates on variscite and strengite. Khan and Bhatnagar (1977) observed that Aspergillus niger solubilized P from eight rock phosphate sources. They found that the presence of sodium fluoride in the medium or in the rock phosphate did not inhibit the solubilization of rock phosphates except at high levels. Agnihotri (1970) studied the ability of fungi occurring in nursery seedbeds to solubilize apatite and fluorapatite in solution culture and found the list of fungi able to do so to be quite limited in scope. Thomas ef al. (1985) found that the PS fungi isolated from coconut (Cocos nuciferu) plantation

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soils belonged to the Aspergillus or Penicillium genera. Kucey (1983) similarly found that most of the PS fungal isolates from prairie soils were either Penicillium or Aspergillus. This indicates that PS abilities are not common to soil fungi in general.

B. OCCURRENCE AND NUMBERS OF PS ORGANISMS IN SOIL Under soil conditions, potential benefits of adding P-solubilizing organisms would depend on several factors, one of the most important being the activity of the PS microbial population already in the soil. In almost all cases, the major sources of PS isolates have been soils. Another source of PS isolates has been the surfaces of seeds. Phosphate-solubilizing microorganisms have been found in almost all soils tested, although the numbers vary with the soil climate and history (Sperber, 1958a; Katznelson and Bose, 1959; Katznelson et a / . , 1962; Chhonkar and Subba-Rao, 1967; Agnihotri, 1970; Khan and Bhatnagar, 1977; Banik and Dey, 1982; Kucey, 1983; Kim et al., 1984; Thomas et al., 1985; U et al., 1985). Katznelson et ul. (1%2) found that 6 7 0 % of bacterial isolates obtained from seed coat surfaces of many plants showed the ability to solubilize P in agar media, but only 10% of the isolates from the rhizoplane and rhizosphere showed this ability. Further work, however, indicated the importance of the soil-borne PS organisms, since the colonization of plant roots was determined to be by soil-borne organisms rather than by seedborne organisms. Sperber (1958a) found that PS organisms in the rhizosphere of subterranean clover (Trfolium subterruneum L.), ryegrass (Lolium perenne), and wheat (Triticumaestivum L.) roots constituted 263% of the microbial population (Table I). Khan and Bhatnagar (1977) and Sperber (1958a) both concluded that there was no preferential effect of rhizospheric conditions on the incidence of PS organisms, since the general rhizospheric microbial population increased in proportion to the increases in the number of PS organisms. Katznelson and Bose (1959) also found approximately onethird of the bacteria from the rhizoplane of wheat to show P-solubilizing abilities, and although no preferential stimulating effect of the roots on PS bacteria was observed, these organisms were found to be more active metabolically than other bacteria isolated from the same soils. Baya et al. (1981) also observed the rhizospheric P-solubilizing bacteria to be more active than those isolated from nonrhizosphere soil. Although there appears to be no preferential effect of the rhizosphere on PS organisms, there is some evidence that legumes support greater numbers of PS organisms than do nonlegumes (Sobieszczanski, 1%1) and that some legumes support more PS organisms than others (Paul and Sundara Rao, 1971).

MICROBIALLY MEDIATED INCREASES IN PHOSPHORUS

21 I

Table I Numbers of Phosphate-SolubilizingMicroorganisms and Total Microbial Populations in Soils Source Rhizosphere Nonrhizosphere Fallow soil Rhizosphere Root-free soil Cultivated soil Grassed soil Degraded rock Sandy soil Alluvial soil Clayey soil Lateritic soil Seed coat Rhizosphere soil Root-free soil Rhizoplane Alluvial soil Desert soil

Total microbial population" 3.1-51.2 x lo9 0.1-1.5 x

0.02-0.04 x ND ND 1.5-19.0 X 0.65-115 X 0.15-0.30 x ND ND ND ND 0.2-340 X ND ND ND 1.0 x 10' ND

lo9 lo9 lo7

lo7 10'

lo'

P-solubilizing population 26-39% 26-39% 10-17% 4.5-120.0 X lo6 0.6-2.0 x lo6 0.14-4.8 x 10' 0.23-11.5 x lo' 0.08-0.5 x 10' 1.1-3.7 x lo3 0.7-14.5 x lo3 1.1-11.6 X lo' 0.7-23.7 x 10' 0.1-202 x lo4 0.6-500 x lo6 0.3-1.7 x 10' 10-1 I%

1.6 x lo6 107-106

Reference Sperber (1958a) Sperber (1958a) Sperber (1958a) Khan and Bhatnagar (1977) Khan and Bhatnagar (1977) Kucey (1983) Kucey (1983) Kucey (1983) Thomas er a / . (1985) Thomas e r a / . (1985) Thomas er a / . (1985) Thomas er a / . (1985) Katznelson et a / . (1962) Katznelson er a / . (1962) Katznelson et a / . (1962) Katznelson et a / . (1962) Banik and Dey (1982) Saber er a / . (1977)

"ND. not determined.

Phosphate-solubilizing bacteria and fungi constituted 0.5 and 0.1%, respectively, of the general soil microbial population in prairie soils, with PS bacteria outnumbering PS fungi two-fold to 150-fold (Kucey, 1983). Banik and Dey (1982) observed that PS bacteria outnumbered PS actinomycetes and fungi in Indian soils three-fold and 50-fold, respectively. Thomas et al. (1985) were able to isolate more PS fungi from alluvial, lateritic, and clayey soils than from sandy soils. After subculturing of PS isolates, many of the bacterial isolates have been observed to lose their PS activity (Sperber, 1958a; Kucey, 1983). Once the P-solubilizing ability has been lost, it cannot be regained. Fungal isolates have not been observed to lose their P-solubilizing ability over many successive subculturings (Kucey, 1983). Fungal isolates in general showed greater PS activity in agar and liquid media than did bacterial isolates in the studies by Kucey (1983), Banik and Dey (1982), and Guar et al. (1973); however, Taha et al. (1969) reported that aerobic sporeforming bacteria were the predominant P solubilizers in the Egyptian soils tested. In spite of the high numbers of PS organisms in some soils (Table I),

212

R. M. N. KUCEY ET AL.

inoculation of some soils with PS organisms has been shown to result in increases in the rhizospheric population of P solubilizers. Khalafallah et al. (1982) observed an increase in the incidence of PS bacteria following inoculation of fava beans (Vicia faba) with a PS isolate of B. megatherium, and although the population increase slowly declined, the effect remained for the duration of their experiment ( 1 16 days). Saber et al. (1977) found similar results for B . megatherium inoculated onto pea plants (Pisum sativum L). Similar observations were made after addition of a PS isolate of a Pseudomonas spp. to the rhizosphere of red pine (Pinus resinosa) seedlings (Ralston and McBride, 1976), and for Bacillus polymyxa and Pseudomonas striata in the rhizosphere of inoculated wheat (Kundu and Gaur, 1980). Kucey (1988) found that inoculation of soil with Penicillium biluji resulted in a nearly four-fold increase in the number of PS Penicillium spp. in the rhizospheres of wheat plants under greenhouse conditions. Inoculation of partially sterilized soils has produced greater responses than inoculation of unsterilized soils, presumably because the native P-solubilizing organisms were greatly reduced by the sterilization treatment (Taha et al., 1969; Kundu and Gaur, 1980). Not all studies have shown increased numbers of PS organisms following inoculation. Ocampo et al. (1978) observed bacteriostatic action after 5 weeks following inoculation of lavender (Lavendulu spica var. Vera) with PS isolates of Pseudomonas and Agrobacterium. This resulted in cessation of increases in the numbers of rhizospheric PS bacteria. In contrast to the results of Saber et al. (1977), Badr El-Din et al. (1986) did not observe any increase in the incidence of P-solubilizing organisms in the rhizosphere of rice following inoculation of field soils with B. megatherium.

C. EFFECTOF INOCULATION IN SOILS

Because of the presence of PS organisms in the vicinity of plant roots, it could be argued that the addition of a few extra P solubilizers would not be of benefit to plant growth (Brown, 1974; Tinker, 1984). Solubilization of P in soil under greenhouse or field conditions is also much more difficult to prove than solubilization of P in solution culture. Nonetheless, several studies have shown plant growth responses to the addition of PS microorganisms to soils (Tables 11,111, IV). Several groups rely primarily on the use of P-solubilizing bacteria (Taha et al., 1969; Azcon et al., 1976; Ralston and McBride, 1976; Saber et al., 1977; Kundu and Gaur, 1980; Raj et a/., 1981; Khalafallaha et a/., 1982; Azcon-Aguilar et al., 1986; Badr El-Din et al., 1986; Piccini and Azcon, 1987) (Table 11). Other researchers (Gaur ef al., 1980; Banik and Dey, 1981a-c, 1982) used a combination of P-solubilizing bacteria and fungi (Table 111). Still other studies have relied primarily on the use of PS fungi (Kucey, 1987, 1988; Asea et

213

MICROBIALLY MEDIATED INCREASES IN PHOSPHORUS

Table 11 Summary of Effects of Inoculation of Soils with Phosphate-SolubilizingBacteria _____

~~

Conditions

Crop

Greenhouse Greenhouse Greenhouse

Greenhouse

Greenhouse Greenhouse Greenhouse

Field Field

Responses and observations

No effect on available P in soil Pseudomonas spp. increased plant weights and P uptake in soils with added Ca phospate B . circulans No effect Tomato Increased VAM colonization Unidentified Soybean Millet increased plant weight and P B . circulans uptake, increase in P available in soil Increased available P in soil, Brrcillrts spp. Rice increased plant P uptake Peas Increased plant weight and P B. inegatlierium uptake var. phosphuticrrm Fava bean B . inegatheriirm Increased plant weight and P uptake and concentration var. phospliuricctm from superphosphatefertilized soil Lavender Pseitdomonas spp. increased plant weight and P uptake and concentration Agrohacterium sp. from soils amended with rock phosphate Increased plant weight and P B. megatherium Barley uptake and concentration var. phosphaticirm Increased plant weights and P B . polymyxa Wheat uptake and concentration P. striuta Increased plant weight and P Unidentified Alfalfa uptake from rock phosphate amended with sandvermiculite No effect B. megatherium Soybean var. pliosphuticum Increased plant weights from B. jirniirs Rice soils with added rock phosphate

Laboratory None Greenhouse Red pine

Greenhouse Greenhouse Greenhouse

Organism Bacillus spp.

Reference" I

2 3 4 5

6 7 8

9

10

II

12

13 14

"(I) Banik and Dey (1982); (2) Ralston and McBride (1976); (3) Lee and Bagyaraj (1986); (4) Azcon-Aguilar et al. (1986); ( 5 ) Raj et ul. (1981); (6) Banik and Dey (1982); (7) Saber et ul. (1977); (8) Khalafallah et a / . (1982); (9) A x o n et ul. (1976); (10) Taha et ul. (1969); ( I I ) Kundu and Gaur (1980); (12) Piccini and Azcon (1987); (13) Badr El-Din et 01. (1986); (14) Datta et a/. (1982).

al., 1988) (Table IV). Phosphate-solubilizing actinomycetes have been isolated (Rao et al., 1982), but inoculation studies with this group of organisms have not been reported. Comparison of results from different tests is difficult because of the variability in the experimental designs, soils, and PS organisms used.

214

R. M. N. KUCEY ET A L . Table 111 Summary of Plant Responses to Inoculation of Soils with Mixed Cultures of Phosphate-SolubilizingBacteria and Fungi

Conditions

Crop

Organisms

Responses and observations

Flask

None

Greenhouse

Rice

Greenhouse

Rice

Field

Wheat

Aspergillus spp. Penicillium spp. Bacillus spp. Aspergillus spp. Penicillium spp. Bacillus spp. Pseudo. srriara Asp. awamori Pseudo. srriata Asp. awamori

Reference"

No effect on available P content of soil

I

Increased plant weight and P uptake and concentration increased available P in soil Increased plant weight and P uptake and concentration Increased plant weights in soils with added rock phosphate and N

2

3 4

"(I) Banik and Dey (1982); (2) Banik and Dey (1981~);(3) Kundu and Gaur (1984); (4) Gaur er al. (1980).

Generally, however, they can be grouped into those studies that measured the effects of inoculation on soil-available P levels, those that measured plant responses under greenhouse conditions, and those that studied plant responses under field conditions. The availability of P in soils inoculated with PS organisms has been Table IV Summary of Plant Responses to Inoculation of Soils with Cultures of Phosphate-SolubilizingFungi

Conditions

Crop

Organism

Flask

None

Greenhouse Greenhouse

Wheat + beans Wheat

Penicillium spp. Aspergillus spp. Penicillium bilaji P . bilaji

Greenhouse

Wheat

P . bilaji

Field

Wheat

P . bilaji

Field

Wheat

P. bilaji

Responses and observations

Reference"

Increased availability of P in soil

1

Increased plant weight and P uptake

2

Increased plant weight and P uptake in soils with added rock phosphate Increased plant weights and P, Cu, and Zn uptake, increased P availability in soil with added rock phosphate Increased plant weights, yields, and P uptake in soils with added rock phosphate Increased plant weights, yields, and P uptake in soils with added rock phosphate

3

~

~~

"(1) Banik and Dey (1981b); (2) Kucey (1987); (3) Asea er al. (1988); (4) Kucey (1988).

4

2 4

MICROBIALLY MEDIATED INCREASES IN PHOSPHORUS

215

shown to be increased in several cases. Banik and Dey (1981b,c) measured increased levels of available P in soils to which had been added farmyard manure, rock phosphate, and PS isolates of Bacillus, Streptomyces, Penicillium, and Aspergillus spp. The levels of NaHC0,-extractable P were determined to be increased after addition of P . bilaji in soils both with and without added rock phosphate (Kucey, 1988). Banik and Dey (1982), however, did not observe increases in P availability in inoculated soils in response to the addition of PS organisms in conjuction with manure and rock phosphate. In this case, the inoculum used was different from that used in the studies reported in 1981 (Banik and Dey, 1981b,c). The fixation of 32P-labeledsuperphosphate and tricalcium phosphate in soils inoculated with B. circulans has been shown to be decreased relative to that in uninoculated control soils (Raj et a / . , 1981). The majority of plant growth tests on P solubilization in soil have been conducted under greenhouse conditions. In the greenhouse rooting volumes are usually restricted, the contribution of soil-borne P to plant nutrition is reduced, and consequently, if P is solubilized by microorganisms the plant response will be greater. Under these conditions, Ralston and McBride (1976), Kundu and Gaur (1980, 1984),and Khalafallah et al. (1982) all reported increased P uptake and plant growth in various crops inoculated with PS organisms. In these cases, the P uptake from rock phosphate by inoculated plants was equal to or greater than that from superphosphate. Asea et a / . (1988),using a '*P isotope dilution method, found that greenhouse-grown wheat inoculated with P . bilaji was able to obtain 18% of its P from sources unavailable to uninoculated plants and was also able to solubilize added rock phosphate. Kucey (1988) observed that wheat dry matter production and P uptake increased under field and greenhouse conditions in response to P . bilaji inoculation in the absence of added rock phosphate and that addition of rock phosphate resulted in a further increase in dry matter production. Banik and Dey (19814 and Taha et al. (1969) both found increased plant growth and P uptake in response to the addition of PS bacteria. Several studies of PS microorganisms have included the effect of VA mycorrhizal fungi. Since the mycorrhizal fungi play an important role in the ability of the host plant to absorb P, this is a logical inclusion in experiments on P solubilization. The individual effects of mycorrhizal fungi and P-solubilizing Pseudomonas and Agrobacterium spp. have been found to be additive, such that lavender plants with both inocula received greater benefit from rock phosphate addition than plants receiving only one of the organisms (Azcon et al., 1976). Similar findings were reported for finger millet (Eleusine coracana) inoculated with rnycorrhizal fungi and PS Bacillus circulans (Raj et al., 1981). Kucey (1987) and Asea et al. (1988) also observed that maximum plant growth and P uptake in sterilized soils were obtained in treatments in which plants received both mycorrhizal

216

R. M.

N. KUCEY E T A L .

fungi and P. bifaji. Piccini and Azcon (1987) observed similar results for alfalfa grown in a sand-vermiculite mixture and inoculated with VA mycorrhizal fungi and a PS bacterium. Again, however, not all studies show positive responses to inoculation. Azcon-Aguilar et a f .(1986) and Lee and Bagyaraj (1986) found that although mycorrhizal fungi increased plant growth and P uptake, further addition of PS bacteria had no effect. Under field conditions, Kucey (1987, 1988) observed an increase in dry matter production and P uptake by wheat from 10 to 27% and 15 to 34%, respectively, as a result of the addition of P. bifaji to Chernozemic soils with low P availability and observed a further increase in growth (up to 47% greater than control) and P uptake (up to 55% greater than control) when rock phosphate plus fungus was added. Gaur et al. (1980) also observed increased wheat growth and P uptake in response to the addition of rock phosphate and a PS culture of Pseudomonas striata and Aspergiffus awamori. Badr El-Din et al. (1986), however, did not observe any positive responses of soybeans to the addition of B . megatherium to a field soil. D. MECHANISM OF ACTION OF P s MICROORGANISMS Phosphate-solubilizing organisms have been reported to solubilize inorganic forms of P by excreting organic acids that directly dissolve phosphatic materials and/or chelate cationic partners of the P ion (Sperber, 1958b; Katznelson and Bose, 1959). Analysis of culture filtrates of pure isolates of these microorganisms has revealed a number of organic acid products including lactic, glycolic, citric, 2-ketogluconic, malic, oxalic, malonic, tartaric, and succinic acids, all of which have chelating properties and could serve as active components of P solubilization (Sperber, 1958b; Louw and Webley, 1959; Duff et a f . , 1963; Taha et al., 1969; Banik and Dey, 1981a, 1982). Table V shows the predominant organic acids found in culture filtrates of various PS microorganisms tested. The quantities of acids produced by these organisms are in some cases equal to more than 5% of the carbohydrate consumed by the organism (Banik and Dey, 1982). It has been reported that conversion of root exudates into 2-ketogluconate by rhizosphere organisms may account for up to 20% of root exudates released into the rhizosphere (Moghimi et a f . , 1978b). Organic acids have several effects in the media. As acids, they have the effect of decreasing medium pH, e.g., to pH 3.8 for Aspergiffusawamori (Khan and Bhatnagar, 1977) and pH 2.7 for Aspergilfus carbonum (Gaur et a f . , 1973). Asea et al. (1988) found that P . bilaji was able to release more P from Idaho rock phosphate than that released by 0.1 N HCI added to achieve equivalent media pH levels. A lack of correlation between the ability to

MICROBIALLY MEDIATED INCREASES IN PHOSPHORUS

217

Table V Principal Organic Acids Produced by Phosphate-Solubilizing Microorganisms

Organism Arthrobacter sp. Bacillus sp. Bacillus firmus B-7650 Bacillus firmus B-7651 Gram -short rod Pseudomonas sp. B. megatherium B. megatherium var. phosphaticum B. subtilis Bacterium S470 Escherichia freundii Micrococcus sp. Micrococcus sp. Streptomyces sp. Streptomyces sp. Nocardia sp. Aspergillus fumigatus Aspergillus candidus Aspergillus niger Penicillium sp. Soil yeasts

Predominant acids produced

Reference

Oxalic, malonic Oxalic, succinic 2-Ketogluconic, succinic Oxalic, succinic Lactic Citric, gluconic Lactic, malic

Banik and Dey (1982) Banik and Dey (1982) Banik and Dey (1982) Banik and Dey (1982) Taha et al. (1969) Taha et al. (1969) Taha er al. (1%9)

Lactic Lactic, citric Lactic, glycolic Lactic Oxalic Lactic, succinic Lactic 2-Ketogluconic Succinic, glycolic Oxalic, tartaric, citric Oxalic, tartaric Citric, glycolic, succinic, gluconic, oxalic Lactic, glycolic Lactic

Taha et a / . (1969) Taha et a / . (1969) Sperber (l958b) Sperber (1958b) Banik and Dey (1982) Taha et a / . (1969) Banik and Dey (1982) Banik and Dey (1982) Sperber (1958b) Banik and Dey (1982) Banik and Dey (1982) Sperber (l958b) Sperber (3958b) Taha et a / . (1969)

reduce media pH and the ability of the isolates to solubilize P has been observed by Chhonkar and Subba-Rao (1967),Gaur et a f .(1973),and Surange (1985). It thus appears that the PS ability of microorganisms is manifested via mechanisms other than strict acidification of the surrounding environment. Kucey (1988) showed that addition of 0.05 M EDTA to solutions containing insoluble copper and zinc compounds had the same solubilizing effect as inoculation with P. bifaji. Reduction of the solution pH to 4.0 by the addition of 0.1 N HCI did not result in metal ion solubilization. Sperber (1958b) was able to duplicate the effects of inoculation with PS microorganisms by the addition of lactic, glycolic, or citric acids to solutions containing apatite. Irrigation of greenhouse soils with 0.001 M EDTA has been shown to increase the uptake by Italian rye grass (Lolium muftiforum L.) of P, aluminum, and iron (Hartikainen, 1981). Duffer a f . (1963) observed that 2-ketogluconic acid produced by several PS bacteria and fungi effected the release in solution of numerous phosphate and silicate

218

R. M.N. KUCEY E T A L .

materials. Moghimi and Tate (1978) concluded that the main action of 2ketogluconic acid was to act as a source of hydrogen ions in the dissolution of calcium phosphates; however, Berrow et al. (1982) concluded that 2ketogluconate obtained from a batch culture of an Erwiniurn species was able to extract more cobalt, nickel, zinc, iron, titanium, and vanadium than ammonium acetate and was equal to EDTA and DTPA in extracting copper, manganese, molybdenum, nickel, and zinc. Certainly, although 2-ketogluconate may be a major component of rhizosphere products, other acids are produced as well. Thus it appears that a mechanism relying on the production and use of organic acids can be used as a PS system by microorganisms. Doubt regarding the role of PS microorganisms in liberating P under soil conditions has been expressed (Brown, 1974; Tinker, 1980, 1984). These doubts were founded upon theoretical limitations and lack of direct evidence of P solubilization. Indeed, many of the earlier experiments were conducted in nonbuffered conditions such as in solution culture or in sand. Many of these experiments included PS organisms as part of a combined treatment in which organic materials and/or phosphatic materials were added, so the effect of PS organisms, if present, could not be separated from the effects of the other amendments. Theoretical arguments have considered the processes necessary for P solubilization to result in increased plant P uptake (Rovira and Davey , 1974; Hayman, 1975; Tinker and Sanders, 1975). The organisms first would have to cause the release of P from the unavailable sources, then the P would have to be available for plant uptake. The first process has been shown to occur in unbuffered systems many times. Nye and Tinker (1977), however, have calculated the quantity of organic acid that would be necessary to solubilize inorganic P in bulk soil and concluded that microorganisms could not produce the quantities necessary and, further, that the plant could produce the necessary pH drop by releasing acids as exudates without the presence of the microorganisms (Hedley et al., 1982; Bekele et al., 1983). However, it is likely that the nature of the acids released is more important to the amount of P released than is the quantity of acid produced (Sperber, 1958a; Louw and Webley, 1959; Chhonkar and SubbaRao, 1967). In addition, although the plants may be able to release large quantities of exudate, it is unlikely that the exuded materials would remain in the rhizosphere untouched for long enough to affect P release (Hale et al., 1971; Moghimi et al., 1978a). It is possible that PS organisms can produce effective chelating materials in a microenvironment such as in the immediate vicinity of rock phosphate or phosphatic fertilizer materials, or in the rhizosphere (Moghimi et al., 1978a; Tinker, 1980). Under these conditions, P could be solubilized and be present in an available form in high enough concentrations to be avail-

MICROBIALLY MEDIATED INCREASES IN PHOSPHORUS

219

able to plant roots. As for plant uptake of the material, if chelating materials are being released it is unlikely that all the P solubilized would be absorbed by the organisms, so most of it would remain in solution. It has already been shown that mycorrhizal fungi aid in plant uptake of P and that, in many cases, the addition of mycorrhizal fungi along with effective PS organisms increases the effect beyond that observed from the addition of either organism alone (Azcon et al., 1976; Kucey, 1987; Asea e f al., 1988). It thus seems plausible that mycorrhizal plant roots could efficiently absorb solubilized P over a period of time and that the increase could be equivalent to that observed from the addition of phosphatic fertilizers. A suggestion has been made that the action of PS organisms is not due to the release of plant-available P but rather to the production of plant growth substances (Brown, 1974; Tinker, 1980). These substances could cause plant growth stimulation, which would result in a larger plant that naturally would contain larger amounts of P and other materials than a smaller plant. Indeed, a number of Soviet scientists supported this idea with respect to the action of phosphobacterins (Dorosinski, 1%2; Mishustin and Naumova, 1%2; Samtsevich, 1%2; Mishustin, 1963; Voznyakovskaya, 1963), and other authors have stated that the responses they received to the addition of PS bacteria might be due to the production of biologically active substances (Barea e f al., 1975; Azcon e f al., 1976; Kucey, 1988). Barea et al. (1976) measured levels of indoleacetic acid, gibberellins, and cytokinins in culture filtrates of PS bacteria. Datta et al. (1982) were able to obtain positive growth responses of field-grown rice to the addition of a Bacillus firmus isolate that showed both P-solubilizing ability and indoleacetic acid-producing ability. In this case, the response was greater in the presence of Mussoorie rock phosphate, indicating that the PS activity shown in pure culture was also shown under field conditions. Jarrel and Beverly (1981) state that definitive evidence that a factor is increasing the availability of a nutrient for plant uptake is if the amount of nutrient in the plant increases and if the concentration of nutrient within the plant tissues increases as well. As previously shown in several studies, the concentration of P within plants inoculated with PS organisms has been found to be increased relative to uninoculated controls (Taha et al., 1969; Azcon et al., 1976; Kundu and Gaur, 1980, 1984; Banik and Dey, 1981~;Khalafallah et al., 1982). In other cases, in which increased plant growth and P uptake were observed but the concentration of P within plant tissues remained the same, the soils used were specifically chosen for their poor P availability indices, so that increases in plant-available P resulted in increases in plant dry matter production similar to increases observed in response to the addition of P fertilizers (Kucey, 1987, 1988; Asea et al., 1988). As further evidence that one of the mechanisms of PS organisms can

220

R. M. N . KUCEY E T A L .

be the release of plant-available P, Asea et al. (1988) reported that addition of P . bilaji to 32P-labeledsoils resulted in dilution of 32P-labeledphosphate in the plant with unlabeled P, which could only have come from added rock phosphate or from unlabeled sources of soil P unavailabe to the uninoculated plants. These experiments were similar to those used to determine that VA mycorrhizal fungi did not solubilize unavailable forms of P (Mosse et af., 1973; Gianinazzi-Pearson et al., 1981). Asea et af. (1988) also observed that the VA mycorrhizae in their test did not cause 32 P isotope dilution. It is possible that PS microorganisms may produce biologically active substances and that these substances may play a role in the plant responses to the addition of these organisms. Barea et al. (1976) state that whereas plant growth stimulators may affect plant growth, P solubilizers would have at least a secondary role in making extra P available for the increased plant demands, and that inoculants that would produce both plant growth stimulants and be able to provide the extra P necessary would be most beneficial for crop production. Indeed, even mycorrhizal fungi, which have been proven to increase plant P uptake, have been shown to produce biologically active substances that alter plant growth (Barea, 1987). A plant growth-promoting isolate of Pseudomonas putida inoculated onto subterranean clover was shown to increase plant uptake of iron, copper, aluminum, zinc, cobalt, and nickel (Meyer and Linderman, 1986), and it was suggested that this organism was able to solubilize sufficient nutrient to meet the increased plant demand caused by the other growthpromoting factors produced by the bacterium. Kucey (1988) also suggested that P . bifaji may have other effects on plant growth and that the fungus was able to solubilize copper and zinc to meet the increased demands. The evidence points out that release of P in a plant-available form can also be one of the results from inoculation of soils with these organisms. Because of the diversity of organisms and soils used in the studies reported here, it is difficult to generalize about PS organisms. However, it is certain that some of the organisms that have been tested do show PS activity in soils as well as in unbuffered systems.

VI. SULFUR OXIDATION AND ROCK PHOSPHATE-SULFUR MIXTURES Lipman and his associates were among the first to mix ground rock phosphate with sulfur in an attempt to increase the availability of the P contained in the rock (Lipman et a f . , 1916a,b; Lipman and Mclean, 1918). The theory was proposed that the sulfur in the mixtures would be oxidized

MICROBIALLY MEDIATED INCREASES IN PHOSPHORUS

22 1

to sulfuric acid by soil microorganisms (Wainright, 1984) and that the sulfuric acid would dissolve the rock phosphate in situ (Swaby, 1975). Wainright (1984) and others have published excellent reports on the many factors that affect the rates of S oxidation in soils. Addition of sulfur to rock phosphates has been tested under soil conditions. Kittams and Attoe (1965) found that application of phosphatesulfur mixtures increased the yields of ryegrass more than did the addition of superphosphate or rock phosphate alone in one of three soils tested but did not affect P uptake in the other two soils. Nimgade (1968) also observed greater P uptake from rock phosphate-sulfur mixtures than from rock phosphate alone, and Neller (1956) observed increased P availability of soils if sulfur was added along with rock phosphate. Bromfield (1975) found that a rock phosphate-sulfur mixture added to peanuts resulted in greater yields and P uptake than those obtained from ground rock phosphate alone and that the mixture was equal to superphosphate. The availability of P from rock phosphate-sulfur mixtures appears to be greatly affected by many factors, including soil temperature and moisture and the granule size of the final product (Terman et al., 1964, 1969; Kittams and Attoe, 1965; Attoe and Olson, 1966; Li and Caldwell, 1966; Nor and Tabatabai, 1977). Although several groups of soil bacteria are able to oxidize sulfur, the most important are the chemautotrophic bacteria Thiobacilfus thiooxidans and Thiobucillus thioparus (Starkey, 1966). For this reason, Swaby (1975) suggested that rock phosphate-sulfur mixtures could be inoculated with these bacteria to provide increased P availability of the mixtures. Rock phosphate availability was found to be highest at a soil pH of 6.0 or lower (Ellis er a f . , 1955), and the action of these bacteria is able to produce these acidic conditions in microsites. The inoculated mixtures (1S/5RP) used by Swaby (1975) were called biosupers and found to be superior to uninoculated mixtures for pasture production in tropical soils. Other studies in tropical conditions showed biosupers to be effective in tropical pasture conditions (Fisher and Norman, 1970; Gillman, 1973; Jones and Field, 1976; Partridge, 1980; Rajan and Edge, 1980; Rajan, 1981, 1982). Whitehouse and Strong (1977), however, found biosupers to be unsatisfactory as a wheat fertilizer in some Australian soils, and Lee and Bagyaraj (1986) found that the addition of thiobacilli to the soil in their greenhouse study resulted in a decrease in plant growth. They also suggested that the organisms naturally present in their soil were effective sulfur oxidizers and that inoculation with thiobacilli did not increase sulfur oxidation rates. Rajan (1982) also determined that inoculation with thiobacilli did not increase the effectiveness of Chatham Rise or Christmas Island biosupers. ,

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The data on the use of biosupers are somewhat contradictory. As with many systems requiring the use of living organisms, the success of the system probably depends on the specific environmental conditions. In the case of biosupers, the activity of both added thiobacilli and indigenous S-oxidizing organisms would be affected by environmental stresses. In addition, since these systems require two steps, S oxidation followed by P dissolution, the second chemical reaction step will also be affected by the soil environment. The greatest effects of biosupers were observed under tropical conditions, and it may be that the use of biosupers will be restricted to these areas. There is a need, however, to determine accurately the conditions that are essential for the rock phosphate-sulfur system to work.

VII. FUTURE OF TECHNOLOGIES The future of the biological technologies outlined in this review will depend greatly on the state of agricultural economies. Certainly, the development and use of phosphobacterins in the USSR was primarily initiated by the need of that country for increased production and the lack of sufficient phosphate fertilizer production plants. This situation is widespread in the so-called underdeveloped countries of the world today. Many of these countries have undeveloped reserves of rock phosphate that could be used if more economical means of exploiting them were available. At present, however, building phosphate fertilizer plants is too great an expense for the value of most of the deposits. North American agriculture could also benefit from some of these systems, since production costs of phosphate fertilizers would be reduced, which would, hopefully, also reduce the farmer’s costs for crop production. The use of VA mycorrhizal fungi is greatly limited by the inability of scientists to grow the fungi in pure culture to produce large amounts of inoculum (Gianinazzi and Gianinazzi-Pearson, 1986). This problem has been overcome in the use of ectomycorrhizal fungi, and at present, these fungi are commonly inoculated onto container-grown coniferous seedlings to increase growth and survival of the trees. Vesicular-arbuscular mycorrhizal fungi must be grown in a labor-intensive system of pot cultures in which the fungus is propagated in the presence of a host plant. Nonetheless, container-grown seedlings of crops such as oranges are inoculated because the mycorrhizal seedlings survive better than uninoculated ones. If a system of mass culture of VA mycorrhizal fungi were developed and specialized strains of VA mycorrhizal fungi were isolated, then the inoculation of certain soils could be of economic benefit. However, since

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the majority of soils contain native VA mycorrhizae, widespread inoculation of agricultural soils would most likely not be of economic benefit. If VA mycorrhizae could be cultured on synthetic media and strain selection could be performed, then introduction of specialized VA strains might prove useful for responsive crops (Gianinazzi and Gianinazzi-Pearson, 1986). The rock phosphate-sulfur mixtures, i.e., biosupers, appear to be useful in situations in which low amounts of P are necessary over a long period of time, such as in pastures. Their use in temperate areas would probably be limited by the S oxidation rate, which is much less than in tropical areas. Temperate annual crops require the release of P early in the growing season. Nonetheless, biosupers, if proved successful under field conditions, could provide a useful economical fertilizer for tropical pasture production. The use of phosphobacterins in North America was essentially terminated following the reports in the 1960s that tests conducted in the United States did not give positive results. If the bacteria were degrading soil organic matter to mineralize P, then it could be argued that the organic matter serves a much more valuable role as a soil structural component. If this is the case, then the use of these degradative bacteria should be discouraged. The study of B. megatherium var. phosphaticum in phosphobacterins led to their use in systems aimed at solubilization of inorganic P forms. The potential for using P-solubilizing bacteria and fungi for in situ processing of rock phosphates in soil has led to a resurgence of interest in biologically released P that has resulted in the submission of a number of patents dealing with these systems. It remains to be seen whether these biologically activated systems are successful under practical field situations. If so, and if the economics of agriculture in North America and elsewhere show that the production of inocula and addition of unprocessed rock phosphate with PS inocula is more economical than processing of rock phosphate into phosphate fertilizers, then we may see new fertilizer products on the market. In developing countries that have rock phosphate reserves, such fertilizers may provide a system by which these reserves can be developed without the expense of building phosphate fertilizer production plants.

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ENZYMOLOGY OF THE RECULTIVATION OF TECHNOGENIC SOILS S. Kiss, M. Dragan-Bularda, and D. Pasca Department of Plant Physiology Babes-Bolyai University 3400 Cluj-Napoca, Romania

I. 11.

Ill. IV. V. VI . VII.

VIII.

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XI. XII.

XIII.

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Introduction Technogenic Soils from Coal Mine Spoils A. Enzymological Research in the USSR B. Enzymological Research in the United States C. Enzymological Research in Poland D. Enzymological Research in Hungary E. Enzymological Research in the Federal Republic of Germany Technogenic Soils from Power Plant Wastes Enzymological Research in Poland Technogenic Soils from Retorted Oil Shale Enzymological Research in the United States Technogenic Soils from Iron Mine Spoils A. Enzymological Research in the USSR B. Enzymological Research in Romania Technogenic Soils from Manganese Mine Spoils Enzymological Research in the USSR Technogenic Soils from Lead and Zinc Mine Wastes A. Enzymological Research in the United Kingdom B. Enzymological Research in Romania Technogenic Soils from Sulfur Mine Spoils Enzymological Research in the USSR Technogenic Soils from Lime and Dolomite Mine Spoils Enzymological Research in the USSR Technogenic Soils from Refractory Clay Mine Spoils Enzymological Research in the USSR Technogenic Soils from Bentonitic Clay Mine Spoils Enzymological Research in the USSR Technogenic Soils on Sand Opencast Mine Floor Drift and Spoils A. Enzymological Research in Poland B. Enzymological Research in the USSR Technogenic Soils from Overburdens Remaining after Pipeline Construction A. Enzymological Research in the USSR B. Enzymological Research in the United States Recultivation of Soils Remaining after Topsoil "Mining" Enzymological Research in New Zealand Concluding Remarks References

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I. INTRODUCTION Technogenic soils are soils that form during the technical and biological recultivation of overburdens, tailings, and other spoils and wastes resulting from strip (opencast, surface) and shaft (underground, deep) mining and other industrial activities. The evolution of technogenic soils is, by definition, the process of transforming all these wastes into agricultural and forest soils or into soils used for other purposes (parks, sports fields, etc.). The practical importance of this process is growing because the development of mining and other industries leads to increasing amounts of wastes and, therefore, the recultivation of wastelands becomes more and more a major economic necessity. It is estimated that up to 1980 about 1600 x lo9 m3 of mine spoils accumulated on the earth’s surface and this amount has increased yearly by about 40 x lo9 m3. Water erosion affects a smaller amount of soil (about 13 x lo9 m3 per year). The evolution of technogenic soils presents questions of theoretical importance, too, which are related to a better understanding of the evolution of landscape as a whole (Greszta, 1973; Nastea et al., 1973; Khazanov, 1975; Szegi, 1983). The evolution of technogenic soils, which affects the efficiency of recultivation, is studied using many physical, chemical, and biological methods. Enzymological methods have also been applied, and it has been found that the level of enzymatic activity is a good indicator of the degree of evolution of technogenic soils. In the present review, data from the literature concerning the enzymology of technogenic soils will be grouped according to the nature of the raw material from the mining and processing of which the wastes resulted (coal; oil shale; iron, manganese, lead, and zinc ores; sulfur; lime and dolomite; refractory clay; bentonitic clay; and sand). Finally, the enzymology of the recultivation of overburdens remaining after pipeline construction and that of the recultivation of soils remaining after “mining” (removal) of their topsoil for use in landscape improvement will be dealt with.

II. TECHNOGENIC SOILS FROM COAL MINE SPOILS A. ENZYMOLOGICAL RESEARCH IN

THE

USSR

Keleberda (1973) determined the H,O,-splitting capacity in the Aleksandrii spoil heap at the Baidakov brown coal strip mine (located in the northern steppe zone of the Ukraine, Kirovograd region) and in the spoil

THE RECULTIVATION OF TECHNOGENIC SOILS

23 1

heaps of the abandoned Yurkov brown coal strip mine (located in the forest-steppe zone of the Ukraine, Cherkassy region). The spoils consist predominantly of loess. At both mines, a part of the spoils were recultivated with Scotch pine (Pinus sylvestris) for 8 and 3 years, respectively. In some spoil plots the plants grew well, whereas in others their growth was inhibited. The H,O,-splitting capacity, and especially its heat-labile component (catalase activity), was always higher in the plots with welldeveloped plants than in those on which the growth of plants was inhibited. Keleberda has drawn the conclusion that the forest-growing properties of recultivated spoils can be evaluated by catalase activity measurements. Verbin and Keleberda (1974) compared urease and catalase activities in the Y urkov spoil plots recultivated only with black alder (Alnus glutinosa), with black alder plus Scotch pine, or only with Scotch pine. In pure stands, the growth of black alder was good and that of Scotch pine bad. In mixed stands, the black alder stimulated the growth of Scotch pine. After 3 years of recultivation, enzyme activities, and nitrogen (N) content (total, NH4+.NO,-) in the 0-20- and 2040-cm layers of the spoil plots showed the following order: black alder plots > black alder plus Scotch pine plots > Scotch pine plots. The values were highest in the rhizosphere and especially in the zone of root nodules of black alder. Keleberda et al. (1974) and Keleberda and Dan’ko (1975) reported on a new enzymological study of the recultivated spoils at the Yurkov mine. Spoils (sandy loams or loamy sands) of the Strizhev coal strip mine (located in the central forest zone of the Ukraine, Zhitomir region) were also studied. Invertase, urease, and catalase activities were determined in soils of spoil plots cultivated with a green manure crop, perennial lupine (at both mines), or with black alder (at Yurkov). Uncultivated spoil plots served as controls. It was found that the enzyme activities decreased with the sampling depth in each plot. Recultivation led to evident, sometimes manifold, increases of enzyme activities as compared to the uncultivated controls. The effect of perennial lupine was stronger than that of the black alder. Owing to the lupine, the humus and total N contents also increased, 2.54-2.87 and 2.94-3.30 times, respectively. In the alder plots, the increase of humus and N contents of soils was 1.95- and 1.81-fold, respectively. The results indicate that recultivation with black alder or, even better, with the green manure plant perennial lupine makes it possible to increase, in a relatively short period of time, the fertility level of technogenic soils. In another study, Keleberda (1976) found that the soils of the Yurkov spoil plots cultivated with Scotch pine for 9 years or with black locust (Robiniu pseudoacacia) for 4 years also manifested higher invertase activity than did the uncultivated spoil. The activity increase in the Scotch pine soil was close to that in the lupine soil, but in the locust soil the activity increase was lower as compared with that in the lupine soil. The

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humus and total N contents increased in the Scotch pine and locust soils, also. It was also found (Keleberda and Dan’ko, 1975; Keleberda, 1976) that jack pine (Pinus banksiana), Scotch pine, weeping birch (Betula verrucosa), and pedunculated oak (Quercus pedunculata) grew much better on the green (perennial lupine)-manured than on the nonmanured Strizhev spoil plots. Keleberda (1977) has pointed out that in addition to invertase, urease, and catalase activities proteinase and amylase activities also reflect the evolution of spoil heaps toward technogenic soils. The Yurkov spoil heaps recultivated with different tree and shrub species became primitive soils in a relatively short time (9-14 years), acquiring a humus layer and an increased N content and enzymatic potential (Keleberda, 1978) (Table I). The spoils, consisting predominantly of loess, at the Baidakov brown coal strip mine located in the northern steppe zone of the Ukraine, were also studied multilaterally by Mikhnovskaya (1981) and Eterevskaya et al. (1985). They found that invertase, urease, and proteinase activities and respiration (CO, evolution) were very low in the loess forming the walls of the quarry and in the loess heap not covered by vegetation and increased very much with the age of the indigenous vegetation covering the loess heaps, but even 23 years after the growth of indigenous vegetation the activity values were lower than those measured in the undisturbed zonal soil (Table 11). The spoil heaps (medium loams; pH -7) at the brown coal strip mines located in the forest-steppe zone in the Nazarovo Basin (which belongs to the Kansk-Achinsk Fuel-Energetic Complex, Siberia) were studied enzymologically by two research groups. Naprasnikova et al. (1982) and Naprasnikova (1983, 1985a,b, 1987) determined enzyme activities in spoil heaps covered with 3-, 5-, and 15-yearold indigenous plant communities, in spoil heaps recultivated with pines, larches, willows, or sweetclover for a maximum of 15 years, and in zonal (gray forest) soils that had not been affected by the strip mining. Proteinase activity was always lower in the recultivated spoils and much lower in the spontaneously revegetated spoils than in the zonal soils. Peroxidase activity remained low in both recultivated and spontaneously revegetated spoils as compared with that in the zonal soils. Invertase and acid, neutral, and alkaline phosphatase activities in the recultivated spoils and in spoils under 15-year-old indigenous vegetation, however, approached or even exceeded those found in the zonal soils. Polyphenol oxidase and the other enzymes were most active in the rhizosphere of the dominant plant species. The results obtained underline the advantages of recultivation over uncontrolled revegetation of the coal strip mine spoils. Based on the finding that the enzyme activities increased twice in non-

233

T H E RECULTIVATION O F TECHNOGENIC SOILS Table I

Humus and N Contents and Enzyme Activities in Primitive Soils under Forest Vegetation Developed on Spoil Plots at the Yurkov Coal Strip Mine"

Plant stand and its age Scotch pine (14 years)

Black locust (9 years)

Sea buckthorn (9 years)

Black alder (9 years)

Control (uncultivated)

Depth (cm)

Humus

N

(%)

(76)

0-2 2-5 5-10 10-20 20-30 0-2 2-5 5-10 10-20 20-30 0-2 2-5 5-10 10-20 20-30 0-2 2-5 5-10 10-20 20-30 0-2 2-5 5-10 10-20 20-30

4.74 2.33 1.20 0.41 0.36 2.08 1.02 0.73 0.16 0.11 1.65 0.75 0.43 0.26 0.18 1.30 1.03 0.84 0.17 0.19 0.72 0.43 0.24 0.23 0.21

0.24 0.12 0.07 0.02 0.02 0.18 0.09 0.07 0.01 0.01 0.19 0.06 0.04 0.02 0.02 0.13 0.12 0.08 0.03 0.03 0.05 0.02 0.01 0.01 0.01

N in humus (96) 6.0 5.1 5.8

5.0 5.5

8.6 9.0 9.6 6.2 9.9 11.5 8.0 9.5 8.0 11.1

10.0 11.6 9.5 17.6 15.8 6.9 4.6 4.1 4.3 5.0

lnvertaseh

Ureaseh

Proteinaseh

29.70 10.00 6.62 4.42 0.60 22.30 8.02 4.90 1.12 0.60 44.50 11.20 2.25 I .71 I .02 28.50 13.50 7.50 4.70 0.80 10.80 5.20 2.35 0.10 0

1.96 0.56 0.45 0.34 0.26

0.56 0.30 0.20 0.16 0 0.48 0.28 0.28 0.16 0 0.96 0.34 0.26 0. I5 0 0.76 0.36 0.26 0.12 0 0.34 0. I5 0.08 0 0

I.64

0.62 0.33 0.28 0.15 3.68 0.72 0.37 0.16 0.11 2.86 0.84 0.42 0.32 0.25 1.08 0.71 0.32 0.20 0

"From Keleberda (1978). "lnvertase activity is expressed as milligrams of inverted sugar, urease activity as milligrams of NH,' N, and proteinase activity as milligrams of NHZ N produced by I g of soil in 40, 40, and 72 hr, respectively.

topsoiled spoils during their agricultural recultivation for 3.5 years, Naprasnikova and Makarova ( 1986) recommend this recultivation method without covering the spoils with a fertile soil layer. Shugalei et ul. (1984, 1983, and Korsunova and Shugalei (1986) studied the enzyme activity and chemical composition of the topsoil (50-cm layer) stockpiled before surface mining of coal. The stockpiles are 1.5-4 m high and, after 3-5 years of storage, they are used to cover the leveled spoils for their recultivation. Catalase activity in the stored topsoil remained nearly at the same level as in the 0-50-cm layer of the adjacent undisturbed

234

S. KISS ET AL. Table I1

Enzyme Activities and Respiration in Spoils (Loess)at the Baidakov Coal Strip Mine”

Analyzed material

lnvertase”

Urease”

Proteinase”

Respiration”

Loess from the walls of quarry Loess (spoil) heap without vegetation Loess (spoil) heaps spontaneously vegetated. age of vegetation: I year 2 years 4 years 23 years Zonal soil (common chernozem)

0.3 0.3

1.1

I .4

2.3 2.4

4.4 4.6

I .2 2.2 4. I 7.0 20.5

I .5 2.0 2.3 4.5 15.6

2.7 3.8 5.1 9.1 28.7

5. I N.D.‘ 28.7 35.3 217.0

.

~~

“From Eterevskaya et ul. (1985). ”Invertase activity is expressed as milligrams of inverted sugars produced by I g of material, urease activity as milligrams of NH3 produced by 100 g of material, proteinase activity as milligrams of tyrosine produced by 100 g of material in 24 hr, and respiration as milligrams of CO, evolved from I kg of material/hr. ‘Not determined.

leached chernozem, but urease activity decreased and proteinase activity increased to some extent in the stored topsoil. The quantity and quality of humic substances in the stored topsoil and in the undisturbed soil were nearly the same. The topsoil, after its replacement on the leveled spoils and recultivation with perennial grasses, showed catalase and urease activities approaching the seasonal mean values recorded in the 0-50-cm layer of the undisturbed soil, whereas proteinase activity was three times higher in the replaced topsoil than in the undisturbed soil. In different raw spoil samples, Shugalei and Yashikhin (1985) registered high variation coefficients of catalase, urease, and proteinase activities, respiration (CO, evolution) rate, organic matter, total N , NH,’ N , and NO,- N contents (77, 90,66, 58, 69, 72,82, and 151%, respectively) which was attributed to nonhomogeneous mixing of the different overburden layers. Leveling of raw spoils led to diminution of the variation coefficients, except that of the urease activity which increased. Shugalei et al. (1985) determined enzyme activities also in the 1-1.5cm humus layer that had formed in 10 years on leveled spoils not covered with stored topsoil but cultivated with Scotch pine or Siberian larch. It was found that catalase, urease, and proteinase activities in the newly formed humus layer exceeded by 5-20 times the activities measured in the 0-20-cm layer of the undisturbed soil.

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235

According to Naplekova et al. (1983) and Trofimov et af. (1986), the soil-forming processes in the technogenic landscapes created by the coal strip mining in the Kuznetsk Basin (Kuzbass, Siberia) take place very vigorously, owing to the spontaneous revegetation and forest recultivation of spoil heaps (slightly alkaline, calcareous sandy loams and clay shales). In 18-year-old spoil heaps in the mountain taiga zone and in 8-12-yearold spoil heaps in the steppe zone the number and composition of microflora approach those in the zonal soils (pseudopodzolic soil and leached chernozem, respectively). This causes the intensification of respiration (CO, evolution); catalase, dehydrogenase, amylase, and phosphatase activities; gelatin and cellulose decomposition; and amino acid accumulation in the technogenic soils and leads, finally, to an increase in their fertility. In the forest-steppe zone, Naplekova ef al. (1985) determined the polyphenol oxidase and peroxidase activities of the technogenic soils formed during the development of spontaneous indigenous plant communities on the spoil heaps. They established that the nature of the dominant plant had a decisive influence on these activities and on their ratio. The polyphenol oxidase-peroxidase ratio, expressed as a percentage, is considered an indicator of the intensity of humification processes (humification coefficient). High values of the humification coefficient were registered in technogenic soils under perennial legumes and grasses. Klevenskaya et al. (1986) emphasize that the associations between microorganisms and plants, owing to accumulation of enzymes, especially of polyphenol oxidase and peroxidase in the rhizosphere, speed up the elementary pedogenetic processes in the technogenic soils. Some enzymological aspects of the recultivation of coal shaft mine spoils situated in the valley of the Samara River (western Donets Basin, Donbass) have been dealt with by Gel’tser and co-workers (Gel’tser and Tsvetkova, 1982; Tsvetkova et af., 1982; Ras’kova et al., 1984; Gel’tser et al., 1986). The four experimental plots had the following structure (from top to bottom): I , mine spoils (silts and clay shales with sands and coal inclusions); 11, 0.5 m of loess plus 0.5 m of sand plus mine spoils; 111, 0.5 m of sand plus 1 m of loess plus mine spoils; IV, 0.5 m of chernozem plus 0.5 m of sand plus 0.5 m of loess plus mine spoils. The thickness of the mine spoil layer was 7-10 m. The plots had all been planted to black locust 5 years before. In plot I the growth of locust was unsatisfactory. In the other plots the locust grew well. For comparison, a zonal soil (common chernozem) under natural vegetation or a 15-year-old black locust plantation was used. Five enzyme activities were measured in the 0.5-m layer of the plots. In plot I the activities were present only in traces. The highest activities were found in the litter of plots I1 and IV. Proteinase, urease, and dehydrogenase activities were no lower in the 0-2 and 2-10-cm layers of these plots than in the humus-enriched horizon of the zonal soil. The opposite was true for the invertase and catalase activities. In the 10-50-cm

236

S. KISS ETAL.

layer of plot I1 (loess) the activities decreased sharply, whereas in the same layer of plot IV (chernozem) they remained, after a slight initial decrease, practically constant. The enzymological measurements made possible a more accurate differentiation of the layers and places of soilforming processes within the soil profile than did its morphological description. In another study, Gel’tser et al. (1985) have found that proteinase and invertase activities were nearly twice as high in the litter of the black locust plots on loess and chernozem overlying mine spoils as in the litter of the black locust plantation on zonal soil. B. ENZYMOLOGICAL RESEARCH IN

THE

UNITED STATES

Reviewing the role of microorganisms in the revegetation of strip-mined land in the western United States, Cundell (1977) emphasized the observations of Pancholy et al. (1979, according to which the low urease and dehydrogenase activities in a completely denuded area near an abandoned zinc smelter in Oklahoma were an excellent indicator of the inability of this soil to revegetate. Therefore, Cundell advocated enzyme activity determinations for studies of mine soil recovery. Miller ( 1978) described complex recultivation investigations in which enzymological methods were also applied. Four sites were studied. The first three were strip coal mine sites and the fourth, a coal refuse site, resulted from deep coal mining. The sites are located in different parts of the United States. At the Indian Head mine (Zap, North Dakota), proteolytic and dehydrogenase activities were determined in topsoil stockpiled prior to replacement on the spoil. In the 0-2.5-cm layer of a single topsoil storage pile at 10 and 22 months of age, the proteolytic activity was only -30 and 25%, respectively, in comparison with that measured in the same surface layer of the adjacent undisturbed soil. At 30-300-cm depths, enzymatic activity was also lower in the topsoil pile at 22 months of age than at 10 months of age. Dehydrogenase activity in the 0-2.5-cm layer was 20% greater in the 29-month-old topsoil pile examined than in the undisturbed soil, but in the 30-400-cm depths of the pile, dehydrogenase activity represented only -1-15% of that found in the 0-2.5-cm layer of the same pile. In other words, during storage the enzymatic potential diminishes to a large extent in most parts of the topsoil pile. At the Jim Bridger mine near Rock Springs, Wyoming, 3 plots were studies enzymologically. In the first plot the spoil was covered with stored topsoil. In the second plot a native soil was immediately reapplied to the spoil without prior storage. Adjacent to these plots was the third (control) plot, an undisturbed native area composed of an Atriplex community and

THE RECULTIVATION OF TECHNOGENIC SOILS

237

an Artemisia community. All plots had a west-facing aspect and a 30% slope. Plots 1 and 2 were established during the spring and fall of 1976, with the reapplied stored topsoil plot having extra growing season. They were seeded with wheatgrasses (Agropyron spp.) and fourwing saltbush (Atriplex canescens). No fertilizers were applied. On both plots, the dominant vegetation was Halogeton glomeratus followed by Salsola kali. Almost no wheatgrass survived, and no fourwing saltbush was evident. However, volunteer Atriplex gardneri and Atriplex confertifolia were encountered, with A . gardneri predominating. In July 1977, soils were sampled to determine their cellulase, urease, and dehydrogenase activities, which were found to decrease in the following order: undisturbed soil > stored topsoil applied on spoil > native topsoil applied on spoil. It should be mentioned that in a sample from the native topsoil plot, cellulase activity was not detectable at all. Proteolytic and dehydrogenase activities were assayed in the surface layer of orphaned (nonrecultivated) spoil heaps at the Big Horn mine and at the abandoned Hidden Water Creek mine near Sheridan, Wyoming. The spoils were composed of semiconsolidated shale and sandstone. Surface soil samples collected from undisturbed grazed rangeland sites near the Big Horn mine served for comparison. The mean value of the proteolytic activity was -7.5 times lower in the spoil than in the soil samples and dehydrogenase activity was lacking in the spoil samples. A coal refuse pile generated in the coal-cleaning process at a deep coal mine near the city of Staunton in Macoupin County, Illinois was submitted, many decades after the abandonment of the mine, to technical and biological recultivation. The enzymological analyses performed at the end of the growing season in the first year of recultivation (1977) indicated the presence of two activities (urease and dehydrogenase) and the absence of proteolytic and cellulase activities in the coal refuse. These four activities gave high values in the adjacent field soil. During the summers of 1975 and 1977, Hersman and Temple (1979) collected samples from six reclamation plots of the coal strip mine spoils in the Colstrip area (eastern Montana) and determined their ATP content, phosphatase and pectinolyase activities, and rate of respiration (0,uptake). ATP content correlated significantly with respiration rate and pectinolyase activity for the 1975 samples, and with respiration rate and phosphatase activity for the 1977 samples. Of the two enzymes tested, phosphatase gave more positive values for correlation. Nevertheless, its positive correlation with both ATP content and respiration rate in one set of samples, but not in the other, suggests that phosphatase activity is more variable than either respiration rate or ATP content. Pectinolyase is more specifically related to plant decomposition and may have some implications for spoils that are in the early stage of pedogenesis. Pectinolyase activity showed only one signiticant correlation, suggesting that it may be primarily

238

S. KISS ET AL.

useful as an adjunct measurement, but not as a general indicator of microbial activity. We think that this conclusion of Hersman and Temple’s would not be valid if the evolution of a given reclamation spoil plot were considered, because the six plots studied by these investigators, as they emphasize, were distributed over an area of many square miles and had been submitted to a variety of treatments, but the differences between plots (and treatments) were not examined. At the San Juan strip coal mine near Farmington, New Mexico, Fresquez and Lindemann (1982) determined dehydrogenase activity and several microbiological parameters (numbers of aerobic heterotrophic bacteria, streptomycetes, ammonium oxidizers, azotobacter, and fungi, and distribution of fungal genera) in representative samples from four sites: (1) a spoil bank (-1 year in age; nonvegetated); (2) a topsoil stockpile (at least I year in age); (3) a reclaimed area (revegetated 3 years earlier by grading and leveling the spoil, spreading stockpile topsoil 18 cm deep, mulching with native hay, fertilizing with 67 kg of N and 100 kg of P,O,/ ha, seeding with native grasses and shrubs, and irrigating for 2 years); (4) an undisturbed surface soil (near the reclaimed area). At each site the samples were collected to a depth of 18 cm. The results of determinations showed that dehydrogenase activity decreased in the following order: undisturbed soil >> reclaimed area > stockpiled topsoil > nonvegetated spoil. Microbial numbers and distribution of fungal genera were greater in the undisturbed soil and reclaimed area than in the stockpiled topsoil or nonvegetated spoil, the lowest values being registered in the nonvegetated spoil. Azotobacter was not found at any of the sites. One can draw the conclusion that stockpiling of topsoil leads to the diminution of enzymactic activity, whereas reclamation of spoils leads to an increase of their enzymatic and microbial potential. In agreement with this conclusion, Fresquez et al. (1985) found that, except for arylsulfatase, the other enzymes analyzed (dehydrogenase, nitrogenase, urease, phosphatase, amylase, cellulase, invertase, and protease) were less active in an older, 3- to 4-year topsoil stockpile than in the undisturbed soil. In February 1979, Fresquez and Lindemann (1982) initiated a greenhouse experiment to study the influence of amendments on the enzymatic and microbial parameters of the nonvegetated spoil. The experimental variants, each in four repetitions, were as follows:

1 . spoil; 2. spoil plus topsoil inoculant (224 t/ha); 3. spoil plus alfalfa hay (22.4 t/ha) plus fertilizers (336 kg of urea and 336 kg of P,O,/ha);

THE RECULTIVATION OF TECHNOGENIC SOILS

239

4. spoil plus alfalfa hay plus fertilizers plus topsoil inoculant; 5. spoil plus sewage sludge (y-irradiated sludge at a rate of 89.6 t/ha); and 6. spoil plus sewage sludge plus topsoil inoculant. The spoil and the spoil-amendment mixtures placed in pots were seeded to blue grama grass (Boutelom grucifis) and kept under favorable humidity and temperature conditions. In June 1979, nonrhizosphere spoil samples were taken for enzymatic and microbial analyses. In September 1979, the same pots were replanted to fourwing saltbush. In April 1980, rhizosphere samples were collected for microbial analyses. The analytical data indicated that dehydrogenase activity remained at the same low level in pots with spoil and in those with topsoil-inoculated spoil but increased in the other pots, the highest increase being found in pots containing spoil plus sludge plus topsoil inoculant. In this experiment, azotobacter could be detected in each pot, both in nonrhizosphere and rhizosphere samples. Comparison of potmixtures 1 and 2 showed that inoculation of spoil with stockpiled soil did not lead to greater microbial numbers in nonrhizosphere and rhizosphere samples, except in the case of fungal number, which increased significantly in the nonrhizosphere sample. Inoculation slightly increased the distribution of fungal genera, also, in the nonrhizosphere sample. In the other mixtures, as compared to those in potmixtures 1 and 2, the microbiological parameters gave higher values (except for azotobacter number in nonrhizosphere samples from mixtures 5 and 6). Under the influence of the treatment with alfalfa hay plus fertilizers, the highest increase occurred in the numbers of ammonium oxidizers and azotobacter in both nonrhizosphere and rhizosphere samples, whereas sewage sludge had the strongest effect on the increase in numbers of aerobic heterotrophic bacteria and streptomycetes in rhizosphere samples. Thus, it is evident that amendment with organic matter (alfalfa hay or sewage sludge) was more effective for increasing the enzymatic and microbial potential of spoil than was topsoil inoculation alone. At the San Juan mine, a reclamation field experiment was also carried out (Lindemann et al., 1984). In May 1979, plots were established on graded and leveled spoil. To increase water movement into the spoil, sterile bottom ash from an electrical generating plant was spread to a depth of 10 cm over the entire area and incorporated to a depth of 20 cm. Nine treatments, each in four repetitions, were applied to the spoil and bottom ash mixtures: 1 . control: unamended spoil; 2. topsoil: stockpiled topsoil at least 1 year old was applied to a depth of 30 cm;

240

S. KISS E T A L .

3. topsoil inoculum: topsoil collected from around plants in an undisturbed area and applied as a source of microorganisms, including spores of vesicular-arbuscular (VA) mycorrhizal fungi, primarily Glomus fasciculatum, at a rate of 14.5 t of topsoiVha; 4. hay: native hay (mostly grass) at a rate of 2.2 t/ha; 5. sludge: dried and y-irradiated sewage sludge at a rate of 2.2 t/ha; 6. Glomus mosseae root inoculum: sorghum roots containing G. mosseae mycelium, vesicles, and spores at a rate of 1.2 t/ha; 7. Glomus mosseae soil inoculum: soil from sorghum pots containing G. mosseae spores at a rate of 9.1 t/ha; 8. Glomus fasciculatum root inoculum: sorghum roots containing G. fasciculatum mycelium, vesicles, and spores a t a density of 0.7 t/ha; and 9. Glomus fasciculatum soil inoculum: soil from sorghum pots containing G. fasciculatum spores at a density of 14.5 t/ha.

In addition to these amendments, 34 kg/ha each of N (NH,NO,) and P,O, (triple superphosphate) was applied to each plot. The plots were planted with a mixture of native grasses and shrubs in May 1979 and replanted in July 1979. In August 1979 and May 1980, nonrhizosphere spoil (or soil) samples were taken from the 0-20-cm depth of plots 1-5 for enzymatic and microbial analyses. In March and September 1980, samples were collected from roots of fourwing saltbush (Atriplex canescens) and alkali sacaton (Sporobofus aeroides) growing on plots 1-5 for enumeration of rhizosphere dcroorganisms. In March and September 1980, the mycorrhizal formation was also evaluated in all plots. The analyses of nonrhizosphere samples indicated that dehydrogenase activity, numbers of streptomycetes and fungi, and distribution of fungal genera were higher in the hay- and sludge-amended and topsoiled plots than in the unamended or topsoil-inoculated plots. The number of ammonium oxidizers increased significantly only in the hay- and sludgeamended plots (August 1979) or in the sludge-amended plots (May 1980). The aerobic heterotrophic bacteria were least affected by treatments. It is clear from the results of this field experiment that hay or sludge amendments were more effective in increasing dehydrogenase activity and microbial parameters than was topsoil inoculation, which is in good agreement with results of the greenhouse experiment of Fresquez and Lindemann ( 1982). The analyses of rhizosphere samples showed that microbial numbers and distribution of fungal genera in the rhizosphere of A. canescens and S . aeroides were not significantly affected by any of the treatments. Azotobacter was absent in the nonrhizosphere samples and in those from the

THE RECULTIVATION OF TECHNOGENIC SOILS

24 1

A . c a n e x e n s rhizosphere, but was present in great number i n the S. aeroides rhizosphere; its growth was strongly stimulated in the organically amended and topsoiled plots. Formation of VA mycorrhizae on the planted grass species was, in general, much weaker in plots 3 and 6-9 (spoil inoculated with topsoil or with sorghum soil and roots containing mycorrhizal fungi) than in plot 2 (spoil covered with stockpiled topsoil). Practically no mycorrhizal infection occurred on plants from plots 1, 4, and 5 . Stroo and Jencks (1982) have studied 11 coal strip mine soils within 3 km of each other in Preston County, West Virginia. These mine soils were or were not excessively acid depending upon how the pyritic and nonpyritic materials were mixed when replaced. They tended to be high in sandstone fragments. The eleven mine soil sites studied were varied in age, type, and degree of plant cover and in the type of postmining treatments. Two adjacent native soils were included for comparison. Amylase, phosphatase, and urease activities and respiration (0, uptake) rate were measured in samples taken in early April 1980. The top layer (0-10 cm) was collected after removing the loose unincorporated litter at the surface. Selected physical and chemical soil properties (pH, clay, oxidizable C, total N, C:N, mineralizable N , acid extractable P and K) were also analyzed. Descriptions of the study sites and the results of the enzyme activity and respiration measurements are presented in Tables I11 and IV. One can deduce from these tables that the enzyme activities and respiration rate were generally lower in the mine soils than in the adjacent native soils. The activities and respiration recovered with time, which was attributed to organic matter and N accumulation, but these indices in a 20-year-old mine soil were still lower than in native soils. Vegetation was critical to the recovery of activities and respiration. As long as legumes were present and actively fixing N2, there seemed to be little difference between grassland and locust vegetation. The only vegetated site with activity levels as low as the barren sites was a mine soil (L-2) with a high clay content that was heavily compacted, resulting in slow organic matter and N accumulation. On the unamended locust sites, amylase and phosphatase activities and respiration rate were all significantly correlated with each other. These three indices were dependent on the levels of oxidizable C and total and mineralizable N. Significant correlations were found between amylase and phosphatase activities and mine soil age. Amylase activity was also correlated with clay content, whereas urease activity correlated only with respiration rate. There were no significant correlations between activities or respiration rate and pH or acid-extractable P or K. When all sites were considered together, amylase activity correlated

Table Ill Description of Mine Soil Sites in Preston County, West Virginiia"

Treatments

Age (years)

B- 1 B-2

17 18

Barren Barren

L- 1 L-2 L-3 L-4 L-5 L-6

9 11

Black locust, goldenrod Black locust, blackberry Black locust, tall fescue Black locust, tire cherry Black locust, blackberry Black locust, joe-pye weed

Locust mine soils Sandy loam Unamended, Clay loam Unamended, Silty clay Unamended, Clay loam Unamended, Clay loam Unamended, Clay Unamended,

Tall fescue, birdsfoot trefoil Tall fescue, orchard grass, mixed clovers Same as above

Grass-legume mine soils Sandy clay loam Unamended, planted to grass-legume mixture Limed, fertilized, topsoil replaced, planted to grass-clover mixture; Clay cut for mulch Clay Same as above

Red oak, black cherry Maize

Silt loam Silt loam

G- 1

11 17

18 20

G-2

17 3

G-3

5

N-l N-2

-

Dominant vegetation

Surface texture

Site

"From Stroo and Jencks (1982).

Barren mine soils Sandy clay loam Unamended, unplanted Limed and fertilized after mining, planted to tall fescue and birdsfoot Sandy loam trefoil; the latter died out within the first 5 years after seeding planted planted planted planted planted planted

to locust to locust; heavily compacted to locust to locust to locust to locust

Native soils Native soil, undisturbed, forest Limed, fertilized, manure added annually; continuous corn production with annual ploughing

E v1 v1

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243

Table IV Average Enzyme Activities and Respiration of Mine Soils in Preston County, West Virginia”

Site”

Amylase‘

Urease‘

Phosphatase‘

Respiration‘

0.6 a” 1.0 b 1.8 c 1.1 b 2.3 d 3.4 e 2.4 d 3.5 e 3.6 e 6.5 h 4.4 f 4.6 f

13.6 bc 13.6 bc 20.4 de 6.8 a 23.6 fg 19.3 def 17.0 cd 11.2 b 18.6 de 17.2 cd 19.1 def 17.0 cd 25.1 g

1.2 b 0.8 a 2.3 d 0.6 a 1.9 c 3.6 f 2.6 de 4.8 g 2.8 e 2.9 e 2.8 e 8.6 h

0.58 a 1.65 b 1.88 b 0.84 a 1.80 b 2.83 e 1.59 b 1.96 b 2.07 bc 5.17 g 3.53 f 2.77 cd 2.74 cd

~

B- 1 8-2 L- I L-2 L-3 L-4 L-5 L-6 G- I G-2 G-3 N- 1 N-2

5.5 g

5.1 g

“From Stroo and Jencks (1982). “See Table 111. ‘Expression of enzyme activities: amylase as pmoles of reducing sugars produced by 100 g of soil/hr; urease as pgrams of urea hydrolyzed by I g of soil/ hr; phosphatase as pmoles of p-nitrophenol produced by I g of soillhr. Respiration is given as pliters of O2 consumed by I g of soillhr. “Numbers followed by the same letter in a column are not significantly different at p = 0.05.

with urease activity, pH, and acid-extractable P. Correlations also appeared between urease activity and oxidizable C and total N , whereas respiration rate correlated only with amylase activity, pH, and P. The results obtained suggest several potential problems in reclamation. Compaction, particularly of fine-textured overburdens, should be avoided because it slows recovery. Phosphatase activity was too low in the studied mine soils, which indicates possible difficulties in phosphorus mineralization. The case of the B-2 barren mine soil suggests that the goal of recreating productive, self-sustaining ecosystems is not being achieved on such sites. For a further study, Stroo and Jencks (1985) selected the B-2 barren minesoil (see Tables 111 and IV). This acidic, infertile soil was sparsely vegetated with tall fescue (Festuca arundinacea). Different plots of this mine soil were treated in four replications with lime, fertilizers, lime plus fertilizers, and lime plus fertilizers plus sewage sludge. Limestone sufficient to raise the pH from 4.6 to 6.2 was broadcast in late February. Ammonium nitrate, triple superphosphate, and KCI sufficient to raise N , P, and K to

244

S. KISS E T A L .

average levels of 40, 34, and 66 kg per ha, respectively, were broadcast in mid-March. Fifty or 100 kg of air-dried, aerobically digested sewage sludge were added per hectare, 14 days after fertilization, as a slurry. In August, soil was sampled from top 10 cm of each plot for determining enzyme activities and respiration rate. The yield of tall fescue was also estimated. The data of Table V show that the treatments, except for liming, led to significantly increased amylase activity as compared to that in the untreated control. Urease and phosphates activities increased significantly only in the plots treated with lime plus fertilizers plus sludge applied at the higher rate. All treatments produced signif cant increases in respiration rate and tall fescue yield, this effect being strongest in the plots treated with lime, fertilizers, and sludge at the higher rate. It is evident from this study that lime, mineral fertilizers, and sewage sludge might be used profitably to restore seeded vegetation diminished over time because of acidity and low fertility in mine soil. As amylase activity was in general strongly, whereas urease and phosphatase activities were poorly, related to respiration rate, it can be concluded that respiration rate andor amylase activity might serve as a reliable indicator of microbial activity in amended mine soils, although urease and phosphatase activities appear to be of little value in evaluating mine soil microbial activity. This Table V Average Enzyme Activities, Respiration, and Tall Fescue Yield in an Amended Mine Soil in Preston County, West Virginia" Treatment

Amylaseh

Ureaseh

Phosphatase'

Respirationh

Yieldh

Control Lime Fertilizers Lime fertilizers Lime fertilizers 50 kg of sludge per ha Lime + fertilizers 100 kg of sludge per ha

26 a' 24 a 43 bc 40 b 48 cd

35.5 cd 35.5 cd 20.9 b 9.0 a 45.0 d

2.2 bc 0.6 a 1.5 b 1.9 bc 2.5 c

0.89 a 1.31 b 2.17 c 2.71 c 4.42 d

217 a 337 b 669 c 703 c 838 d

55 d

66.6 e

5.3 d

6.38 e

1043 e

+ +

+ +

"From Stroo and Jencks (1985). "Expression of enzyme activities: amylase as pmoles of reducing sugars produced by 1 kg of soil/hr; urease as milligrams of urea hydrolyzed by 1 kg of soil/hr; phosphatase as pmoles of p-nitrophenol produced by I g of soil/hr. Respiration is given as milliliters of O2 consumed by 1 kg of soiVhr. Yield is recorded as kg/ha. 'Numbers followed by the same letter in a column are not significantly different at p = 0.05.

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245

conclusion is not surprising, because only a part of the activity of these enzymes is due to the proliferating microorganisms that determine the respiration of soil: the other part is the result of the accumulated enzymes, which are independent of the momentary proliferation of soil microorganisms (Kiss et al., 1975). Persson and Funke (1986) studied, enzymologically and microbiologically, the topsoil pile R26T03, which is located on the Baukol-Noonan lignite strip mine near Center, North Dakota. The pile was formed in July 1983 and vegetative cover was established on its surface. Samples were taken from depths of 0-7.6 and 114-122 cm in May 1984 and July 1985. Alkaline phosphatase and dehydrogenase activities were found to decrease at increasing depths and to show further declines during the storage of the topsoil pile. Counts of bacteria and actinomycetes were similar in all samples, whereas the number of fungi decreased with depth but increased with storage time.

c. ENZYMOLOGICAL RESEARCH IN POLAND Gilewska and Bender (1978, 1979) carried out detailed studies of the Patnow strip mine spoil heap located in the Konin Brown Coal Basin. They have determined the cellulolytic, invertase, urease, and catalase activities in the ploughed layer (0-25 cm) of spoil plots (medium-heavy loam; pH in H,O 7.6) cultivated with spring barley (in the third year of their recultivation) and not fertilized or fertilized with NH,NO,, triple superphosphate, and potash salt in single ( 1 NPK: N, 130; P205,240; and K 2 0 , 140 kg/ha) or double (2 NPK) dosage. An adjacent arable soil that had not been affected by the strip mining was used for a control. Each activity was demonstrable, although at a low level, in the unfertilized plots and each gave higher values in the fertilized plots, particularly in those treated with the double NPK dosage. Cellulolytic activity in these plots exceeded, whereas invertase and urease activities approached, that found in the arable soil. Catalase activity in fertilized plots increased to a lesser extent, reaching only 50% of that measured in the arable soil. The barley grain yields were also higher in the fertilized spoil plots than in the unfertilized ones. The increased enzyme activities and barley yields were accompanied by the accumulation of humus and the beginning of cloddy structure formation in the ploughed layer of the spoil plots (Bender and Strzyszcz, 1978). Mineral fertilization, especially with the double NPK dosage, increased cellulolytic, invertase, urease, and catalase activities and led to humus accumulation also in those spoil plots that were cultivated with other plants (winter rape, winter wheat, alfalfa) (Bender, 1980).

246

S. KISS E T A L .

Other plots of the Patndw spoil heap as well as plots of other spoil heaps in the Konin Brown Coal Basin (Wschodnie and Goslawice) were also studied by Bender and Gilewska (1980, 1984a,b,d) and Gilewska and Bender (1983, 1984). The newly studied Patnow plots were brought into agricultural recultivation 3 or 4 years before the studies began. (The last two crops were winter wheat in some plots and alfalfa with orchard grass, Dactylis glomerata, in some others.) Both the wheat and the alfalfa-orchard grass plots included unfertilized, 1-NPK-and 2-NPK-fertilized variants. The Wschodnie plots had been recultivated for 10 years (last crop: winter wheat) and all were 1-NPK-fertilized.The Goslawice plots were 6-year-old mine spoils without any plant cover, fertilized with the smaller NPK dosage. “Raw and fresh” (unplanted and unfertilized) Patnbw spoils and an adjacent arable soil fertilized with 1 NPK and cultivated with wheat served as controls. Four activities (cellulolytic, invertase, urease, and catalase) were measured in the ploughed layer (0-25 cm) of the spoil plots and arable soil. The lowest activities were registered in the raw spoils. The main factor determining the increase of enzymatic activity in coal mine spoils proved to be mineral fertilization with a high NPK dosage. The presence of plants strongly enhanced the effect of fertilizers, which increased with recultivation time. Thus, after 10 years of recultivation the enzymatic potential of the mine spoils reached that of the arable soil. The nature of the crop plant used for recultivation had also an evident influence: e.g., invertase activity of the recultivated spoils was highest under the alfalfa-orchard grass mixture; the highest urease activity was recorded in the wheat plots. The optimum moisture content for the enzyme activity was in the 4040% range of the maximum water-holding capacity of the spoils. The recultivated spoils containing higher amounts of clay minerals showed higher catalase activity. In parallel with increasing enzyme activity, the production of plant biomass, humification of plant residues, accumulation of microbial biomass, soil respiration (CO, evolution), N, fixation, and formation of a humus layer have intensified. The C:N ratio in spoil decreased from 22-26 (raw spoil) to 15 (spoil recultivated for 10 years). Owing to the agricultural recultivation techniques applied, it became possible to transform the spoils into fertile soils in a relatively short period of time (10-15 years) (Bender and Gilewska, 1980, 1984b,d,e; Golebiowska and Bender, 1980, 1983). Bender and Gilewska (1983) have demonstrated that invertase, urease, and catalase activities are good indicators of the soil-forming processes occumng even in those Patndw spoils that had been submitted to technical recultivation only 1 year before. Samples were taken from the 0-25, 2550, 50-75, and 75-100-cm depths of 15 representative profiles. Invertase and urease activities were always highest in the 0-25-cm layer and de-

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247

creased with depth. Catalase activity of the 0-25-cm layer did not give the highest value in each profile and showed only a trend to decrease with depth. This differentiation of the enzyme activity is a noticeable symptom of the soil profile variation and indicates that the soil-forming processes are most intense in the top layer, which is due to the microorganisms penetrating the spoils from the surrounding agroecosystem. These soilforming processes are, however, limited because of the shortage of biogenic elements, especially N and P, in the spoils. Therefore, the initiated soil-forming processes would develop very slowly; the transformation of spoils into soils would take many decades. This is why the NPK fertilization and biological recultivation of spoils are required. Some enzymological aspects of the forest recultivation of strip mine spoils in the Konin Brown Coal Basin have also been studied (Bender and Gilewska, 1984c; Gilewska and Wojcik, 1984). Black locust was used as a pioneer forest plant on the slopes of the Goslawice and Nieslusz spoil heaps (medium-heavy loam and sandy loam). At the time of study, the locust plantations were 15 years old. The average tree height was 10.8 m at Goslawice and 11.6 m at Nieslusz. The stand density was medium on both spoil heaps. Black cherry (Prunus serotina), purging buckthorn (Rhamnus cathartica), golden elder (Sambucus nigra), and aspen (Populus tremula) occur sporadically in the plantations. Twenty profiles were examined; Nos. 1-10 are located on Goslawice and Nos. 11-20 on Nieslusz. The results showed that invertase activity in all profiles and urease and catalase activities in most profiles were highest in the 0-25-cm layer (see Table VI, containing the analytical data obtained in profiles 1, 8, 10, 15, and 20). In general, organic matter, total N , P,05, and K,O content were also highest in this layer, but in most cases, there was no distinct outline of the humus horizon in contrast to the agriculturally recultivated coal strip mine spoils, in which the 0-25-cm humus layer, after 10 years of recultivation, was clearly outlined. Six enzyme activities (dehydrogenase, catalase, invertase, p-glucosidase, urease, and asparaginase) were determined in the Smolnica black coal mine spoil heap (Silesia), which had been submitted to three recultivation experiments (Osmariczyk, 1980; Osmanczyk-Krasa, 1984a,b). The spoils consisted of sandstones and, mainly, of shales; pH in H,O was 4-5. In the first experiment, which began in 1973, 10 mixtures of grasses and legumes (Arrhenatherum elatius, Bromus inermis, Ductylis glomerata, Festuca capillata, F. heterophylla, F. pratensis, F. rubra, Lolium multijlorum, L. perenne, L . westerwolicum, Lotus corniculatus, and Trifolium repens) were used for revegetating spoil plots. The second and third experiments started in 1975, and the spoil plots were recultivated with four tree species: European ash (Fraxinus excelsior), red oak (Quercus rubra),

248

S. KISS ET AL. Table VI Enzyme Activities in Technogenic Soils Formed in Some Plots Planted to Black Locust on the Goslawice and Nieslusz Coal Strip Mine Spoil Heaps‘

Profile No.

Depth (cm)

Invertase”

Ureaseh

CataIaseh

I

0-5 (A,) 5-25 25-40 40-70 70-100 0-5 (A,) 5-25 25-40 40-70 70-100 0-25 25-40 40-70 70- I00 0-25 25-40 40-70 70- I00 0-5 (A,) 5-25 25-40 40-70 70-100

322 156 35 45 47 565 240 80 57 65 I I5 22 65 75 175 60 55 47 520 233 83 70 87

38.0 18.2 10.0 9.5 8.7 90.0 49.4 19.0 9.5 7.7 15.5 9.0 13.2 18.2 20.5 11.8 8.7 10.0 80.0 41.8 7.4 10.0 8.2

132 22 26 20 20 340 108 10 2 4 14 14 10 26 40 16 6 6 312 68 12 2 4

8

10

15

20

“From Gilewska and W6jcik (1984). ”Expression of enzyme activities: invertase as milligrams of “glucose” produced by 100 g of soil in 24 hr; urease as milligrams of NH,’ N produced by 100 g of soil in 3 hr; catalase as milliliters of 0, consumed by 100 g of soil in 15 min.

sycamore (Acer pseudoplatanus), and black alder (Alnus glutinosa). The enzymological analyses were carried out in 1977 and 1978 (every 6 weeks during the vegetation period). The plots submitted to recultivation with grass-legume mixtures were fertilized with 120 kg of N, 75 kg of P205,and 25 kg of K,O/ha in the first year (1973), only with N and P in the next years, and with the N dosage being reduced to 60 kgha in the third year; later no fertilizers were applied. For enzymological analyses samples were taken from the 0-10-cm layer of each spoil plot. The results show that each of the six enzymatic activities determined increased in the recultivated plots as compared to the control (unreclaimed, stored raw spoil). The increase was highest in the case of

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invertase and P-glucosidase activities. Of the 10 plant mixtures tested, that consisting of Bromus inermis, Festuca pratensis, F. rubra, Lolium perenne, and Lotus corniculatus manifested the strongest positive effect on the enzymatic potential in recultivated spoil (Osmanczyk, 1980). The plots submitted to recultivation with tree species were fertilized with 100 kg of N , 50 kg of P205,and 25 kg of K20/ha in the first year (1975), with the same N and P dosages but without K in the second and third years, and with doubled N and P dosages in the fourth year. The samples for enzyme analyses were collected from below the root system (at 30-cm depth). In the case of the control plot (not recultivated, not revegetated), the samples were taken from the 0-10-cm layer. It appears from the analytical data that the enzyme activities, except for catalase activity, increased in the recultivated plots in comparison with those measured in the control. The highest increase occurred in dehydrogenase activity. The increase was remarkable in the invertase and P-glucosidase activities also. The trees enhanced the enzymatic potential of the recultivated spoil plots in the following order: alder > sycamore = ash > oak (OsmaAczyk-Krasa, I984a). Tree seedlings were planted on spoil plots in three different ways. The first tree seedlings were planted directly into the spoil. The second ones were planted in holes made on the spoil surface and filled with about 6 liters of podzolic soil taken from pine forests. The third ones were planted in holes filled with about 6 liters of fly ash derived from a slag dump. The fertilizers applied were 100 kg of N and 50 kg of P,O,/ha in 1975 and 1976, 200 kg of N and 100 kg of P,O,/ha in 1977, and 100 kg of N and 40 kg of P,O,/ha in 1978. As in the second experiment, the samples were taken from below the root system (at 30 cm depth). Comparison of enzyme activities in the untreated, soil-treated, and flyash-treated spoils has shown that treatment of spoils with podzolic soil or fly ash led to an increased dehydrogenase activity. The effect of soil, as compared with that of the fly ash, was stronger in the plots with ash tree and oak, weaker in the plot with sycamore, and identical in the alder plot. Catalase activity also increased under the influence of soil and fly ash treatments. The increase was more pronounced in the fly-ash-treated plots than in those treated with soil, except for the alder plot. Invertase and P-glucosidase activities behaved like catalase activity. Urease activity was not influenced by treating the spoils with soil or fly ash; the only exception was the soil-treated ash tree plot, in which urease activity increased. Asparaginase activity was favorably influenced by both soil and fly ash treatments in each plot, but the effect of fly ash was stronger than that of the soil. Of the four species planted, alder caused the greatest increase in the activity of the majority of enzymes in both untreated plots and those treated with soil or fly ash. The effect of alder should be

250

S . KISS ET AL.

attributed to the N,-fixing capacity of its root nodules, which finally leads to improved nutritional conditions for microorganisms and, implicitly, to increased enzyme production. The enzyme activities were high also under ash trees planted in fly ash (Osmanczyk-Krasa, 198413). Reviewing the results of these experiments, Osmanczyk-Krasa ( 1987) emphasizes that the enzyme activities appearing in unreclaimed spoil after its 3- to 5-year storage indicate the initiation of the soil-forming process in it, while the increase in enzyme activities of the revegetated spoil proves further development of pedogenesis. D. ENZYMOLOGICAL RESEARCH IN HUNGARY In a pot experiment carried out by Sulyok and Voros (1983), refuse soils (yellow sand, yellow clay, gray clay, and andesitic tuff), resulted from the strip mining of lignite in Gyongyos-Visonta and the original topsoil (brown chernozem) were fertilized with NPK (at a rate equivalent to 309 kg of N, 189 kg of P,05, and 180 kg of K,O/ha) or with NPK plus 10 t of wheat straw per ha and sown with alfalfa. The controls were: (1) unfertilized and unsown and (2) unfertilized but sown refuse soils and topsoil. After 2 years of recultivation, invertase activity of both controls and fertilized variants was highest in the topsoil and lowest in the yellow sand. Rate of respiration (CO, evolution) and intensity of cellulose decomposition were also higher in the unfertilized and unsown topsoil than in the unfertilized and unsown refuse soils. In the case of the sown controls and fertilized variants, however, the largest amount of CO, was produced by the yellow clay. Cellulose decomposition was most intense in the yellow clay fertilized with NPK and straw, whereas the highest alfalfa yields were obtained on the yellow clay fertilized with NPK and on the gray clay fertilized with NPK and straw. The results suggest that enzyme (invertase) accumulation in the recultivated sandy refuse soil is a very slow process. In the recultivated clayey refuse soils this process is somewhat faster but not so fast as the microbial cellulose decomposition or CO, production. Therefore, enzyme accumulation should be considered a more reliable indicator of the transformation of mine spoils into soils than cellulose decomposition or CO, evolution.

E. ENZYMOLOGICAL RESEARCH IN THE FEDERAL REPUBLIC OF GERMANY Schroder et al. (1985) studied, from physical, chemical, microbiological, enzymological, and micromorphological viewpoints, seven profiles of

THE RECULTIVATION OF TECHNOGENIC SOILS

25 I

technogenic soils formed during the agricultural recultivation of spoils (loess; pH in 0.01 M CaCI, solution was 7.1-7.9) that resulted from the strip mining of brown coal in the Rhine region. The soils studied were located near Grefrath and Berrenrath. The recultivation started about 20 years ago through redeposition of stored loess either as dry material, followed by leveling (five soils), or as wet material (pumping of water-loess mixture into depressions, i.e., slurry poldering), followed by partial evaporation of water (two soils). Recultivation led to humus and K accumulation and to Na and Mg loss but due to tillage, the 30-50-cm layer of soils became compacted and impermeable. In the soils formed through redeposition of dry loess, the 50-120-cm layer also frequently became compacted. Dehydrogenase activity was detectable, in general, only from the 0-30cm layer but even in this layer, the activity reached only about 10% of that of the native soils. Respiration (CO, evolution) was more intense in the upper layers than in the deeper ones, the lowest values being recorded in the compacted layers. In each layer of the native soils, respiration rate was greater as compared with the corresponding layers of the seven technogenic soils. Similar results were obtained also in regard to cellulose decomposition. For improving the compacted soils deep loosening and drainage were recommended. In the same coal mine region, but in another locality (Gustorf near Grevenbroich), Lessmann and Kramer (1985) determined, in 1983, some enzymological and microbiological parameters in a leveled spoil (loess) recultivated with alfalfa for 2 years. For comparison they used a native, vegetated loamy riverside soil. This study site was located at Kirchhoven (at approximately 33 km from Gustorf). The Gustorf plot was never fertilized and the Kirchhoven plot was not treated with mineral fertilizers in the last 2 years. Both plots are serving as controls for an experiment on organic fertilization. The studies will continue for many years, and the organically fertilized plots will also be analyzed. The unfertilized plots differed from each other in the enzymological and microbiological parameters of their 10-20-cm layer. In this layer, the loess plot had lower dehydrogenase and urease activities, contained less bacteria, respired less strongly (produced less CO,), and degraded the cellulose more slowly than the plot of native soil. In the 3545-cm layer, there were no significant differences between the two plots. The profile differentiation as indicated by enzymological and microbiological data was reduced in the loess plot. Dealing with the problems of soil assessment in the recultivated brown coal area located in the “Niederrheinische Bucht,” Schroder (1986) compared a native soil, characteristic for this area, with two representative technogenic soils on loess, the agricultural recultivation of which began in 1970. Recultivation of one of these two soils was considered good and

252

S. KISS ET AL.

that of the other bad. Thus, in each horizon, the bulk density was lower than 1.65 g/cm3in the good recultivated soil and had a higher value in the bad recultivated soil. In the 0-40-cm layer, dehydrogenase activity, like CO, production and cellulose decomposition, showed the following order: native soil > good recultivated soil > bad recultivated soil. Haubold et al. (1987) performed a similar comparison using 15 good and 15 bad recultivated soils on loess, also from the brown coal area in the Rhine region. They found that dehydrogenase activity, microbial biomass, and cellulose decomposition in the 0-40-cm layer were, in general, about 50-100% lower in the recultivated soils than in the native soils. At the same time, the mean values of these microbial parameters and those of the chemical parameters analyzed (C, N, Na, K, Mg, and Ca contents, and cation-exchange capacity) indicated no remarkable differences between the good and bad recultivated soils. We think that this finding, although valid for dehydrogenase activity (an activity depending on the momentary proliferation of microorganisms), cannot be applied to those enzymes that are able to accumulate in soil and to be, in the accumulated state, independent of the momentary microbial proliferation. The activity of such enzymes was not assayed by these investigators. In other studies carried out in the same brown coal strip mining area, Schroder et af. (1987) and Schroder (1988) have determined, among other things, the dehydrogenase activity in the 0-40 cm layer of 13 technogenic loamy-silty loess soils formed after slurry poldering of spoils followed by their agricultural recultivation for 6-25 years, and have found that the activity was significantly higher in the older soils than in the younger ones. The C, N, K, and Ca contents, cation-exchange capacity, and microbial biomass also increased significantly over time, while the time-dependent decrease of carbonate, Na, and Mg contents and increase of cellulose decomposition were insignificant.

Ill. TECHNOGENIC SOILS FROM POWER PLANT WASTES ENZYMOLOGICAL RESEARCH IN POLAND Field experiments were carried out over 10 years to plant the ash-containing wastes of the Halemba power plant with different species. Plots fertilized and cultivated with potato, barley, field bean, and flax and an unfertilized and uncultivated plot were selected by Balicka and Wegrzyn ( 1984) for microbiological and enzymological analyses. For this purpose, the 0-10- and 10-20-cm layers were sampled two or three times yearly. Owing to cultivation, the bacterial counts increased in each plot; the

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253

highest increase occurred in the potato plot. The increase of dehydrogenase and catalase activities and of nitrification potential was similar in plots with different crop plants. It has also been found that the low biological activity in the unfertilized and uncultivated plot is the result of the lack of N ; these wastes are not toxic for soil microorganisms.

IV. TECHNOGENIC SOILS FROM RETORTED OIL SHALE

ENZYMOLOGICAL RESEARCH IN

THE UNITED STATES

Hersman and Klein (1979) studied, under laboratory conditions, the effects of retorted oil shale additions on some enzymological and microbiological characteristics of surface soils. The retorted oil shale used was produced by the Paraho process. Samples of this retorted oil shale transported to the study site (the Piceance Basin, northwestern Colorado) and samples of surface soil collected from the study site were sieved and mixed in different proportions by weight, obtaining five variants: 100% soil (control); 95% soil plus 5% shale; 90% soil plus 10% shale; 75% soil plus 25% shale; 100% shale (control). The mixtures were stored at 23-25°C with the soil-water content maintained at 10%weight and sampled and analyzed at 2-week intervals over a 10-week period. The results showed that retorted shale additions caused, for all sampling times, significant decreases in acetylene reduction (N, fixation) rate, dehydrogenase activity, ATP content, mineralization rate of uniformly I4Clabeled glucose, respiration (0,uptake) rate, and fungal counts, although counts of bacteria and actinomycetes were not significantly affected by the presence of up to 25% retorted shale. In another experiment (Klein et al., 1982) retorted oil shale produced by the Lurgi process was used. Its addition to soil samples in 5, 10, and 25% proportions also brought about significant diminutions in dehydrogenase and phosphatase activities as compared to those in the control soil. These results have a great importance for the revegetation programs in areas disturbed by oil shale processing. If retorted oil shales are covered with stored surface soils, it may be necessary to insure that little physical mixing of the surface soil and retorted oil shale occurs. It may even be necessary to consider the construction of capillary barriers below the surface soil column to minimize upward movement of soluble fractions from the retorted oil shale. These suggestions were verified by Sorensen et al. (1981) and Klein et af.(1982, 1985), under field conditions using experimental plots installed

254

S. KISS ETAL.

in the Piceance Basin in the fall of 1977 and designed to allow the study of vegetation succession on surface soil (a fine loam) of various depths (thicknesses) overlying Paraho retorted oil shale and on surface soil separated from retorted shale by a gravel barrier. The profile configuration for five panels included panels with soil depths of 30,61, and 91 cm over compacted shale, a capillary barrier panel (61 cm of soil over 30 cm of fine and coarse gravel), and a control panel (soil). The surface of each panel was divided into three replicate plots. Within each of these plots nine possible combinations of three seed mixtures (native species, introduced species, and native plus introduced species) and three fertilization rates (1 12 kg of N plus 56 kg of P per ha, 56 kg of N plus 28 kg of P per ha, and no N and P) were applied randomly to subplots. In the summers of 1979, 1980, and 1981, samples were taken from the 5-10-cm depth after removal of the 0-5-cm layer. The samples were analyzed to determine their acetylene reduction, dehydrogenase, and phosphatase activities. The average yearly activities across the panels from 1979 to 1981 are shown in Fig. 1, from which one can see that over the 3 years acetylene reduction decreased to a large extent in all experimental variants. In each year, this activity was most intense in the control soil and weakest in the 30- or 61-cm deep soil over retorted shale. In 1979, this activity in soil overlying the capillary barrier was not significantly lower than that in the control soil.

h

-

C D E 1981

1979 1980 FIG.1. (a) Acetylene reduction, (b) potential dehydrogenase, and (c) phosphatase activities of soil of various depths over retorted oil shale and capillary banier. The error bars indicate least significant differences between means. A, control; B, capillary barrier; C, 91 cm; D, 61 cm; E, 30 cm. (Redrawn from Klein et al., 1985.)

THE RECULTIVATION OF TECHNOGENIC SOILS

255

Potential (zymogenous) dehydrogenase activity and phosphatase activity behaved contrarily over the 3-year period: potential dehydrogenase activity increased, whereas phosphatase activity showed a consistent downward trend since 1979 in all variants. In each year, both activities had the highest values in the control soil and in the soil overlying the capillary barrier, whereas the soil 30 cm deep over shale was, in general, the least active. The acetylene reduction data manifested a significant interaction

A B C D E

t B C D E

1979

1980

A B C D E

1979

A B C D E

I C D E

1981

A B C D E

1980 1981 Panel treatment and year FIG. 1. (conrinued)

256

S. KISS E T A L .

between seed mixture (or the ensuing plant community) and fertilization in 1979, although in 1980 and 1981 only a simple effect of fertilizer on acetylene reduction occurred. In 198 1, potential dehydrogenase activity showed a significant three-way interaction between seed mixture, fertilization, and soil-shale arrangements. In the case of phosphatase activity, a significant interaction between seed mixture, fertilization, and soil depth was observed when the activity values from 1979, 1980, and 1981 were analyzed together. Measurements of percentage mean plant cover over the 3-year period in the control plot, corresponding capillary barrier plot with 61 cm of soil, and plots with 91, 61, and 30 cm of soil directly over shale indicated no significant decrease in percentage mean plant cover when comparing the control plot with the plot with only 30 cm of soil. These measurements also suggested an improved plant development in the plot with the capillary barrier. In comparison, acetylene reduction, potential dehydrogenase, and phosphatase activities decreased to a greater extent than the corresponding values for percentage mean plant cover on the plots with less surface soil overlying the retorted shale. Thus, this field study confirms the laboratory results of Hersman and Klein ( 1979). Appreciable diminution of microbial and enzymatic activities occurred in soil up to 91 cm in depth when it was placed directly over retorted oil shale. Suppression of these activities in such soils might lead to a long-term reduction in productivity. A capillary barrier composed of fine and coarse gravel helped maintain enzyme activities in soils placed over retorted oil shale during revegetation. In 1981, the retorted shale over which soil had been placed was also analyzed enzymologically . Potential dehydrogenase and phosphatase activities were essentially absent in the shale. In contrast, actual dehydrogenase activity in the shale showed values equivalent to those found in the control soil (Klein et al., 1982), but the possibility that this activity was due to nonenzymatic factors is not excluded. Also in 1981, Klein ef al. (1982) analyzed enzymologically a stored topsoil pile. The planted north end of the soil pile in comparison with its unplanted south end manifested higher dehydrogenase and phosphatase activities. Contrarily, numbers of actinomycetes, fungi, and bacteria and the soil water content were higher in the unplanted south end of the pile. In several industry-constructed reclamation plots in which the surface soil was placed over shale materials retorted by different processes (TOSCO 11, USBM, Union B, Union Decarbonized), dehydrogenase activity decreased with soil depth and the shale-to-surface had the lowest activity. The TOSCO I1 shale-to-surface was less dehydrogenase-active than the USBM shale-to-surface. The plots were constructed in western Colorado during 1970-1975 and analyzed enzymologically in 1981 (Klein et al., 1982, 1985).

THE RECULTIVATION OF TECHNOGENIC SOILS

257

V. TECHNOGENIC SOILS FROM IRON MINE SPOILS A. ENZYMOLOGICAL RESEARCHIN THE USSR Catalase and invertase activities in the revegetated spoil heaps around the Lebedin strip mine (located in the Starooskol iron ore zone within the Kursk Magnetic Anomaly region) were determined by Sviridova and Panozishvili (1979). The spoil heaps here are of three types: sandy, loamy, and cretaceous-marly. They were revegetated with spontaneous and introduced grasses or with forest plants (sea buckthorn, black locust, and oleaster). After 8-10 years the heaps revegetated with grasses were covered with a 1.5-2-cm sod, protecting them against erosion. In the 8-10year-old forest stands the mass of litter reached densities of 0.5-0.9 t/ha. Catalase activity was measurable in the soil of all revegetated heaps. This activity, like respiration (CO, evolution) and humus accumulation, was more intense under herbaceous vegetation than under forest. Invertase activity was influenced not only by the nature of vegetation but also by the nature of spoil heaps. This activity was highest in the rhizosphere of sea buckthorn growing on the cretaceous-marly heap. Zasorina (1985a,b) has studied, enzymologically, the spoil heaps near the Stoilensk iron strip mine (located within the Kursk Magnetic Anomaly region). These spoil heaps are of two types: sandy and cretaceous. The age of natural vegetation growing on heaps varied between 3 and 20 years. Invertase, urease, and catalase activities in the 0-5-cm layer of spoil heaps increased in parallel with the age of vegetation. The increase was more pronounced in the sandy spoils than in the cretaceous one. During the growing season, the maximum activity values were registered in midsummer. In both young and old spoil heaps, after their revegetation with a mixture of six grasses and legumes (bromegrass, fescue, wheatgrass, alfalfa, clover, and sainfoin), the enzymatic activities increased l .5-3 times.

B. ENZYMOLOGICAL RESEARCHIN ROMANIA Blaga et al. (1981) compared dehydrogenase, catalase, and invertase activities in spoils (calcareous sandy loams or clays; pH in H,O was 7.78.3) leveled with the aim of their agricultural recultivation in the northern zone of the iron strip mine in C%pus(Cluj district) and in adjacent soils. In the soils the activities decreased with depth (0-70 or 0-80 cm), whereas in the spoils the activities were approximately the same in the 0-20- and 50-80-cm layers. In the 0-20-cm layer each activity was many times lower in the spoils than in the soils. In the 50-80-cm layer the differences between spoils and soils in dehydrogenaseactivity were great, but differences

258

S. KISS ET A L .

were not so pronounced in the case of their catalase and invertase activities. In another study carried out in the same zone by Bunescu and Blaga (1980), similar results (i.e., very low and higher activities, respectively) were registered in different spoil and soil profiles, except for a spoil profile that showed relatively high dehydrogenase and catalase activities. The activities correlated with the total N content of spoils and soils. On the leveled spoils in the southern zone of the CBpus iron strip mine, recultivation plots were installed. Some plots were recultivated with sainfoin (Onobrychis viciaefolia) and others with orchard grass (Dactylis glomerata). After 3 years of recultivation the spoils of these plots were analyzed enzymologically by Drggan-Bularda et al. (1983). For comparison, the 0-15-cm layer of an adjacent native soil (rendzina) and the same layer of a spoil plot not submitted to recultivation were also analyzed. Some of the results are presented in Fig. 2. They show that recultivation led to increased enzyme activities in iron mine spoils during their transformation into technogenic soils. Potential dehydrogenase activity increased to a lesser extent than phosphatase activity, which reached values similar to that of the native soil. Both activities were higher in the sainfoin plots than in those recultivated with orchard grass. In each case, the 020-cm layer was more active than the 2040-cm one. In the Satra zone of the Cilpus iron strip mine, Blaga et al. (1984) recorded very low values of dehydrogenase, catalase, and invertase activities in the 0-20- and 50-80-cm layers of three profiles of spoils leveled for their recultivation. In the fall of 1985, the ninth year of a fertilization and crop rotation experiment on the southern zone of the CBpus iron strip mine, samples were taken for enzymological analyses from the 5-20-cm depth of unfertilized, farmyard-manured (40 t/ha), lightly or heavily NPK-fertilized ( 100 kg of N as NH,NO, plus 60 kg of P as simple superphosphate plus 40 kg of K as potash salt or 300 kg of N plus 180 kg of P plus 120 kg of K/ha, respectively), and complexly fertilized (40 t of farmyard manure plus 100 kg of N plus 60 kg of P plus 40 kg of K/ha) spoil plots planted in maize, oats, or sainfoin. The enzymatic and nonenzymatic catalytic potential (actual and potential dehydrogenase, invertase, phosphatase, urease, and nonenzymatic H,O,-splitting capacity), like the yield of crops, was highest in the complexly fertilized spoil plots and lowest in the unfertilized ones. Significant correlations were found between invertase activity and corn yield and between phosphatase activity and sainfoin yield. But under the influence of long-term fertilization, the crop production capacity of the studied technogenic soil increased to a larger extent than its biological potential reflected by its enzymatic activities. This means that long-term fertilization is able to greatly enhance the crop production capacity of the

T H E RECULTIVATION OF TECHNOGENIC SOILS

259

I 2 RG.2. Potential dehydrogenase and phosphatase activities in recultivated iron strip mine spoils. (I)Adjacent native soil; (2) spoil not submitted to recultivation; (3) spoil recultivated with sainfoin (depth: 0-20 cm); (4) spoil recultivated with sainfoin (depth: 2 0 4 0 cm); ( 5 ) spoil recultivated with orchard grass (depth: 0-20 cm); (6) spoil recultivated with orchard grass (depth: 20-40 cm). (Redrawn from Dragan-Bularda et a/.,1983.) 1

2

3

4

5

6

0

technogenic soil, but the increasing effect of fertilization on the biological potential of the technogenic soil, as reflected by its enzymatic activities, is the result of much slower processes (Dragan-Bularda et al., 1987).

VI. TECHNOGENIC SOILS FROM MANGANESE MINE SPOILS ENZYMOLOGICAL RESEARCHI N

THE

USSR

According to Keleberda (1973), the 0-20-cm layer of the spoil heap (consisting of medium loams) at the Aleksandrov manganese quarry (Dnepropetrovsk region, Ukraine) contains more humus and shows higher H,O,-splitting (enzymatic plus nonenzymatic) capacity than the 20-40-cm

260

S . KISS E T A L .

layer. In another study, carried out in the same area, Keleberda (1978) has found that recultivation of spoil plots with oleaster for 11 years resulted in the formation of a technogenic soil with increased humus and N contents and invertase, urease, and proteinase activities in the 0-5-, 5-lo-, and 1020-cm layers as compared with the uncultivated control plot. At the same manganese quarry, Uzbek (1986) determined several enzyme activities in different layers of a 20-year-old spontaneously revegetated spoil plot, now covered with a stable phytocoenosis made up of meadowgrass (Poa angustifolia) and sagebrush (Artemisia austriaca) and found that the activities were much higher (catalase 1.5, phosphatase 13, urease 36, invertase 46, and dehydrogenase 72 times) in the 1-cm-deep surface layer rich in roots than at a 6-cm depth and in deeper layers of the spoil plot. Microbial counts as well as humus, total N, mobile P, and exchangeable K contents were also highest in the surface layer. The enzymological properties of the recultivated strip mine spoils in the Chiatura manganese ore zone (situated in the Kvirila Basin, western Georgia) were described by Daraseliya and Kalatozova (1973, 1976) and Daraseliya (1979). The experimental variants were the following: (1) adjacent native (brown forest) soil; (2) mine spoils without plants; (3) perennial grass with legume (ryegrass-alfalfa) on mine spoils; (4) grapevines on mine spoils; and (5) grapevines on stored topsoil (40-45-cm layer) reapplied on mine spoils. The mean values of the analytical data over 3 years (1969-1971) indicated that the native soil had much higher enzyme activities than the uncultivated spoils (calcareous sands and clays; pH in H,O was 8.4). The activities increased in the recultivated spoils but did not reach the values registered in the native soil. The perennial grasslegume mixture was more efficient than grapevines. Reapplying the stored topsoil on the surface of spoils had beneficial effects on the accumulation of enzymes (invertase, phosphatase) in the grapevine plots. The 0-20-cm layer was more active and richer in humus and microorganisms than the 2040-cm one. No relationship was found between catalase activity in spoils and the recultivation measures applied.

VII. TECHNOGENIC SOILS FROM LEAD AND ZINC MINE WASTES A. ENZYMOLOGICAL RESEARCH IN

THE

UNITED KINGDOM

Studying the decomposition of vegetation growing on metal mine waste, Williams et al. (1977) also carried out enzymological analyses. The waste studied was located around the disused mine at Y Fan (Powys, Wales)

THE RECULTIVATION OF TECHNOGENIC SOILS

26 1

and contained high concentrations of lead and zinc. After the abandonment of the mine (1928), the waste was partially colonized naturally by metaltolerant Agrostis tenuis. An evenly colonized area was selected for study. A similar but uncontaminated area was also selected on a pasture situated about 500 m from the mine. The vegetation on this site consisted primarily of A . tenuis and Festuca ovina. Urease activity in soil, microbial populations in litter, and soil and microfauna in litter from both sites were compared. Accumulation of litter was greater on the waste, which also contained significantly less humic and fulvic acids in the soil immediately beneath the litter layer. Urease activity was also significantly lower in the mine soil than in the nearby pasture soil (Table VII). Microbial counts from litter at the two sites were not markedly different, although numbers of fungi were lower on litter from the mine waste, while those of bacteria and actinomycetes were higher. In contrast, counts of all groups in the mine soil were considerably lower than those in the pasture soil. Similarly, there were fewer animals in the litter on the waste. The low biological activity in the litter and soil of the studied mine waste, caused by the high Pb and Zn concentrations, explains the retarded decomposition of vegetation growing on this site. Clark and Clark (1981) have applied soil-enzymological methods, in order to determine the reasons for the differences in the floras of adjacent species-poor and species-rich areas of a limestone terrace in the lead-mining complex on Grassington Moor, in the Yorkshire Pennines (England). The northern half of the terrace received drainage water and fine-textured, Pb- and Zn-containing mine waste from abandoned mine workings up slope, and the vegetation there was sparse, floristically impoverished, and composed of species typical of heavy-metal mine areas in the British

Table VIl Urease Activity in a Mine Soil and a Pasture Soil at Y Fan, Powys, Wales"

mg of NH,' N released from 100 g of soil (on air-dry basis) at 37" C in 3 hr

Reaction mixture Soil Soil

+ urea solution + water

Urease activity ~~

Mine soil'

Pasture soil'

2.55 2 0.14 1.97 t 0.25

42.62 t 2.38 3.32 2 0.58

0.58

39.31

2

0.19

?

2.17

~

"From Williams p t ul. (1977). 'Means of two soils significantly different at p

=

0.05.

262

S. KISS ETAL.

Isles, i.e., Minuartia verna, Agrostis tenuis, and Festuca ovina. There was no direct input of mine waste on the southern half of the terrace, and there the vegetation was floristically rich and continuous, except where the limestone cropped out. The mean number of species per 0.25 m2 on the species-poor area was 2.4, in contrast to the species-rich area, where it was 10.1. The soils of the species-poor area had lower pH values and contained less humus, N03--N, NH,+-N, available P, and exchangeable K, compared with those of the species-rich area. The total lead content averaged in the soils of the species-poor and species-rich areas 78,000 and 8,000 pg/g of soil, respectively, far above the 350 pg/g threshold value above which lead levels are anomalously high. The soils of both these areas would therefore normally be expected to be toxic to all but tolerant races. The average level of ammonium acetate-extractable Pb was 21,800 pg/g of soil on the species-poor area and only 311 pg/g of soil on the other area. Zinc levels were mainly lower than those of lead on both areas and the difference in the levels of total and ammonium acetate-extractable Zn between the areas was less marked than for Pb. Acid phosphatase, dehydrogenase, and urease activities and respiration (CO, evolution) were measured in soil samples taken in the root zone, 29 cm below the surface. When expressed on an air-dry soil basis, they were higher in the species-rich soil. However, when expressed on an airdry organic matter basis, the differences were reduced or eliminated (Table VIII). In each half of the terrace there were significant correlations between density of species, amounts of plant nutrients, and enzyme activities, and all were related inversely to the levels of extractable lead. The conclusion has been drawn that nutrient enrichment is involved in the formation of the species-rich area on Grassington Moor; the higher enzyme activities in the species-rich area indicated that metal detoxification was taking place there, and the higher organic matter content of this area is related to the enzyme activities. B. ENZYMOLOGICAL RESEARCHIN ROMANIA The raw and the revegetated wastes at the Sgsar mine (Baia Mare, Maramures district), the ores of which contain Pb and Zn as well as Cu, Cd, and some other heavy metals, were studied enzymologically by Soreanu (1983). An adjacent native meadow soil served for comparison. The revegetation experiment started in 1975 and comprised unfertilized and NPKfertilized mine waste plots seeded with a mixture of perennial grasses and legumes or with individual grass and legume species or sunflower. In 1980, samples were taken from the 0-20-cm depth of each plot and native soil

263

THE RECULTIVATION OF TECHNOGENIC SOILS Table VIII

Enzyme Activities and Respiration in Soils from a Limestone Terrace Contaminated by Pb- and ZnContaining Mine Waste on Grassington Maor, Yorkshire" Area Species-poor (SP) Activity or respiration

Acid phosphatase (pg of p-nitrophenol) Dehydrogenase (pg of triphenylformazan) Urease (mg of urea) Respiration at current moisture content' (mg of C) Respiration at field capacity (mg of C)

Sb 9.9 (2.7)" 38.3 (2 I .5) I .5 (0.4) 0.085'

0.067

OM' 67.4 260.4

-

Species-rich (SR) Sh

18.1 (2.2) 2503.O

10.0 0.578

6.9 (2.7) 0.093

0.456

0.090

OM'

Ratio (SWSP) S'

OM'

1.8

1.2

65.4

42.3

30.4 0.409

4.6

3.0

1.1

0.7

0.396

1.3

Ok9

79.5 11026.4

"From Clark and Clark (1981). 'Activity or respiration registered in 1 g of air-dry soil in 24 hr. 'Activity or respiration reported for I g of air-dry organic matter in 24 hr. dFor activity values, standard deviations are given in parentheses. 'For respiration values, the least significant difference is 0.016 at p < 0.05. 'Considerably less than field capacity.

for determining invertase, dehydrogenase, phosphatase, and urease activities in wastes and soil, respectively. It was found that revegetation caused an increase in each activity as compared with those measured in the raw waste, but except for urease activity, the other activities did not reach the values recorded in the native soil. The fertilized plots revegetated with the grass-legume mixture gave the best results in respect of plant cover percentage, herbage yield, and enzyme activities of wastes.

VIII. TECHNOGENIC SOILS FROM SULFUR MINE SPOILS ENZYMOLOGICAL RESEARCHIN THE USSR Peterson et al. (1976, 1979) determined the actual dehydrogenase and catalase activities, counts of microorganisms, and respiration (COz evolution) in sulfur strip mine spoils (the Podorozhnen mine, Rozdol, Ukraine), submitted to agricultural recultivation. Spring wheat, pea, a vetch-oats

264

S. KISS E T A L .

mixture, spring barley, sweetclover, and trefoil were used for recultivation in unfertilized and NPK-fertilized plots. Unfertilized spoil heaps (clays and sandy loams; pH in KCI solution was 4.6-5.6) under ruderal vegetation served as controls. The published analytical data were obtained with spoil samples collected in spring, summer, and fall during the first 3 years of recultivation (19761976). In the first year, dehydrogenase activity was lacking in the control heaps. In the recultivated plots, the activity was measurable in samples collected in summer. In the second and third years, the activity in control heaps appeared in summer and fall, whereas it was present in the recultivated plots in spring also, the highest values being found in summer. In general, the fertilizers applied in spring caused a decrease of activity in spring and an increase in summer and fall. Of the crop plants tested, sweetclover gave the best results in increasing the dehydrogenase activity of spoils. This activity was strongly related to the number of heterotrophic microorganisms growing on starch-ammonium-agar and of the oligotrophic ones growing on soil extract-agar. There was no significant correlation between dehydrogenase activity and respiration rate of the spoils and between their catalase activity and plant species or fertilization rate.

IX. TECHNOGENIC SOILS FROM LIME AND DOLOMITE MINE SPOILS ENZYMOLOGICAL

RESEARCHIN

THE

USSR

Keleberda (1973) collected samples from the G20- and 2 W - c m depths of the spoil heaps at the Ol’gin lime and dolomite quarry (Novotroitsk, Donetsk region, Ukraine). The spoils are sandy loams mixed with lime and dolomite in form of rubbles. Both humus content and H,O,-splitting (enzymatic plus nonenzymatic) capacity were higher in the upper than in the deeper layer.

X. TECHNOGENIC SOILS FROM REFRACTORY CLAY MINE SPOILS

ENZYMOLOGICAL RESEARCHIN

THE

USSR

Keleberda and Dan’ko (1975) studied the Dneprov spoil heap that resulted from the strip mining of the Chasov-Yar refractory clays (Donetsk region). The spoils are loamy sands. This spoil heap was recultivated with sweetclover (Melilotus volgicus) as a green manure plant. Uncultivated

T H E RECULTIVATION OF TECHNOGENIC SOILS

265

plots served as controls. It was found that invertase, urease, and catalase activities and respiration (CO, evolution), like humus and total N contents, increased significantly in the 0-5- and 10-20-cm layers of the recultivated spoil heap as compared with the control plots. Invertase activity of the 20-30-cm layer was also higher in the sweetclover plots than in the controls (Keleberda, 1976), and proteinase activity also increased in the top layer of the sweetclover plots (Keleberda, 1977). Afforestation of some spoil plots in this area was performed with black locust and oleaster. It has been established (Keleberda, 1978) that after 11 years the spoil was transformed into a primitive soil, characterized by increased humus and N contents and invertase, urease, and catalase activities in its 0-20-cm layer as compared with the uncultivated control plot (Table IX). In the same area, Keleberda (1979) has also studied the influence of black locust on the development of other tree species: green ash (Fraxinus viridis), small-leaf linden (Tifiu corduta), and elm (Ufmuspinnato-rumosa). When these species were planted in rows having contact with locust, they developed better, even in the first years of their plantation, than the plants having no contact with locust. Their better development was accompanied by increased invertase, urease, and proteinase activities (Table X); humus

Table IX Humus and N Contents and Enzyme Activities in Primitive Soils under Forest Vegetation Developed on a Spoil Heap Resulting from the Strip Mining of the Chasov Yar Refractory Clays"

Plant stand and its age Black locust ( I I years)

Depth (cm)

Humus

N

N in humus

(%)

(%)

(%)

Invertase'

Ureaseh

Proteinaseh

0-5 5-10

2.23 0.87 0.60 0.54 2.61 1.03 0.46 0.33 0.98 0.67 0.57 0.49

0.23 0.11 0.04 0.03 0.41 0.17 0.05 0.03 0.08 0.06 0.02 0.02

10.3 12.6 6.6 5.4 15.7 17.0 10.8 9.0 8.1 8.9 3.5 4.0

26.40 10.00 7.15 4.20 39.30 16.70 8.31 2.32 10.80 5.30 3.25 0.20

2.02 0.82 0.49 0.42 3.92

0.76 0.35 0.26 0.20 I .09 0.81 0.22 0. I4 0.36 0.15 0.04 0

10-20 20-30

Oleaster ( I I years)

0-5 5-10 10-20 20-30

Control (uncultivated)

0-5 5-10 10-20 20-30

1.60

0.77 0.73 I .06 0.57 0.34 0.16

"From Keleberda (1978). "Invertase activity is expressed as milligrams of inverted sugar, urease activity as milligrams of NH,' N, and proteinase activity as milligrams of NH, N produced by I g of soil in 40, 40, and 72 hr, respectively.

266

S. KISS E T A L . Table X Enzyme Activities in Technogenic Soils Resulting from the Recultivation of the Chasov Yar Refractory Clay Strip Mine Spoils“

Contact with black locust

Depth (cm)

Green ash

t

Small-leaf linden

+

0-5 5-10 10-20 0-5 5-10 10-20 0-5

Tree species

5-10 -

+

Elm

10-20 0-5 5-10 10-20 0-5 5-10 10-20 0-5 5-10

Black locust

Pure stand

10-20 0-5 5-10

10-20

Invertaseh

Urease”

Proteinase”

24.8 5.1 1.3 2.9 2.3 2.1 28.9 5.5 5.4 17.9 7.2 6.3 21.9 14.7 11.7 19.8 10.0 9.6 26.4 10.0 7.2

I .97 0.78 0.52 0.72 0.56 0.47 1.79 0.66 0.63 1.10 0.64 0.64 I .79 0.71 0.65 1.14 0.67 0.63 I .89 0.74 0.50

0.69 0.32 0.09 0.02 0.07 0.02 0.68 0.32 0.24 0.51 0.10 0.03 0.68 0.3 I 0.09 0.45 0.33 0.09 0.76 0.26 0.35

“From Keleberda (1979). ”Invertase activity is expressed as milligrams of inverted sugar, urease activity as milligrams of NH,’ N, and proteinase activity as milligrams of NH2 N produced by 1 g of soil in 40, 42, and 72 hr, respectively.

levels; amounts of total and hydrolyzable N ; and mobile P and K contents of their soils (especially in the 0-5-cm layer).

XI. TECHNOGENIC SOILS FROM BENTONITIC CLAY MINE SPOILS

ENZYMOLOGICAL RESEARCHIN

THE

USSR

Daraseliya et al. (1978) applied enzymological and microbiological methods to evaluate the efficiency of the recultivation of spoils that resulted from the strip mining of bentonitic clays (gumbrine, askanite, etc.) at Gumbra (Tskhaltubo district, Georgia). The spoil heaps were reculti-

THE RECULTIVATION OF TECHNOGENIC SOILS

267

vated with maize (unfertilized or fertilized with 300 kg of N, 240 kg of P, and 90 kg of m a ) or with a ryegrass-alfalfa mixture. Naturally revegetated spoil heaps and an adjacent native yellow soil under forest cover were used for comparison. The results have shown that the spoils, in comparison with the original soil, are characterized by lower invertase and phosphatase activities, which are related to the reduced humus content of spoils. Owing to recultivation, the activities increased. Fertilization of the spoil heaps recultivated with maize had a beneficial effect on the enzyme activities. The highest activities were found in the spoil heaps recultivated with the ryegrass-alfalfa mixture. The activities in the 0-20-cm layer of these spoil heaps approached those measured in the native soil. In each variant, the 0-20-cm layer was more active than the 20-40-cm one. Recultivation also led to substantial increases in the number of the main groups of microorganisms in the spoil heaps. Studying the same spoil heaps, Rtskhiladze et al. (1981) found that dehydrogenase activity behaved like invertase activity.

XII. TECHNOGENIC SOILS ON SAND OPENCAST MINE FLOOR DRIFT AND SPOILS A. ENZYMOLOGICAL RESEARCHIN POLAND Hazuk (1%7) was the first to utilize enzymological methods for studying the recultivation of the floor drift of sand opencasts formerly used for the mining of filling sand (necessary for the hydraulic filling of the workings in coal mines). The study area, the Szczakova filling sand quarry, is situated in the western part of the Little Bledowska Desert (Cracow region). The alluvial “soil” of this former sand opencast consists of deep (thick), medium-grain loose sands (pH in H,O was 8.3). Only the top layer (1-10 cm) of these sands contains nutrients ( P , K), but in very small amounts. For recultivation, a fertilization experiment was carried out, in five variants. It has been established that fertilization with a sorbent based on bentonite and NPK without or with the addition of peat led to increased enzyme activities in the “soil”: invertase, urease, and asparaginase activities approximately doubled, whereas catalase activity increased twofour times as compared with the unfertilized control. These investigations were continued, developed, and described in detail by Greszta and Olszowski (1974). They studied seven experimental variants (each in four plots): 1. controls (no fertilization); 2. mineral fertilizers;

268

S. KISS E T A L .

3. humus (60 t/ha) plus mineral fertilizers; 4. sorbent fertilizer (30 t/ha); 5 . sorbent fertilizer (60 t/ha); 6. sorbent fertilizer (60 t/ha) plus peat dust (12 t/ha); and 7. cinders (60 t/ha plus mineral fertilizers). The dosage of mineral fertilizers in variants 2, 3, and 7 was: N 20, P,O, 65, and K,O 60 kg/ha. The humus used came from the caprock of the sand pit hole and consisted of the material from the humus-mineral horizon mixed with forest leafmold. The sorbent was produced from bentonite containing 73-74% clayey components (montmorillonite, kaolin, illite). The sorbent fertilizer consisted of bentonite to which lime was added in the proportion 1 part of lime per each 20 parts of clay. It was subsequently mixed with mineral fertilizers (N 4, P,O, 16.2, and K,O 24 kg/ha). The cinders came from a power plant. Following these treatments, a seed mixture of perennial plants, predominantly legumes, were sown in all the plots. Prior to sowing, the seeds of leguminous plants were treated with “Nitragina” (specific Rhizobium culture). The mixture consisted of: Lupinus luteus 100, Lupinus polyphyllus 10, Melilotus albus 10, Lotus corniculatus 5 , Trifolium repens 5, Anthyllis vulneraria 3, Festuca ovina 1, and Bromus inermis 1 kg/ha. For 2 years (1966-1967), soil samples were collected throughout the vegetative season at 3-week intervals for the determination of enzyme activities (invertase, P-glucosidase, urease, asparaginase, and catalase) and respiration (CO, evolution) rate. Counts of bacteria, actinomycetes, and fungi as well as several physical and chemical properties were also determined periodically. The results have shown that the activity of enzymes was highest in the soil of the plots with a sorbent fertilizer, especially when this was applied with added peat. In plots fertilized with cinders the activity recorded during the first year did not differ from the activity values determined from plots treated with the sorbent fertilizer, but it appeared to be lower in the second year, showing a tendency to decrease steadily. Treatment with humus containing an addition of mineral fertilizers and with mineral fertilizers alone did not cause any significant change in the activity of enzymes. The highest enzyme activities were found in the 1-5-cm layer. At the 6-10-cm depth a marked decrease in activities was observed. The results obtained for the plots with the sorbent fertilizer containing added peat indicate that it is advisable to combine mineral and organic fertilization. The results also indicate that there exists a causative relationship between the activity of soil enzymes and the herbage yield of the recultivated

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269

plots. Thus, the highest enzyme activities and the best yields were recorded for plots treated with sorbent fertilizer containing added peat. This correlation seems to be the result of a stimulating effect of the applied fertilizer treatment on the metabolism of soil microorganisms expressed by the activity of the enzymes studied, due to which the growth of the plants was better. Another possible explanation is that the sorbent fertilizer retained more available nutrients, so the plants grew better and their residues stimulated the microorganisms. The highest rate of CO, evohtion and the largest number of microorganisms, especially bacteria and fungi, were recorded also in the soil of plots treated with the sorbent fertilizer containing an addition of peat. In a recultivation experiment started in 1977 on the area of an exhausted sand mine in Cheszczdwka, dehydrogenase and urease activities in the technogenic soil formed were analyzed in 1984 and it was found (Zukowska-Wieszczek et al., 1985) that the enzyme activities like the biomass of grasses were highest in the experimental variants treated with clay plus sewage sludge, fly ash plus sewage sludge, and waste fungal mycelia from pharmaceutical plants. The variants treated with sewage sludge alone, municipal refuse, and compost were less efficient.

B. ENZYMOLOGICAL RESEARCHIN THE USSR Enzymological and microbiological study of the spoils resulted from the opencast mining of quartz sand in the Chiatura district (western Georgia) and recultivated with grapevine, fruit trees, forest trees, and perennial grasses has proved (Rtskhiladze et af., 1981) that dehydrogenase and invertase activities and counts of microorganisms from different physiological groups can be used as indicators of the efficiency of the recultivation measures applied.

XIII. TECHNOGENIC SOILS FROM OVERBURDENS REMAINING AFTER PIPELINE CONSTRUCTION A. ENZYMOLOGICAL RESEARCHIN THE USSR The enzymological study of the recultivation of overburdens remaining after pipeline construction was initiated by ldrisova (1984). In 1981, she modeled, in laboratory and field, a recultivation technology based on the

270

S. KISS E T A L .

spreading of overburdens on the surface of adjacent agricultural fields, in different proportions: overburdens (calcareous loam) remaining after the construction of 100 m of pipeline (diameter 1420 mm) were spread on 0.5, 1, 1.5, and 2 ha of agricultural field (leached chernozem on the foreststeppe of the Bashkir Pre-Urals). The control field was not treated with overburdens. Analysis of the ploughed (0-30-cm) layer of soil showed that catalase activity was practically unaffected by overburdens. In contrast, invertase and phosphatase activities and respiration (CO, evolution) rate of soil decreased in parallel with the diminution of the field surface on which the overburdens were spread. Thus, the maximum values were found in the control soil and the minimum ones in the field with the smallest surface (0.5 ha) on which the overburdens were broadcast. The contents of humus, mineral N, mobile P, and exchangeable K in soil as well as crop yields (in 1981: vetch-oats mixture; in 1982: winter rye) decreased in a similar manner. The decrease in crop yields was lower in the plots fertilized with 40 t of farmyard manure plus 90 kg of N as NH,NO, plus 60 kg of P as double superphosphate plus 45 kg of K as KCl/ha than in the unfertilized plots. In addition to the technology of spreading overburdens on soil surface, in 1981 Ishem’yarov ef al. (1984) and Idrisova et al. (1986a,b, 1987) applied, under both laboratory and field conditions, another technology: mixing of the ploughed soil layer with overburdens in the following proportions: 87.5, 75.0, 62.5, 50.0, 37.5, 25.0, and 12.5% soil plus overburdens up to 100%.

As expected, in the soil-overburden mixtures, in comparison with the control soil, catalase, invertase, phosphatase, and urease activities and the contents of humus, mineral N, mobile P, and exchangeable K decreased with increasing proportions of overburdens. In this case, too, some plots were fertilized with the same amounts of farmyard manure plus NPK as specified above. Other unfertilized plots served for comparison. In the 1981-1984 period, the crop yields were estimated in a rotation composed of vetch-oats mixture, winter rye, spring wheat, and maize. In the soiloverburden mixtures, the crop yields decreased, mainly in the unfertilized plots. Comparison of crop yields after application of the two recultivation technologies (spreading or mixing of overburdens on or with soil, respectively) indicated that diminution of crop yields was lowest with the spreading technology applied in association with organic and mineral fertilization. In addition, the cost of recultivation was also lower when this technology was used. At the same time, it has been emphasized that the ratio between the amounts of overburden and soil (ploughed layer) should never exceed 1:1.

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27 1

B. ENZYMOLOGICAL RESEARCHIN THE UNITED STATES The work edited by Redente and Cook (1986) contained information concerning revegetation of soils disturbed by pipeline construction in the Piceance Basin, northwestern Colorado. In 1985, three high-elevation sites (2250 m) and a low-elevation site (2040 m) disturbed by pipeline construction 2-27 years ago were compared with undisturbed native controls. The soils were sampled from the 5-10-cm depth. The data included in Table XI indicate distinct direct relationships between i.icreases in soil organic matter, mineralizable N, and phosphatase activity that occurred in relation to plant community age. In contrast, dehydrogenase activities showed a tendency to decrease in the older sites, which may be due to the fact that some of the areas that were sampled have been heavily grazed.

Table XI Influence of Revegetation on Some Parameters of Sites Disturbed by Pipeline Construction as Compared to Native Sites in the Piceance Basin, Colorado’

High-elevation sites Disturbed (years ago)

Low-elevation sites Native

Disturbed

Native

(23 years ago)

Parameters

2

4

27

PH Organic matter (%) Phosphatase activity (pg of p-nitrophenoll g of soil/hr) Mineralizable N (wg of NH,’ N/g of soil) Deh ydrogenase activity (pg of triphenylformazan/g of soil in 24 hr) Actual (autochthonous) Potential (zymogenous) Litter cover (%) Total plant cover (%)

8.36 0.95 193

7.72 1.49 467

7.35 1.04 690

6.70 2.66 781

8.54 0.53 70

7.13 0.93 377

47.4

63.7

62.7

91.9

29.2

46.1

37.1

17.7

11.5

13.6

13.7

3.8

25.3

25.9

9.8

15.4

13.9

8.0

2 19

0 20

63 67

6 38

“From Redente and Cook (1986).

47 60

4 20

272

S. KISS E T A L .

XIV. RECULTIVATION OF SOILS REMAINING AFTER TOPSOIL “MINING” ENZYMOLOGICAL RESEARCH IN NEWZEALAND Topsoil is “mined”, i.e., removed, around many urban areas for use in landscape improvement. Recultivation of the remaining soil for restoring its fertility is required with the same emphasis as the recultivation of wastelands resulted from mining or other industrial activities. This problem was enzymologically studied by Ross et al. (1982). The soil studied, the Judgeford silt loam (Wellington area), was originally under grazed grassclover pasture, then used for topsoil “mining”, removal to depths of 10 cm (S,, plots) and 20 cm (S,, plots) in March 1978. The remaining soil was treated with lime and urea and with high amounts of P and K fertilizers, and resown in pasture species. Plots without topsoil removal (So) served for comparison. Between March 1978 and March 1981, seven enzymatic activities and several other biochemical as well as chemical and physical properties were determined periodically in the 0-10-cm layer of all the plots. Herbage yield was also recorded and taken as the criterion for soil fertility. The herbage yield and enzyme activity values obtained are presented in Fig. 3. They show that these values increased rapidly in the S,, plots but had not reached So levels after 3 years. Similar results were obtained in the measurements of CO, and mineral N production, biomass C, mineral N flush, and ATP. Organic C and total N contents increased only slowly. Enzymatic and other biochemical activities were, in general, significantly correlated with herbage yields in the S,, or S,, plots, but not in the So plots; organic C and total N contents were not generally correlated significantly with yields over the first 2 years. Overall, invertase, and then sulfatase, activity appeared to be the best indicators of soil fertility status in the stripped (“mined”) soil studied.

XV. CONCLUDING REMARKS The literature reviewed shows that application of enzymological methods makes it possible to indicate the degree of evolution of technogenic soils, the transformation of overburdens and other spoils and wastes into agricultural or forest soils, the efficiency of the recultivation measures applied. In comparison with microbiological parameters, the enzymes are

THE RECULTIVATION OF TECHNOGENIC SOILS

30

273

Urease

P

'2.

0 -0

> .-c > .-c

2

10

y" 2

0

0 lnvertase 0 1 Y

I

;250k

Amylase

.

^^^

loool /--

50

H

I

2000

I

I

0

.750

Phosphatase

v)

P 0

-k

I

I

I

1

A-1

I

I

1978 I1979 I198011981

1978 11979 I1980 11981

.-2

-3

FIG.3. Herbage yield and enzyme activities from plots of Judgeford soil that had been stripped of 0, 10, and 20 cm of the original topsoil in March 1978, reestablished in a grassclover pasture, and sampled periodically over the following 3 years. (1) Soplots (no topsoil removed); (2) S,, plots (10 cm of topsoil removed); (3) S,, plots (20 cm of topsoil removed). Herbage yield is expressed on a dry matter basis. Enzyme activities are given as pmoles of product released by 1 g of soil per second (products-urease: NH,' N; invertase, amylase, and cellulase: "glucose"; xylanase: "xylose"; phosphatase and sulfatase: p-nitrophenol). (Redrawn from Ross et al., 1982.)

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more synthetic indicators of the evolution of technogenic soils because they reflect due to their accumulation in form of humic complexes, the past of technogenic soils, and due to their catalytic activity, which plays a key role in nutrient cycles, the present biological status of these soils. REFERENCES Balicka, N., and Wegrzyn, T. 1984. In “Soil Biology and Conservation of the Biosphere” (J. Szegi, ed.), pp. 819-825. Akademia, Budapest. Bender, J. 1980. Miedzynar. Czas. Roln. (3), 50-55. Bender, J., and Gilewska, M. 1980. Sb. Dokl. Mezhdunar. Simp. Rekul’tivatsiya Landshaftov, Narushennykh Promyshlennoi Deyatel’nost’yu, 7th (Katowice-Zabrze-Konin)3, 178184. Bender, J., and Gilewska, M. 1983. I n “Recultivation of Technogenous Areas” (J. Szegi, ed.), pp. 193-198. MBtraalja Coal Mining, Gyongyos. Bender, J., and Gilewska, M. 1984a. Arch. Ochr. hodow. (I),163-176. Bender, J., and Gilewska, M. 1984b. In “Soil Biology and Conservation of the Biosphere” (J. Szegi, ed.), pp. 45-52. Akademia, Budapest. Bender, J., and Gilewska, M. 1984c. In “Soil Biology and Conservation of the Biosphere” (J. Szegi, ed.), pp. 827-835. Akadkmia, Budapest. Bender, J., and Gilewska, M. 1984d. Sb. Dokl. Mezhdunar. Simp. Razrabotka Sposobov Rekul’tivatsii Landshaftov, Narushennykh Promyshlennoi Deyatel’nost’yu. 8th (Bucharest-Craiova-Tg.Jiu) pp. 155- 162. Bender, J., and Gilewska, M. 1984e. Sb. Dokl. Mezhdunar. Simp. Razrabotka Sposobov Rekul’iivatsii Landshaftov, Narushennykh Promyshlennoi Deyatel’nost’yu, 8th (Bucharesf-Craiova-Tg.Jiu) pp. 295-303. Bender, J., and Strzyszcz, Z. 1978. Sb. Dokl. Soveshch. Rekul’iivatsiya Tekhnogennykh Landshaftov (Gyongyos-Visonta) pp. 113-127. Blaga, G., Nastea, S., RButB, C., Bunescu, V., and Dumitru, M. 1981. Lucr. Conf. Nut. Stiinla Solului (Bra$ov),1979. Publ. Soc. Nat. Rom. Stiinla Solului (Bucharest) (19D). 125-132. Blaga, G . , Nastea, S., RBuIB, C., Bunescu, V., MiclBuq, V., and Dobai, R. 1984. Sb. Dokl. Mezhdunar. Simp. Razrabotka Sposobov Rekul’tivatsii Landshaftov, Narushennykh Promyshlennoi Deyatel’nost’yu, 8th (Bucharest-Craiova-Tg.Jiu)pp. 169-176. Bolewski, A., and Skawina, T. 1972. Pr. Mineral. (30), 1-69. Bunescu, V., and Blaga, G. 1980. Bul. Inst. Agron. Cluj-Napoca, Ser. Agr. 34, 17-20. Clark, R. K., and Clark, S. C. 1981. New Phytol. 87, 799-815. I Range . Manage. 30, 299-305. Cundell, A. M. 1977. . Daraseliya, N. A. 1979. “Biologicheskaya Aktivnost’ Osnovnykh Pochv Zapadnoi Gruzii,” pp. 235-267. Metsniereba, Tiblis. Daraseliya, N. A., and Kalatozova, G. B. 1973. Soobshch. Akad. Nauk Gruz.SSR 70,429432. Daraseliya, N. A., and Kalatozova, G. B. 1976. Tr. Nauch.-lssled. Inst. Pochvov.. Agrokhim. Melior. (Tiblis) 17, 195-213. Daraseliya. N. A., Kalatozova, G. B., and Lapanashvili, E. F. 1978. Soobshch. Akad. Nauk GruzSSR 92, 425428. Dragan-Bularda, M., Kiss, S., Paqca, D., and Olar-Gherghel, V. 1983. Lucr. Conf. Nal. Stiinla Solului (Brdila), 1981. Publ. Soc. Nat. Rom. Siiinta Solului (Bucharest) (21B). 109-1 17.

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Dagan-Bularda. M., Blaga, G., Kiss, S., PaSca, D., Gherasim, V., and Vulcan, R. 1987. Stud. Univ. Babes-Bolyai, B i d . 32, (2). 47-52. Eterevskaya, L. V., Lekhtsier, L. V., Mikhnovskaya, A. D., and Lapta, E. I. 1985. I n “Tekhnogennye Ekosistemy: Organizatsiya i Funktsionirovanie” (A. A. Titlyanova, ed.), pp. 107-135. Nauka, Sib. Otd., Novosibirsk. Fresquez, P. R., and Lindemann, W. C. 1982. Soil Sci. Soc. Am. J . 46, 751-755. Fresquez, P. R., Aldon, E. F., and Sorensen, D. L. 1985. Proc. Natl. Meet. Am. Soc. Surface Mining Reclamation (Denver, Colorado) pp. 340-345. Gel’tser. Yu. G., and Tsvetkova, L. A. 1982. I n “Biogeotsenologicheskie Issledovaniya Stepnykh Lesov, Ikh Okhrana i Ratsional’noe Ispol’zovanie” (A. P. Travleev. ed.), pp. 103-1 12. Gos. Univ., Dnepropetrovsk. Gel’tser. Yu. G., Tsvetkova, L. A., and Trofimov, S. Ya. 1985. I n “Voprosy Stepnogo Lesovedeniya i Nauchnye Osnovy Lesnoi Rekul’tivatsii Zemel” (A. P. Travleev, ed.), pp. 63-70. Gos. Univ., Dnepropetrovsk. Gel’tser, Yu. G., Travleev, A. P., Tsvetkova, L. A,, and Utinova, I. S. 1986. Tez. Dokl. S’ezda Pochvov. Agrokhim. Ukr.SSR, 2nd (Kharkov) pp. 110-1I I . Gilewska, M.,and Bender, J. 1978. Sb. Dokl. Sovcshch. Rekul’tivatsiya Tekhnogennykh Landshafiov (Gyongyos- Visonta) pp. 189-203. Gilewska, M., and Bender, J. 1979.Arch. Ochr. Srodow. ( I ) , 49-56. Gilewska, M., and Bender, J. 1983.Arch. Ochr. Srodow. (34).157-169, 171-178. Gilewska, M., and Bender, J. 1984.Arch. Ochr. srodow. (2). 125-132. Gilewska. M.,and Wojcik, A. 1984.Arch Ochr. Srodow. ( 3 4 ,141-156. Golebiowska, J.. and Bender, J. 1980. Sb. Dokl. Mezhdunar. Simp. Rekul’tivatsiya Landshaftov, Narushennykh Promyshlennoi Deyatel’nost’yu, 7th (Katowice-Zabrze-Konin)

3, 249-262. Golebiowska, J., and Bender, J. 1983.Arch. Ochr. srodow. (l-2), 65-75. Greszta, J. 1973.I n “Protection of Man’s Natural Environment,” pp. 396-417. Pol. Acad. Sci., Warsaw. Greszta, J., and Olszowski. J. 1974.Ekol. Pol. 22, 339-368. Haubold. M.,Henkes, L., and Schroder, D. 1987.Mitt. Drsch. Bodenkd. Ges. 53, 173-178. Hazuk. A. 1967.Ref.-Samml. I n [ . Simp. Rekultivierungcn der durch den Bergbau beschiidigten Biiden, 3rd (Prague) (taken from Bolewski and Skawina, 1972). Hersman, L. E.. and Klein. D. A. 1979.J . Environ. Qual. 8, 520-524. Hersman, L. E..and Temple. K. L. 1979.Soil Sci. 127, 70-73. Idrisova. Z. N. 1984.I n “Pochvennye Usloviya i Effektivnost’ Udobrenii” (F. Sh. Garifullin, ed.), pp. 43-50. Sel’skokhoz. Inst., Ufa. Idrisova. Z. N., Garifullin, F. Sh., and Ishem’yarov, A. Sh. 1986a. Vestn. Sel’skokhoz. Nauki (6).53-59. Idrisova, Z.N., Garifullin, F. Sh., and Ishem’yarov, A. Sh. 1986b. Agrokhimiya (12).1419. Idrisova, Z.N., Canfullin, F. Sh., and Ishem’yarov, A. Sh. 1987. Pockvovcdenie (6),8288. Ishem’yarov, A. Sh., Garifullin, F. Sh.. and Idrisova, Z. N. 1984.I n Povyshenie Effektivnosti Rekul’tivatsii Zemel’. Narushennykh pri Stroitel’stve Truboprovodov” (A. P. Iofinov, ed.), pp. 26-31. Sel’skokhoz. Inst., Ufa. Keleberda, T . N. 1973. Lesn. Zh. (3). 158-161. Keleberda. T. N. 1976. Pochvovedenie (lo), 126-131. Keleberda. T. N. 1977. Tez. Dokl. Deleg. S’ezda Vses. Obshch. Pochvov., 5th (Minsk) (2). 271-272. Keleberda, T. N. 1978. Pochuovedenie (9),109-1 15. Keleberda, T. N. 1979. Vestn. Sel’skokhoz. Nauki (2). 87-90.

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Keleberda. T. N., and Dan’ko, V. N. 1975. Tez. Dokl. Koord. Soveshch. Rekul’tivatsiya Zemel’, Narushennykh pri Dobyche Poleznykh Iskopaemykh (Tartu) pp. 245-249. Keleberda, T. N., Verbin, A. E., and Zharoms’kii. V. Ya. 1974. Visn. Sil’s’kogospod. Nauki (12). 53-56. Khazanov, M. 1. 1975. “lsskustvennye Grunty, Ikh Obrazovanie i Svoistva.” Nauka, Moscow. Kiss, S., DrZlgan-Bularda, M., and RBdulescu, D. 1975. Adv. Agron. 27, 25-87. Klein, D. A., Sorensen, D. L., and Metzger, W. 1982. In “Revegetation Studies on Oil Shale Related Disturbances in Colorado” (E. F. Redente and C. W. Cook, eds.), pp. 27-44. Dept. Range Sci., Colorado State Univ., Fort Collins. Klein, D. A., Sorensen, D. L., and Redente, E. F. 1985. In “Soil Reclamation Processes: Microbiological Analyses and Applications” (R. L. Tate, 111 and D. A. Klein, eds.), pp. 141-171. Dekker, New York. Klevenskaya, I. L., Trofimov, S. S., and Kandrashin, E. R. 1986. Tez. Dokl. Vses. Nauch. Konf. Mikroorganizmy v Sel’skom Khozyaistve, 3rd (Moscow), pp, 89-90. Korsunova, T. M., and Shugalei, L. S. 1986. Tez. Dokl. Konf.Zemel’no-Otsenochnye Problemy Sibiri i Dal’nego Vostoka (Barnaul) Part 2, pp. 156-157. Lessmann, U., and Kramer, F. 1985. Landwirt. Forsch. 38, 110-114. Lindemann, W. C., Lindsey, D. L., and Fresquez, P. R. 1984. Soil Sci. Soc. A m . J . 48, 574-578. Mikhnovskaya, A. D. 1981. Tez. Dokl. Deleg. S’ezda Vses. Obshch. Pochvov., 6th (Tiblis) 2, 190-191. Miller, R. M. 1978. In “Land Reclamation Program. Annual Report (July 1976-October 1977);’ pp. 95-1 19. Argonne Nat. Lab., Argonne, Illinois. Naplekova, N. N., Kandrashin, E. R., Trofirnov, S. S., and Fatkulin, F. A. 1983. In “Recultivation of Technogenous Areas” (J. Szegi, ed.), pp. 177-183. Mtttraalja Coal Mining, Gyongyos. Naplekova, N. N., Trofimov, S. S., Kandrashin, E. R., Fatkulin, F. A., and Barannik, L. P. 1985. In “Tekhnogennye Ekosistemy: Organizatsiya i Funktsionirovanie” (A. A. Titlyanova, ed.), pp. 38-69. Nauka, Sib. Otd., Novosibirsk. Naprasnikova, E. V. 1983. I n “Dinamika Veshchestva v Geosistemakh” (V. A. Snytko, ed.), pp. 55-61. Inst. Geogr. Sib. Otd. Akad. Nauk SSSR, Irkutsk. Naprasnikova, E. V. 1985a. Stud. Humus, Trans. Int. Symp. Humus et Planta, 8th (Prague) pp. 484-486. Naprasnikova, E. V. I985b. Tez. Dokl. Deleg. S’ezda Vses. Obshch. Pochvov., 7th (Tashkent) Part 2, p. 192. Naprasnikova, E. V. 1987. Abstr. Int. Symp. Interrelationships between Microorganisms and Plants in Soil (Liblice, Czechoslovakia) p. 81. Naprasnikova, E. V. and Makarova, A. P. 1986. Tez. Dokl. Vses. Nauch. Konf. Mikroorganizmy v Sel’skom Khozyaistve, 3rd (Moscow) p. 128. Naprasnikova, E. V., Nikitina, Z. I., and Makarova, A. P. 1982. Muter. Vses. Simp. Mikroorganizmy kak Komponent Biogeotsenoza (Alma-Ata) pp. 177-179. Nastea, S., RZlufZl, C., Marin, N., and Blaja, I. 1973. Stiinia Solului (4), 17-26. Osmanczyk, D. 1980. Arch. Ochr. Srodow. ( 3 4 , 175-181. Osmariczyk-Krasa, D. 1984a. In “Soil Biology and Conservation of the Biosphere” (J. Szegi, ed.), pp. 837-846. Akademia, Budapest. Osmanczyk-Krasa, D. 1984b. Arch. Ochr. Srodow. (l), 177-182. Osmariczyk-Krasa, D. 1987. Proc. Int. Symp. Soil Biology and Conservation of the Biosphere, 9th (Sopron) pp. 671-678. Pancholy, S . K., Rice, E. L., and Turner, J. A. 1975. J. Appl. Ecol. 12, 337-342.

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Persson, T., and Funke, B. 1986. Proc. North Dakota Acad. Sci. 40, 122. Peterson, N. V., Kurylyak, E. K., and Panas, R. N. 1976. Tez. Dokl. Vses. Soveshch. po Fizio1.-Biokhim. Osnovam Vzaimodeistviya Rastenii v Fitotsenozakh, 4th (Kiev) pp. 117-1 18. Peterson, N. V., Kurylyak, E . K., and Dubkovetskii, S. V. 1979. Mikrobiol. Zh. (Kiev)41, 129- 134. Ras’kova, N. V., Gel’tser, Yu. G., Tsvetkova, L. A., and Trofimov, S. Ya. 1984. Tez. Dokl. Vses. Shk. Vliyanie Promyshlennykh Predpriyatii na Okruzhayushchuyu Sredu (Zvenigorod) pp. 157-158. Redente, E. F., and Cook, C. W. (eds). 1986. “Structural and Functional Changes in Early Successional Stages of a Semiarid Ecosystem.” Dept. Range Sci., Colorado State Univ., Fort Collins. Ross, D. J., Speir, T . W., Tate, K. R., Cairns, A., Meyrick, K. F., and Pansier, E. A. 1982. Soil Biol. Biochem. 14, 575-581. Rtskhiladze, T. G., Rostiashvili, K. A., and Lapanashvili, E. F. 1981. Tez. Dokl. Deleg. S’ezda Vses. Obshch. Pochvov., 6th (Tiblis) 2, 189. Schroder, D. 1986. Z. Kulturtech. Flurberein. 21, 318-325. Schroder, D. 1988. Z. Pjlanzenernaehr. Bodenkd. 151, 3-8. Schroder, D., Stephan, S., and Schulte-Kamng, H. 1985. Z. Pjlanzenernaehr. Bodenkd. 148, 131-146. Schroder, D., Haubold, M., and Henkes, L. 1987. Landw. Z. (Bonn), 154, 1466-1469. Shugalei, L. S., and Yashikhin, G. I. 1985. In “Biologicheskaya Aktivnost’ Lesnykh Pochv” (V. M. Korsunov, ed.), pp. 80-88. Inst. Lesa i Drevesiny Sib. Otd. Akad. Nauk SSSR, Krasnoyarsk. Shugalei, L. S., Yashikhin, G. I., and Korsunova, T. M. 1984. Tez. Dokl. Vses. Shk. Vliyanie Promyshlennykh Predpriyatii nu Okruzhayushchuyu Sredu (Zvenigorod) pp. 224226. Shugalei, L. S., Korsunova, T. M., and Yashikhin, G. 1. 1985. Izv. Sib. Otd. Akad. Nauk SSSR, Ser. Biol. Nauki (13/2), 7 6 7 6 . Soreanu, 1. 1983. Bul. Stiiny. Inst. Invdtrimint Sup. Baia Mare, Ser. B 6, 93-98. Sorensen, D. L., Klein, D. A., Ruzzo, W. J., and Hersman, L. E . 1981. J. Environ. Qual. 10, 369-37 I. Stroo, H. F., and Jencks, E. M. 1982. Soil Sci. Soc. Am. J . 46,548-553. Stroo, H. F., and Jencks, E. M. 1985. J. Environ. Qual. 14, 301-304. Sulyok, L., and Voros, 1. 1983. In “Recultivation of Technogenous Areas” (J. Szegi, ed.), pp. 207-212. Mhtraalja Coal Mining, Gyongyos. Sviridova, 1. K., and Panozishvili, K. P. 1979. Tez. Dokl. Vses. Soveshch. Biol. Produktivnost’ Pochv i Ee Uvelichenie v Interesakh Narodnogo Khozyaistva (Moscow) pp. 141-142. Szegi, J. 1983. In “Recultivation of Technogenous Areas” (J. Szegi, ed.), pp. 105-1 11. Matraalja Coal Mining, Gyongyos. Trofimov, S. S., Naplekova, N. N., Kandrashin, E. R., Fatkulin, F. A., and Stebaeva, S. K. 1986. “Gumusoobrazovanie v Tekhnogennykh Ekosistemakh.” Nauka, Sib. Otd., Novosibirsk. Tsvetkova, L. A., Ras’kova, N. V., and Gel’tser, Yu. G. 1982. Mater. Vses. Simp. Mikroorganizmy kak Komponent Biogeotsenoza (Alma-Ata) pp. 109-1 10. Uzbek, I. Kh. 1986. Tez. Dokl. S’ezdu Pochvov. Agrokhim. Ukr.SSR, 2nd (Kharkov) p. 112.

Verbin, A. E., and Keleberda, T. N. 1974. Pochvovedenie (2), 116-120. Williams, S. T., McNeilly, T., and Wellington, E. M. H. 1977. SoilBiol. Biochem. 9, 271275.

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ADVANCES IN AGRONOMY, VOL. 42

EFFECTS OF NITRIFICATION INHIBITORS ON NITROGEN TRANSFORMATIONS, OTHER THAN NITRIFICATION, IN SOILS' K. L. Sahrawat International Crops Research Institute for the Semi-Arid Tropics ICRISAT Patancheru P.O. Andhra Pradesh 502 324, India

I. Introduction 11. Effects of Nitrifcation Inhibitors on Physical and Chemical Processes Relevant to Nitrogen Transformations A. Transport and Movement of Nitrogen B. Ammonium Fixation and Release C. Ammonia Volatilization 111. Effects of Nitrification Inhibitors on Biological Nitrogen Transformations A. Mineralization and Immobilization B. Denitrification C. Nitrous Oxide Emission via Nitrification and Denitrification D. Urea Hydrolysis IV. Other Effects V. Perspectives References

1. INTRODUCTION Interest in nitrification inhibitors stems from the fact that retardation of nitrification reduces loss of nitrogen by leaching and denitrification following nitrification. This helps in some situations to achieve more efficient use of nitrogen for crop production and may also help in minimizing fertilizer nitrogen-related environmental stresses, especially accumulation of nitrate in surface and ground waters. Nitrification is generally used to mean biological oxidation of ammonium to nitrate via nitrite effected, respectively, by Nitrosomonas and Nitrobacter species of nitrifying bacteria, although nitrification inhibitors are defined as compounds or materials that specifically retard the oxidation of ammonium to nitrite without 'Approved for publication as ICRISAT Journal Article No: 705. 279 Copyright 0 1989 by Academic Press. Inc. All rights of reproduction in any form reserved.

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affecting the subsequent oxidation of nitrite to nitrate. Inhibition of nitrification is referred to as retardation of nitrification because complete inhibition is seldom achieved with the use of nitrification inhibitors. The literature on nitrification inhibitors is very extensive (e.g., see Gasser, 1970; Prasad et al., 1971; Hauck, 1972, 1983,1984; Huber et al., 1977; Meisinger et al., 1980; Sahrawat, 1980, 1986; Hauck and Behnke, 1981; Mulvaney and Bremner, 1981; Slangen and Kerkhoff, 1984; Sahrawat and Keeney, 1984, 1985; Amberger, 1986). These reviews cover various aspects of the effects of nitrification inhibitors on (i) retardation of nitrification in soil, and (ii) crop production and some aspects of crop quality (e.g., see Sahrawat and Keeney, 1984). The interest in nitrification inhibitors followed the development of nitrapyrin [2-chloro-6-(trichloromethyl) pyridine] by the Dow Chemical Company of the United States as an effective inhibitor of nitrification (Goring, 1962a,b). Research has suggested that in addition to retarding nitrification, nitrification inhibitors may affect certain other processes of the nitrogen cycle in soils such as mineralization-immobilization, nitrous oxide production, ammonia volatilization, and denitrification (e.g., see Table I). The capacity of nitrification inhibitors to affect these processes depends on their bioactivity in soil, which is affected by soil texture, temperature, and the amount of inhibitor added. The half-lives of nitrification inhibitors such as nitrapyrin may vary from a few days to several weeks depending on the rate of application, soil type, and season (temperature) (e.g., see Meisinger et al., 1980; Sahrawat, 1980). This review summarizes the literature on the effects of nitrification inhibitors on nitrogen transformations other than nitrification in soil and identifies future directions for research. This field of research is developing in importance because of increasing interest in the use of these chemicals.

II. EFFECTS OF NITRIFICATION INHIBITORS ON PHYSICAL AND CHEMICAL PROCESSES RELEVANT TO NITROGEN TRANSFORMATIONS Since retardation of nitrification increases the persistence of ammonium in soils, it must be expected that retardation of nitrification affects ammonium nitrogen transformation processes such as fixation or adsorption and volatilization in some situations. Also, retardation of nitrification may result overall in less movement and transport of mineral nitrogen because of higher NHJNO, ratios in soils caused by retardation of nitrification (e.g., see Sahrawat and Keeney, 1984).

EFFECTS O F NITRIFICATION INHIBITORS

28 1

Table I Recent References on the Effects of Nitrification Inhibitors on Nitrogen Transformations, Other Than Nitrification, in Soils

Aspect of N transformation processes

References

Physical and chemical processes Huber et a/. (1969); Keeney et a/. (1979); Owens (1981); Papendick and Engibous (1980); Hergert and Wiese (1980); Onken (1980); Timmons (1984) Gin et a/. (1982); Juma and Paul (1983); Aulakh and Ammonium fixation and release Rennie (1984) Cornforth and Chasney (1971); Bundy and Bremner Ammonia volatilization (1974); Smith and Chalk (1978, 1980); Jain e r a / . (1981); Rodgers (1983); Simpson et a/. (1985); Magalhaes and Chalk (1987); Prakasa Rao and Puttanna (1987)

N transport and movement

Mineralization and immobilization Denitrification

Nitrous oxide production

Urea hydrolysis

Biological processes Dubey and Rodriguez (1970); Laskowksi et a / . (1975); Malhi and Nyborg (1979a, 1983); Juma and Paul (1983, 1984); Aulakh and Rennie (1984) Mitsui et a / . (1964); Sandhu and Moraghan (1972); Henninger and Bollag (1976); McElhannon and Mills (1981); Notton er a/. (1979); Yeomans and Bremner (1985a,b); Bremner and Yeomans (1986); Mills (1984); Mills and McElhannon (1984) Bremner and Blackmer (1978, 1979); Freney el a/. (1979); Smith and Chalk (1978, 1980); Bremner er a/. (1981); Aulakh ef a / . (1984); Magalhaes er a / . (1984); Yeomans and Bremner (1985a,b); Casella et a / . (1986); Bremner and Yeomans (1986); Davidson et a/. (1986); Magalhaes and Chalk (1987) Goring (1962b); Bremner and Douglas (1971); Bundy and Bremner (1974); Bremner and Bundy (1976); Reddy and Prasad (1975); Ashworth et a/. (1977, 1979, 1980); Amberger and Vilsmeier (1979); Guthrie and Bomke (1981); Rodgers (1983); Sahrawat (1979a,b); Mishra and Flaig (1979); Mishra et a/. (1980); Lethbridge and Burns (1976); Malhi and Nyborg (1979b); Coos (1985)

A. TRANSPORT AND MOVEMENTOF NITROGEN Keeping nitrogen in the ammonium form by retarding nitrification reduces movement of mineral nitrogen because (i) ammonium is retained by soil particles and thus is less mobile and (ii) if less nitrate is formed, this results in reduced amounts of nitrate N leached. For example, in a

282

K. L. SAHRAWAT

3-year study Huber et al. (1969) showed that in the field inhibition of nitrification of fall-applied ammonium sulfate with nitrapyrin (0.56 kg/ha) prevented the movement of applied nitrogen below a depth of 30.5 cm (Table 11). These results show that the inhibition of nitrification reduced the amounts of nitrate formed and its subsequent leaching in the soil profile. A greater proportion of mineral N was present in the ammonium form in the inhibitor-treated plots. Keeney et al. (1979) found that nitrapyrin inhibited nitrification of ammonium in a loamy sand soil and also reduced the amounts of nitrate leached in soil columns over 2-5 weeks. However, by 20 weeks, the period for which the experiment was run, the amounts of nitrate leached were similar in soil columns with and without nitrification inhibitor treatment. This might have been due to degradation of nitrapyrin. Similarly, Owens (1981) showed that nitrapyrin reduced the amounts of mineral N (mostly nitrate) leached in 1-m-long soil columns. After 91 days, 1.0 and 9.7% of the applied urea nitrogen had leached from nitrapyrin-treated and untreated soil cores; however, after 144 days, 41.9 and 53.0%, respectively, of applied N had leached. Studies in the United States on the effect of nitrapyrin on transformations and movement of fertilizer nitrogen in soils indicated that in some situations it reduced the movement of fertilizer nitrogen over winter o r during irrigation (Hergert and Wiese, 1980; Onken, 1980; Papendick and Engibous, 1980; Timmons, 1984). In a 3-year field lysimeter study, Timmons, (1984)

Table I1 Extractable Ammonium and Nitrate N Content (kg/ba) in Southwick Silt Loam Soil in the Spring Following Fall N Fertilization with and without Nitrapyrin"'*

NO, N at depth (cm) Treatment

NH4 N at depth (cm)

0.0-30.5

30.5-61.0

Total

30.5-61.0

Total

17.3 21.2

18.9 18.9

36.2 40. I

2.9 8.5

8.3 5.4

11.2 13.9

19.3 35.3

22.5 20.7

41.8 56.0

11.3 8.8

7.6 8.1

18.9 16.9

28.7

22.2

50.9

27.6

9.2

36.8

0.0-30.5

~~

Control Calcium nitrate Calcium nitrate plus nitrapyrin Ammonium sulfate Ammonium sulfate plus nitrapyrin

"From Huber et a / . (1969). The study was conducted in the fall of 1965, 1966, and 1967. Data are average of 3 years. bNitrogen was applied at the rate of 67.5 kg N/ha in the fall of 1965 and 1966, and 84 kg N/ha in the fall of 1967. Nitrapyrin was added annually at a rate of 0.56 kg/ha.

EFFECTS O F NITRIFICATION INHIBITORS

283

found that nitrapyrin application with urea reduced the 10s of NO, N leached at the 1.2-m depth in soil planted to corn (Zea mays L.) (Table 111). In a 6-year lysimeter study, Owens (1987) found that nitrapyrin (1.12 kg/ha) application with urea (336 kg N/ha) reduced the loss of inorganic N in percolation water from Rayne silt loam (fine loamy, mixed, mesic; Typic Hapludult) planted to no-tillage corn. The average annual N loss by leaching in the untreated lysimeter was 160 kg N h a , which was reduced to 117 kg N/ha by nitrapyrin application. Nitrapyrin was found to be effective in reducing the leaching loss of inorganic N in spring, summer, autumn, and winter (Table IV).

Table 111 Leaching Loss of Nitrate N in a Field Lysimeter Sandy Loam Soil (Typic Hapludoll) Fertilized with Urea, with and without Nitrapyrin" _

_

~

Treatmentb Year 1977 1978 1979 Average I977 1978 1979 Average

Urea

Urea plus Nitrapyrin

Percolation (mm) 337 233 218 263

313 236 234 26 1

NO, N leached (kg/ha) I94 I48 161 157 127 142 161 I49

Flow-weighted NO, N Conc' (mmoVliter) 1977 4. I 3.4 1978 4.9 4.7 1979 4.2 4.3 Average 4.4 4.1 "From Timmons (1984). Each value is an average of three replications measured at 1.2-m depth. "Lysimeters were fertilized with 224 kg urea N/ha before planting to corn and nitrapyrin was added at the rate of 0.56 kg/ha. 'Flow-weighted concentration is total NO, N leached divided by total water percolated and converted to mmoVliter.

284

K. L. SAHRAWAT Table IV

Effect of Nitrapyrin on Nitrogen Loss in Percolation Water from Rayne Silt Loam (Typic Hapludult) in Lysimeters Fertilized with Urea and Planted to No-Tillage Corn"

Inorganic N lossb (kg Nlha) ~

Lysimeter

Treatment

Spring

Summer

Autumn

Winter

Annual

A

Urea Urea plus nitrapyrin Urea plus nitrapyrin

66.6 5 1.7 49.3

12.0 10.2 9.8

18.8 12.6

62.6 45.2 40.9

160.0 119.7 114.0

B C

14.0

"From Owens (1987); results presented are averages of 6 years' data, 1978-1984. Urea was applied at a rate of 336 kg N/ha and nitrapyrin at the rate of 1.12 kg/ha annually. 'Spring, April-June; Summer, July-September; Autumn, October-December; Winter, January-March.

B. AMMONIUM FIXATION AND RELEASE The retardation of nitrification can enhance immobilization of fertilizer nitrogen because the persistence of ammonium increases (i) its incorporation in the organic nitrogen fraction or (ii) its migration to fixed or nonexchangeable sites on clay minerals. For example, Juma and Paul (1983) found that under field conditions treatment of "N-aqueous NH, and "Nurea with a nitrification inhibitor, ATC (4-amino-l,2,4-triazole), caused enhanced recovery of fertilizer N in the soil surface layer (52-55% versus 28-30%). Between 5 and 8% of the fertilizer N was recovered in the nonexchangeable ammonium form in the A horizon of the soil treated with ATC as opposed to about 1% in the non-ATC treatments (Table V). Laboratory study these soil samples further revealed that the nonexchangeable "NH, was released at rates equivalent to a half-life of 38 weeks and the rate constant was 0.0Wweek at 28 ? 1°C at a soil water potential of - 34 kPa (kPascal). The clay fraction of the soil, consisting of mica, vermiculite, and smectites, contained 49% of the labeled nonexchangeable NH, whereas the coarse silt fraction accounted for 26% of the labeled nonexchangeable NH,. Aulakh and Rennie (1984) showed that nitrapyrin did not increase the fixation of NH, initially, but the release of recently fixed NH, was decreased and delayed by nitrapyrin application in a 2-year study of fallapplied "N-labeled urea in Canadian chernozemic soils (Typic Udic Haploborrolls) (Table VI). In another experiment, nitrapyrin application significantly increased the amount of fertilizer urea recovered as fixed NH, after 8 months of application to a clay loam soil (see Table XV). In some situations the changes in the amounts of fixed NH, could influence N loss and availability to plants.

285

EFFECTS O F NITRIFICATION INHIBITORS Table V

Percentage Recovery of "N-Labeled Aqueous Ammonia and Urea in a Loam Soil (0.43%total N, pH 7.4) with and without ATC Nitrification Inhibitor after Harvest of the Wheat Crop''b

"N recovered in soil depth

Treatments NH, OH

0-15 cm 15-30 cm 30-60 cm Total Nonexchangeable NH,'

+ ATC

NH, OH

28 10 2

56 3

40 1

60 5

Urea 31 10 2 43 I

1

Urea

+ ATC 52 2 I 55 8

"From Juma and Paul (1983). bThe fertilizers were added at a rate of 56 kg N/ha and the inhibitor at a rate of 4% of fertilizer N. The "N excess of each fertilizer was 5.6% 'Nonexchangeable NH, expressed as percentage of the remaining ''N.

C. AMMONIAVOLATILIZATION Retardation of nitrification in soil results in accumulation of ammonium and higher soil pH (Cornforth and Chasney, 1971; Hauck and Bremner, 1969; Bundy and Bremner, 1974; Smith and Chalk, 1978; Magalhaes and Chalk, 1987), which are conducive t o ammonia volatilization. In fact, Cornforth and Chasney (1971) showed in the field that application of AM (2-amino-4-chloro-6-methyl pyrimidine) nitrification inhibitor with ammonium sulfate (168 kg N h a ) increased the ammonia loss by volatilization Table VI Changes in Ammonium Fixation (kg N/ha) of Fall-Applied Urea in the Soil Profile to 30-cm Depth without and with Nitrapyrin Applied to Baline Lake Clay Loam Soil"

Treatmentb Sampling date

Urea

30 September 1981 20 October 1981 I December 1981 23 March 1982 27 April 1982 27 May 1982

10.6 8.6 9.6 6.9 6.2 I .o

Urea

+ Nitrapyrin 10.8 9.7 9. I 7.4 8.1 4.6

"From Aulakh and Rennie (1984). "Urea was applied at a rate of 100 kg N/ha and nitrapyrin at a rate of 1% of fertilizer N on 30 September 1981.

286

K. L. SAHRAWAT

from bare soil. The inhibitor increased by nearly eightfold the amount of ammonia volatilized from grass-covered soils in comparison with the control during 28 days of study, and nearly 22 kg N/ha was lost as ammonia. Less ammonia was lost when unamended ammonium sulfate or urea was applied to grass rather than to bare plots. Bundy and Bremner (1974) showed that nitrapyrin [2-chloro-6-(trichloromethyl) pyridine], ATC (4-amino-l,2,4-triazole) and CL-1580 (2,4-diamino-6-trichloromethyl-s-triazine) nitrification inhibitors retarded nitrification of urea in soil but increased the volatile loss of ammonia from soils in a laboratory study (Table VII). However, it should be mentioned that these losses were experienced when a sandy clay loam soil was treated with a relatively high rate of urea (400 pg N/g soil). This study, nevertheless, indicates the potential of high loss due to ammonia volatilization when nitrification inhibitors in conjunction with urea are surface-applied to coarse-textured calcareous soils. The increased ammonia volatilization from soils treated with nitrification inhibitors was due to the persistence of ammonium and higher soil pH (Table VIII), which created a soil environment conducive to ammonia volatilization. In another laboratory study, Rodgers (1983) determined the loss by ammonia volatilization from three soils fertilized with urea prills or urea prills containing 7% by weight of DCD (dicyandiamide), a nitrification inhibitor. It was found that the volatile loss of ammonia was less when urea or urea and DCD was incorporated than when it was applied to the surface. Soil type influenced the volatile loss of ammonia during 4 weeks of testing. The volatile loss of ammonia from a soil that did not nitrify was not affected by DCD application but volatilization was increased in the two other soils (Table IX). In general, the soils were quite slow in nitrification, and by Table VII

Effects of Three Nitrification Inhibitors on Nitfieation and Volatile Loss of Ammonia from a Sandy Clay Loam Soil (pH 7.2; organic C 1.65%)at 14 Days of Incubation‘** Inhibitor

Inhibition of nitrification (%)

Volatile loss as ammonia (% of urea N added)

None Nitrapyrin ATC CL-1580

94 92 88

9 34 30 28

“From Bundy and Bremner (1974). bSoil samples (10 g) were treated with 4 mg of urea N and with 0 or 100 pg of nitrification inhibitor and incubated at 30°C and 60% WHC (water holding capacity) moisture.

EFFECTS OF NITRIFICATION INHIBITORS

287

Table VIIl Effect of Nitrapyrin on Soil pH in a Sandy Clay Loam Soil Treated with UreaEvb

Soil pH (1:2.5 H,O)

Time (days)

-

With nitrapyrin

Without nitrapyrin

0 2 4 6 8 10 12 14 21

7.2 8.2 8.2 8.2 8.1 8.0 8.0 8.0 7.5

7.2 8.0 7.3 6.3 6.2 6.1 6.2 6.2 6.2

“From Bundy and Bremner (1974). ’Soil samples (I0 g) were treated with 4 mg of urea N and with 0 or 100 pg of nitrapyrin and incubated at 30°C and 60% WHC moisture.

4 weeks only 1-21% of the urea N added was recovered as nitrate N in soil samples not treated with DCD. The effects on ammonia volatilization due to retardation of nitrification in this study are not as dramatic as those obtained by Cornforth and Chasney (1971) in the field and Bundy and Bremner (1974) in the laboratory. These differences are probably due to the difference in nitrifying capacity of soils and persistence of ammonium in soil samples with and without the nitrification inhibitor treatment. Smith and Chalk (1978) found that in a calcareous soil treated with ammonia, nitrapyrin application only slightly increased the volatile loss of ammonia in 28 days. The volatile loss of ammonia amounted to 86 and 92 pg/g soil in treatments without and with nitrapyrin when the soil was fertilized with 1127 kg/g ammonia N. The pH of the nitrapyrin-treated soil was higher, as was the extractable NH, N, and nitrification was at a low ebb (Table X). The losses due to ammonia volatilization by retardation of nitrification were similar and small in the studies reported by Rodgers (1983) and Smith and Chalk (1978) although they used high rates of urea application (Tables IX and X). This could additionally be due to the different method of urea application used by these researchers (soil incorporation) as opposed to Bundy and Bremner (1974). Also, Rodgers (1983) used urea prills and Bundy and Bremner (1974) applied urea solution to the soil surface, and this might have affected urea hydrolysis and subsequent nitrification. As

K . L. SAHRAWAT

288

Table IX Effect of Dicyandiamide (DCD) on Urea Transformations in Three Soils"*b

Treatment Form of urea N recovered

Soil ~

~~

Urea

Urea

+ DCD

~

Rothamsted (PH 5.2)

Saxmundham (PH 7.7)

Woburn (PH 5.4)

Urea N

0.0

0.0

NH4 N NO2 N NO3 N NH3 N Urea N

17.8 0.0 I .o 15.4 0.0

78.2 0.0

NH, N NO, N NO, N NH, N Urea N

72.4 2.6 20.9 9.2 0.0

74.9 0. I 2.1 11.8 0.0

NH, N NO, N NO3 N NH, N

56.2 0.8 16.0 31.2

58.9 0.0 2.0 31.3

0.5.

14.6 0.0

"From Rodgers (1983). %oil samples (50 g) were treated with 50 mg urea N or urea containing 7.2% by weight DCD and incubated at 30°C under aerobic conditions for 4 weeks.

Table X Effects of Nitrapyrin on Inorganic N and Gaseous N Evolution ( p g N/g soil) from a Calcareous Soil (pH 8.5, organic C 1.3%)Treated with Ammonia"'*

Inorganic N (28 days)

Gaseous N evolved (28 days)

Treatment

Soil pH

NH4+

NO,-

NO,-

N2

N20

No nitrapyrin Nitrapyrin

7.8 8.2

792 1012

70 44

154 0

76 13

57 0

NO

+ NOz 9 1

NH3 86 92

"From Smith and Chalk (1978). "Soil samples were incubated at 30°C and 0.33 bar soil water potential after treatment with I127 pg ammonia N/g soil, and 0 t o 10 pg nitrapyridg soil.

289

EFFECTS OF NITRIFICATION INHIBITORS

mentioned earlier, the soils in these studies differed greatly in their capacity to produce nitrate from hydrolyzed urea. Simpson er al. (1985) studied the effects of phenylphosphorodiamidate (PPD), a urease inhibitor, and dicyandiamide, a nitrification inhibitor, on nitrogen losses, transformations, and recovery of nitrogen, when urea was applied to a flooded rice field. It was found that although PPD delayed urea hydrolysis and decreased loss via ammonia volatilization, DCD, the nitrification inhibitor, had no significant effect on nitrate concentrations in the flood water and ammonia loss. Of the 80 kg of urea N added, 20.6% was lost through ammonia volatilization from the control, followed by 18.8%from the urea plus DCD treatment, and 12.5% from the urea plus PPD treatment during the I I days after application of the fertilizer (Table XI). These results show that DCD was not effective in inhibiting nitrification in the flooded soil, in contrast to its effectiveness as a nitrification inhibitor in aerobic soils (Amberger, 1986). The pattern of ammonia loss from the urea plus PPD treatment was very different from that of the Table XI Effects of DCD and Phenylphosphoradiamidate (PPD) on Ammonia Volatilization Losses (kg N/ha/day) from Flooded Clay Soil (Pelloxerert, pH 8.2Fb

Treatment Days after urea application

Urea

0 I 2 3 4 5 6 7 8 9 10 I1 Total loss Loss as % of

0.11 2.70 3.00 3.67 1 .OO 1.08 1.30 1.12 0.49 0.77 0.86 0.39 16.49 20.6

Urea

+ DCD

0.28 2.24 1.32 2.38 I .06 0.96 1.12 1.47 1.11 1.14 1.43 0.56 15.07 18.8

Urea

+ PPD

0.00 0.07 0.05 0.22 0.20 0.50 0.94 1.88 I .74 I .35 1.81 1.21 9.97 12.5

applied N "From Simpson er a / . (1985). "Prilledurea applied at the rate of 80 kg N h a by uniformly broadcasting into the flood water. DCD was added at the rate of 10% urea N and PPD at the rate of 1% of urea N (w/w).

290

K. L. SAHRAWAT

control in that the losses were small in the beginning during the first 6 days but increased during the last 5 days. Prakasa Rao and Puttanna (1987) conducted laboratory and field experiments to study the effects of DCD on nitrification and ammonia volatilization from a sandy loam soil (pH, 7.3; organic C, 0.5%) fertilized with urea. It was found that in laboratory experiments 15 or 20 mg/kg of DCD effectively retarded the nitrification of urea but increased the volatile loss of ammonia and also extended the period of ammonia emission. In the field experiment, DCD (15 or 20% of urea N) application greatly increased the loss via ammonia volatilization when DCD-treated urea was surface-applied; however, the loss was minimized when the inhibitortreated urea was applied at a 5-cm depth. More than 30% of the applied urea (187 kg N/ha) was lost as ammonia when DCD-amended urea was applied to the soil surface, but this was decreased to less than 5% when DCD-amended urea was placed at a 5-cm depth in the soil. Placement of unamended urea also greatly reduced the volatile loss of ammonia as compared to its surface application. It would appear that if the benefit of retardation of nitrification is not to be offset by enhanced ammonia volatilization loss in light-textured calcareous soils, placement of inhibitor-amended urea or ammonium fertilizers at least at a 5-cm depth in soil would be a better strategy for efficient nitrogen management.

Ill. EFFECTS OF NITRIFICATION INHIBITORS ON BIOLOGICAL NITROGEN TRANSFORMATIONS A. MINERALIZATION AND IMMOBILIZATION Nitrification inhibitors may effect mineralization of soil nitrogen in some situations. They can also influence immobilization of nitrogen due to persistence of ammonium, which is preferentially immobilized over nitrate by soil microorganisms (Alexandra, 1977). Not only is ammonium preferentially immobilized over nitrate, but also remineralization of immobilized ammonium is relatively slower than that of immobilized nitrate (Bjarnason, 1987). Dubey and Rodriquez (1970) found that the fungicides dyrene [2,4-dichloro-6-(O-chloroanilino)-s-triazine] and maneb (manganese ethylene bisdithiocarbamate) did not affect ammonification of soil nitrogen at 60 pg/ g soil concentrations although nitrification of ammonium was greatly retarded. Only at a high rate of application (960 Kg/g soil) did dyrene and maneb retard ammonification. Laskowski et al. (1975) showed that 6-

29 1

EFFECTS OF NITRIFICATION INHIBITORS

chloropicolinic acid (GCPA), a hydrolysis product of nitrapyrin, had no effect on net mineralization of soil organic nitrogen at up to 1000 pg/g soil concentrations. Some data on the effects of nitrapyrin and 6-CPA on general microbiological activity of soils, as indicated by CO, production, are shown in Table XII. These results clearly establish that nitrapyrin has no effect on the general microbial activity of soils. Although 6-CPA did not affect general microbial activity in two soils, it significantly reduced the production of CO, in the loam soil that was low in organic carbon, even at the lowest concentration. The compound, however, did not affect the production of CO, at a loo0 pg/g soil concentration, although it significantly reduced CO, production at lower concentrations in the low-organic matter loam soil. Such an effect of 6-CPA is important to note because it would usually be attributed to nitrapyrin. Nitrification inhibitors can also influence mineralization of soil nitrogen. For example, Malhi and Nyborg (1979a,b, 1983) found that nitrapyrin, ATC (4-amino-I ,2,4-triazole), and CS, (carbon disulfide) reduced the amounts of nitrate formed but also reduced the amounts of ammonium released, i.e., ammonification in soils. The effect of ammonification inhibition was greater with ATC and CS, than with nitrapyrin. ATC and CS, at concentrations of 22 kglha suppressed both ammonification and nitrification of soil in the field and thus reduced nitrate formation during a wet spring (Table XIII). Juma and Paul (1984) used the soil samples from plots previously fertilized with "N-labeled urea or aqueous ammonia (NH,OH) with and

Table XI1 Effect of Nitrapyrin and 6-Chloropicolinic Acid (6-CPA) on General Microbiological Activity as Indicated by CO, Production in Soils in 291 Days of Incubation"'b Amounts of C 0 2 (mg) evolved 6-CPA (mg/kg soil)

Nitrapyrin (mg/kg soil) Organic C I

Texture

pH

(%)

0

1

10

Loam 7.6 Clay 7.7 Loam 7.2 Average all soils

0.5 0.8 2.4

221 320 422 321

242 274 403 306

236 188 335 253

100 lo00

241 298 384 310

204 209 270 228

0 384 469 421 425

I

10

100

lo00

243' 262' 406 441 524 446 391 383

257' 301 356 305

366 312 419 366

"From Laskowski e t a / . (1975). bSoil samples (50 g) were treated with dust formulation of nitrapyrin or solutions of K salt of 6-chloropicolinic acid to achieve the specified concentrations and incubated (optimum water content) at 20°C. 'Significantly different from control at p = 0.05.

292

K. L. SAHRAWAT

Table XIII The Effects of Nitrification Inhibitors on the Release of Mineral N over the Winter in Mdmo Silty Clay Loam Soil (pH 6.0; O.M. 9.7%) in 1978-1979" NH, N and NOp N in the 0-30-cm layer' NO, N (kg/ha)

NH4 N (kg/ha) Treatmentb 27 Oct

16Mar 10 May

Control ATC Nitrapyrin CS,

22 c 30 b 25 bc 38 a

14

-

-

22 b 36a 24 b 40a

27 Oct

16Mar

14

60a 33 b 56a 21c

-

(NH,

10 May 27 Oct 18a 19a 16a 19a

28

-

+ NO,) N (kg/ha) 16 Mar

10 May

82 a 63 b 81 a 59 b

40 b 55 a 40 b 59 a

"From Malhi and Nyborg (1983). bThe nitrification inhibitors were added a t a rate of 22 kg/ha. ATC and nitrapyrin were mixed into the soil to a depth of 10 cm, and carbon disulfide (CS,) was injected 10 cm deep in bands 23 cm apart. 'In each column, the values not followed by the same letter are significantly different ( p = 0.05).

without ATC nitrification inhibitor (Juma and Paul, 1983) to study the effect of the nitrification inhibitor on N mineralization during 2 weeks of incubation at 28 1°C and - 34 kPa soil moisture tension in the laboratory and on NH, released during a 10-day incubation of fumigated soil. It was found that although the nitrification inhibitor did not affect the mineral N released during 2 weeks of incubation, the amounts of NH, N released in fumigated soils were higher in the inhibitor-treated samples. The extractability ratios (ratio of atom percentage "N excess of extracted N to atom percentage ''N excess of total N) were higher for the samples treated with the nitrification inhibitor compared to those treated with fertilizer alone. Juma and Paul (1983) made a detailed study of the effect of ATC on immobilization of I5N-labeled aqueous ammonia and urea N and found that ATC caused a greater immobilization of fertilizer "N (see Table V) and also increased the rate of release of "N-labeled microbial biomass following fumigation and incubation for 12 weeks (Table XIV). Aulakh and Rennie (1984) found that immobilization of fall-applied labeled urea and KNO, was minimal under fallow conditions (7%) but ranged from 1521% and from 2626% of the applied N as KNO, and urea, respectively, in wheat-stubble fields. Nitrapyrin did not affect the immobilization of fertilizer N, and the amounts of fertilizer N recovered in the organic and in the inorganic N pools were similar in urea and urea plus nitrapyrin treatments 8 months after fertilizer application (Table XV). Other studies have suggested an interesting pathway of nitrite incor-

*

293

EFFECTS O F NITRIFICATION INHIBITORS Table XIV

Effect of ATC Nitrification Inhibitor on Decay of Microbial Biomass in Loam Soil in a 12-Week Laboratory Incubation"

Treatment

"NH, released on fumigation and incubation (ng/g soil)

"N in biomass (ng IsN/g soil)

Decay rate constant'

(weeks)

NH,OH I b NH,OH 2' NH,OH plus ATC Urea plus ATC

18 14 38 35

60 47 I27 1 I7

0.028 0.026 0.020 0.026

24.7 27.2 33.9 26.2

tlnd

~

"From Juma and Paul (1983). "Similar treatments incubated at separate times. 'Decay rate constant expressed as net decaylweek, setting the initial pool sizes to 100%. dHalf-lives for biomass "N.

poration into the organic nitrogen fraction via nitrite self-decomposition and fixation on organic matter in a humic-rich acidic forest soil (pH, 4.5; organic matter, 46%) (Boudot and Chone, 1985). Nitrapyrin application not only reduced the loss of nitrite via chemodenitrifcation (Nelson, 1982) but also decreased the incorporation of nitrite into the organic N fraction (Boudot and Chone, 1985). In later studies, Azhar et al. (1986a) reported that nitrite formed from ammonium oxidation in grassland soil (pH, 6.5; organic C, 4.09%) was incorporated into the organic matter fraction following the pathway suggested by Boudot and Chone (1985). Nitrapyrin application checked the fixation of nitrite into organic matter. It is Table XV Recovery of Fall-Applied "N-Labeled Urea in May 1981 in the Soil Profile (kgN/ha) to 30-cm Depth of Baline Lake Clay Loam (Typic Udic Haploborolls)" Treatment" Urea Urea plus nitrapyrin

Organic N 12.2 a (24.4)d 14.5 a (28.9)

(NH,

+ NO, + NO,) N 32.4 a (64.7) 32.7 a (65.3)

Fixed NH, N'

Total N'

0.7 a

45.3 b (90.5) 48.3 b (96.6)

(1.4 1.2 b (2.3)

"From Aulakh and Rennie (1984). "Urea was applied at the rate of 50 kg N/ha and nitrapyrin at a concentration of 1% of active ingredient per weight of fertilizer N on 27 September 1980. 'In each column, the values differ significantly ( p < 0.05) when not followed by the same letter. dValues in parentheses represent the percentage recovery of fertilizer N.

294

K. L. SAHRAWAT

important to note that this mechanism of nitrite fixation in organic matter has been reported in soils in which nitrification occurred and nitrite accumulated only in small amounts (Azhar et al., 1986b,c). It has been proposed that nitrite formed reacted with phenols, forming nitro- and nitrosophenols. Nitrosophenols tautomerized to form quinone oxime, which could be reduced or oxidized chemically or enzymatically ultimately to form gaseous products of nitrogen. Results from these studies suggest an interesting pathway such that nitrification could lead to incorporation of mineral N (NO,) into organic N. Nitrapyrin has been found to block this pathway by checking NO, accumulation in soils. It should be made clear here that nitrification inhibitors increase immobilization of N by increasing the persistence of NH,. Also, nitrification inhibitors check NO, accumulation in soils and thus block fixation of NO, into organic matter. These two examples are simply two different aspects of the N immobilization process. Nitrite accumulation and its fixation into organic matter occurs under specific soil conditions (Chalk and Smith, 1983), whereas immobilization of mineral N is a more general process, but both are influenced by nitrification inhibitors. B. DENITRIFICATION It has been reported that nitrification inhibitors can inhibit denitrification is soils. For example, Mitsui et al. (1964) showed that nitrapyrin, dicyandiamide, and sodium azide retarded denitrification of nitrate N in wetland rice soils. Similarly, Henninger and Bollag (1976) found that sulfathiazole (ST), potassium azide, and phenylmercuric acetate (PMA) inhibited denitrification by soil microorganisms, but they could not confirm the inhibitory effect of nitrapyrin on denitirification. Other compounds, such as AM (2-amino-4-chloro- 6-methyl pyrimidine), ATC, and anilines also had no effect on denitrification. Some pesticides and nonspecific inhibitors of nitrification may also retard the denitrification process in soil (e.g., see Hauck, 1980, 1983; Goring and Laskowski, 1982). Yeomans and Bremner (1985a,b) found that none of the several herbicides, fungicides, and insecticides tested had any significant effect on denitrifcation of nitrate when added at 10 m a g soil concentration. Some of them had small effects when added at 50 mg/kg soil concentration. These results suggest that commonly used pesticides will have little effect on denitrification when added at normal rates. McElhannon and Mills (1981) investigated the effect of nitrapyrin on denitrification of nitrate in a field planted to sweet corn in a 2-year study. It was found that nitrapyrin reduced the loss of nitrate by denitrification in situations in which a readily oxidizable carbon substrate was available, for example, in the rhizosphere of a living plant, and when nitrapyrin was

EFFECTS OF NITRIFICATION INHIBITORS

295

applied to the nitrogen fertilizer band rather than by broadcast application. Contrary to these findings, Notton et a l . , (1979) found that nitrapyrin stimulated denitrification of nitrate, particularly in the presence of carbon sources such as root debris or acetone in sand culture used for growing turnip, cauliflower, and radish plants. Acetylene, which is an effective inhibitor of nitrification (Walter et al., 1979; Sahrawat et al., 1987),also inhibits nitrous oxide reductase enzyme, which converts N,O to N, (Federova et al., 1973; Yoshinari and Knowles, 1976; Yoshinari et al., 1977), and, consequently, the gaseous product of denitrification is released largely as N,O. In fact, the acetylene block technique is used to measure denitrification loss in soils by measuring N 2 0 emissions on a short-term basis (Yoshinari et al., 1977; Ryden and Rolston, 1983; Keeney, 1986). Bremner and Yeomans (1986) evaluated the effects of 28 nitrification inhibitors on denitrification of nitrate in soil by determining their influence on the amounts of nitrate lost and the amounts of nitrite, nitrous oxide (N,O), and N, produced when soil samples were incubated anaerobically after treatment with nitrate. The inhibitors evaluated included nitrapyrin (N- Serve); etridiazole (Dwell); potassium azide; 2-amino-4-chloro-6methyl pyrimidine; sulfathiazole(ST);4-amino-l,2,4-triazole;2,4-diamino6-trichloromethyl-s-triazine; potassium ethylxanthate; sodium diethyldithiocarbamate; phenylmercuric acetate (PMA); caffeic acid; and dicyandiamide. It was found that only potassium azide of the nitrification inhibitors studied retarded denitrification of nitrate when added at the rate of 10 m a g soil. Some results of this study are given in Table XVI. When added at the rate of 50 mg/kg soil, only potassium azide and 2,4-diamino6-trichloromethyl-s-triazine of the compounds tested inhibited denitrification. The other inhibitors either had no appreciable effect on denitrification or enhanced it when added at the rate of 10 or 50 mg/kg soil. The inhibitory effects of nitrapyrin and etridiazole (Dwell) on denitrification reported earlier (Mitsui et al., 1964; Mills and McElhannon, 1983, 1984; Mills et al., 1976; McElhannon and Mills, 1981; Mills, 1984) could not be confirmed because these compounds had no effect on denitrification when added at the rate of 10 mg/kg soil and enhanced denitrification when they were added at the rate of 50 or 100 mg/kg soil (Bremner and Yeomans, 1986). c . NITROUS OXIDE EMISSION VIA NITRIFICATION AND DENITRIFIC ATION It is generally believed that nitrous oxide (N,O) in soils is produced only through denitrification (CAST, 1976) but other research has clearly established that N,O is also produced during nitrification of ammonium

296

K. L. SAHRAWAT Table XVI

Effects of Some Nitrification Inhibitors on Denitrification of Nitrate in Soil"-b N produced (mg/kg soil) NO3 N lost (mg/kg soil) N 2 0 N

Nitrification inhibitor

N2 N

(NOz + N 2 0 + Nz) N ~

None Nitrapyrin (N-serve) Potassium azide 2-Amino-4-chloro-6-methyl pyrimidine (AM) 2-Mercaptobenzothiazole Sulfathiazole (ST) Etridiazole (Dwell) Potassium ethylxanthate Thiourea 4-Amino-] ,2,4-triazole (ATC) Sodium diethyldithiocarbamate Phenylmercuric acetate (PMA) Dicyandiamide (DCD) 2.4-Diamino-6-trichlorornethyls-triazine (CL- 1580) Caffeic acid

I 09 109 88 108

34 36 I 32

74 72 87 75

108 108 88 107

I10 108 I09 107 109 109 110 I16 I08 I08

38 39 33 26 35 39 38 20 39 31

72 68 75 81 74 70 73 78 69 76

I10 107 108 I07 109

109

33

74

107

109

Ill I17 108 I 07

"From Bremner and Yeomans (1986). "Thirty-gram samples of Canisteo soil (Typic Haplaquoll) were incubated at 30°C with 15 ml water under He atmosphere after treatment with 9 mg nitrate N as KNO, and 0 . 3 mg of the inhibitor (10 mg/kg soil) specified.

(Bremner and Blackmer, 1978; Freney et af., 1978, 1979; Goodroad and Keeney, 1984;Aulakh et al., 1984; Sahrawat et al., 1985). The mechanism of N,O production via nitrification is not clearly understood. The production of N,O via denitrification of nitrate and nitrification of ammonium can be represented as follows: Nitrate reductase

- *

NO3

NH,

NHzOH

Nitrite reductase

Nitrous oxide reductase

'

NOz

I

- '

NzO

Unidentified(H,N,O,?) compound

NO,

(1)

Nz

NO,

(2)

N2O

Because nitrification inhibitors retard oxidation of ammonium to nitrite, it is not surprising that they also retard N,O emissions through nitrification of ammonium. Bremner and Blackmer (1978) showed that nitrapyrin

297

EFFECTS O F NITRIFICATION INHIBITORS

greatly reduced emission of N,O from soils during nitrification of ammonium (Table XVII). Acetylene (C,H,), which retards nitrification of ammonium, also greatly reduces emissions of N,O from soils during nitrification of ammonium (Table XVIII) (Bremner and Blackmer, 1979; Aulakh et al., 1984). Smith and Chalk (1978, 1980) studied the effect of nitrapyrin addition on evolution of N 2 0 , N,, nitric oxide (NO), and nitrogen dioxide (NO,) gases from a calcareous soil treated with ammonia. Nitrapyrin largely reduced the gaseous loss of N, and oxides of N including N,O from soil. Nitrite accumulation occurred in soil treated with ammonia but was prevented by nitrapyrin (see Table X). It is recognized that nitrification inhibitors such as nitrapyrin check accumulation of nitrite N in soils and thus are likely to reduce N,O emissions via chemodenitrification or microbial denitrification of nitrite N indirectly (e.g., see Bremner and Blackmer, 1980; Nelson, 1982; Hauck, 1983; Chalk and Smith, 1983). Freney et al. (1979) found that N 2 0 emitted from soils, apparently via nitrification, at water contents ranging from air-dry t o field capacity was inhibited by HgCI, and toluene. Field studies have shown that nitrapyrin added at field rates of application reduced N,O emissions induced by fertilization of soils with urea and anhydrous ammonia (Table XIX) (Bremner et a / . 1981; Aulakh et al., 1984). In a field study of N,O emission from Australian soils, it was found that under fallow conditions, nitrapyrin significantly reduced anhydrous ammonia-induced loss of N,O only from a calcareous soil (pH, 8.5, organic C, 1.3%) but not from another soil (pH, 7.5; organic C, 2.0%). The inhibitor

Table XVII

Effect of Nitrapyrin on Emission of N 2 0from a Clay Loam Soil (pH 7.8; organic C 4.4%) Incubated under Aerobic Conditions after Treatment with Different Forms of NEVb

Form o f N added None None Ammonium [(NHJ2S041 Ammonium Urea Urea Nitrate (KN03) Nitrate

Nitrapyrin added ( p d g soil)

Amount of N,O N evolved in 20 days (pg/g soil) 4

4 148 10 122 4 6 4

"From Bremner and Blackmer (1978). "Different forms of N were added at a rate of 100 mg/kg soil and incubated at 60% WHC moisture and 30°C.

298

K. L. SAHRAWAT Table XVIII Effects of Acetylene on Nitrification and N20 Production in a Clay Loam Soil (pH 8.1, organic C 4.2%) Treated with Ammonium under Aerobic Conditions""

Treatment ~

~~~

~~~

Ammonium added (wdg soil)

C2H2added (%, v/v)

(NO2 + NO,) N produced in 12 days (mg/kg soil)

N 20 N evolved in 12 days (ng/g soil)

0 0 100 100

0 0.1 0 0.1

11 <1 105
-

208 <1

"From Bremner and Blackmer (1979). bSoilsamples (30 g) were treated with (NHJ2S04and incubated at 100 cm water tension under air or air containing C2H2(0.1%. v/v) at 30°C. Atmospheres in incubation vessels were renewed at 3-day interval.

had little effect on nitrification in the non-calcareous soil, probably due to its high content of organic matter (Magalhaes et al., 1984). In a laboratory study, Aulakh and Rennie (1985) found that potassium azide (KN,), a nitrification inhibitor, caused a severalfold increase in N,O emissions from soils under aerobic conditions when added at a rate of 1 mM. Nitrogen added either as NH4 or NO, had no effect on azide-induced N,O production (Table XX).Azide also inhibited nitrification, as reported

Table XIX Effects of Nitrapyrin on N,O Emission from Soil Fertilized with Anhydrous Ammonia"*b

Amount of N 2 0 N evolved (kg N/ha) in 167 days Treatment

Fall application

Spring application

None Anhydrous ammonia Anhydrous ammonia plus nitrapyrin

0.16 1.55

0.43 1.37

1.04

0.55

"From Bremner et al. (1981). bAnhydrousammonia (180 kg Nha) and nitrapyrin (0.56 kg/ha) were applied on 9 October 1979 for the fall application and on 15 April 1980 for the spring application.

EFFECTS OF NITRIFICATION INHIBITORS

299

Table XX Effects of KN3 on N,O Emission and CO, Evolution in Elston Clay Loam during 96 Hours of Incubation'" N Source

KN,'

(NHMO,

-

KNO, KNO2

+ + +

N 2 0 N evolvedd ( p d g soil)

C 0 2 C evolvedd ( p d g soil)

84212 a 1121 40 b 2023 a 1674 2 101 b 3556 2 247 a 1346 646 b

5729 a 8524 b 57k9 a 8756 b 131 2 14a 111 2 11 a

* *

~~

"From Aulakh and Rennie (1985). "Soil samples (100 g) were treated with 50 pg N/g soil as (NH,),SO,, KNO,, or K N 0 2with and without KN, (1 mM) and incubated at 60% moisture saturation at 25 2 1°C for 96 hr. ' - , KN, not added; + , KN, added at I mM rate. q h e values in the same column within each source in the absence and presence of KN, are significantly different at p c0.05 when not followed by the same letter.

previously. Addition of nitrapyrin or C2H2to azide showed that they had no effect on azide-induced N,O emissions. It was postulated that KN, stimulated denitrification by possibly enhancing the synthesis of denitrifying enzymes. Azide stimulated general microbial activity in soil treated with (NH,),SO, and KNO,, as measured by CO, evolution (Table XX). Nitrapyrin has been found to have little direct effect, if any, on N,O evolution through denitrification of nitrate N in soils, although it greatly reduces the production of N,O via nitrification (e.g., see Bremner and Blackmer, 1980). A study by McElhannon and Mills (1981), however, showed that nitrapyrin reduced nitrate fertilizer-induced N 2 0 emissions from soil planted to sweet corn. Bremner and Yeomans (1986), however, could not confirm the effect of nitrapyrin on denitrification of nitrate in a laboratory study in which mineral N as well as gaseous products of denitrification were determined. Casella et al. (1986) used 0.1% C2H, (v/v) under aerobic and anaerobic conditions to determine N 2 0 production via nitrification of ammonium and denitrification of nitrate and concluded that N losses by denitritication may potentially be higher than those occurring via nitrification. Davidson et al. (1986) have developed a technique based on the effects of low and high concentrations on C,H, on nitrification and denitrification for distinguishing between nitrification and denitrification as sources of nitrous oxide production in soils. The measurements of denitrification N,O were made

300

K . L. SAHRAWAT

from 24-hr laboratory incubations in which nitrification was inhibited by 10 Pa C2H,. Nitrification N,O was estimated from the differences between N,O production in the absence of C,H, and that determined for denitrification. Denitrification N,O was estimated from the differences between N,O production at 10 kPa C,H, and that at 10 Pa C,H,. It was found that the laboratory estimates of N,O were significantly correlated with field measurements in two forested watersheds during a 10-month period. These authors suggested that this technique is suitable for distinguishing between N,O production during nitrification and denitrification, which is important because the source of N,O produced in soil is often uncertain due to the possibility that denitrification and nitrification can occur simultaneously in the soil. This technique may also be suitable for qualitative study of the environmental parameters that regulate gaseous N loss via nitrification and denitrification (Davidson and Swank, 1986). There is an obvious need for further research to clarify the effects of nitrification inhibitors on N20 production through denitrification of nitrate in the presence of growing plants.

D. UREAHYDROLYSIS It has been generally found that the compounds proposed as nitrification inhibitors have little effect, if any, on urea hydrolysis when added at normal recommended rates (Mulvaney and Bremner, 1981). For example, Goring (1962b) found that nitrapyrin did not affect urea hydrolysis and was a specific inhibitor of the first step of the nitrification process, i.e., conversion of ammonium to nitrite. Similiarly, Bremner and Douglas (1971) Bundy and Bremner (1974), and Bremner and Bundy (1976) showed that of the several nitrification inhibitors tested (nitrapyrin, AM, ST, ATC, and several substituted anilines) for their effect on urea hydrolysis, only potassium azide (KN,) retarded urea hydrolysis to some extent when applied at 10 or 50 pg/g soil rates. However, Reddy and Prasad (1975) reported that nitrapyrin (1% of urea N) retarded urea hydrolysis in soil. It was found that potent nitrification inhibitors such as carbon disulfide (CS,) and sodium trithiocarbonate (Na,CS,), which release CS, upon decomposition, and nitrapyrin had little effect on urea hydrolysis in soil (Ashworth et al., 1977). Studies have also shown that DCD, a nitrification inhibitor, does not affect urea hydrolysis in soil (Amberger and Vilsmeier, 1979; Hauck and Behnke, 1981; Rodgers, 1983). Guthrie and B o d e (1981) also showed that ATC and nitrapyrin had no effect on urea hydrolysis at 2 or 20 pg/g soil rates of application. However, in other studies thiourea and ammonium thiosulfate were found to retard urea hydrolysis in addition to retarding nitrification in

30 1

EFFECTS OF NITRIFICATION INHIBITORS

soils (Malhi and Nyborg, 1979a,b; Goos, 1985). Ammonium thiosulfate retarded urea hydrolysis in soils but did not affect urea hydrolysis by jackbean urease (Table XXI). Thiourea retarded urea hydrolysis in soil (Table XXII) at very high concentrations (urea:thiourea, 2: 1). Such high rates of thiourea may unfavorably affect its practical use. It would appear that both thiourea and ammonium thiosulfate are general metabolic inhibitors rather than specific urease inhibitors (Goos, 1985). In an earlier study, Sahrawat (1979a) found that thiourea had a small effect on urea hydrolysis (10% inhibition) in a sandy clay loam soil when added at a 50 mg/kg rate. Ashworth et al. (1979) showed that potassium ethylxanthate, a nitrification inhibitor, was also a moderately effective inhibitor of urease activity in soils. Further studies showed that xanthates of unsubstituted alcohols of low molecular weight were very effective inhibitors of nitrification in soil at 20°C when added at the rate of 20 mg/kg of soil. The xanthates were also found to retard urease activity in soils when added at 200 mg/ kg of soil (a concentration I0 times higher than that used for inhibition of nitrification) (Ashworth et al., 1980). However, the xanthates were comparatively less effective urease inhibitors than benzoquinone in Maywood clay loam (Mollic Cryoboralf, pH 6.2, organic matter 2.6%) (Table XXIII). The effectiveness of xanthates in retarding nitrification and urease activity was not well correlated with the amounts of carbon disulfide evolved from soil treated with xanthates in sealed chambers. Mishra et Table XXI Inhibitory Effect (%) of Ammonium Thiosulfate (ATS) on Urea Hydrolysis of Urea Ammonium Nitrate (UAN) by Soil Urease and Jackbean Urease"

Amounts of ATS added to UAN (%, v/v) 0

1

2

2 days 4 days

0 (20)d 0 (62)'

William loam soil' 20 28 35 45

1 hr

0 (75)d

Jackbean urease' 0 0

5

10

SE"

45

50 52

5

47 0

0

-

3

"From Goos (1985). 'Standard error. 'Soil samples (25 g) were treated with aqueous solutions containing different volumes of UAN and ATS and incubated at 25°C for 2 or 4 days. *dues in parentheses show the percentage of the original urea hydrolyzed in the control (no ATS added). 'THAM buffer containingjackbean urease was treated with different volumes of UAN and ATS and incubated at 25°C for I hr.

302

K. L. SAHRAWAT Table XXII

Effects of Formulation of Urea with Thiourea and Pellet Size on Its Hydrolysis In the Field on Malmo SUty Clay Loam (Black Chernozem)”

Treatmentb

(9)

Urea N hydrolyzed’ at 8 days in 0-15-cm soil (apparent %)

Urea Urea plus thiourea (2: 1) Urea Urea plus thiourea (2: 1) Urea Urea plus thiourea (2: 1)

0.01 0.01 0.21 0.21 2.26 2.51

98 a 49 d 84 b 36 e 63 c 25 f

Pellet size

“From Malhi and Nyborg (1979a). ”Urea and thiourea were pelleted together and added at a rate of 112 kg N h a considering N both in urea and thiourea. Urea plus thiourea was added at a rate of 178.5 kg urea and 89.2 kg of thioureaha. ‘Values not followed by the same letter are significantly different (p <0.05).

Table XXIII

Effects of Xanthates, Benzoquinone, and Sodium Trithiocarbonateon Urease Activity in a Clay Loam Soil“’*

Compound

Inhibition of urease activity after 24 hr (%)

Benzoquinone Sodium trithiocarbonate Potassium methyl xanthate Sodium methoxymethyl xanthate Potassium ally1 xanthate Potassium ethyl xanthate Potassium 2-methoxyethyl xanthate Potassium isopropoxyethyl xanthate Potassium ethylene glycol xanthate Potassium 2-dimethylaminomethylxanthate Sodium 2-nitrilo-2-propyl xanthate

82 22 74 49 48 29 49 21 18 29 19

“From Ashworth et al. (1980). ”Soil samples (18 g) were treated with 400 mg of urea N and 200 mg of the inhibitor per kilogram of soil and incubated at 15% water content under aerobic conditions at 23°C.

EFFECTS OF NITRIFICATION INHIBITORS

303

af. (1980) studied the effects of some quinoid and phenolic compounds on urease activity and found that 1,4-naphthoquinone; 2-methyl- 1,4napthoquinone; 2,3- dichlorohydroquinone; 4,6-ditert-butyl-o-benzoquinone; 4,6-di-tert-butylpyrocatechol; and 4-tert-butylpyrocatechol added at 10 and 20 mg/kg soil rates retarded urea hydrolysis to varying degrees in addition to retarding nitrification. Some organophosphorus insecticides were also found to retard urea hydrolysis (e.g., see Lethbridge and Burns, 1976; Sahrawat, 1979b) in addition to retarding nitrification in soil (Sahrawat, 1980).

IV. OTHER EFFECTS In addition to the effects of nitrification inhibitors discussed above, they may also affect soil-borne plant diseases (Huber and Watson, 1974; Huber et af., 1977;White et af., 1978) and growth of leguminous and cereal crops due to phytotoxicity (for review see Sahrawat and Keeney, 1984). It has been found that in general nitrapyrin and 6-CPA were more phytotoxic to dicotyledenous plants than to grasses (Geronimo et af., 1973a,b). Nitrapyrin has also been found to be toxic to leguminous plants, such as soybean and alfalfa (McKell and Whalley, 1964; Riley and Barber, 1970), and to cotton and ryegrass (Parr et af., 1971). The effects of nitrification inhibitors on plant disease, phytotoxicity, and plant quality and composition were discussed by Sahrawat and Keeney (1984). Janzen and Bettany (1986) studied the effect of ammonium thiosulfate on nitrification of ammonium in Weyburn loam soil (Typic Cryoborolls; pH, 7.2; organic C, 2.8%) It was found that unlike nitrification inhibitors such as nitrapyrin, thiosulfate inhibited the second step of nitrification, the oxidation of nitrite to nitrate, and resulted in the accumulation of high concentrations (as high as 42 mg NO, N/kg soil) of nitrite N. The accumulation of nitrite was very conspicuous at higher concentrations of thiosulfate. The accumulation of nitrite may thus pose a serious drawback in the use of ammonium thiosulfate as a nitrification inhibitor, because it is known that small nitrite concentrations, as low as 2 mg N/kg can adversely affect plant growth (Keeney, 1982). Additionally, thiosulfate has been reported to be toxic to plants (Audus and Quastel, 1947)and may also retard other beneficial microbial processes (Schmidt, 1982). The inhibitory effect of thiosulfate on soil nitrification may be due to the toxic effect of thiosulfate or its oxidation products, tetrathionate and sulfite, on Nitrobacter, resulting in slowed nitrate formation (Janzen and Bettany, 1986). It is also possible that the effect on ammonium oxidation

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could be a secondary one due to accumulation of nitrite, which is known to be toxic to ammonium oxidizers (Labedo and Alexander, 1978). The accumulation of nitrite could be further influenced by soil pH in the alkaline range and by ammonium concentrations (Chapman and Liebig, 1952). In a greenhouse study, Morris et al. (1980) found that nitrapyrin increased soybean dry matter at 0.1 and 1 mg/kg concentrations but had an adverse effect on growth at a concentration of 10 mg/kg. Nitrapyrin had little effect on nitrogenase activity of soybean at soil concentrations less than 10 m/kg but the enzyme activity was increased by 0.1 mg/kg concentration of nitrapyrin. Root and nodule dry weights were not affected by nitrapyrin. (Table XXIV). Application of nitrapyrin with ammonium sulfate retarded nitrification and also corrected iron chlorosis in peanuts growing on a calcareous soil in a pot-culture study (Kafkafi and Neumann, 1985). It was suggested that retardation of nitrification resulted in preferential uptake of NH,, which caused an efflux of H’ from plant roots that solubilized iron near the root surface and helped correct iron chlorosis. It is known that plants grown in a nitrate medium compared to ammonium show chlorosis due to an increase in pH accompanying nitrate uptake (for review see Haynes and Goh, 1978; Sahrawat and Keeney, 1984).

Table XXIV Effect of Nitrapyrin on Nitrogenase Activity and Nodule Weight of Soybeans Grown on Pak Chong Clay (Oxic Rhodic Paleustult) in Greenhouse Pots”

Nitrapyrin added (mg/kg soil)

Nodule dry wt”

(g)

Nitrogenase activity‘ (MC2H,/nodule/hr)

2.91 3.80 3.42 2.35 3.23 3.28 3.16 20.40

0.05 bc 0.10 a 0.06 ab 0.03 bc 0.02 bc 0.01 bc 0.04 57.90

0. I5 0.34 0.24 0. I7 0. I6 0.18 0.21 43.00

Root dry wth

(€9

~

0 0. I I 10 20 30 Average CVd (%)

“From Morris et a / . (1980). Two soybean plants, cultivar SJ4, were grown in each pot containing 5 kg soil. The data presented are means of two soybean plants obtained at flowering stage. ”Root and nodule dry weight differences are not significant at p = 0.05. “Values not followed by the same letter are significantly different at p = 0.05. of variation. d C ~coefficient .

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305

V. PERSPECTIVES There has been considerable research during the past two decades on the effects of nitrification inhibitors on retardation of nitrification in soils. However, in contrast, the effects of these chemicals on other processes involved in the nitrogen cycle in soils have received relatively little attention. Recent research has focused on some of these aspects. For example, results summarized in this chapter clearly establish that nitrification inhibitors such as nitrapyrin are able to reduce the movement of nitrate N formed as a result of nitrification of ammonium-forming fertilizers in the soil profile, which has relevance not only to efficient use of ferttilizer nitrogen for crop production but also to nitrate pollution of surface and ground waters. There is a need to generate more data for temperate and tropical soil conditions in which intensive agriculture is prevalent under high fertilizer N inputs. Work by Juma and Paul ( 1983) has suggested that nitrification inhibitors can influence nitrogen cycling in soils by affecting fixation of ammonium chemically by clay minerals and by microbial immobilization of ammonium. Both these processes affect the subsequent release of mineral N and its availability to plants. Further research is needed to characterize the effects of nitrification inhibitors on ammonium fixation and microbial immobilization in relation to organic matter, clay mineralogy, and other soil characteristics. Immobilization of N and ammonium fixation as affected by retardation of nitrification could be important factors in temporarily holding the mineral N against its loss by leaching and ammonia volatilization. However, information is needed to elucidate the fate of fixed ammonium and immobilized nitrogen following retardation of nitrification in relation to its subsequent release in mineral N form and availability to plants. Ammonia volatilization seems to be an important consideration in the use of nitrification inhibitors. Based on the information available, it can be inferred that when ammonium fertilizers amended with nitrification inhibitors are applied to the surface of a light-textured calcareous soil, loss due to ammonia volatilization would increase. More information, however, is needed as to how the placement of ammonium fertilizers amended with nitrification inhibitors affects the volatile loss of ammonia, and also how the presence of growing plants with established root systems affects the loss due to ammonia volatilization. This would help in evolving a suitable N management practice so that the advantage of retardation of nitrification is not offset by the volatile loss of ammonia. It is important to note especially that although nitrification inhibitors such as nitrapyrin reduce the emission of N,O via nitrification in soils,

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they have little, if any, direct effect on denitrification and associated N 2 0 production. There is an urgent need to develop chemicals that can control N,O production in soils associated with both nitrification and denitrification. Better understanding of the enzyme system involved in N20 production via processes of nitrogen transformations should aid in developing chemicals that could block the specific enzyme system that is responsible for N 2 0 production (Sahrawat and Keeney, 1986). The best way, of course, of reducing N 2 0 emissions from soils is by increasing the efficiency of N in crop production. It should be emphasized that the retardation of nitrification affects the microsite chemistry of soils by increasing the persistence of ammonium, with a concomitant rise in soil pH. These two changes greatly affect the subsequent nitrogen transformations such as ammonium fixation, ammonia volatilization, immobilization, and N 2 0 production in soils. High soil pH at the microsite level may also greatly affect the solubilization of organic matter, and this could change the course of the release of mineral nitrogen and other plant nutrients. It is very important to follow the changes in soil pH, particularly at the microsite level, in studies investigating the effects of nitrification inhibitors on nitrogen transformation processes in relation to soil physical and chemical characteristics. There is an obvious need to generate information as to how the microsite soil pH changes brought on by retardation of nitrification affect the physical, chemical, and biological processes relevant to nitrogen transformations in soils. The more important processes that are likely to be influenced and have relevance to nitrogen cycling in soils and its availability to plant, are obviously nitrogen mineralization, immobilization, and remineralization and ammonium futation; these should receive research priority. It is hoped that this review will stimulate research on the vaned effects of nitrification inhibitors on nitrogen transformations in soil. This area of research should develop in importance with the increasing interest in the use of nitrification inhibitors.

REFERENCES Alexander, M. 1977. “Introduction to Soil Microbiology,” 2nd Ed. Wiley, New York. Amberger, A. 1986. Z . Pflanzenernaehr. Bodenkd. 149,469-484. Amberger, A., and Vilsrneier, K. 1979. Landw. Forsch. 32, 409415. Ashworth, J., Briggs, G . G . , Evans, A. A., and Matula, J. 1977. J . Sci. Food Agric. 28, 673-683. Ashworth, J., Rodgers, G . A., and Briggs, G . G . 1979. Chem. Ind. (London) 17,90-92. Ashworth, J., Akerboorn, H. M., and Crepin, J. M. 1980. Soil Sci. SOC.A m . J . 44, 12471249. Audus, L. J . , and Quastel, J. H. 1947. Nature (London) 159, 263-264. Aulakh, M. S., and Rennie, D. A. 1984. Soil Sci. SOC.Am J . 48, 1184-1189. Aulakh, M. S., and Rennie, D. A. 1985. Can. J . Soil Sci. 65, 205-212.

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Aulakh, M. S., Rennie, D. A., and Paul, E. A. 1984. Soil Biol. Biochem. 16, 351-356. Azhar, EISayed, Vehre, R., Proot, M., Sandra, P., and Verstraete, W. 1986a. Plant Soil 94, 369-382. Azhar, EISayed, Vandenabeele, J., and Verstraete, W. 1986b. Plant Soil 94, 383-399. Azhar, EISayed, van Cleemput, 0.. and Verstraete, W. 1986~.Plant Soil 94, 401409. Bjarnason, S. 1987. Plant Soil 97, 381-389. Boudot. J. P., and Chone, Th. 1985. Soil Biol. Biochem. 17, 135-142. Bremner, J. M., and Blackmer, A. M. 1978. Science 199, 295-296. Bremner, J. M., and Blackmer, A. M. 1979. Nature (London) 280, 380-381. Bremner, J. M., and Blackmer, A. M. 1980. In “Biochemistry of Ancient and Modern Environments” (P. A. Trudinger, M. R. Walter, and R. J. Ralph, eds.), pp. 279-291. Australian Academy of Science, Canberra. Bremner, J. M., and Blackmer, A. M. 1981. I n “Denitrifcation, Nitrification and Atmospheric Nitrous Oxide” (C. C. Delwiche, ed.), pp. 151-170. Wiley, New York. Bremner, J. M.,and Bundy, L. G. 1976. Soil Biol. Biochem. 8, 131-133. Bremner, J. M., and Douglas, L. A. 1971. Soil Biol. Biochem. 3, 297-307. Bremner, J. M., and Yeomans, J. C. 1986. Biol. Ferfil. Soils 2, 173-179. Bremner, J. M., Breitenbeck, G. A., and Blackmer, A. M. 1981. Geophys. Res. Lett. 8, 353-356. Bundy, L. G., and Bremner, J. M . 1974. Soil Biol. Biochem. 6, 369-376. Casella, S., Leporini, C., and Picci, G. 1986. Biol. Fertil. Soils 2, 65-70. CAST (Council for Agricultural Science and Technology) 1976. Report 53, Iowa State University, Ames. Chalk, P. M., and Smith, C. J. 1983. In “Gaseous Loss of Nitrogen from Plant-Soil Systems” (J. R. Freney and J. R. Simpson, eds.), pp 65-89. (Dev. Soil Plant Sci. 9.) Martinus Nijhoff, The Hague. Chapman, H. D., and Liebig, G. F. 1952. Soil Sci. SOC.A m . Proc. 16, 276-282. Cornforth, I. S., and Chasney, A. D. 1971. Plant Soil 39, 497-501. Davidson, E. A., and Swank, W. T. 1986. Appl. Environ. Microbiol. 52, 1287-1292. Davidson, E. A., Swank, W. T., and Pery, T. 0. 1986. Appl. Environ. Microbiol. 52, 12801286. Dubey, H. D., and Rodriguez, R. L. 1970. Soil Sci. SOC.A m . Proc. 34,435439. Federova, R. I., Milekhina, E. I., and II’Yukhina, N. I. 1973. Akad. Nuuk SSR Izv. Ser. Biol. 6, 797-806. Freney, J. R., Denmead, 0. T., and Simpson, J. R. 1978. Nature (London) 273, 530-532. Freney. J. R., Denmead, 0. T., and Simpson, J. R. 1979. Soil Biol. Biochem. 11, 167-173. Gasser, J. K. R. 1970. Soils Fertil. 33, 547-554. Geronimo, J., Smith, L. L., Jr., and Stockdale, G. D. 1973a. Agron. J. 65, 692-693. Geronimo, J., Smith, L. L., Jr., Stockdale, G. D., and Goring, C. A. I. 1973b. Agron. J . 65, 689-691. Gin, T., Saha, D., and Mukhopadhyay, A. K. 1982. Indian Agric. 26, 11 1-1 14. Goodroad, L. L., and Keeney, D. R. 1984. Soil Biol. Biochem. 16, 3943. Goos, R. J. 1985. Soil Sci. SOC.A m . J . 49, 232-235. Goring, C. A. I. 1962a. Soil Sci. 93, 211-218. Goring, C. A. I. 1962b. Soil Sci. 93,431439. Goring, C. A. I., and Laskowski, D. A. 1982. I n “Nitrogen in Agricultural Soils” (F. J. Stevenson, ed.), pp. 689-720. Am. SOC.Agron., Madison, Wisconsin. Guthrie, T. F., and Bomke, A. A. 1981. Can. J. Soil Sci. 61, 529-532. Hauck, R. D. 1972. In “Organic Chemicals in the Soil Environment” (C. A. I. Goring, and J. W. Hamaker, eds.), Part B, pp. 633-690. Dekker, New York. Hauck, R. D. 1980. In “Nitrification Inhibitors-Potentials and Limitations” (J. J. Meisinger, G. W. Randall, and M. L. Vitosh, eds.) Special Publ. No. 38, pp. 19-32. Am. SOC. Agron. Madison, Wisconsin.

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Hauck, R. D. 1983. In “Gaseous Loss of Nitrogen from Plant-Soil Systems” (J. R. Freney, and J. R. Simpson, eds), pp. 285-312. (Dev. Plant Soil Sci. 9.) Martinus Nijhoff, The Hague. Hauck, R. D. 1984. In “Nitrogen in Crop Production” (R. D. Hauck, ed.), pp. 551-560. Am. SOC.Agron., Madison, Wisconsin. Hauck, R. D., and Behnke, H. (eds.) 1981. Proc. Workshop on Dicyandianide. SKW Trostberg AG, West Germany. Hauck, R. D., and Bremner, J. M. 1969. In “Biology and Ecology of Nitrogen,” pp. 3139. National Academy of Sciences, Washington, D. C. Haynes, R. J., and Goh, K. M. 1978. Biol. Rev. 53, 465-510. Henninger, N. M., and Bollag, J. M. 1976. Can. J . Microbiol. 22, 668-672. Hergert, G. W., and Wiese, R. A. 1980. In “Nitrification Inhibitors-Potentials and Limitations” (J. J. Meisinger, G. W. Randall., and M. L. Vitosh, eds.), pp. 89-105. Special Publ. No. 38, Am. SOC.Agron., Madison, Wisconsin. Huber, D. M., and Watson, R. D. 1974. Annu. Rev. Phyroparhol. 12, 139-165. Huber, D. M., Murray, G. A., and Crane, J. M. 1969. Soil Sci. SOC.A m . Proc. 33, 975976. Huber, D. M., Warren, H. L., Nelson, D. W., and Tsai, C. Y. 1977. BioScience 27, 523529. Jain, J. M., Sarkar, M. C., and Deori, M. L. 1981. J . Indian SOC. Soil Sci. 29, 97-100. Janzen, H. H., and Bettany, J. R. 1986. Soil Sci. SOC.A m . J . 50, 803-806. Juma, N. G . , and Paul, E. A. 1983. Can. J . Soil Sci. 63, 167-175. Juma, N. G., and Paul, E. A. 1984. Soil Sci. SOC.Am. J . 48, 76-80. Kafkafi, U., and Neumann, R. G. 1985. J . Plant Nurr. 8, 303-309. Keeney, D. R. 1982. In “Nitrogen in Agricultural Soils” (F. J. Stevenson, ed.), 605-649. Am. SOC.Agron., Madison, Wisconsin. Keeney, D. R. 1986. In “Field Measurement of Dinitrogen Fixation and Denitrification” (R.D. Hauck, and R. W. Weaver, eds.), pp. 103-115. Special Publ. 18, Soil Sci. SOC. Am., Madison, Wisconsin. Keeney. D., Walter, H., Goodroad, L., and Kelling, K. 1979. Commun. Soil Sci. Plant Anal. 10, 1505-1512. Labedo, D. P., and Alexander, M. 1978. J. Environ. Qual. 7, 523-526. Laskowski, D. A., O’Melia, F. C., Griffith, J. D., Regoli, A. J., Youngson, C. R., and Goring, C. A. I. 1975. J. Environ. Qual. 4,412-417. Lethbride, G., and Burns, R. G . 1976. Soil Biol. Biochem. 8, 99-102. McElhannon, W. S., and Mills, H. A. 1981. J . Am. SOC.Hortic. Sci. 106, 673-677. McKell, C. M., and Whalley, D. B. 1964. Agron. J. 56, 26-28. Magalhaes, A. M. T., and Chalk, P. M. 1987. Fertil. Res. 11, 173-184. Magalhaes, A. M. T., Chalk, P. M., and Strong, W. M. 1984. Fertil. Res. 5, 411-421. Malhi, S. S., and Nyborg, M. 1979a. Plant Soil 51, 177-186. Malhi, S. S., and Nyborg, M. 1979b. Proc. Alberta Soil Science Workshop, Lethridge, Alberta, Canada, pp. 113-131. Malhi, S. S., and Nyborg, M. 1983. Soil Biol. Biochem. 15, 581-585. Meisinger, J. J., Randall, G. W., and Vitosh, M. L. (eds.) 1980. “Nitrification InhibitorsPotentials and Limitations.” Special Publ. No. 38, Am. SOC.Agron., Madison, Wisconsin. Mills, H. A. 1984. Commun. Soil Sci. Plant Anal. 15, 1007-1016. Mills, H. A., and McElhannon, W. S. 1983. HortScience 18,740-741. Mills, H. A., and McElhannon, W. S. 1984. HortScience 19, 54-55. Mills, H. A., Barker, A. V., and Maynard, D. N. 1976. Agron. J . 68, 13-17. Mishra, M. M., and Flaig, W. 1979. Plant Soil 51, 301-309.

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Moms, D. R., Boonkerd, N., and Vasuvat, Y. 1980. Plant Soil 57, 31-39. Mulvaney, R. L., and Bremner, J. M. 1981. Soil Biochem. 5 , 153-196. Nelson, D. W. 1982. In “Nitrogen in Agricultural Soils” (F. J. Stevenson, ed.), pp. 327363. Am. SOC.Agron., Madison, Wisconsin. Notton, B. A., Watson, E. F., and Hewitt, E. J. 1979. Plant Soil 51, 1-12. Onken, A. B. 1980. I n “Nitrification Inhibitors-Potentials and Limitations” (J. J. Meisinger, G. W. Randall, and M. L. Vitosh, eds.), pp. 119-129. Special Publ. No. 38. Am. SOC. Agron., Madison, Wisconsin. Owens, L. B. 1981. J. Environ. Qual. 10, 308-310. Owens, L. B. 1987. J. Environ. Qual. 16, 34-38. Papendick, R. I., and Engibous, J. C. 1980. In “Nitrification Inhibitors - Potentials and Limitations” (J. J. Meisinger, G.W. Randall, and M. L. Vitosh, eds.), pp. 107-117. Special Publ. No. 38. Am. SOC.Agron., Madison. Wisconsin. Parr, J. F., Carroll, B. R., and Smith, S. 1971. Soil Sci. SOC.A m . Proc. 35, 469473. Prakasa Rao, E. V. S. , and Puttanna, K. 1987. Plant Soil 97, 201-206. Prasad, R., Rajale, G. B., and Lakhdive, B. A. 1971. Adv. Agron. 23, 337-383. Reddy, R. N. S., and Prasad, R. 1975. J. Soil Sci. 26, 304-312. Riley, D., and Barber, S. A. 1970. Agron. J . 62, 550-551. Rodgers, G. A. 1983. Fertil. Res. 4, 361-367. Ryden, J. C., and Rolston, D. E. 1983. I n “Gaseous Loss of Nitrogen from Plant-Soil Systems” (J. R. Freney and J. R. Simpson, eds.) pp. 91-132. (Dcv. Soil Plant Sci. 9.) Martinus Nijhoff, The Hague. Sahrawat, K. L. 1979a. Fertil. Technol. 16, 244-245. Sahrawat, K. L. 1979b. Plant Soil 53, 11-16. Sahrawat, K. L. 1980. Plant Soil 57, 335-352. Sahrawat, K. L. 1986. Proc. Nat. Symp. Curr. Trends Soil Biol. Haryana Agric. Univ., Hisar, India, 25-27 Feb., 1985. pp. 85-97. Sahrawat, K. L., and Keeney, D. R. 1984. J. Plant Nutr. 7, 1251-1288. Sahrawat, K. L., and Keeney, D. R. 1985. Commun. Soil Sci. Plant Anal. 16, 517-524. Sahrawat, K. L., and Keeney, D. R. 1986. Adv. Soil Sci. 4, 103-148. Sahrawat, K. L., Keeney, D. R., and Adams, S. S. 1985. For. Sci. 31, 680-684. Sahrawat, K. L., Keeney, D. R., and Adams. S. S. 1987. Plant Soil 101, 179-182. Sandhu, M. S., and Moraghan, J. T . 1972. Commun. Soil Sci. Plant Anal. 3, 439447. Schmidt, E. L. 1982. I n “Nitrogen in Agricultural Soils” (F. J. Stevenson, ed.). pp. 253288. Am. SOC.Agron., Madison, Wisconsin. Simpson, J. R., Freney, J. R., Muirhead, W. A., and Leuning, R. 1985. Soil Sci. Soc. A m . J . 49, 1426-1431. Slangen. J. H. G., and Kerkhoff, P. 1984. Fertil. Res. 5 , 1-76. Smith, C. J., and Chalk, P. M. 1978. In “Plant Nutrition 1978” Proc. I n t . Colloq. Plant Anal. Fertil. Problems. Sth, Auckland pp. 483-490. Smith, C. J., and Chalk, P. M. 1980. Soil Sci. SOC.A m . J . 44,277-282. Timmons, D. R. 1984. J. Environ. Qual. 13, 305-309. Walter, H. M., Keeney, D. R., and Fillery, I. R. 1979. Soil Sci. Soc. A m . J . 43, 195-196. White, D. G., Hoeft, R. G., and Touchton, J. T. 1978. Phytopathology 68, 81 1-814. Yeomans, J. C., and Bremner, J. M. 1985a. Soil Biol. Biochem. 17, 447452. Yeomans, J. C., and Bremner, J. M. 1985b. Soil Biol. Biochem. 17, 453-456. Yoshinari, T., and Knowles, R. 1976. Biochem. Biophys. Res. Commun. 69, 705-710. Yoshinari, T., Hynes, R., and Knowles, R. 1977. Soil Biol. Biochem. 9, 177-183.

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ADVANCES IN AGRONOMY. VOL. 42

COMPACTION EFFECTS ON SOIL STRUCTURE' Satish C. Gupta,' Padam P. Sharma? and Sergio A. DeFranchi3 *Department of Soil Science University of Minnesota St. Paul, Minnesota 55108 'lnstituto di Agronomia Universita Degli Studi Della Basilicata, Italy I. II. III. IV.

V. VI.

Introduction Soil Structural Parameters A. Pore Geometry B. Soil Matrix Mechanisms of Soil Structure Changes during Compaction Guidelines on Water Contents and Mechanical Stresses Conducive to Irreversible Changes in Soil Structure A. Degree of Saturation at Minimum Pore Water Pressure B. Mechanical Stress Corresponding to a Minimum Pore Water Pressure Summary Future Research Needs References

I. INTRODUCTION The term soil structure refers to the arrangement of primary soil particles into secondary particles or aggregates. This definition can be further extended to include the arrangement of aggregates or clods in a soil body. Simply stated, soil structure is the result of the interactions between soil solids and soil pores. Soil solids could be in the form of primary or secondary particles. Soil structure can be described at a macroscopic or a microscopic level. A macroscopic description of soil structure includes a measure of the relative proportion of soil solids to soil pores, often characterized as bulk density, void ratio, or total porosity. The microscopic description of soil 'Contribution from the Department of Soil Science and the Minnesota Agricultural Experiment Station, University of Minnesota, St. Paul, MN 55108. Paper No. 15636, Science Journal Series. 31 I Copyright 8 1989 by Academic Press, Inc. All rights of reproduction in any form reserved.

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structure covers interactions of soil solids with other soil solids or soil pores. These interactions are interpreted from soil behavioral properties like soil strength or water and air flow characteristics. Soil compaction refers to the compression of unsaturated soil, during which the density of the soil body increases and there is a simultaneous reduction in fractional air volume. In other words, the effect of compaction on a soil body is a change in its structure. That is why soil compaction is often described by the measures of bulk density, void ratio, or total porosity, parameters that grossly quantify soil structure. Other parameters used to describe soil compaction include applied force or applied stress. There is an increased concern regarding the effect of compaction on crop production in mechanized agriculture. Compaction effects on crop yield are due to changes in soil physical, chemical, and biological processes that in turn are dependent upon the structure of the soil. To separate beneficial from harmful compaction and to provide guidelines on the range of applied stresses and water contents not conducive to excessive compaction, we need to understand and quantify changes in soil structure upon compaction (Gupta and Allmaras, 1987). Previous reviews on soil structure and soil compaction covered the following topics: (a) the concepts of soil structure and its influence on physical properties and plant growth (Russel, 1971; Hamblin, 1985); (b) forces causing soil compaction and the soil compaction process (Barnes et al., 1971); (c) models to assess the behavior of soils undergoing compaction (Gupta and Larson, 1982a; Gupta and Allmaras, 1987); and (d) effect of compaction and tillage on various physical properties and plant growth (Barnes et al., 1971; Unger et al., 1982). In this paper, we review the literature on (a) the effects of compaction on parameters that describe soil structure at a microscopic level, i.e., soil solid-soil pore interactions (soil pore geometry) or soil solid-solid interactions (soil matrix), and (b) mechanisms by which microscopic soil structural changes result during soil compaction. The former objective could be simply stated as the study of relationships between macroscopic and microscopic changes in soil structure.

II. SOIL STRUCTURAL PARAMETERS Depending upon the experimental objectives, soil structure studies in the literature could be grouped into those describing (1) pore geometry or (2) soil matrix. Scientists concerned with water, solute, and gas movement have characterized soil structure in terms of pore geometry, i.e., pore size distribution and pore continuity. Soil morphologists and scientists

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313

working in soil erosion research have concentrated on the understanding of soil matrix, i.e., aggregate size distribution and aggregate strength.

A. POREGEOMETRY Measurements that describe pore geometry include the water retention characteristic curve, permeability/infiltration rate, soil-water diffusivity, sorptivity, air permeability, and gas diffusion.

1 . Water Retention Characteristic The water rentention characteristic (WRC) curve is the relationship between the quantity of water in soil pores to the energy with which this water is held in these pores. Soil structure greatly affects the water retention characteristics of soils. Aggregated soils generally retain more water than sands at a given soil matric potential (Fig. 1). In aggregated soils, a large proportion of soil water at high matric potential is in the voids formed by aggregates, whereas soil at low matric potential is in voids formed by soil particles. Since soil compaction alters the aggregate size distribution, we expect a shift in the proportion of inter- and intra-

1-2-MM CLAY

SOIL AGGREGATES

1-2-MM SAND

u

0

-0.5 -1.0 -1.5 -2.0 -2.5

MATRIC POTENTIAL, kPa

FIG. 1. Water retention curves of 1- to 2-mm fractions of sand and Gault clay soil aggregates. (Adapted from Childs, 1957.)

314

SATISH C. GUPTA ET AL.

aggregate voids during compaction. Thus, the shape of the WRC curve is a good indicator of alterations in soil structure or soil pore geometry due to compaction. From studies of water retention characteristics, Croney and Coleman (1954) concluded that increasing the dry density of soils modifies the pore size distribution such that there results (a) a decrease in the amount of water held at high potentials, (b) a decrease in the potential at which air first enters the soil, and (c) an increase in the amount of water held at low potentials (Fig. 2). The same authors also noted that the compaction of soil partially closes the hysteresis loop between the wetting and drying curves (Fig. 2). Figure 3 shows WRC curves of Nicollet clay loam at four levels of compaction (Gupta and Larson, 1982b). With an increase in the level of compaction, the amount of water held at a high soil matric potential decreases whereas the amount of water held at a low soil matric potential increases. This shift in the WRC curves reflects a change in the relative proportion of soil pores (Table I). Figure 3 and Table I indicate the following changes in soil structure due to compaction: (a) a reduction in total volume of soil, as indicated by

MOISTURE CONTENT, % g g-' Relationships between suction and water content for a silty sand at two densities. FIG.2. (Adapted from Croney and Coleman, 1956.)

315

COMPACTION EFFECTS ON SOIL STRUCTURE Table 1 Pore Size Distribution in a Nicollet Clay Loam at Four Levels of Soil Compaction

Pore volume"

(%I Applied stress (kPa)

>I50

15-150

Pore sizeb (pm) 1-15


5.8 14.8 45.0 113.8

21.1 12.5 9.4 6.2

35.7 33.9 23.7 12.2

10.9 13.6 19.2 16.7

32.3 40.0 47.7 64.9

"Pore volume (%) of pore size < lpm = 100 - sum of pore volumes (%) of other pore size fractions. bPore size (pm) = 150/matric suction (kPa).

water retention at the highest potential (JI, = - 0.1 kPa); (b) a reduction in the proportion of large pores (water retention at soil matric potentials between -0.1 and - 1.0 kPa); and (c) an increase in the proportion of smaller pores (water retention at soil matric potentials of - 1.0 kPa and lower). Curves in Fig. 3 show that the sigmoid shape of the WRC curve changes to a straight line with an increase in compaction. This indicates the presence of single size pores (corresponding to the lowest potential) at a higher level of compaction. 0.6

? * E

0

E0.5

m W"

5 0.4

P

& 0.3 c

3 g 0.2 c

0

E 0.1 3 0

- 0.1

-1.0 MATRIC POTENTIAL, kPa

-10.0

-100.0

RG.3. Water retention characteristics of Nicollet clay loam at four bulk densities.

3 16

SATISH C. GUPTA ET AL.

Hill and Sumner (1967) studied the effects of compaction on the WRC of nine different-textured soils and grouped WRCs into three categories based on the soil texture (Fig. 4). I. Sand: An increase in bulk density resulted in an increase in capacity to retain water at a given matric potential. The magnitude of this effect decreased with a decrease in soil water potential. 2. Sandy loams and sandy clay loams: An increase in bulk density resulted in a decrease in the capacity to retain water at high matric potentials and the reverse at low matric potentials. 3. Clay loams and clays: An increase in bulk density resulted in an increase in water retention, and the magnitude of this effect increased with decreasing matric potential. Hill and Sumner (1967) explained these differences as follows: In sands, large pores predominate and, consequently, compaction has the greatest effect in the high matric potential range. In clays there are many more smaller pores and compaction, therefore, has an effect over a wide range of matric potentials but particularly in the low matric potential range. In the case of sandy loam, there is an even distribution of various particle size fractions. In such soils, compaction results in a reduction of total porosity rather than a large decrease in the number and relative volume of large pores or large increase in the number and relative volume of small pores. Thus, changes in the WRC curves are minimal in sandy loams and sandy clay loams (Fig. 4). 2 . Permeabilityllnfiltration Rate

Laliberte and Brooks (1967) measured the effect of compaction on permeability of three soil materials. Figure 5 shows the change in relative

-10

-100

-1000

MATRIC POTENTIAL, kPa

FIG.4. Relationship between moisture content (% g g-'), matric potential (kPa) and bulk density, pb, (Mg K3)(Adapted from Hill and Sumner, 1967.)

COMPACTION EFFECTS ON SOIL STRUCTURE 1

0.1 T

t A

z

T

't

h

0.01

W

317

n

4 = 0.395

W

1

4

KO=

0.177/.im

$b= 89.5 mb 7.1

W K

v=

10

'

4 = 0.423

4

KO=0.257pm

&,=

75.6 mb

0.449

KO=0 . 3 2 8 p $b' 63.5 mb 6.8

v

1= 6.9

4 = 0.478 KO= 0.563 pm $ b= 50.7mb I 6.4

v

4 = 0.503 KO=0 . 6 9 5 w $13'41.3 mb = 5.1

v

1 A I L I 100 loo( I0 100 100

FIG.5. Relative permeability (K,) as a function of capillary pressure ($I.) for Touchet silt loam packed at five different porosities. porosity, KO,saturated permeability, Jib. air entry value, and q, pore size distribution index. (Adapted from Laliberte and Brooks, 1967.)

+,

permeability (K,) of Touchet silt loam at different porosities representing five levels of compaction. Relative permeability is defined as the ratio of permeability to saturated permeability. Since curves in Fig. 5 can be represented by a power function proposed by Brooks and Corey (1964), Laliberte and Brooks (1967) explained the changes in permeability ( K ) of soil due to compaction in terms of Brooks and Corey parameters. Brooks and Corey functions are

and

ab

where KOis saturated permeability, 9, is the capillary pressure, is the bubbling pressure or air entry suction, and q is the pore size distribution index. Inset tables in Fig. 5 show the effect of compaction (+, porosity) on KO,+b, and q. With a light hydrocarbon oil as wetting fluid, q changed only slightly over a wide range of porosities. KOand $br however, changed severalfold over the same range of porosities. As expected, KOdecreased and $b increased with an increase in the level of compaction. The decrease in KO is associated with a decrease in the size of pores conducting fluid

3 18

SATISH C . GUPTA ET A L .

at higher levels of compaction. Similarly, the increase in Jlb is due to a decrease in the size of the larger continuous pores. Since pore geometry is determined by the net result of applied forces versus soil strength, it is expected that water content at the time of compaction will have a strong influence on the resulting soil structure. This was illustrated by Akram and Kemper (1979), who measured infiltration rates on soil cores that had been prepared by subjecting moist soils to four levels (43, 87, 173, and 346 kPa) of applied stresses. Water content of the soils ranged from air dryness to fractional field capacities of 0.5, 0.75, 1.0, 1.2, and in some cases, 1.4. Figure 6 shows the effect of compacting loads and water content at the time of compaction on infiltration rate, percentage volume reduction, and bulk density of Akron sandy loam (Akram and Kernper, 1979). For a given water content at the time of compaction, the infiltration rate decreased as compacting forces increased. However, at a given compacting load, infiltration rate increased (in some cases) slightly, then decreased, and increased again as the water content at the time of compaction increased. Figure 6 indicates that the infiltration rate may vary severalfold for the same level of macroscopic compaction (percentage volume reduction or bulk density). For example, an airdry soil compacted at an applied stress of 173 kPa develops a similar bulk density as a soil packed at a water content corresponding to field capacity and an applied stress of 87 kPa. However, the infiltration rate is 30 times higher for soil packed at air-dry water content and an applied stress of 173 kPa than for a soil packed at field capacity water content and an applied stress of 87 kPa. This suggests that macroscopic measurements like bulk density do not truly reflect the pore size distribution of a soil. A soil at a given bulk density can have several different pore geometries, depending upon the water content and the applied stresses at the time of compaction. In addition to these two factors, methods of soil compaction also have important effects on the changes in soil pore geometry. Davies et al. (1973) measured the effects of normal applied loads versus wheelslip on infiltration rates in Boxworth clay loam. The infiltration rate decreased from 71.0 to 7.6 and 2.7 c d h r (Table 11) when untreated land was subjected to an applied mechanical load of 1710 and 3140 kg, respectively. For a given applied load, wheel-slip further reduced the infiltration rate. The reduction in the infiltration rate was much greater due to wheel slippage (7.6 to 0.01 c d h r at an applied load of 1710 kg) than to an increased load (71.0 to 7.6 cm/hr). Decrease in the water entry rate due to wheel slippage is a result of increased compaction by realignment of particles in an orientation parallel to the direction of shear forces.

COMPACTION EFFECTS ON SOIL STRUCTURE

319

WATER CONTENT AT TIME OF COMPACTION (Mg mm3)

1 l.OoLuulAA 3 0.4 0.0 WATER CONTENT AT TIME OF COMPACTION AS A FRACTION OF FIELD CAPACITY

FIG.6. Effect of compacting load and water content at the time of compaction on infiltration rate, compaction, and bulk density. (Adapted from Akram and Kemper, 1979.)

320

SATISH C. GUPTA ET AL. Table I1

Influence of Applied Load and Wheel Slip on Water Entry Rates in a Boxworth Clay Loam" Massey-Ferguson 135'

Ford 5000'

Untreated land

No slip

31.4% slip

No slip

30.3% slip

(cdhr)

71

7.6

0.01

2.7

0.06

Range of values

18-128

4.0-17.2

0-0.4

1.0-7.0

0.04-0.1

Water entry rate

"Adaptedfrom Davies er nl. (1973). bRearaxle weight 1710 kg. 'Rear axle weight 3140 kg.

3. Soil-Water Dijfusivity

The soil-water dfisivity versus soil-water content relationship is often needed to describe non-steady state water movement in soils. Soil-water diffusivity as a soil-water transmission property reflects the pore size distribution of soil. Jackson (1963) studied the effect of compaction on the soil-water dflusivity function of Adelanto loam, Pachappa loam, and Pine silty clay. Figure 7 shows that for a given change in bulk density (approximately 0.12-0.14 Mg/m3),Pachappa loam exhibited little change in soil-water diffusivity , but Adelanto loam and Pine silty clay exhibited a two- to threefold change in diffusivity at the same water content. The silt content of the three soils was similar whereas the clay content was in the order Pine silty clay > Adelanto loam > Pachappa loam. Jackson (1963) concluded that high-clay soils (Adelanto loam and Pine silty clay) showed greater change in soil water diffusivity due to compaction than low-clay soils like Pachappa loam. Using data from the literature, Libardi et al. (1982) modified an existing equation (Miller and Bresler, 1977) to describe the effect of compaction (bulk density) on the soil-water dflusivity function. The modified equation is of the form Di ( 8 , pb)

(ai - 0 . 4 6 4 ~ exp ~ ) ~(pe)

(3) where Di is the soil-water diffusivity of soil i at a relative water content 8 and a bulk density Ph; 0: and p are constants, and ai is the slope of wetting front to t' function at a known bulk density. Brutsaerts (1979) suggested values of and 8 for 0: and p, respectively. Based on Eq. (3), Libardi et al. (1982) concluded that the smaller the Di, the larger is the effect of bulk density on soil-water diffusivity. This is the case for =

0:

COMPACTION EFFECTS ON SOIL STRUCTURE

32 1

PINE SILTY CLAY

'ce 3.0

- 3.00 0

o 1.37 1.45

0

O Oo

a 0.6Y 0.4

z :

i5 0.3 K

0

o o of

-

0.2-

I 0.1-5 .08

0.b

0

-

e o

a.

3.0

2.01.0

I

0.4: (b!

-

0.30.2 -

0

o

1.34 1.40

0.4 0.3 0.2

b

* A b

-

0.1.08

I

I

0.1 .08

0.5

RELATIVE WATER CONTENT,

@=

0.6

0.7 0.8 0.9 1.0

(e-ei) (Qs-Qi)

FIG.7. Soil-water diffusivity versus relative water content of Adelanto loam, Pachappa loam, and Pine silty clay at three bulk densities. (Adapted from Jackson, 1963.)

fine-textured soils with a large number of micropores. These soils, already slow in water flow, are most affected by compaction. This conclusion was similar to the findings of Jackson (1963).

4. Sorptivity Sorptivity, So, is a measure of the uptake of water by soil without gravitational effects (Philip, 1957). Sorptivity values depend upon the structure and the antecedent water content of the soil. Walker and Chong (1986) measured sorptivity from horizontal cumulative infiltration in laboratorycompacted soil columns of Alford silt loam (Typic Hapludalfs). Compaction treatments covered six levels of applied stresses (38,65,92, 138,321, and 458 kPa) at four soil water contents (10, 15, 20, and 25% kg/kg). Figures 8 and 9 show the changes in void ratio and sorptivity as influenced by different levels of applied stress and antecedent soil water contents. Starting from a dry condition, both void ratio and sorptivity increased with increasing soil water content, reached a peak, and then decreased. The shape of curves in Figs. 8 and 9 is similar to a typical water content-density curve obtained in a Proctor test. Because void ratio versus water content (Fig. 8) and sorptivity versus water content (Fig. 9) curves were similar in shape, sorptivity was linearly related to void ratio (Fig. 10). Sorptivity increased linearly with an increase in the void ratio of the soil for all six levels of applied stress and four levels of antecedent water contents. Sorptivity can be a useful index that measures the combined effect of applied stress and water content on pore geometry, provided a relationship like that of Fig. 10 holds true for other soil types.

322

SATISH C. GUPTA ET AL. 1.6

1.4 1.5

Pa

E

65

2E 1.3 -

d 1.2 QK Q

9

1.1 -

1.0

-

0 5 10 15 20 25 30 SOIL WATER CONTENT, % kg I kg FIG. 9. Sorptivity as a function of soil-water content at various levels of applied static pressure. (Adapted from Walker and Chong, 1986.)

-

323

COMPACTION EFFECTS ON SOIL STRUCTURE 30 (v

-v-. -ul.

E

-

cn"

20

d

9

G I-

2

10

li

$ 0 0.75

0.95

1.15

1.35

1.55

1.75

1.95

VOID RATIO, e , m3/m3

FIG. 10. Relationship between sorptivity and void ratio. (Adapted from Walker and Chong, 1986.)

Since soil structure may change (swell or shrink) when soils become wet, hydraulic properties may not reflect the structure of soil in its original form. Fluids like air, which does not interact with the soil matrix, are thus better in describing the pore geometry of soils in its original condition. Two properties that describe the pore geometry of soil without the complications of fluid interaction with the soil matrix are gas diffusion and air permeability.

5. Gas D n o s u ' i It has been well recognized that the main mechanism of gaseous exchange between soil and atmosphere is molecular diffusion (Currie, 1984). The coefficient of mutual diffusion for a pair of gases interdiffusing within air-filed pores of the soil is conveniently expressed as a fraction of Do, the coefficient of diffusion in the absence of impeding solids. The ratio DID,is, within broad limits, independent of the nature of diffusing gases, and it is a measure of pore continuity and tortuosity (Currie, 1984). In addition to being affected by pore geometry, DID, is strongly influenced by the amount of air or water content. According to Currie (1961), the decrease in DID, with increase in water content is large for a simple pore system, i.e., one with a unimodal pore size distribution, such as sand or gravel. In a compound pore system (bimodal size distribution), such as a packing of soil crumbs or a tilled soil,

324

SATISH C . GUPTA ET A L .

the decrease in DID, is small as the intracrumb pores are wetting but becomes large, as in the simple system, when intercrumb pores are wetted. Figure 11 shows a hypothetical relationship of DID, to air content for a packing of soil crumbs (Currie, 1983). Curves 1-3 and 4 represent the diffusion versus air content relationship of intra- and intercrumb pores, respectively. Figure 11 shows that as the crumbs wet to saturation (Curves 1-3), the decrease in diffusion is small. However, as more of the continuous intercrumb pore system wets, diffusion decreases in proportion to the fourth power of the remaining air-filled pore space (Curve 4). Currie (1984) studied the diffusion of hydrogen through air in I-2-mm aggregates of Batcombe clay loam that had been subjected to various levels of compaction. Figure 12 shows the change in diffusion coeffkient at five levels of compaction. The two-part relationship between diffusion coefficient and air content was similar over a bulk density range from 0.86 to 1.29 Mg/m3, but the two parts became less distinct with increased compaction (Fig. 12). Also, the range of air content over which the diffusion coefficient is exponentially related to air content decreased with an increase in the level of compaction. This reflected a shift in the proportion of large (interaggregate) to small (intraaggregate) pores during compaction. 6 . Air Permeability

Air permeability is the ability of soil to allow convective transport of air in response to a total pressure gradient. Like hydraulic conductivity or permeability, air permeability reflects the size and continuity of airfilled pores. Bowen (1966) studied the effect of vertical applied stresses (0, 7, and - 1

0

lntercrumb

0.1

+-I

0.2 0.3 (cm3 cm-3)

Crumb+

0.4

0.5

0.6

E

Frc. 11. Generalized reiationship of DID,, versus air content (E) in packing of soil crumbs. (After Cume, 1983.)

325

COMPACTION EFFECTS ON SOIL STRUCTURE

0.3

-

80.2 -

\

P

0.1

1 0.99 2 1.06 3 1.12 4 1.20 5 1.29

-

0

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

AIR CONTENT, E (cm3 cm-3)

FIG. 12. Relationship of DIDo versus air content of 1-2-mm soil aggregates at five levels of compaction. (Adapted from Currie, 1984.)

35 kPa) on air permeability of Ruston loamy fine sand. Figure 13 shows relatively little influence of compaction on air permeability when the soil was near air-dry water content. However, air permeability changed significantly due to compaction when soil water was 6% or more. Bowen (1966) suggested that higher air permeability in soils compacted at water contents of 6% or more was due to lower bulk density of these soils; in other words, to larger pores.

40

?

30

X

g 20 t d

zw

0

10 0

0

2

4

6

8

10

12

MOISTURE CONTENT, % 9g-l

FIG. 13. Air permeabilitiesversus moisture content for a Ruston fine loamy sand. (Adapted from Bowen, I%.)

326

SATISH C. GUPTA ET AL.

B. SOILMATRIX Effects of soil compaction on soil solid-solid interaction have been characterized in terms of aggregate size distribution, aggregate density, and wet and dry aggregate stabilities. 1 . Aggregate Size Distribution

Voorhees et al. (1979) studied the effect of field traffic (compaction)on aggregate size distribution and random roughness following tillage. Figure 14 shows the aggregate size distribution following tillage in wheel-tracked and nontracked areas of a Nicollet silty clay loam. Wheel traffic from all field operations was restricted to the same wheel path for 5 years. The field equipment weights, including tractor weight, ranged from 3700 to 7300 kg. Data in Fig. 14 show an increase in the proportion of large aggregates when compacted Nicollet silty clay loam soil was subsequently tilled. In the same experiment, Voorhees et al. (1979) also found that the density of clods taken from tilled wheel-tracked areas was higher than from nontracked areas (Table 111). Mechanical crushing strength of 50mm clods from tilled wheel-tracked area increased from 13 to 56 kPa. In a study on response of winter cover crops to soil compaction, Flocker et al. (1958) measured the changes in the physical condition of a Yo10 fine sandy loam seedbed at three levels of compaction (Table IV). Compaction treatments included (a) severely compacted plots, on which a jeep weighing about 908 kg was driven across the field, followed by a tractor with 182

--_--_ -

NONTRACKED

WHEEL- TRACKED

AGGREGATE DIAMETER, mm

FIG.14. Aggregate size distribution of subsequently tilled nontracked and wheel-tracked Nicollet silty clay loam, after planting, May 1975. (After Voorhees et a/., 1979.)

COMPACTION EFFECTS ON SOIL STRUCTURE

327

Table Ill Clod Density as Affected by Wheel Traffic" Clod density (Mdm') Date sampled

Untracked

Wheel-tracked

After planting, May 1975 After planting, May 1977

1.56 1.44

I .72 1.66

"Adapted from Voorhees et al. (1979).

kg of water in each rear tire, two trips with a cultipacker pulled by the same tractor, and finally, by a jeep carrying a load of 363 kg of sand; (b) moderately compacted plots, on which a tractor was driven across the field once; and (c) lightly compacted plots, on which no traffk other than subsequent tillage operations was allowed. The seedbed was prepared by double-disking all plots once and harrowing with a spike-toothed harrow twice. Bulk density, clod population, clod density, and clod shear strength increased as the level of compaction increased (see Table IV).

2. Aggregate Stability Power and Skidmore (1984) studied the effect of compaction on wet and dry aggregate stability. Compaction treatments included an application of vertical stress of 2.45 MPa in the laboratory to undisturbed surface samples that had been brought to a soil matric potential (Jl,,,) of -33 or - 100 kPa. Surface samples corresponded to cultivated and uncultivated fields of Reading silt loam (Typic Argiudols). Power and Skidmore (1984) defined dry aggregate stability as the energy needed to crush the compacted sample between two parallel plates. Wet stability is defined as the amount Table IV Physical Properties of Yolo Fine Sandy Loam as Influenced by Compaction Treatment" Bulk density (Mdm3) Compaction treatment Light Moderate Severe

0-3 cm

3-6 cm

Clod population (kg)

1.22 1.39 I .56

1.29 1.42 1.54

8.4 21.8 43.7

"Adapted from Flocker et a!. (1958).

Clod density (Mdm')

Clod shear strength (kPa)

I .49

0.49 0.75 0.87

1.50 1.64

328

SATISH C. GUPTA ET AL. Table V

Dry and Wet Aggregate Stabilities of Uncompressed and Compressed Readings Silt Loam"

Compressed (2.45 MPa) Treatments

Uncompressed

Soil-water pressure -33 kPa - lOOkPa

Dry aggregate stability,

Cultivated Uncultivated Cultivated Uncultivated

(J/m2) 7.18 33.54 12.52 40.25 Wet aggregate stability. (kdkg) 0.48 0.30 0.86 0.85

30.46 38.97

0.28 0.75

"Adapted from Powers and Skidmore (1984).

of soil left on a 0.25-mm sieve (60-mesh) after a sample has been lowered and raised through a distance of 27mm, 25 times per minute, in a tank of water. Table V shows that dry aggregate stability of Reading silt loam increased as a result of compaction for both cultivated and uncultivated samples. Differences in soil-water content (Jim = - 33 and - 100 kPa) at the time of compaction had a minimal effect on dry aggregate stability. Power and Skidmore (1984) attributed the increase in dry aggregate stability of compacted soils to an increase in bonding of particles because these particles were forced into closer proximity during compaction. Wet aggregate stability of Reading silt loam was slightly lower or unchanged due to compaction (Table V). This indicated that bonds between particles created during compaction were not stable to wet sieving. Power and Skidmore (1984) also concluded that compaction of samples breaks bonds formed during natural aggregation. These bonds are more resistant to the disruptive action of differential swelling and entraped air exploding off the water-submerged aggregates.

111. MECHANISMS OF SOIL STRUCTURE CHANGES DURING COMPACTION In the previous section, we quantified parameters that describe the effects of compaction on pore geometry or soil matrix. In this section, we will review possible mechanisms that control changes in soil structure (pore geometry or soil solid-solid interactions) during compaction.

COMPACTION EFFECTS ON SOIL STRUCTURE

329

Day and Holmgren (1952) microscopically examined the nature of changes occuring in moist 1-2-mm aggregates after application of a mechanical stress. Figure 15 shows photomicrographs of compressed samples of Yolo silty clay loam. Yolo soil with a 25% water content (corresponding to the lower plastic limit) when slowly compressed to a terminal stress of 49 kPa showed distinct aggregate boundaries with inter-aggregate spaces contributing to an appreciable proportion of the total volume. However, the aggregates are crowded together rather than (as normally) found in a loose pack and appear to be somewhat deformed because many of the interaggregate contacts are line segments rather than points (Fig. 15a). At a later stage of compression (Fig. 15b), the interaggregate pore spaces remaining at 49 kPa had almost completely disappeared at 148 kPa. Although traces of aggregate boundaries are visible in Fig. 15b, each aggregate has come into contact with adjacent aggregates over its entire periphery. This indicated that plastic flow has occurred extensively. The effect of water content on the state of compaction is shown in Fig. 1%; In the case, Yolo (I-2-mm) aggregates containing 15% water were subjected to an applied mechanical stress of 148 kPa. At this water content, plastic deformation, which was prominent at the lower plastic limit (25%), was much reduced. It can be seen from Fig. 15c that a number of aggregates are braced against the others, giving mechanical strength against further compression. These *observationsindicate that plastic flow occurred in the contact zone between aggregates, but not extensively. During compaction, the forces exerted upon an individual soil aggregate by surrounding aggregates produce a complicated force system. Theoretically, these forces can be resolved into direct and shear stresses. Plastic flow will occur only if shearing stress exceeds the shearing strength of aggregates. With these theoretical bases, Day and Holmgren (1954) suggested the following mechanisms for the changes in soil structure upon application of mechanical load. At the beginning of compaction, contact

FIG. 15. Photomicrographs of compressed samples of Yolo silty clay loam at the following combinations of water content and applied stresses: (a) W = 25%. ua = 49 kPa, (b) W = 25%. u, = 148 kPa. and (c) W = 15%. u= = 148 kPa. (Adapted from Day and Holmgren, 1952.)

330

SATISH C. GUPTA ET AL.

area between the aggregates is small. Due to large localized stresses in the inter-aggregate contact zones, shear may occur in the vicinity of the contact points, even though the shearing strength may be relatively high. With further compaction, the contact area between aggregates increases. However, the flattening process is self-degenerating because it causes more uniform distribution of load. Eventually the plastic flow ceases when the shearing stress falls below the shearing strength. If shear stress due to applied load is greater than shear strength, there will be a plastic deformation, and the pores between aggregates will disappear. Since shear stress versus shearing strength of soil controls the plastic deformation in soils, mechnically applied loads and water content at the time of compaction have strong influence on soil structural changes. Larson and Gupta (1980) interpreted the mechanism of soil structural changes during compaction from pore water pressure measurements. The experimental system consisted of a uniaxial compression of 60 g of moist soil (<2 mm) placed on a ceramic plate connected to a pressure transducer. Measurements included applied mechanical stress, sample length, and pore water pressure. Figures 16a and 16b are examples of macroscopic (bulk density) and microscopic (pore water pressure) changes in the sample as m

AQUIC HAPLUDOLL “l41 WATER CONTENT,

- 2.0 -1.6 -1.6

-1.2 F < -1.0 a -0.8

- 0.6

APPLIED STRESS,

P

a ,kPe

FIG.16. Bulk density (p), void ratio (e), and pore water pressure (&I, as influenced by applied stress (03 for an Aquic Hapludoll at three water contents. (Adapted from Larson and Gupta, 1980.)

COMPACTION EFFECTS ON SOIL STRUCTURE

33 1

a function of applied mechanical stress. Restricting the discussion to microscopic changes, Fig. 16b shows that as the applied mechanical stress increased, the pore water pressure became more negative, reached a minimum, and then increased until all the pores were filed with water. Drawing conclusions similar to those of Day and Holmgren (1954), Larson and Gupta ( I 980) explained these changes in pore water pressure as follows: 1. During the initial application of mechanical stress, the number of contacts of each aggregate increases, leading to the production of new menisci, which act to pull water from within the aggregate and thus result in a lower pore water pressure. At this stage, the aggregates are flattened at contact points. 2. As additional mechanical stress is added and the contact area increased, little change in pore water pressure occurs until pores between the aggregates become small enough for the menisci to coalesce, i.e., these pores fill with water and pore water pressure increases. 3. As the pore water pressure approaches zero, aggregate strength decreases rapidly and application of further mechanical stress results in shearing or plastic deformation of aggregates. When pore water pressure is nearly zero, soil has a massive structure.

IV. GUIDELINES ON WATER CONTENTS AND MECHANICAL STRESSES CONDUCIVE TO IRREVERSIBLE CHANGES IN SOIL STRUCTURE Mechanical stress corresponding to a minimum pore water pressure is an indicator of the bearing capacity of soil above which irreversible changes (shearing or plastic deformation of aggregates) occur in soil structure. To prevent degradation of soil structure due to compaction, it is necessary to develop guidelines on threshold water contents and applied stresses above which irreversible changes occur in the soil structure. Larson and Gupta (1980) presented two criteria based on pore water pressure measurements during compaction that define soil conditions under which irreversible changes in soil structure start to occur. A. DEGREE OF SATURATION AT MINIMUM PORE WATER PRESSURE When pore water pressure in Fig. 16b was normalized and plotted against the degree of saturation, the results formed a single curve (Fig. 17) for each soil, irrespective of the initial water content. Also, the degree

332

SATISH C. GUPTA ET AL.

I

COMPUTED

em = .479 \I

AQUlC HAPLUDOLL IM41 WATER CONTENT,g/g 0 0.187

I

a 0.232 0 0.277 A 0 0 0

n o

0

A

W

A

0

3

A

.20

0

e I

I

.20

I .40

I

60

I

.ao

%

I 1.00

es

DEGREE OF SATURATION,

FIG.17. Normalized pore water pressure (U,/U,) versus degree of saturation, OP,for an Aquic Hapludoll at three water contents. 8, is the minimum pore water pressure. (Adapted from Larson and Gupta, 1980.)

of saturation corresponding to the minimum pore water pressure (0,) was a unique value for a given soil. Since the minimum pore water pressure indicates a stage at which irreversible changes start to occur in the microscopic structure, Larson and Gupta (1980) suggested that 0, can be a useful macroscopic index to delineate threshold water contents above which soil structure deteriorates for a given applied load. From the compression data of 82 soils, Gupta et al. (1985) determined a relationship between Om and clay content (Fig. 18). The degree of saturation at minimum pore water pressure increases with clay content and then levels off at a clay content between 30 and 40%. This observation suggests that the clay fraction dominates both the aggregate and pore structure above a clay content of 3040%. In other words, soils with a clay content of 30% or more behave as a pure clay system (Larson and Gupta, 1980).

COMPACTION EFFECTS ON SOIL STRUCTURE

333

-

3 0.6 m

0 0 0

O

0.3

0.2 0.1

em=

0 . 3 8 5 3 + 0 . 7 0 9 0 ~ 1 0 - ~Clay0.5671 x Clay' r = 0.68 SE = 0.068 n = a2

1 10 20 30 50 60 70 80

0

40

Clay, % FIG. 18. Degree of saturation at minimum pore water pressure (0,) versus percentage of clay. (Adapted from Gupta et a / . , 1985.)

B. MECHANICALSTRESSCORRESPONDINGTO A MINIMUMPORE WATERPRESSURE Figure ;6b shows that the mechanical stress corresponding to a minimum pore water pressure increases with a decrease in the water content of the soil. This is expected because smaller pores (greater mechanical stress) are needed to cause the menisci to coalesce at lower water contents. When normalized pore water pressures were plotted against the normalized mechanical stresses, the results formed a single curve for each soil type irrespective of the water content (Fig. 19). Also, the normalized mechanical stress at the minimum pore water pressure was a unique value for a given soil. Using the same reasoning as described earlier for Om, Larson and Gupta (1980) suggested normalized mechanical stress at minimum pore water pressure (a,,)as a macroscopic index of soil structure that can define critical applied stress (uc),at which irreversible changes in soil structure begin to occur. The normalized mechanical stress at a minimum pore water pressure is defined as un = log a,/ log us

(4)

where usis the mechanical applied stress at zero pore water pressure (all pores are filled with water). Larson and Gupta (1980) also presented a method to estimate usfrom soil compression characteristics.

334

SATISH C. GUPTA E T A L . U, at U n

\

E1.00 l x

\

2

a

A

=1

4-

A 4A

Y

1 S.E.

B A

a

w" .80U

0.

3

v)

8

$I f

a

o

A

0

SO-

0

W

- Uw/Um

5.13

+ 17.2

#

AQUIC HAPLUDOLL IM41

t?N

4 .20-

0

WATER CONTENT, g/g 8 0.187 A 0.232 00.277

z

P V

L

A 0

0 4

I

I

I

I

I

.5

.6

.7

.8

.9

O L l

1.0

FIG. 19. Normalized pore water pressure (U,/U,) versus normalized stress (log uJog for an Aquic Hapludoll at three water contents. (Adapted from Larson and Gupta, 1980.)

0,)

o-0.9 l*

an=

r

0.4 0.3 0

+

0.5806 0 . 8 6 5 3 ~ 1 0 - ~Clay0.8752 x 10 Clay

-' '

= 0.74

SE = 0.046 n = 82 10

20

30

40 50 Clay, YO

60

70

FIG. 20. Normalized stress (u,) at U, = 1 versus percentage of clay. (Adapted from Gupta ef al., 1985.)

COMPACTION EFFECTS ON SOIL STRUCTURE

335

Figure 20 shows the change in cr, with clay content. Like 6, in Fig. 18, u, increases with clay content and then levels off. Again, the clay content at which u, levels off is between 30 and 40%. Figure 20 can be used to define mechanical stresses above which irreversible changes in soil structure begin to occur at a given water content. The information needed to use Figs. 18 and 20 is the particle size distribution (Gupta and Larson, 1982a).

V. SUMMARY There is increased concern regarding the effect of compaction on crop production in mechanized agriculture. Compaction effects on crop yield are due to changes in soil physical, chemical, and biological processes. These processes, in turn, are dependent on the soil structure. In order to provide guidelines for appropriate soil management, we need to understand and quantify changes in soil structure due to compaction. Soil structure refers to the arrangement of primary soil particles into secondary particles or aggregates. Soil structure, per se, is a qualitative concept. Efforts to quantify soil structure have involved the interpretation of soil behavioral properties like water and air flow characteristics, aggregate size distribution, and aggregate stability. This is done in the belief that soil behavioral properties will reflect the variations in soil structure at a microscopic level. An operational definition of soil structure could be an interaction between soil solids and soil pores. Soil solids could be in the form of primary or secondary particles. Soil compaction refers to the compression of unsaturated soils, during which an increase in the density of the soil body and a simultaneous reduction in fractional air volume occurs. In addition to mechanical load, soil compaction is often characterized in terms of bulk density, void ratio, or total porosity. These are also the parameters that quantify soil structure macroscopically, i.e., the relative proportion of soil solids to soil pores. This review summarizes previous research on (a) the effects of compaction on soil structure at a microscopic level, or in other words, the relationships between macroscopic and microscopic soil structure parameters, and (b) the mechanisms by which microscopic soil structure changes during compaction. Properties that reflect microscopic soil structure under the broad categories of pore geometry and soil solid-solid interactions are water retention characteristics, hydraulic conductivity and permeability, infiltration, sorptivity, gas diffusion, air permeability, aggregate size distribution, and aggregate stability. The effect of compaction on pore geometry is a

336

SATISH C . GUPTA E T A L .

decrease and an increase in the proportion of large and small pores, respectively. The rate of change in the proportion of various pore classes due to compaction depends on the soil type, primarily the clay content. The major compaction effects on soil matrix are an increase in the proportion of large aggregates when compacted soils are subsequently tilled and an increase in the dry stability of soil aggregates. Soil compaction may also lead to lower wet stability due to breakage of stable bonds formed during natural aggregation. Mechanisms that control changes in pore geometry or soil matrix during compaction are hypothesized to be (a) increased contact among aggregates, (b) deformation of aggregates at contact points, and (c) plastic flow at large applied loads or high soil water contents. Plastic flow occurs when shearing stress exceeds the shearing strength of aggregates. This results in a massive soil structure. Procedures to study mechanisms of soil deformation during compaction include photomicrographs or measurements of changes in pore water pressure. Both procedures reflect changes in soil structure at a microscopic scale. Laboratory measurements show that the pore water pressure increases and then decreases with an increase in applied mechanical load. It has been suggested that the minimum pore water pressure indicates a soil condition above which irreversible changes (aggregate shearing and plastic deformation) begin to occur in soil structure on a microscale. For practical applications, it is tedious to make measurements of pore water pressures in order to define soil conditions or machinery loads that are conducive to further deterioration of soil structure. Easily measurable parameters are needed that indicate both macroscopic and microscopic changes in soil structure. Laboratory compression experiments have shown that variables like degree of saturation and normalized stress are uniquely related to normalized pore water pressure for a given soil (initial pore water pressure between - 5 and -60 kPa). Since pore water pressure describes the microscopic soil structure and degree of saturation and the measurement of normalized stress indicates the macroscopic soil structure, it is suggested that the degree of saturation and normalized stress at minimum pore water pressure (both easily measurable parameters that reflect macroscopic and microscopic soil structure) may be useful indicators of the point at which irreversible changes start to occur in soil structure. Because the degree of saturation and normalized stress at minimum pore water pressure are unique for each soil irrespective of the initial soil water content, these measures are further adaptable to routine calculations. Compression data also show that the degree of saturation and normalized stress at minimum pore water pressure are related to clay content of soils. These relationships, together with the data base on compression characteristics, could make routine particle size analysis a useful tool for de-

COMPACTION EFFECTS ON SOIL STRUCTURE

337

fining threshold water contents or applied machinery stresses that are conducive to irreversible changes in soil structure.

VI. FUTURE RESEARCH NEEDS 1. Parameters that can define soil structure in terms of pore geometry and soil solid-solid interaction must be identified. 2. A data base on the effects of applied stress on parameters (macroscopic and microscopic) that define soil structure should be developed. Based on this data base, generalization and quantification of the relationships between macroscopic and microscopic soil structural parameters can proceed. 3. Clarification of the mechanisms of soil deformation during soil compaction should be pursued. 4. Determination of the suitability of degree of saturation or stress at minimum pore water pressure as parameters that identify soil conditions when plastic deformation (irreversible change in soil structure) begins to occur is necessary.

ACKNOWLEDGMENTS The work was supported by the Binational Agricultural Research Development Fund between the United States and Israel under Grant No. US-973-85 and by the International Agricultural Research Center (IARCS) Collaborative Research under grant No. USAID/ USDA 86CRSR--2-2899.

REFERENCES Akram, M., and Kemper, W. D. 1979. Soil Sci. Soc. A m . J . 43, 1080-1086. Barnes, K. K., Carleton, W. M., Taylor, H. M., Throckmorton, R. I., and Vanden Berg, G . E. (eds). 1971. Compaction of agricultural soils. A m . SOC.Agric. Eng. Monogr. (1). Bowen, H . D. 1966. Trans. Am. SOC.Agric. Eng. 9, 725-735. Brooks, R. H., and Corey, A. T. 1964. Hydrology paper No. 3, Colorado State University, Fort Collins. Brutsaert, W. 1979. Water Resour. Res. 15, 481-483. Child, E. C. 1957. In “Drainage of Agricultural Lands” (J. N. Luthin, ed.), pp. 1-78. Amer. SOC.Agron., Madison, Wisconsin. Croney, D., and Coleman, J. D. 1954. J. Soil Sci. 5, 75-84. Cume, J. A. 1961. Br. J . Appl. Phys. 12, 275-281. Cume, J. A. 1983. J. Soil Sci. 34, 217-232. Cume, J. A. 1984. J. Soil Sci. 35, 1-10. Davies, D. B., Finney, J. B., and Richardson, S. J. 1973. J. Soil Sci. 24, 399-409.

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Day, P., and Holmgren, G. G. 1952. Soil Sci. SOC.Am. Proc. 16,73-77. Flocker, W. J., Vomocil, J. A., and Vittum, M. T. 1958. Soil Sci. SOC.A m . Proc. 22, 181184.

Gupta, S. C., and Allmaras, R. R. 1987. Adv. Soil Sci. 6, 65-100. Gupta, S. C., and Larson, W. E. 1982a. In “Predicting Tillage Effects on Soil Physical Properties and Processes” (P. Unger, D. M. Van Doren, Jr., F. D. Whisler, and E. L. Skidmore eds), pp 151-178. Am. SOC.Agron., Spec., Publ. 44.Madison, Wisconsin Gupta, S. C., and Larson, W. E. 1982b. Int. Soil Sci. Congr. 12th, New Delhi, 8-16 Feb. Vol. 6 , 3 (Abstr.). Gupta, S. C., Hadas, H., Voorhees, W. B., Wolf, D., Larson, W. E. and Schneider, E. C. 1985. Res. Rep. Submitted to BARD Fund on completion of Project Grant No. US337-80.

Hamblin, A. P. 1985. Adv. Agron. 38, 95-157. Hill, J. N. S., and Sumner, M. E. 1%7. Soil Sci. 103, 234-238. Jackson, R. D. 1963. SoilSci. SOC.Am. Proc. 27, 123-126. Laliberte, G. E., and Brooks, R. H. 1967. Soil Sci. SOC.Am. Proc. 31, 451-454. Larson, W. E., and Gupta, S. C. 1980. Soil Sci. SOC.A m . J . 44, 1127-1 132. Libardi, P. L., Reichardt, K., Jose, C., Bazza, M., and Nielsen, D. R. 1982. Water Resour. Res. 18, 177-181. Miller, R. D., and Bresler, E. 1977. Soil Sci. SOC.Am. Proc. 41, 1021-1022. Philip, J. R. 1957. Soil Sci. 84, 257-264. Power, D. H., and Skidmore, E. L. 1984. Soil Sci. SOC.Am. J . 48, 897-884. Russel, E. W. 1971. J. Soil Sci. 22, 137-151. Unger, P. W., Van Doren, D. M., Whisler, F. D., and Skidmore, E. L. (eds.) 1982. Predicting tillage effects on soil properties and processes. Am. SOC.Agron. Special Publ. 44. Voorhees, W. B., Young, R. A., and Lyles, L. 1979. Trans. A m . SOC. Agric. Eng. 22,786790.

Walker, J., and Chong, S. K. 1986. Soil Sci. SOC.Am. J . 50, 288-291.

ADVANCES IN AGRONOMY, VOL. 42

TISSUE CULTURE IN RICE IMPROVEMENT: STATUS AND POTENTIAL’ Satish K. Raina Biotechnology Centre Indian Agricultural Research Institute New Delhi 110012. India

I. Introduction Embryo Culture Anther, Pollen, and Ovary Culture A. Factors Affecting Anther/Pollen Culture Efficiency in 0. sativa B. Anther Culture of Other Oryza Species and Interspecific Hybrids C. Genetic Evaluation and Utilization of Anther-Derived Plants D. Practicability of Anther Culture Breeding E. Ovary Culture IV. Somatic Cell Culture A. Significance of Somatic Cell Culture for Rice Improvement B. Callus Induction and Single-Cell Cultures C. Somatic Embryogenesis and Regeneration D. I n Vitro Mutant Selection E. Somaclonal Variation V. Protoplasts A. Areas of Potential Usefulness in Rice Improvement B. Isolation, Culture, and Regeneration of Protoplasts C. Genetic Manipulation of Rice Protoplasts VI. Overview and Strategies for the Future References 11. 111.

1. INTRODUCTION Rice, the major source of calories for about 40% of the global population, is ranked as the world’s most important food crop. Consequently, it is ‘Abbreviations: ABA, abscisic acid; BA, 6-benzyladenine; BPH, brown plant hopper; CH, casein hydrolysate; CIAT, International Centre of Tropical Agriculture, Colombia; CW, coconut water; 2,4D, 2,4-dichlorophenoxyaceticacid; IAA, indole-3-aceticacid; IBA, indole3-butyric acid; IRRI, International Rice Research Institute, Manila; K, kinetin; MCPA, 2-methyl-4-chlorophenoxyacetic acid; MS, Murashige and Skoog’s medium; 5-MT, 5-methyltryptophan; NAA, naphthaleneacetic acid; PEG, polyethylene glycol; S-AEC, (Sj-2-aminoethyl-L-cysteine; 2,4,5,-T, 2,4,5-trichlorophenoxyaceticacid; TCCP, Tissue Culture for Crops Project, Colorado; TCP, 4-amino-3,5,6-trichloropicolinicacid (Picloram); TIBA, triidobenzoic acid; YE, yeast extract. 339 Copyright 8 1989 by Academic Press,he. All rights of reproduction in any form reserved.

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also one of the most intensively investigated crop plants. The accumulated knowledge base has been of considerable value in elevating rice productivity and, in turn, in the betterment of millions of people who depend on it as a source of food and income. As is the case for other crop plants, rice breeders have mainly pursued the exploitation of natural variation for tailoring cultivars suitable to the demands of agriculture. The processes of hybridization and recombination have been the main vehicles of this delivery system. The achievements have been impressive. Taichung Native-I, IR-8, Mashuri, IR-36, and several other products of this system are internationally known for their impact on rice cultivation. In spite of this, every now and then the attention of breeders is drawn to innovations that claim to offer speedier, or even immediate, solution to a whole range of problems. Among these innovations, a maximum of attention has been drawn by the techniques of plant tissue culture because they promise to offer a number of approaches to the challenges of crop improvement. Tissue culture is a blanket term used to denote in vitro culture of plant cells, tissues, and organs. Rice tissue culture had its beginning in Japan when Ameniya et al. (1956) succeeded in in vitro culture of immature rice embryos. Furuhashi and Yatazawa (1964) were the first to regenerate rice callus derived from nodal segments. Maeda (1967, 1968, 1969), Yatazawa et al. (1967,1968), Saka and Maeda (1968, 1969), and Yamada et al. (1967) contributed greatly to the assessment of the nutritional requirements for growth and differentiation of rice callus tissues. Nishi et al. (1968) were the first to succeed in regenerating whole plants from rice calli. This was also the first such success with monocots. What attracted much more attention, however, was another report, in the same year, of the production of haploid plants from rice pollen through in vitro culture of immature anthers (Niizeki and Oono, 1968). Thereafter, owing to the significance of haploids in speeding up breeding programs, rice tissue culture, which had seemed thus far a Japanese concern only, gained considerable importance. Niizeki and Oono’s success was promptly repeated, first in Japan (Nishi and Mitsuoka, 1969), then in Korea (Harn, 1969), in India (Guha et al., 1970), and by several others, especially in China. Chinese workers have pursued rice tissue, in particular anther, culture both intensively and extensively. By 1986, more than 100 varieties and lines were reported to have been developed in China by what came to be known as anther culture breeding (see Zhang and Chu, 1986). Considerable progress has also been made in other areas of rice tissue culture. In this chapter, an assessment of the current status and future prospects of tissue culture in rice improvement is presented. An attempt has been made to analyze in some detail the present state of the art and to suggest such areas as warrant future attention.

TISSUE CULTURE IN RICE IMPROVEMENT

34 1

II. EMBRYO CULTURE This technique involves excision of embryos (generally immature) and their germination into viable seedlings under artificial conditions. The technique is generally used to rescue hybrid embryos of otherwise unsuccessful interspecific crosses. Ameniya et at. (1956) examined the requirements of culture conditions of immature rice embryos and obtained seedlings from even 5-day-old cultured embryos. F, hybrids from otherwise unsuccessful rice crosses were subsequently obtained by Bourhamont (1961), Li et a/. (1961), Iyer and Govila (1964), and, more recently, Guzman (1983). The study of Guzman was the most elaborate. Germination of viable seedlings from early embryos (even 4-day-old), and also from a number of intra- and interspecific crosses, was achieved. Growth was best in ‘/4 MS medium, and addition of CW was found to be inhibitory at all embryo ages. Guzman’s studies have been extended at IRRI (Jena and Khush, 1984) to interspecific crosses between improved plant-type, BPH-susceptible lines of 0. sativa and BPH-resistant 0. officinalis, 0. australiensis, and 0. brachyanrha. The problem of interspecific hybrid embryo abortion was successfully overcome by embryo rescue methods. More recently, at IRRI, embryo rescue techniques have been successfully utilized for raising interspecific F, hybrids involving 0. sativa (Agenome), 0. australiensis (E-genome), 0. eichingeri (C-genome), 0. minufa (BC-genome), and 0. latifoliu (CD-genome) (K. K. Jena, personal communication). In Russia, Kostylev and Yatsyna (1986) have also successfully utilized embryo rescue techniques for raising interspecific rice hybrids. The alien species involved were 0. uustraliensis and 0. malampuzhaensis. In this study, however, full strength MS medium supplemented with K and IAA (0.5 mg/liter each) was used.

111. ANTHER, POLLEN, AND OVARY CULTURE Anther culture remains the most extensively investigated aspect of rice tissue culture. More than a hundred papers have been published, most of them from China. The reason that this technique has attracted such wide attention is the possibility of obtaining haploid cells, tissues, and complete organisms, which have only a single dose of genetic information. Instant homozygous lines could be obtained by doubling the chromosome complement in these haploids without having to resort to a lengthy and timeconsuming program of inbreeding, which is indispensable for attaining

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SATISH K. RAINA

homozygosity by conventional methods. Mutants could also be more readily discovered because only one set of chromosomes, and only one copy of each gene, is present in the haploids. The advantages that anther culture-derived doubled haploids offer in rice improvement have been a subject of discussion in several papers. The reviews of Rush and Shao (1981), Rush et al. (1982), Khush and Virmani (1985), Baenziger ef al. (1984), Pulver and Jennings (1986) and Han and Bin (1987) have discussed the subject in some detail. The main attraction of anther culture is the possibility of early fixation of recombinants. Pollen plant populations derived from anther culture of F, hybrids will express segregation patterns otherwise expressed in the F, or F, generation. Diploidization of haploids will render them homozygous, resulting in rapid establishment of pure lines that otherwise would require several generations of selfing. The savings of time and resources would be higher in long-duration compared to short-duration cultivars, in which homozygosity can be achieved rather speedily through conventional methods such as single seed descent (see Baenziger et al., 1984). In rice, therefore, anther culture breeding would be more advantageous in situations involving temperate environment, photosensitivity, or environmental stresses, because only one generation can be raised in a year. Even for short-duration cultivars, in which a generation can be raised twice a year, however, the anther culture route can save two to three seasons’ breeding and allow a more efficient use of time an2 resources, Increased selection efficiency is the other advantage of anther culture breeding, especially when dominance variation is significant. In conventional breeding, early generation (F2-F4)lines show phenotypic differences to which both additive and dominance effects contribute. In contrast, doubled haploid lines derived from F, anther cultures will show only additive variance and, therefore, high heritability due to the elimination of dominance effects. Thus, compared to the F, population, fewer doubled haploid plants will be required for purposes of selection of the desired recombinant(s). To elaborate this point, Baenziger and Schaeffer ( 1983) cited the hypothetical example of a cross segregating for three recessive genes and for three dominant genes. In the case of three recessive genes, 1/8 of the doubled haploids derived from such a cross would be selected as compared to 1/64 of the F,. In the case of three dominant genes, whereas 1/8 of the doubled haploids, all of which would breed true, would be selected, 27/64 of F, plants would be selected, on the basis of phenotype, but only 1/64 would be true-breeding. Chinese scientists have estimated that about 150 pollen plants derived from F, anthers would be enough, instead of 4-5000 F, plants, for the purposes of selection of desirable genotypes (Shen et al., 1983).

TISSUE CULTURE IN RICE IMPROVEMENT

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Doubled haploid breeding would be particularly useful in situations in which extensive recombination is not important (Chaleff, 1980; Chu, 1982). In rice, efforts to combine the desirable traits of indica and japonica cultivars have often been frustrated by the high degree of sterility of the hybrids and vigorous segregation toward the parental types in F, and subsequent generations. Doubled haploids produced through anther culture of the F, hybrid, or of a promising segregant, would ensure stabilization and thereby preclude or arrest segregation in successive generations. Chu (1982) outlined schemes (Fig. 1) indicating how homozygous diploid substitution and addition lines could be obtained through anther culture of interspecific hybrids. This could considerably ease the transfer of 0. sativa / 0. officinalis (12 IIS) (12 110) 0. sativa (12 11s)

0.officinalis (12 110)

J

Dihaploid hybrid (1215 + 1210)

D ihaploid hybrid (121s

+

c

1210)

Anther culture

Ha loid substitutions (n + n I O , n =0-12)

IJ

Chromosome doubling

c

Diploid substitutions (n IIS+nllO, n =0-12)

J

Chromosome doubling

1 Amphidiploid / 0. sativa (12 IIS + 12 110) (12 IIS)

Backcross hybrid (12 IIS + 12 10)

4

Anther culture

Haploid alien addition lines (121S+n 10, n =0-12)

Chromosome doubling

Diploid alien addition lines (12IIS+n 110,n =0-12)

FIG. 1. Schemes for production of alien addition (right) and substitution lines through anther culture of interspecific hybrids. (From Chu, 1982.)

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SATISH K. RAINA

desirable genes of alien sources across the barriers of genomic incompatibility. In addition to 0. officinalis, which is known to show resistance to all biotypes of brown plant hopper (Khush, 1984), 0. longistaminata, 0. australiansis, and 0 . brachyantha are among the more important genetic sources of resistance to diseases and could be effectively utilized through Chu’s scheme. In fact, use of anther culture for solving the problems arising from intersubspecific and interspecific hybridization can be regarded as the most important area of its application in rice improvement. Another area of use of doubled haploids was pointed out by Rush and Shao (1982). In the selection of the races of pathogen or insect biotypes, use of “pure line” varieties or breeding lines may cause considerable experimental error due to residual heterozygosity of the genotypes investigated. Anther culture could be used to obtain homozygous lines for utilization in such situations, as well as in screening for resistance to herbicides, in physiology studies, and in purification of A, B, and R lines for hybrid-rice programs. AFFECTING ANTHE~POLLEN CULTURE A. FACTORS IN 0. sativa EFFICIENCY Briefly, the anther culture technique consists of collecting panicles when the anthers are predominantly in the uninucleate pollen stage. Florets containing anthers at the mid- to late uninucleate pollen stage are collected and surface-sterilized. Anthers are removed aseptically and planted on a callusing medium. After about a month of incubation, generally in the dark at about 25”C, some of the anthers show pollen calli emerging through the anther lobes. After about a week of their emergence, anther calli are transferred to the regeneration medium under light, where shoot buds appear in some of them. Pollen plantlets reaching a height of 5-6 inches are transferred to a liquid medium for hardening for a week or so, and then to soil. However, the efficiency of the technique may vary considerably. Several factors have been identified that influence anther response (a percentage of anthers showing visible pollen calli) and the subsequent regeneration of plants from pollen calk The following factors implicated in response modulation deserve elaboration. 1 . Genotype

Since the very first report of success by Niizeki and Oono (1968), many researchers have reported genotypic variation in anther response (Guha et al., 1970; Iyer and Raina, 1972; Wang et al., 1974; Raina and Iyer, 1974;

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Tsai and Lin, 1975; Chen and Lin, 1976; Sun et al., 1978a; Chu, 1982; Rush and Shao, 1982; Ding et al., 1983; Zapata, 1985; Hu, 1985; Miah et al., 1985; Karim et al., 1985; Boyadzhiev and Fam, 1986; Guiderdoni et al., 1986; Mikami and Kinoshita, 1988). In our initial studies (Iyer and Raina, 1972) of the 15 cultivars screened for response only 5 showed callusing and of these, only in one (IARI-5788)regenerated roots and shoots. However, with the improvement of methodology over the years, the position has improved considerably. Genotypic variation is now seen mostly in the extent of response, rather than in the existence of some responsive genotypes and some unresponsive genotypes. In general, japonica rice varieties are known to respond better than indicas (see Chen ,and Lin, 1976; Hu, 1978; Rush and Shao, 1982; Chu, 1982; Woo and Chen, 1982; Gun, 1982; Zapata, 1983, and F, hybrids perform better than their inbred parents (Chen, 1986). Not only anther response, but also the subsequent regeneration of green plants, is better in japonica cultivars. Reviewing the progress of anther culture in China, Hu (1984, 1985) has reported that although the induction frequency of green plants was more than 10% (of the number of cultured anthers) in japonica cultivars, it was only 1% in indica rices. Among the more than 50 indica varieties screened in our laboratory, anther response of up to 40% was achieved in a purple-pigmented cultivar, Crossa-2. For the remaining varieties, it was generally around 20%. Finegrain varieties in general respond more poorly (<10% response) than those with coarse grains (unpublished data). Ouyang et al. (1983) attempted to ascertain the basis of genotypic differences in anther response and green plantlet formation in wheat and reached three conclusions from their study:

(i) The production of pollen calli and green plantlets are heritable characteristics controlled by multiple genes. (ii) Induction frequency is influenced by the genotype of the anther wall and not that of the pollen. (iii) Transfer of anther culturability to F, hybrids is independent of the maternal cytoplasm In rice, Miah et al. (1985) found that callus induction ability was inherited as a recessive character conditioned by a single block of genes. Japonica types, in comparison to indica cultivars, appeared to be better combiners for callus induction. 2. Physiological Status of the Donor Plants Growth conditions, particularly the physiological state of the anther donor plants at the time of anther excision, are known to affect anther

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response in a number of plant species (Engvild et al., 1972; Dunwell and Perry, 1973; Foroughi-Wehr and Mix, 1979). In rice, Hu et al. (1978) found that temperature and sunshine during flowering affected anther response remarkably. Donor plants that came to flower during continuously cloudy and rainy weather, at very low temperatures (16-18"C), or at high temperatures (26-30°C) in a greenhouse callused less frequently. Best results were obtained when plants were grown with bright sunshine and a temperature range of 18-20°C. Plants raised at high temperatures not only showed poor anther callusing and regeneration of pollen plants, but also a high rate of albinism (Hu et al., 1978). However, indica types, cultivable predominantly in tropical and subtropical conditions, may have different requirements. This appears so from our recent study of a fine-grain, aromatic indica variety, Basmati-370, which gave rather better responses from donor plants that came to flower when the average max-min temperature range was 34.2-23.3"C, than from those that flowered when the average max-min temperature was 29.1-16.4"C (Raina et al., 1987). Several workers have recorded seasonal variation in anther response in rice (Tsai and Lin 1977; Chaleff and Stolarz, 1982; Chen, 1986a). Pandey (1973) suggested that anther response might be improved by enhancing endogenous hormone levels through direct treatment of donor plants with hormones. Although Wang et a f . (1974) reported some increase in anther response following treatment of rice panicles with 4000 ppm etheral(2-chloroethyl phosphonic acid) for 48 hr at 10°C Chaleff and Stolarz (1982) did not find any improvement in anther response in etheraltreated donor plants. On the other hand, Cheng and Zhou (1986) found significantly increased response when young panicles were sprayed with 600 ppm etherel or 10 ppm gibberellic acid. 3. Pre- and Postinoculation Treatment of Anthers

Following upon the observations of Nitsch and Noreel (1973), using Datura innoxia, and Nitsch (1974), using tobacco, that cold shock treatment of flower buds prior to anther inoculation increased the induction frequency significantly, several researchers have reported a stimulatory effect of cold pretreatment in rice. Chaleff et al. (1975), using the rice variety Dungham Shali as the test material, obtained a callusing frequency of 22.2% from anthers cold pretreated at 6°C for 5 days, compared to only a 10.5% frequency from the untreated anthers. Hu et a f . (1978) found a significant increase in response when a cold treatment of 4-8 days at 10°C was given immediately after inoculation of rice anthers. Genovesi and Magill (1979) studied the cold-shock effect in more detail. Anthers of the rice variety Norin-21 treated at 5 or 10°C for 7 or 10 days, or at 13°C for

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10 or 14 days, gave an anther response of up to 73.6%, compared to a 17.9% response of untreated anthers. Rush and Shao (1982) confirmed these results at IRRI, using the variety Taipai-309. Chaleff and Stolarz (1982) also obtained increased anther response when rice anthers (cv. Minehikari) were pretreated at 7°C for 3 days. Zhao (1983) found that indica and japonica varieties differed in their requirements for cold-shock treatment. They also pointed out that when cold treatment duration exceeded a certain limit, the induction frequency decreased markedly. Investigating further, Zhou et al. (1983) observed that cold treatment not only significantly increased anther response but also enhanced green plantlet production. When anthers of the otherwise low-responding indica variety Shan-You No.2 were cold pretreated at 910°C for 20 days, production of green plantlets increased from 0.37 (untreated anthers) to 12.3/100 anthers inoculated. The efficiency of cold-shock treatment is striking when anthers are plated on liquid medium; whereas Chen et al. (1980a) obtained an anther response of 15 times more than the control, Zapata et al. (1983, 1985) found cold shock for 8 days at 8°C to be the best. Pulver and Jennings (1986) also found that giving a cold shock to anthers at 8°C for 4-8 days stimulates callus formation in liquid nutrient cultures. That cold shock treatment is beneficial seems abundantly clear. The requirements, however, may vary. Differences arise not only from genotypic differences but also from differences in the physiological state of the donor plants at the time of anther excision. Additional nuclear divisions of the nongametophytic type are known to occur during cold shock treatment, and these facilitate the induction process in the dedifferentiating culture medium (Nitsch, 1974). In rice cold pretreatment is also known to delay degeneration of pollen (Qu and Chen, 1983) and anther wall tissues (Fang and Liang, 1985). Heat treatment instead of cold pretreatment is known to be stimulatory in Brassica species (Keller and Armstrong, 1978). In rice also, Rush and Shao (1982) claimed a doubling of callus induction frequency when anthers were pretreated at 40°C for 15 min. Zapata and Tomzo (1986) also recorded increased frequency of callusing caused by heat treatment of anthers at 35°C for 15 min, or even 5 min, prior to cold pretreatment at 10°C for 7 days. However, we could not repeat these results in our laboratory using two high-responding indica rice varieties as the test material. Heat pretreatment at 35°C for 5, 10, or even 15 min did not seem to make any difference to the panicles of the two varieties, which, however, were exposed to about 30-35°C temperatures in the field (Raina and Hadi, unpublished data). Aside from temperature, pretreatment with CO, for 6 days at 8°C (Xu and Tao, 1983, centrifrugal treatment of rice panicles at 2000 rpm for 10

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min (Zhu and Wang, 1982), and y-irradiation of anthers prior to culture (T. E. Sun et al., 1978; Yin et al., 1984; Zapata et al., 1986) have been found beneficial. Of these, CO, pretreatment is claimed to have increased the callusing efficiency by more than 28 times. 4 . Stage of Anther

In rice, anthers inoculated at mid- to late uninucleate pollen stages have been found to be the most suitable for culture. Oono (1975), Chen and Lin (1976), Chen (1977), Raina (1977), and Genovesi (1978) conducted detailed studies and reached the following similar conclusions: (i) Anthers at the tetrad stage do not response at all. (ii) Early uninucleate pollen may respond poorly. (iii) Mid- to late uninucleate pollen responds the best. (iv) Anther response falls sharply after the first pollen mitosis. The switch towards embryoid-callus development seems to occur more readily when the process of nuclear division has already been initiated than when it has to be initiated in culture. The difficulties with the older stages of pollen appear to be due to their commitment to differentiation into a male gametophyte. Chen (1977) observed a relationship between anther stage and plant regeneration. Calli from late uninucleate pollen appeared to show less regeneration potential and produced more albinos (see also Section 111, A,9).

5 . Callusing Medium Niizeki and Oono (1%8) used Blaydes’ (1966) medium in the first success with rice anther culture. Subsequent researchers until about 1975 used the same formulation or attempted only slight changes. Oono (1975) examined several other media and recommended MS as the most suitable. However, by 1978, several media formulations, reportedly high-responseinducing, were in use. N,, Heh-2, Heh-5, SK-3, SK-8, Szechuan medium, Medium V , and Chaleffs R-2 were among the prominent ones; most of them came from Chinese laboratories (see Liang, 1978; Rush and Shao, 1982). Although N, was recommended for japonica varieties, Heh-5 was recommended for indica and SK-8 for indica x japonica hybrids. N, remains the best-known medium for rice anther culture and has been found highly useful for other crops also. Developed by Chu and co-workers of the Institute of Botany, Kwangtung, China, N, medium is based on investigations of individual medium components. Chu’s studies (Chu et a f . ,

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1975; Chu, 1978) established that the level of ammonium nitrogen is a crucial factor. Also, the ammonium requirement for indica varieties is different from that of japonica, as it is from that of japonica x indica hybrids. An optimal NH, content of 7.0 me (milligram equivalent)/liter forjaponica varieties, 4.76 me/liter for japonica x indica hybrids, and 3.0 me/liter for indica varieties was recommended (see Chen, 1983). In our studies (Raina, 1983) with the indica rice variety Crossa-2, N, was found to be significantly superior to MS, Blaydes’s (1966), SK-8, and Chaleff s R,. Reddy et a / . (19851, using eight indica cultivars as test materials, have found Heh-2 and Heh-5 to be better than N,. Sucrose has been found to be the most suitable carbon source (Oono, 1975; Raina, 1977); generally, 3-5% is used. However, following observations by Clapham (1971) that better anther response was obtained with 12% sucrose in Hordeum vulgare anther cultures, high sucrose concentrations have been tested in rice also. Wang et a / . (1974) found 6% to be the optimal concentration for rice. Mercy and Zapata (1986) have found that 6 and 12% sucrose concentrations induced greatly enhanced anther callusing compared to 2% sucrose. Perhaps the main reason that high sucrose levels are not generally used is the general impression that the higher concentrations induce production of a greater number of albinos (see also Section 111, A,9). The increased anther response induced in media supplemented with high sucrose levels may not necessarily be a consequence of greater carbon availability but due to its influence on the osmotic potential of the medium. This seems possible, as the addition of mannitol (1%) also stimulates anther callusing (Zapata and Torrizo, 1985). A wide variety of growth-stimulating hormones have been tried, singly and in numerous combinations. Initially (Niizeki and Oono, 1968; Harn, 1969; Guha et a / . , 1970; Iyer and Raina, 1972), a combination of auxins (IAA and 2,4-D), a cytokinin (k), and complex growth substances (CW, YE and/or CH) was used. A rather elaborate study was undertaken in our laboratory (Raina, 1977) using the high-responding indica variety Crossa2. It was found that 2,4-D (2 mg/Iiter) induced a high response. Addition of zeatin (0.01 mg/liter) or YE (2 g/liter) enhanced the response slightly. Auxins (IAA, IBA, 2,4-D, NAA), cytokinins (K, zeatin) individually or in several combinations, and complex growth substances (CW, YE, CH) individually or in several combinations with an auxin and/or a cytokinin typically induced a lower response than 2,4-D (2 mg/liter) alone. The findings were more or less similar to that of Oono (1975). Hu er al. (1978), using japonica varieties and hybrids, observed that NAA (2 mg/liter) induced a higher frequency of callusing than 2,4-D at the same concentration. However, plant regeneration was lower in NAA-induced pollen calli than in those induced with 2,4-D. Chou er al. (1978) looked for chemicals that might be more effective than 2,4-D and found that 2,4,5-T, MCPA, and

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TCP (Picloram), of the 22 chemicals tested, were effective in callus induction. In fact, MCPA proved significantly better than 2,4-D at callus induction in the three japonica varieties tested. Hu ef al. (1978) claimed increased callusing efficiency when the medium was supplemented with 2-5 mg/l etheral, without any detrimental effects to the regeneration potential of the calli obtained. Yang et al. (1980) recommended use of a modified N, medium containing NH,H,PO, instead of (NH,),SO, and KH,PO, and a combination of NAA, 2.4-D, and K at concentrations of 4, 1, and 3 mglliter, respectively, for better results, especially with indica varieties. Of late, the strategies in rice anther culture have been changing from one of using 2,4-D (2mglliter) alone, to that of using a combination of 2,4-D + K + NAA, keeping 2,4D at 1 mg/liter or lower, and using K at 1-3 mglliter and NAA at 2-4 mg/ liter. Such a combination has been found especially suitable for the subsequent regeneration of green plants from the anther calli (see Huang et al., 1985; Y. Chen, 1986a; C. C. Chen et al., 1986). Also, two new media formulations have come up from China: “L-8” and “General” or “Universal’’ (see Chen, 1986a; Loo and Xu, 1986). Another one, N,-Y,, by Chung and Sohn (1986) has come from Korea (see Table AI). N,-Y, is essentially the same as N,, but its nitrogen sources consist of 28 mM KN03, 1.75 mM (NH,), SO, and 1.75 mM L-Glutamine, instead of 28 mM KNO, and 3.5 mM (NH4)2 SO, of N,. Anthers of indica varieties are reported to have a lower alanine content (Zhou ef al., 1983). The authors claimed that incorporating D,L-alanine ( 2 4 mglliter) caused the frequency of anther response of indica varieties to more than double. Potato extract medium, in which the main constituents of a synthetic medium are replaced by potato extract (10 or 20%), was reported to have greatly enhanced callusing of rice anthers (Chen et al., 1978). Potato extract (20%) is prepared by boiling 200 g of potato tubers (cleaned, sprouts removed, and cut into small pieces) for about 30 min in an equal volume of distilled water and then filtering the mixture through two layers of cheesecloth. To this extract, sucrose, agar, and 2,4-D are added and pH adjusted to 5.8 after raising the volume to 1 liter. Potato extract medium is said to be widely used in China for its low cost, simple production, and high efficiency. However, not all potato varieties are known to have a stimulatory effect. This seems to account for the rather poor response we observed in our laboratory with extracts from two potato varieties. Also, reproducibility with potato medium was found to be poor. It seems that certain companies are now marketing potato extracts (see Wenzel and Foroughi-Wehr, 1984). An attempt was made to examine the constituents of potato extracts by separating the fractions by a Dextran Gel (3-50 method (see Chen, 1983). While some of the fractions promoted cell division, others were found to have inhibitory effects.

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In addition to potato extract, a wide variety of complex growth substances have been tested for their effect on anther callus production. These include CH, CW, YE, and even mashed banana. Though some of them have been found to show some stimulatory effect, mashed banana was found to inhibit anther callus induction (Zapata et al., 1985). Chen and Tsay (1986) have reported delayed anther browning and increased callusing ability of rice anthers cultured in medium supplemented with 500-1000 ppm of Ginseng powder. The use of conditioned medium has been found to enhance anther culture efficiency significantly in barley float another cultures (Xu et al., 1981). To condition media, mature anthers are cultured in liquid callusing medium for about a week. The mature anthers are then removed and the medium is used as a conditioned medium or diluted by adding fresh callusing medium. Zapata and Torrizo (1985) tested the effect of conditioned medium on callus production and plant regeneration in rice anther cultures. Some enhancement of callusng frequency as well as of green plant regeneration was observed in one of the two rice varieties tested. However, in the other, callusing as well as plant regeneration efficiency were adversely affected. 6 . Float Anther Culture

Callus induction frequency in rice can be improved considerably by resorting to float anther culture (Chen, 1983; Zapata et al., 1983). First introduced by Sunderland and Roberts ( 1977), using Nicotiana tabacum, the technique involves floating anthers on shallow layers of liquid medium in petri dishes. The anthers dehisce and the developing pollen embryoids and multicellular pollen are shed into the nutrient medium. From time to time, embryoids and calli of appropriate size are removed from culture and planted on differentiation medium for plantlet regeneration. The main advantages that the technique is supposed to offer are: (i) It allows better gaseous exchange and nutrient availability to pollen, which is restricted in agar medium. (ii) It offers greater scope for alleviating the effects of competetive inhibition among the potentially embryogenic pollen grains. (iii) Inhibitors or toxic compounds produced by degenerating anther wall tissues diffuse into the liquid medium rather than remaining localized around the anthers in the agar medium. Float anther culture was first successfully attempted in japonica rice culture by Chen et al. (1979); success was also achieved in indica rice culture by Zhu De-yao and co-workers (see Chen, 1983). Chen (1983) and

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Zapata et al. (1983) reported high-frequency anther callusing in rice by float culture. Anther response and particularly anther productivity (number of calli per anther) are found to be significantly better. However, the regeneration efficiency of calli was lower than that obtained when solid medium was used for callusing (Chen, 1983). Recently, Chen (1986a) has reported better callus induction as well as green plant regeneration frequency when the liquid medium containing 20% potato extract was filter sterilized. Zapata (1985) found float anther culture to be still the most efficient and the technique, therefore, is routinely used for anther culturing at IRRI. The float culture technique is also in use for large-scale anther culturing at CIAT because of its overall superior performance (Pulver and Jennings, 1985).

7 . Culture of Isolated Pollen Isolated pollen culture, which has proved so successful in Nicotiana tabacum and, to some extent, in certain other members of the Solanaceae (Nitsch, 1981), generated a lot of hope in view of the opportunities it offers not just for production of haploids for breeding use but especially in studies relating to androgenesis, mutagenesis, genetic manipulation, and transformation. However, as in many other crops, isolated pollen culture has been of only limited value in rice (Chen et al., 1980b, 1981; Cornejo-Martin and Primo-Milla, 1981; Chen, 1983). The technique involves isolation of pollen from anthers by mechanical means (crushing or magnetic stirring) and incubation of isolated microspores in shallow layers of liquid medium. Chen et al. (1980a) were successful in raising green rice pollen plants from isolated pollen, but the rate of success was very low. Cold-shock of panicles at 10°C for 10-15 days and preculture of anthers for 3-4 days before pollen isolation were found to be helpful (Chen, 1983). In fact, preculture of anthers was found to be very important as its absence resulted in very few pollen undergoing division and subsequent development (Chen, 1986a). The major problem with isolated pollen culture is the very low regeneration frequency and most of the regenerants are albinos. Jia et al. (1987) has had success with isolated rice pollen culture. Pollen plants were raised from three japonica varieties: Sasanishiki, Nipponbare, and Reimei. At IRRI also, success has been obtained in raising plants from a japonica variety, Taipai- 309, and also an indica variety, IR-43 (F. J. Zapata, personal communication). Efforts are being made to enhance efficiency. 8. Regeneration

When anther response is in the form of pollen calli instead of embryoids and plantlets, problems of further growth are concerned largely with shoot

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differentiation. Not all pollen calli regenerate shoots. In rice, shoot differentiation from 50% of the calli is considered very good. Best regeneration response is observed when calli are transferred after 10-20 days (about 2-mm size) of their emergence through the anther lobes. Older or younger calli show a lower regeneration potential (Tsai and Lin, 1977; Chen, 1986a). Shoot regeneration could be achieved by transferring 2,4-D-induced pollen calli to IAA plus K medium (Niizeki and Oono, 1968), or to a hormone-free medium (Harn, 1969; Nishi and Mitsuoka, 1969; Wang et al., 1974). Callus induced in a medium supplemented with CW or CH had better potential for root-shoot regeneration when transferred to IAA plus K regeneration medium (Anonymous, 1974). When NAA was used for callus induction, organ formation could occur without a change of medium (Oono, 1975; Heszky and Pauk, 1975; Chen and Lin, 1976; Chaleff and Stolarz, 1982; Zapata ef al., 1982). In our laboratory, MS medium with CW (15%) was found to be the most suitable for regeneration of 2,4-Dinduced calli (Raina and Iyer, 1974; Raina, 1977). Chaleff and Stolarz (1982) claimed a high rate of regeneration (about 70%) using a modified MS medium (R3) supplemented with NAA (2 mglliter) and K (0.3 mg/liter) for callusing as well as regeneration. Similarly, Zapata et a f . (1983), using Gamborg et al.’s (1968) medium supplemented with NAA and K ( 1 mg/ liter each), achieved plant regeneration efficiency of 40% Float anther culture, which is currently much in vogue due to its high efficiency in producing anther callusing, suffers from poor regenerating ability of the pollen calli (see Chen, 1983; Hu, 1984, 1985). Chen (1983) noted that regeneration in pollen calli from float cultures was much influenced by the callusing medium used. She found that the best results could be obtained when potato extract (20%) plus KNO, (1500 mg/liter) plus 2,4-D (2 mg/liter) was used for callusing, and MS plus IAA (2 mgl liter) plus K ( 1 mg/liter), gelled with 0.58% agar, was used for plant regeneration. Green plantlet differentiation of as much as 50% was achieved. Zapata and his group at IRRI also found float culture to be more efficient and have adopted the procedure of transferring pollen calli of about I mm in size onto an M-shaped paper bridge wetted with MS medium containing NAA plus K ( 1 mg/liter each) for differentiation of shoots (Zapata et al., 1983). In their studies also, regeneration efficiency was found to be greatly influenced by the callus-inducing medium. The two-step method of ABA, which induced high shoot regeneration in rice somatic cultures (Inoue and Maeda, 1981), has been successfully extended to rice anther culture. ABA used in the preculture medium at rather high concentrations of 10 or 25 mg/liter caused a significant increase in average green plant regeneration in all three rice varieties tested (Torrizo and Zapata, 1986). Chung and Sohn (1986) also noted that regeneration was better in calli induced in a medium containing NAA, K, and ABA than those induced

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in a 2,4-D medium. Further, Chung (1987) found that addition of ABA prolonged the regeneration potential of anther calli which otherwise drops markedly 20 days after emergence. 9. Albinos and Incubation Temperature One of the major problems with rice anther culture is the occurrence of albino plants in large numbers. In a few genotypes the frequency may remain low (up to 20%); in the majority of cases, however, the frequency may be more than 50%. Studies have revealed the existence of proplastids in leaf cells of albinos (Wang et al., 1978), but there is a lack of ribosomes and an absence of fraction-I protein (C. S. Sun et al., 1978). No definite relationship could be established between the occurrence of albinos and the media components (Wang et al., 1978; Chu, 1982). However, the temperature during incubation seems to affect the frequency of albino production. When calli were incubated to differentiate at 2r12SoC, fewer albinos were produced; the frequency increased with an increase in temperature (Song et al., 1978). The findings of Wang et al. (1978) were similar and further indicated that the low-temperature effect on albino plant production occurred mainly during the early stages of divisions in the pollen. The studies of Qu and Chen (1983) have shown that even pretreatment of rice panicles at 35°C for 3 to 5 days causes increased albino production. On the other hand, low temperature (10°C) pretreatment increased green plant formation. Furthermore, when donor plants came to boot at 38"C, as opposed to 25°C the frequency of albinos increased from 13.w0 to 34.7% (Huang et al., 1983). Perhaps temperature affects the orientation of the first division(s) in pollen, which could be significant in that the embryo so produced could be deficient in cytoplasmic contents. The possibility of cytoplasmic insufficiency as the reason for the high incidence of albinism has gained attention because the frequency of green plants obtained by ovary culture is higher than that obtained from anther culture (see Liu and Zhou, 1984). Use of a high sucrose concentration (9%) (Woo and Chen, 1982) and high levels (10 mglliter) of 2,4-D or NAA (Chen, 1983) are also reported to increase the frequency of albinos. Only albinos are reported to have regenerated from the calli induced on a medium having 2,4-D at 20 mglliter (Wang et al., 1977). Mercy and Zapata (1986) have observed that although higher sucrose concentrations (6 or 12%) resulted in higher frequencies of albino plants, the frequency of green plant production was also higher. They have concluded that higher sucrose concentrations can be advantageously used to increase green plantlet regeneration. Guo (1983) obtained increased green plant regeneration frequency when the strength of iron salts (N, medium) in the callus induction medium was raised six times.

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The fact that the large number of albinos produced in rice anther cultures are not a result of gene mutations induced in culture was investigated by He and Ouyang (1983). Anther culture of a diploid and a tetraploid rice variety was carried out and it was found that the frequency of albinos produced was more or less similar. The authors argued that if gene mutation was the cause of albinism then few albinos should have resulted from the tetraploid rice. Moreover, no segregation of green and albino plants was observed among the self progeny of green regenerants. However, as pointed out by Dunwell (1989, the arguments are applicable only for nuclear and not cytoplasmic mutations. It seems that most of the albino regenerants have a deletion (up to 80%) of their plastid genome (Day and Ellis, 1984). Whether the deletions are caused by the process of culture and/or pre-exist in the pollen remains to be resolved. However, it seems likely that the deletions correspond to the metamorphosis of plastids which Huang (1982) observed in the microspores as they develop into bicellular pollen. The internal structure and the number of ribosomes in the proplastids decreases until they completely disappear in the bicellular pollen. This seems to explain why most plants from binucleate pollen are albinos.

10. Direct Pollen Plants

Unlike Nicotiana or Datura, in which pollen embryoids develop directly into plantlets by passing through stages similar to those of zygotic embryos, in rice pollen plant development involves an intervening callus phase. Callus formation introduces problems not only of plant regeneration but also of genetic instability. Attempts made to induce direct plants growth in rice (Chu et al., 1976; Ouyang et al., 1983) met with some success but frequency of response remained very low. Lin Gong-song and his group (see Chen, 1983) made rather elaborate studies involving mainly auxins and cytokinins and claimed a direct plantlet production frequency of 4% for japonica and I% for indica-japonica hybrids, using 2,4-D at 0.1 mgl liter and NAA and K at 2-3 mdliter. However, the possibility of some callus development prior to plantlet formation could not be ruled out. Several hormone combinations, involving mainly NAA, K, and/or BAP were tested in our laboratory for their effects on direct plantlet differentiation (Raina, 1977). Though an occasional case of possible direct plantlet formation was observed in some combinations, the frequency as well as the reproducibility was poor. Ling et al. (1984b) have claimed a high rate of direct pollen plantlet production in japonica as well as in indica varieties. However, in this case the term direct plantlet formation has been used to describe a situation in which both pollen calli formation and subsequent regeneration of plantlets occur in the same medium. A combination of 2,4-D (0.01 mg/liter), kinetin ( 3 4 . 5 mg/liter), and NAA (3 mg/

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liter) has been found to be the most suitable for such direct pollen plantlet formation. The hormonal combination, however, did not prove successful with some of the indica rices that we tested (unpublished). Therefore, we still follow the two-step method for a better recovery of regenerants. B. ANTHERCULTUREOF OTHEROryza SPECIESAND INTERSPECIFIC HYBRIDS Other than 0. sativa, which has been intensively investigated, the African rice 0. glaberrima, the wild species 0. perennis, and certain interspecific hybrids have also been tested for anther culture suitability. Wakasa and Watanabe (1979) found 0. perennis (Spontanea type) to be a lowresponding (0.5%) species. Out of 2421 anthers inoculated, only one haploid and two diploid plants were obtained. Woo and Huang (1980), however, obtained high anther response (about 43%) as well as high regeneration of plants in 0. glaberrima. Woo et al. (1978) studied anther culture efficiency of single crosses and backcrosses involving 0. sativa (Taichung 65) and 0. perennis Moench. Although only albino plants were derived from anthers of reciprocal single-cross hybrids, hybrids backcrossed to 0. sativa produced some green plantlets in addition to albinos. Poor anther response (3-5%) was recorded even from reciprocal hybrids of 0. glaberrima X 0 . sativa (Taichung 65) although 0. glaberrima otherwise gave high anther response (see Woo and Chen, 1982). Among the regenerants, albinos were twice as numerous as the green plantlets. Hybrids having 0. glaberrima as the female parent produced more albinos than those having 0. sativa as female parent. C. GENETICEVALUATION AND UTILIZATION OF ANTHER-DERIVED PLANTS Regenerants from rice anther calli consist of haploids, diploids, polyploids, and mixoploids. Most of the plants (more than SO%), however, are diploids (Iyer and Raina, 1972; Oono, 1975; Yin et al., 1976; Chen and Li, 1978; Huang et al., 1978; Shen et al., 1983; Zapata, 1985; Chen et al., 1985, Loo and Xu, 1986). of the remaining, the majority are haploids. Huang et al. (1978) examined a total of 2496 rice anther-derived plants. The frequencies of haploids, diploids, polyploids, and mixoploids were found to be 35.3, 53.4, 5.2, and 6.0%, respectively. Genetic investigations of the diploids have shown that over 90% of them were stable for all major economic traits (Tang, 1978; Sun et al., 1978; Chu, 1982; Chen and Li, 1978; Zhang, 1982). Tang (1978) studied 429 dip-

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loid anther-derived plants. He found that 91 3% were uniform, 7.8% had slight variation, and 0.7% exhibited significant variations. Similar results were reported by several other investigators (see Rush and Shao, 1982). According to Loo and Xu (1986), among more than 2000 pollen plants examined at the Institute of Crop Breeding and Cultivation Research, Shanghai, only about 3% were found unstable. Zhang (1982), Shen et af. (1983), and several others studied the stability of diploid anther-derived plants over several generations. The investigations revealed that in the majority of cases, the coefficient of variance did not increase with increasing numbers of generations. Obviously, the diploids arise as a result of spontaneous chromosome doubling of the haploid complement (see She e? al., 1984). As regards the small number of unstable diploid lines, possibly these arise due to mutations occuring during the course of in vitro culture. Those occurring prior to chromosome doubling appear as homozygous mutations; whereas those that occur after chromosome doubling appear as heterozygous mutations. These have been referred to as gametoclonal variants and considered as a novel source of genetic variation (Evans et al., 1984). Attempts have been made to utilize gametoclonal variation for induction of “subtle” genetic change(s)in an otherwise desirable cultivar. Schaeffer (1983) succeeded in isolating a dwarf variant of the rice variety Calrose-76 through an example of gametoclonal variation. Similarly, Xue et af. (1984) identified two lines among several anther culture-derived doubled haploid lines from a local cultivar (Nonghu 3-2) that were earlymaturing and higher-yielding in comparison to the donor plants. Enhanced gametoclonal variation could be obtained by using anthers irradiated with y rays (Kuo, 1986). From among plants derived from irradiated anthers, the variety 2205 was obtained; it had improved disease resistance, better grain quality, and higher yield than the parent cultivar. Similarly, Yin and Yu (1986) developed a new strain, R462, from the japonica variety 501 Xuan, following irradiation of anthers with 2000 rad y-rays. Studies conducted mainly in different Chinese laboratories have indicated (see Hu, 1978; Chen and Li, 1978; Shen e? al., 1983) that the pollen plant populations obtained from anther culture of F, hybrids show a spectrum of segregation that is similar to that observed in a conventionally raised F2 generation. No significant selection of recombinants was found to have occurred during the course of in vitro culture. Chen et af. (1982), Zhu et al. (1984), and, more recently, Chen (1986b) also found that the results obtained from genetic analysis of an F, population were in good agreement with those from anther-derived plants. However, in indica x japonica hybrids, when anthers of F, hybrids were cultured on N, medium, which is suitable for japonica rice, most of the pollen plants were of japonica type (see Chen and Li, 1978; Shen et al., 1983). Chen (1986b) has cited several examples of how using anther culture technique in China,

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in comparison to pedigree technique, has made selection of the desirable types possible in a shorter time while using a smaller number of pollen plants. Utilization of anther culture techniques in rice breeding programs has been in practice, mostly in China, for more than 15 years. Since the release for cultivation of Hua-Yu Noa I and Hua-Yu No. 2, the first rice varieties developed through anther culture breeding in 1976, Chinese workers are reported to have developed over 100 rice varieties and lines (Zhang and Chu, 1986).Anther culture breeding in China is being utilized extensively for development of breeding lines for earliness, yield, resistance to diseases, and grain quality (Shen et al., 1983; Hu, 1985). Figure 2 shows the general working procedure used for rice breeding through anther culture, and Fig. 3 summarizes the schemes of development of some of the popular anther culture-bred rice varieties in China (cf. Shen et al., 1983). The variety Hua-Yu No. 1 combines the disease resistance trait of Nihan-bare and the desirable attribute of good yield under saline conditions of Qian-jun-bang. Hua-Yu No. 1 is said to have fared very well in field trails of up to 0.3% salinity. Therefore, it proved to be a popular variety in the Tian-jin district. The varieties Zhong Hua-8 (Fig. 4), Zhong Hua-9, and Hua-jian 7902 have incorporated the blast resistance genes of Toride No. 1 and No. 2 with the high yields of local varieties. Similarly, Hua-han-zao was developed as a cold-tolerant variety. In this case, anther culture was utilized at two stages involving multiple recombination with cultivars or pollen plants possessing desirable traits.

Genetic resource and

t

\

+Regionalc

I

test]

1

Variety

FIG. 2. General working procedure used for rice breeding through anther culture in China. H, refers to the anther-derivedregenerants. H2refers to the fmt generation of selfregenerants. (From Shen et al., 1983.)

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He - jiao 752

Pu-xuan 10 Pedigree of He - dan 76-085 Toride No. 2

Jing -xi No. 17 Jing - xi No. 17

H,-

y

Pedigree of Zhong - hua No. 8 and No. 9 Nang - hua NO. 6

Toride No. 1

Shi - jian - dao

Pedigree of Hua Jia - nong 485

No. 7902

Ke - C 1699

Fl

Labellt

- jian

H,--H,-

line

Tai-nan 13 Pedigree of Hua - han - zao Nihan - bare

Qian - jun -bang Pedigree of Kua - yu No. 1

FIG. 3. Pedigree of some of the anther culture-bred popular varities in China. (From Shen e r a / . , 1983.)

Two new rice varieties, Hua Ju No. 1 and No. 2, were developed through a procedure involving multiple hybridization and anther culture (Li et al., 1984). The varieties developed consisted of desirable characters drawn from several parents. Compared to the conventional composite hybridization, utilization of anther culture offered the advantages of relatively

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FIG. 4. Zhong Hua 8, a popular anther culture-bred variety in China. Courtesy of Dr. Hu-Han, Institute of Genetics, Beijing, China.

smaller- and shorter-scale breeding operations and higher selection efficiency. Among the high-yielding varieties developed through anther culture in China, Qian-hua No. 1 appears to be the most notable example. It is a selection from a pollen plant population of the variety Nan-you No. 2. Qian-hua No. 1 is reported to have yielded 10.3 tons/ha under moderately fertile conditions and is a recommended variety in Guizhou province, where it was developed (Shen et al., 1983). Another notable example is the variety Xin-Xiu, developed at the Shanghai Academy of Agricultural Sciences by Zhang Z-H, who utilized intervarietal hybridization and anther culture (Zhang, 1982). The variety is reportedly being grown on over 100,000 ha in eastern China (Zeng, 1983; Hu, 1985). Chinese workers have also successfully extended anther culture techniques to certain challenging areas of rice breeding.

(i) The anther culture approach is reported to have tackled successfully the problem of sterility and vigorous segregations commonly encountered otherwise in indica-japonica hybrids (Chen and Li: 1978; Hsu, 1978; Chu, 1982; Zhang, 1982; Shen et al., 1983). The rice variety Huafeng No. 10 was developed from indica-japonica hybrids and released for cultivation in the Hupeh province of China (see Chu, 1982). (ii) Stable tines with improved characters (large spikes and high fertility)

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were reportedly developed, only after the anther culture approach was attempted, from such hybrids as rice x sorghum (Sun, 1978). (iii) Anther culture was extended to hybrid rice (Chen and Li, 1978). Yang Hsien-ming and co-workers obtained several thousand pollen plants from hybrid rice and identified many improved lines (See Chen and Li, 1978). Some of these were claimed to be “even more ideal than the hybrid rice.” Hu (1985) reported the selection of six strains from anther culturederived plants of the hybrid rice Shan-you No. 2. The strains are said to have the potential of replacing Shan-you No. 2 and, thereby, eliminating the troublesome necessity of producing hybrid seed every year. One of the lines, Shan-Hua 369, showed a yield increase of up to 11% and early maturity. Outside China, utilization of anther culture in rice breeding programs has been comparatively of a lesser magnitude. In Taiwan, Woo and coworkers (Woo and Tung, 1972; Mok and Woo, 1976; Woo et al., 1978; Woo and Huang, 1980; Woo and Chen, 1982) studied application of anther culture mainly to intersubspecific (indica x japonica) and interspecific (0.sativa X 0 .perennis and 0 . sativa x 0 . glaberrima) F, hybrids. A number of pollen-derived lines were obtained that exhibited genetic stability. However, in the case of interspecific hybrids, the problem of sterility was not completely overcome. Woo and Chen (1982) isolated brown plant hopper-resistant lines from pollen plants derived from F, hybrids of Tainan5 x Tainan Sen-yu 30. The selections yielded significantly more (up to 50%) than Tainan-5 in sites where the disease was prevalent. In Korea, the use of anther culture techniques has remained concerned mainly with the development of blast-resistant varieties (Gun, 1982). Several promising lines have been identified, one of which, HB 1 IB, showed blast resistance and gave more yield than the standard variety Suweon. However, it had the drawback of an increased amylose content that was higher than the acceptable level. Of late, the anther culture program has been intensified at the Yeongnam Crop Experiment Station in Korea (Chung, 1987). More than 2000 lines are reported to be under evaluation trials. Of these, Milyang-90, a selection having good grain quality and resistance to brown plant hopper and stripe virus disease, has shown an overall good performance. Earlier, in 1985, the rice-breeding team at the Crop Experiment Station developed the first Korean rice anther culture variety named “Hwaseongbyeo” (Kim, 1986). According to the author, it took just 4 years to breed this variety. IRRI’s program of anther culture application is currently focused mainly on the production of cold-tolerant rice varieties using F, hybrids of japonica-indica and indica-japonica crosses. The development of cold-tolerant lines is otherwise severly hampered due to problems of sterility and poor heritability of the desired trait (Zapata, 1985). In 1983, a collaborative

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project between IRRI and the Rural Development Administration of Korea was initiated with the objective of developing cold-tolerant lines through anther culture of F, hybrids. Under the program, which involves anther culture work at IRRI, as many as 8670 anther culture-derived plants had been produced by December 1985. Of these, 1270 progeny lines have been sent to Korea for screening for cold tolerance (Zapata et al., 1986). In India, true-breeding, promising recombinants (for earliness and yield) were selected from a large number of pollen-derived lines (Fig. 5 ) raised from potentially desirable indica F, hybrids (Raina, 1983). It was demonstrated that the development of stable promising lines took just three seasons and would have otherwise required at least six, thereby saving considerably on time and resources. Perhaps the biggest program outside China is the one at CIAT (Pulver and Jennings, 1986). As many as 20,000 diploid R, lines are reported to have been raised from approximately 100 triple crosses. The authors have estimated that it took just 1 year to go from anther inoculations to announcing availability of lines for observation nurseries. The period includes the time involved for raising R, plants and subsequent testing for blast resistance, Fe tolerance, and white-belly determination. Using conventional breeding methods, at least 4 years would have been required for the project at CIAT, where two generations are raised per year.

FIG.5. Pollen-derived pure line plots from indica F, hybrids showing variation for height and maturity.

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D. PRACTICABILITY OF ANTHERCULTURE BREEDING That anther culture techniques have considerable potential to speed breeding programs is now well established. As discussed earlier, application of the technique would be most advantageous in early establishment of fertile and stable addition-substitution lines from intersubspecific and interspecific crosses. In the intervarietal breeding programmes, the technique would be most advantageous in situations involving long-duration, temperature-sensitive, and/or photosensitive cultivars, or when breeding for resistance to environmental stresses like drought or disease, in which case raising two crops in a year may not be possible or feasible. Even for breeding short-duration, photoinsensitive varieties, the anther culture route could save two or three seasons’ time and resources. Although considerable progress has been made in rice anther culture techniques during the last decade or so, the net yields of pollen plants are still rather low. Even in China, where the level of expertise is considered very high, the induction frequency of green plants is about 10% (on the basis of anther number) for japonica and only about 1% for indica rice varieties (Hu, 1985). It is perhaps for this reason that Chinese successes with breeding new varieties through anther culture are largely in japonica varieties. It needs to be mentioned, however, that better anther response in japonicas is not due merely to genetic reasons but also to the fact that considerably more attention has been devoted to them. Comparatively, the problems of indica rices have not been intensively investigated. Outside China, however, even for japonica rice breeding, examples of anther culture utilization are few. One reason for this indifference could be the necessity of large-scale anther inoculations, which must be made even for japonicas. The other reason appears to be the lack of adequate awareness and/or technical expertise. Although the former might be the case in developed countries, which may not find anther culture technology sufficiently attractive so long as it remains labor-intensive, the latter may be the case in developing countries. Moreover, in most of the rice-growing developing countries, indica rice is grown; as already mentioned, indicas respond more poorly than japonicas. In India, for example, where the anther culture technique was identified (Guha and Maheshwari, 1966), the poor response of indica rice is one of the main reasons that anther culture has not, as yet, attracted wider acceptance for application purposes. In our anther culture studies with F, hybrids, raising a population of 100150 pollen plants from 5000-10,000 anthers has been rather readily achieved in a few cases (Raina, 1983). However, from certain others, with as many anthers we have ended up with an embarrassing dozen or so pollen plants. In one study, only nine green plants were obtained from over 21,000 anthers of the fine-grain rice variety Basmati-370 (Raina et

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al., 1987).Though Basmati-370 is one of a few cases of very low response, it does call attention to the limitations of the technology. For japonica rices, Pulver and Jennings (1985) have performed an interesting exercise in evolving procedures for applications of anther culture using minimal resources, to “high-volume rice breeding.” The procedures, said to be in practice at CIAT, are based on a detailed consideration of such aspects of the technique as impede its wider application. Among the novel aspects of these procedures are the methods described for mass extraction of anthers from rice florets and also for mass transfer of anther calli to the regeneration medium, which requires just one person. Instead of dissecting out anthers from each floret, the mass extraction method involves dissecting en masse appropriate florets of a panicle at the base. Cut florets are collected in a test tube containing the induction medium and the tube is shaken until the anthers are observed floating in the medium. The liquid is then poured into a small flask through a filter that retains the glumes but allows the anthers to pass with the medium. The small flask (the induction flask) containing anthers from about 25 florets of a panicle is sealed and incubated for callus induction. The authors claim that one person can easily prepare about 150 flasks per day. Similarly, the authors have devised a mass transfer method for transfer of anther calli to the regeneration medium. The entire contents of the induction flask are filtered on a Buchner funnel apparatus using a flat vacuum filter to produce uniform dispersion of the calli. The filter paper, which should be of the same size as the regeneration medium container, is inverted on the regeneration medium and tapped lightly. The filter paper is then removed, and the calli remain embeded in the medium. The authors claim that one person can easily prepare about 150-200 flasks per day. We have tried the mass anther extraction method mentioned above. However, we have found it inconvenient. Even with vigorous shaking of the liquid medium containing the cut florets, not many anthers come out. Moreover, anthers that do come out tend to cling to the cut florets. This, in turn, makes separating anthers a difficult task. We have evolved a procedure using a simple device for mass extraction of anthers (Raina and Hadi, 1987). The device is made from an animal cage feeding bottle (125ml capacity), which is made of polypropylene or polymethylpentane. Fitted in the screw cap of the bottle is a length of about 3.5-cm steel tubing. For making the device, from the base upward to about three-fourths of the height of the bottle, horizontal slits of about 1.5 mm in width and 3 cm in length are made 4 mm apart in four horizontal rows. Slits of one row correspond to the gaps of the adjacent row. For anther extraction, cut florets are collected in the bottle. A piece of Teflon-coated magnet (about 2 cm) is introduced and the device is closed and transferred to a 500-ml

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beaker containing about 300 ml of liquid medium. The beaker is placed on a magnetic stirrer and, while the steel tubing is held so as to keep the device in position, the stirrer is switched on for a minute or more as required. The slits allow the anthers to pass through, leaving the empty glumes behind in the bottle. We have recorded anther yields of over 80% when cut florets used were dissected at the distal end in addition to the cut at the base. However, with only the basal dissection, yields were only about 60%. This method is useful when large-scale anther culturing is required especially in liquid media. For semisolid media, the technique currently in vogue in China is perhaps the most suitable. Florets are dissected at the basal end, just below the anthers, and collected in a petri plate. Then, using a small forcep, the florets are lifted by their distal end and tapped on the rim of the culture plate or culture flask to allow the anthers to drop on the surface of the medium. However, it is somewhat unsuitable for culture tubes since the anthers tend to cling to the walls of the tubes. With some practice, the technique can be efficiently used and provides clean cultures since the anthers drop straight onto the medium and thus require no physical handling. So as to get a longer period of flowering, Pulver and Jennings (1985) have suggested staggered planting and ratooning (cutting back to a little above the root to induce regrowth) of the F, hybrids. Though such practices are already in use by those engaged in large-scale anther culturing, the suggestions are note worthy for the beginners and the uninitiated. It should be mentioned, however, that in situations in which the day-night temperatures drop rather sharply after the usual phase of flowering, ratooning may cause a significant drop in anther response and subsequent shoot regeneration, to the extent that the whole exercise may not be worth the trouble. E. OVARY CULTURE Asselin de Beauville (1980) and Zhou and Yang (1980), investigating independently, obtained haploids from cultures of unpollinated ovaries. Embryological studies traced the origin of proembryos and calli from the embryo sac (Zhou and Yang, 1981). Investigating further, Yang er a / . (1984) found that gynogenic embryoids originated mainly from the synergids. The technique of ovary culture (C. Zhou et al., 1983) involves excising ovaries from florets having anthers at late uninucleate pollen stage. Ovaries, together with stamens, receptacle, and the sterile glumes, are inoculated in N, liquid medium supplemented with MCPA (0.125 mglliter) and incubated in the dark at about 25°C. Gynogenic calli develop after about 35 days. For plant regeneration, the calli are dissected o u t and

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transferred to the same but solidified medium with a reduced level of MCPA (0.033mglliter). Picloram was also found to be effective (Zhou et af., 1986). In ovary culture also, japonica cultivars displayed a better induction frequency of gynogenic calli than the indicas. However, a high-responding anther culture cultivar was not necessarily remarkable in ovary culture. Similarly, a low-responding anther culture cultivar could do very well in ovary culture (Zhou et af., 1986). Although the efficiency of ovary culture (1.5-12%, japonicas) compared to anther culture is still rather poor, most of the plants that regenerate from ovary culture are green (Zhou and Yang, 1981;Kuo, 1982;Yang et al., 1984;Liu and Zhou, 1984;Zhonglai and Chang, 1984).Liu and Zhou (1984)compared 159 ovary culture-derived plants with 231 from anther culture of the cultivar Zao-Geng-19.As many as 89.3% ovary culture plants were green compared to only 36.4% of anther culture plants. This seems to lend support to the contention that the occurrence of the high frequency of albino plants in anther cultures may possibly be due to some cytoplasmic insufficiency of the pollen or pollen embryo. Also, in contrast to anther culture-derived plants, which show a wide range of ploidy, most of the ovary culture-derived plants are haploids plus a few diploids (Zhonglai and Chang, 1984).With improvement of culture techniques, mainly through the research of Yang and Zhou of Wuhan University, China, ovary culture in rice appears to be gaining recognition as a potential technique for production of haploids and doubled haploids in numbers sufficient for utilization in applied programs (see Zhou et al., 1986).

IV. SOMATIC CELL CULTURE In addition to the techniques of embryo culture, anther or pollen culture, and ovary culture, somatic cell culture techniques offer the possibility of extending microbiological techniques to crop improvement. Areas of potential usefulness include large-scale plant regeneration for rapid propagation of specific genotypes and rapid screening at the cellular level for variants. Large cell populations can be screened using limited facilities and time in comparison to the process of evaluation of a large population of plants in field, which is considerably more resource-intensive. Moreover, it has been demonstrated in several cases that when higher plant tissues undergo a process of dedifferentiation and cell proliferation in vitro, mutations occur at a frequency much higher than expected (see Orton, 1984). Increased mutation frequency and the possibility of screening millions of cells in cell cultures permit isolation of even rare mutants. The notable example is that of nitrate reductase-deficient mutants, which occur

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at a very low frequency (Muller, 1983) and therefore could not be isolated in populations of whole plants. Several rare auxotrophic phenotypes have been isolated from cell cultures. These include auxotrophs for adenine, histidine, tryptophan, leucine, isoleucine, nicotinic acid, etc. (See Maliga, 1985). That these changes in culture are not mere epigenetic events but true mutations has been confirmed in several cases through inheritance studies. Maliga (1985) listed nine such cases in which mutants have been “satisfactorily characterized genetically.” The existence of heritable variation among populations of plants regenerated from cell cultures has been known since the report of Heinz and Mee (1969) was published. Larkin and Scowcroft (1981) reviewed the literature on such genetic variability and termed it somaclonal variation. The authors discussed at some length the probable underlying mechanisms involved in bringing about somaclonal variation and its potential usefulness as a novel source of genetic variability. The subject has attracted considerable attention and has been a regular topic of discussion during the past few years (Larkin and Scowcroft, 1983; Meinz, 1983; Orton, 1983, 1984; Scowcroft et al., 1985; Larkin, 1985). Genetic changes that have been reported effect several morphological and biochemical characteristics, including some of potential agronomic importance such as yield, resistance to disease, protein content, and plant height. Scowcroft et al. (1985) listed 10 economically important species in which somaclonal variation has been demonstrated. From the viewpoint of its utilization in crop improvement, perhaps the main attraction of somaclonal variation is the rather frequent occurrence in tissue cultures of chromosomal rearrangements, including reciprocal and nonreciprocal interchanges. This provides an opportunity for introgression of alien gene(s) into the genome of a cultivar across barriers raised by the impossibility of sexual recombination. The other advantages that somaclonal variation seems to offer and that are distinct from those realized through induced mutagenesis include (i) the alteration of cytoplasmically encoded variation in chloroplast and mitochondria1 genomes; (ii) occurrence of mutations that change an allele from dominant to recessive and vice versa; and (iii) greater frequency of homozygous mutations and of the total spectrum of mutations.

A. SIGNIFICANCE OF SOMATIC CELLCULTUREFOR RICE IMPROVEMENT

In annual, population-intensive crops like rice in general cultivation, mass propagation of elite materials through tissue culture cannot be considered as a viable proposition for economic reasons. For maintenance

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of a desired genotype and propagation of an elite material to a limited extent, one could readily resort to vegetative means. However, in the cases of maintaining male-sterile lines or specific F, hybrids, which require rather rapid and large-scale propagation, tissue culture methods could serve the purpose. I n vitro induced adventitious budding or somatic embryogenesis of meristematic tissues and recycling of some of the derived shoots for further bud and embryo formation could provide a system for large-scale plantlet production. One of the areas in which conventional plant breeding techniques have remained rather handicapped is the upgrading of nutritive aspects of crop plants, especially protein. As a result, vital food crops like rice have remained poor in protein quality and quantity. However, cell culture techniques have opened up the possibility of obtaining cell lines and eventually plants grown to produce increased amino acid levels and a higher total protein content. This could be done by induction and selection of mutants at cellular level for resistance to amino acid analogs and regeneration of plants from the resistant calk Similarly, resistance to disease could be obtained by selection at the cell level for disease toxins or toxin analogs. For example, Carlson (1973) obtained tobacco mutants resistant to the wildfire disease by selecting cell lines resistant to the toxin analog methionine sulfoximine. The analog evokes chlorotic lesions similar to those caused by the wildfire toxin produced by Pseudomonas tabaci. The regenerated plants from resistant calli showed resistance to the bacterial infection and the resistance was inherited. Likewise, Gengenbach et al. (1977) selected for Helminthosporium T-toxin tolerance in maize cell cultures. The regenerated plants proved to be fully toxin-resistant (Dixon et al., 1982). Development of tolerance for environmental stresses such as soil salinity, soil acidity, metal toxicity, and drought constitute some of the challenging areas of crop improvement. Cell culture techniques offer the possibility of generating plants with increased tolerance to these stresses. Using NaCl as the stressing agent, Nabors et al. (1975) obtained salttolerant cell lines in tobacco and regenerated plants from the tolerant cultures. Later, the tolerance was demonstrated to be inherited (Nabors et al., 1980). In selection for tolerance to aluminium, cells are stressed with aluminium, and for acids, they are stressed with hydrogen ions. Meredith (1978) selected aluminium-resistant cell lines in tomato and Ojima and Ohira, (1982) did the same in carrot. In the opinion of Nabors (1982), drought tolerance could possibly be accomplished by incorporating in the medium high-molecular-weight chemicals like dextran sulfate or PEG (polyethylene glycol), which compete with the cells for available water in much the same way as does dry soil. However, the in vitro expression of a character may not necessarily

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correlate with the expression of the same character in an intact plant. Moreover, epigenetic and transient changes are known to occur frequently in plant tissue cultures and thus, to distinguish a true genetic variant from a physiologically adapted variant may pose a serious problem (see section

IV,D). Whereas embryo culture can be successfully employed for rescuing interspecific rice hybrids, a callus-mediated tissue culture cycle could be also attempted to induce or enhance genetic exchange between the two genomes. Conventional techniques are usually frustrating for the rice breeder due to lack of genetic exchange in hybrids between a cultivated species and an alien genome. Prolonged tissue culturing may be required at times to allow for enhanced genomic exchange and so as to recover the regenerant with the desired “recombination.”

B. CALLUS INDUCTIONA N D SINGLE-CELL CULTURES Since Furuhashi and Yatazawa (1964) first succeeded in inducing callus in rice using nodal segments, a wide range of tissues have been successfully tried, embryo being the most popular material. MS is the most commonly used basal medium and 2,4-D the most widely used hormone for callus initiation. Nishi et al. (1973) and Sekiya et al. (1977) found 2,4-D to be the most effective among the commonly used auxins (2,4-D, NAA, and IAA) for callus initiation. However, the most effective level may vary not only from one cultivar to another (Chung, 1975; Abe and Sasahara, 1982; Fatokan and Yamada, 1984), but also between explant sources (Chou et al., 1980).Wu and Li (1971)found that specific 2,4-D concentrations were necessary for callus induction from different organs of a young rice seedling. This observation was later confirmed by Henke et al. (1978); the lowest 2,4-D concentrations allowing callus initiation from different organs were 0.5 mghter for root tissue, 1 .O mgiliter for scutellar and cotyledonary node regions, and 4.0 mg/liter for leaf tissue. Japonica cultivars show a better efficiency of callusing than indica, javanica, and japonica x indica hybrids (Abe and Futsuhara, 1986a). Although 2,4-D alone may be sufficient for efficient callusing (Nabors, 1982: Siriwardhana and Nabors, 1983: Brar et af., 1985; Kishor and Reddy, 1986b; Raina et al., 1987),one or more of the other supplements like auxins (IAA, NAA), K, and complex growth substances such as CH and/or YE have also been used in addition to 2,4-D (Tsai et al., 1978; Davoyan and Smetanin, 1979; Ling et at., 1983; Abe and Futsuhara, 1984; Abrigo el a[., 1985). 2,4,5-T has also been found to be almost as effective a hormone for callusing as 2,4-D (Nabors, 1982; Siriwardhana and Nabors, 1983: Raina et af., 1987).

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Suspension cultures are generally initiated from actively growing callus cultures. About 100 mg of callus tissue is dispersed in about 3 ml of liquid medium placed on a shaker at 25-50 rpm. Cells in suspension cultures usually grow faster than callus cells and are ideal material for production of protoplasts, biochemical studies, and chemical mutagenesis. Maeda (1965) isolated single cells in suspension cultures from rice seedling callus, and Ohira et al. (1973) developed a defined medium for rapid growth of rice cell cultures. Growth was better with ammonium and nitrate together than with nitrate as the only nitrogen source. The medium developed (R2) was found to be better than several other well-known media. Chun (1984) used N6 and modified B-5 medium (medium 11) and found both suitable, especially the latter, for cell suspension culture as well as for subsequent callus formation. Abe and Futsuhara (1986b) and Zimny and Lorz (1986) established root-derived suspension cultures that were capable of plantlet regeneration. MS medium with 2,4-D was used for initiation of suspension cultures and decamba and picloram for organogenesis (Zimmy and Lorz, 1986).

EMBRYOGENESIS AND REGENERATION C. SOMATIC Shoot regeneration has been found to occur when the 2,4-D level is considerably reduced or substituted for by a weaker auxin such as IAA or NAA (Nishi et al., 1968; Yamada, 1977). Better regeneration efficiency could be achieved in the presence of K and/or CH (Kawata and Ishihara, 1968). Saka and Maeda (1%9) emphasized the role of K in enhancing shoot regeneration. Subsequently, K-induced or -enhanced regeneration was reported by others also (Davoyan and Smetanin, 1979; Zhuang, 1981). Abe and Futsuhara (1986a) examined 60 cultivars, including various ecospecies, for regeneration potential, using callus derived from root sections of 5-7-day-old seedlings. Variability in plant regeneration was significant. Only a few of the indica varieties, japonica-indica hybrids, and the large-grained javanica types showed shoot regeneration potential. In contrast, all the 28 japonica cultivars examined regenerated plants. Japonicas also exhibited better regeneration frequency than the others. Shoot regeneration has been found to occur when the 2,4-D level is considerably reduced or substituted for by a weaker auxin such as IAA or NAA (Nishi et al., 1968;Yamada, 1977). Better regeneration efficiency could be achieved in the presence of K andor CH (Kawata and Ishihara, 1968). Saka and Maeda (1%9) emphasized the role of K in enhancing shoot regeneration. Subsequently, K-induced or -enhanced regeneration was reported by others also (Davoyan and Smetanin, 1979; Zhuang, 1981).

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The frequency of shoot-bud and plantlet differentiation could be further increased in the K-supplemented medium if the preculture medium contained abscisic acid (Inoue and Maeda, 1981). Ling et al. (1984a)obtained similar results. Cornejo-Martin et al. (1979) reported that ethylene (0.36 CLM) promoted shoot regeneration in rice. Higher regeneration frequencies could be achieved with an ethylene-CO, combination in the ratio of 2.5 x 10- 4: 1. Carbon dioxide alone was ineffective. Other studies with rice somatic cell cultures have been concerned not only with high frequencies of plant regeneration but also with extending regeneration capacity over a long period of time. The recognition of such a requirement arose out of the observation that utilization of in vitro cell genetic manipulation techniques, especially in cereal crop improvement, is often hampered due to limitations of plant regeneration. Using embryo derived rice callus cultures, Nabors et al. (1983) observed that white, compact, smooth-surfaced, knobby-looking embryogenic (E) callus could regenerate plants 33 times more frequently on transfer to a regeneration medium than the nonembryogenic (NE) callus, which was yellowish to translucent, loose, rough-surfaced, and crystalline in appearance. The authors claimed that the high regenerative ability of E calli was maintained over eight passages involving a total of 40 weeks. E calli production could be significantly increased if tryptophan (a precursor of IAA in plants) was incorporated in the callusing medium (Siriwardhana and Nabors, 1983). However, of the seven varieties tested, only four (japonica as well as indica) showed tryptophan-promoted E callus formation. The authors also recorded a higher frequency of plant regeneration from E calli in BA- or TIBA-supplemented (0.1-0.5 mg/liter) medium than in the presence of IAA or no added hormone. Enhanced regeneration frequency in BA-supplemented media has been observed in other rice regeneration studies also (Cornejo-Martin et al., 1979; Brar et al., 1985; Jones, 1985). Ling ef al. (1983) reported over 80% plant regeneration in each subculture for 12 passages (1 year) in highly embryogenic calli obtained from immature panicles (5-25 mm) of an interspecific hybrid (0.sativa x 0. latifolia). The highly embryogenic callus-inducing medium consisted of Nitsch’s (1969) medium supplemented with 2,4-D, NAA, K, YE, and CH. For regeneration, the medium was supplemented with NAA and K. On average, as many as 2040 plantlets could be obtained from a 24-mm piece of callus. It seems that the development of highly embryogenic calli and the high efficiency of regeneration, recorded above, however, was due mainly to the source of the explant rather than to the hormones and other complex substances used. This is suggested by the studies of Chou et al. (1983),who also achieved highly embryogenic callus formation and plant regeneration (100%) using young panicles (of about the same size)

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but with simpler media. Moreover, Ling et al. (1984a), working with mature embryos, could achieve regeneration for only up to five passages from certain IR lines; plant regeneration ranged from 1040%. Using the cultivar IR-40, Wang and Zapata (1987) compared the performance of embryogenic calli derived from young inflorescences and mature embryos. It was observed that the inflorescence calli retained the regeneration potential for a longer time and registered less of a drop in the regeneration frequency over the passages. Young inflorescences thus seem to offer an ideal source for raising high-frequency and long-term regenerating cultures-a requirement so vital for a whole range of cell genetic manipulation studies including those involving the establishment of embryogenic cell suspensions for protoplast-related programs. Kishor and Reddy (1986b) have been able to obtain a high regeneration frequencies (5040%)in root and embryo- derived rice callus tissues over a period of 1400 days. This has been possible through the use of a medium containing sorbitol or mannitol. Whereas the calli proliferating on a medium containing 2% sucrose regenerated shoots at a low frequency (20-30%). In contrast, calli grown on a medium containing 2% sucrose plus 3% sorbitol or 3% mannitol exhibited a higher shoot regeneration frequency (5060%) and retained this potential over a period of 1400 days. The promotive effect of sorbitol or mannitol could not be reproduced by making appropriate osmotic adjustments of the medium, using substances such as sucrose, fructose, or glucose. Whatever the mode of promotive effect of sorbitol and mannitol may be, the results are very encouraging from the viewpoint of utilization of in vitro techniques in rice cell genetic manipulation studies. In addition to the genotype, explant source, callusing medium, and the medium for regeneration, certain other factors have been identified as markedly affecting the plant regeneration frequency. Ram and Nabors (1985) obtained a significant increase in regeneration by optimization of the callus-medium volume ratio, which was 6.5 mg embryogenic callus per ml of regeneration medium. Regeneration frequency could be further increased by using regeneration medium previously conditioned for 1-2 weeks by optimal amounts of embryogenic callus. Chun (1984) observed a significant improvement in callus yields as well as in regeneration frequency when sodium hypochlorite instead of mercuric chloride was used for sterilization of the explant. For purposes of propagation, multiple-shoot buds can be readily induced from seeds/mature embryos using a BA-supplemented medium (Hisajima et al., 1987; Kumari e f al., 1988). Clumps of multiple shoots are induced which can be recycled for further shoot induction in order to increase the propagation rate. Replacing BA with IBA (1-2 ppm) ensures good root growth of the derived plantlets.

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D. In Vitro Mutant Selection Following the demonstration of Helsell et al. (1972) that the enzyme aspartokinase (of the aspartate family) from rice is sensitive to feedback inhibition by the lysine analog S-AEC, attempts have been made to use the analog as a selective agent for isolating mutants for lysine overproduction. Schaeffer and Sharpe (1981, 1983) obtained improved rice seed protein and lysine from plants regenerated from anther-derived calli resistant to S-AEC. Lysine content increased by up to 10% and the total protein by up to 6%. Haploid cell cultures offer the advantage that mutations, whether dominant or recessive, can be readily detected. Zapata (1985) repeated the work at IRRI using anther-derived calli of the rice variety Taipai-309. The calli were stressed with 1 x M S-AEC, a concentration that killed normal cells. Variant cells resistant to S-AEC were plated on the regeneration medium and the plants raised and analyzed. One of the lines, AC-1040-64, showed an increase in total protein of as much as 9.3% and an increase in lysine of 13.9% over that of the parent. Schaeffer (1986) stressed rice calli of the cultivar Calrose-76 with inhibitory levels of lysine plus threonine. From among the plants regenerated from the resistant calli, a line was identified that had a significantly higher lysine content in seed proteins than did the parent cultivar. Chen and Meng (1986) also obtained S-AEC-stressed cell lines from rice antherderived calli that showed a twofold increase in lysine and fivefold increase in threonine compared to the control cells. Wakasa and Widholm (1982, 1986) used cultured diploid somatic cells to isolate rice mutants with increased tryptophan levels through selection for cells resistant to the inhibitory levels of the analog 5-methyltryptophan (5-MT). One callus of over 1000 seed-derived calli showed good callus growth in the 5-MT-stressed medium. Six plants that regenerated were selfed and the progeny tested for 5-MT resistance. Some of the seed-derived calli from three of the regenerants showed resistance. However, only in some of the cases was the resistance associated with an increase in tryptophan content. With the objective of generating cell lines that could prove valuable as markers in somatic cell genetics studies, Wakasa ef al. (1984) selected nitrate reductase-deficient rice cell lines using anther-derived pollen calli inoculated on a medium containing 300 mM of sodium chlorate. Of 520 cell lines that were screened for resistance, 2 cell lines from a group of 67 that showed resistance did not exhibit any nitrate reductase activity. Such mutant markers could also prove useful in the development of a selection system for somatic cell hybridization studies. With the same objective, Mikami and Kinoshita (1985) attempted to obtain streptomycinresistant cell lines of rice. Seed-derived calli were exposed to various levels

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of streptomycin and three cell lines were identified that exhibited a significant increase in resistance index. Several groups of researchers (Reddy and Vaidyanath, 1985; Kishore and Reddy, 1986a; Abrigo e l al., 1985; Nabors and Dykes, 1985; Wong et al., 1986) have taken up studies on in vitro selection for salt tolerance using NaCl as the stressing agent. The group at TCCP have reported (Nabors and Dykes, 1985) the regeneration of about 4000 plants from NaCItolerant cell cultures of a number of rice cultivars. It was also claimed that the tolerance was stable and inheritable. The IRRI group (Abrigo et al., 1985) also reported some interesting results. It was found that the growth curves of a salt-tolerant variety (Pokkali) and that of a susceptible variety (IR-28) were similar at different NaCl concentrations. From the results, it was concluded that the salt-tolerant plants are not necessarily tolerant at the cellular level. Screening for salt tolerance was, therefore, carried out using seeds of the cell culture-derived variants (see section IV, E). A similar procedure was also adopted for screening for aluminium toxicity. Although the procedure of exposure of a large population of cells to stress factors has been successfully used in certain cases as the strategy for selection of variants carrying the desired resistance or tolerance, the methodology may prove generally ineffective. The reason for this is that the expression of the character may be dependent upon a large number of factors (see Ingram and MacDonald, 1986). For an efficient use of in vitro techniques it would be appropriate, therefore, to undertake studies aimed at understanding the biochemical and molecular basis of the expression of the character involved and, accordingly, identify suitable biochemical markers whose presence or absence could be used as the basis of selection. For example, a variety of chemical substances such as lignins, toxic phenolic compounds, and a wide range of low-molecularweight toxins known collectively as phytoalexins, have been found to be the primary determinants of disease resistance (Baily, 1983). A number of studies have indicated that the appearance of phytoalexins correlates well with the expression of resistance in the differentiated plant (Helgeson and Deverall, 1983). It may be possible, therefore, to use phytoalexin biosynthesis or other such biochemical markers to identify variant cells resistant to biotic or abiotic stresses. It may also be practical to use molecular biological techniques to identify primary gene products associated with resistance factors and use this information as a basis for cell selection (see Negrutiu et al., 1984). So far as rice is concerned, available information in regard to biochemical or molecular expression of resistance or tolerance characters+ almost nonexistent. Such information must, therefore, be obtained in order to use the in vitro cell selection system as an effective and efficient procedure for the selection of genotypes possessing novel resistance factors.

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E. SOMACLONAL VARIATION Oono (1978, 1981, 1983) recorded a high frequency of somaclonal variation in rice. He examined 1121 regenerants (R,,) derived from 75 seed calli of a doubled haploid pure line. Mutation frequencies for seed fertility, plant height, date of flowering, morphological characters, and chlorophyll deficiency of the progeny of self regenerants (R,) were 63.1, 19.3, 5.4, 1.8, and 17.2%, respectively. With respect to these five characters, 43.9% of the progeny were variant for one character, 28.0% for two or more characters, and 28.1% were considered normal. Evaluation of R, and R2 progenies indicated that mutations affecting plant height, flowering date, grain weight, grain number, panicle length, chlorophyll pigmentation, and salt tolerance were simply inherited. Two of the mutated traits, plant height and glabrous seed, bred true in the R, progeny, suggesting the occurrence of homozygous mutations (Oono, 1983). The dwarf mutants obtained by Oono (1985) from among regenerants derived from rice seed calli cultured in 1% NaC1-supplemented medium have added a new dimension to ideas about the mechanisms involved in generating somaclonal variation. Although the dwarf character was transmitted as a homozygous mutation by selfing through eight generations, the trait disappeared in the F,-F, generations of reciprocal crosses between the mutant and control plants or progeny of normal regenerated plants. Chimeric reversion to the normal height was observed. The revertant phenotype was found to be stable for at least three selfed progenies. Heritable gene inactivation during the course of in vitro culture was the explanation offered for the occurrence of the “putative” dwarf mutations. In a subsequent study (Oono, 1986), evaluation of nuclear DNA sequences from rice callus following restriction endonuclease digestions revealed quantitative variations. Callus nuclear DNA smears showed many bands that were not clear in embryo DNA. There are several other reports of somaclonal variation in rice. Kucherenko (1979) studied 98 somaclones derived from four rice genotypes. R , progeny evaluation showed variations in 36% of the regenerants. Similarly, Suenaga el af. (1982) and Zhao el af. (1982) regenerated plants from callus cultures of various rice varieties. An array of mutations was reported by both the groups. Zhao and co-workers evaluated the progenies derived from selfed regenerants. Mutations recorded included such agronomically important traits as flowering date, grain shape, and fertility, as in Oono’s studies. Most of the variant lines were found to be fixed in the R, generation and subsequently remained stable. Sun et af. (1983) examined a large population of 2000 somaclones derived from 18 rice genotypes including hybrids. Genetic evaluation of 950 R, lines showed that only 24.2% were normal. Variation frequency for different traits ranged from 11.5 to 36.5%. In this investigation also, most

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of the altered traits attained uniformity in the R, generation and bred true. In general, plant height tended to become shorter, the number of productive tillers increased, and the 1000-grain weight decreased. Whereas the average probability among somaclones of decreased plant height was 31.3%, it was only 5.6% for increased height. Similarly, the average probability for decreased grain weight was 36.9%, whereas it was only 2.7% for increased grain weight. It has been suggested that such preferential variation for certain traits might be due to a linkage of some sort between certain characters and the gene for regenerability. From the above studies, a promising somaclone, T-42, that had shorter height, better plant type, more productive tillers, and higher yields was released for cultivation (Zhao et al., 1984). Fukui (1983) and Croughan (1984) also obtained short-culmed, earlyheading, and high-tillering variants among rice somaclones. Fukui studied progenies of 12 plants regenerated from a single seed callus of the rice variety Nipponbare. His studies seem to indicate that Zone is looking for early-flowering and short-culm mutations, these could be rather readily obtained through a short tissue culture cycle involving a brief dedifferentiated period. Longer periods of dedifferentiated state may permit the accumulation of mutations. From the results so far, it seems that a callus-mediated tissue culture cycle could prove to be a novel source of useful genetic variability in rice. In addition, it might also prove useful in situations requiring a subtle change in an otherwise desirable cultivar. For example, the indica rice variety Basmati-370 is perhaps the best example of the high-priced, aromatic, fine-grain rice varieties. However, it has too tall a plant type (about 150 cm) and therefore is highly susceptible to lodging. Attempts to improve the plant type through hybridization and/or induced mutagenesis without disrupting the gene assemblages that determine the fine grain characteristics have been rather frustrating. Desirable semidwarfs could possibly be obtained from a rather short tissue culture cycle involving low doses of phytohormones. Oono (1985) found that use of a high concentration of BA (30 mg/liter) in the culture medium increased genetic variability nearly 50 times as much as a lower concentration of BA (2 mg/liter). Recently, we evaluated 257 Basmati-370 somaclone lines from among over 600 self regenerants, raised from embryo-derived calli (Fig. 6, top). As many as 21 semidwarf lines, ranging in height from 65 to 140 cm (Fig. 6, bottom), were identified. Though all the semidwarfs had smaller panicles and lesser grains per panicle in comparison to the control Basmati-370, some of them had more productive tillers and have retained the “basmati” characteristics of aroma, high-kernel elongation, and fluffiness of the cooked grain (Raina and Hadi, unpublished). Similarly, an early-maturing somaclone of IR-36 was identified from

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FIG. 6. Somatic cell culture of Basmati-370. Top, Somatic embryos in embryo-derived calli. Bottom, Semidwarf variants of Basmati-370; control, extreme left.

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127 R, lines (Ling er al., 1987). However, besides earliness, the line differed from IR-36 in seven other characters including tiller number, panicle size, and grains per panicle. According to the authors, however, even though the line is inferior to IR-36 in several traits, it is still adapted in several areas of China due to its earliness. At IRRI, Abrigo er al. (1985) utilized somaclonal variation in screening for salt and aluminium tolerance. Using seedlings derived from self regenerants, they identified a number of salt-tolerant and Al-tolerant mutants. Some highly Al-tolerant variants showed longer root growth at 30 ppm Al than did the parent at 0 ppm Al. Ling et al. (1985) screened somaclonal variants derived from somatic cells cultured in a medium containing 25% Helminthosporium oryzae toxin and from toxin-free medium for resistance to H. oryzae. One resistant variant was identified from among regenerants derived from toxin-free medium and two from those derived from toxin-containing medium.

V. PROTOPLASTS The possibilities of genetic manipulation thrown open by the success of isolation, culture, and regeneration of plant protoplasts are indeed the most fascinating of all tissue culture techniques. In addition to the opportunities protoplasts offer as single cells, their lack of a cell wall provides opportunities for transformations through incorporation of foreign DNA, cell organelles, etc. Also, as naked cells, protoplasts lend themselves to cell fusions, thereby opening up the possibilities of somatic hybridization. Such hybridization offers unique advantages over sexual hybridization for combining unrelated genomes and creating novel cytoplasmic combinations. Also, amphidiploids can be directly generated. Since Takebe et al. (1971) succeeded in regenerating tobacco plants from protoplasts, considerable progress has been made in protoplast isolation, culture, and regeneration in a large number of species. Vasil and Vasil(l980) listed the wide variety of enzyme preparations that have been utilized for high-efficiency isolation of protoplasts from almost every plant tissue. Procedures have been developed to induce fusions. These involve the use of sodium nitrate, calcium at high pH, dextran, elevated temperature, electrical stimulation, and/or polyethylene glycol. Use of the latter (at high pH eluted with calcium ions) remains the most common method. Carlson et al. (1972) were the first to regenerate hybrid plants from fused protoplasts of Nicotiana glauca and N . 1angsdorfJii. By 1984, somatic cell fusion methods have been used to produce hybrid plants from 26 inter-

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specific and 6 intergeneric combinations. In addition, in 4 cases of interspecific fusions and 25 intergeneric fusions, divisions have been achieved in the fusion products (see Collins et al., 1984). Parasexual hybridization through protoplast fusion eliminates the prezygotic barriers and maternally induced inhibitions that may occur in sexual hybridization. However, hinderance due to genomic incompatibility remains a challenge. Frequently, in somatic hybrids involving distantly related species, genome incompatibility results in chromosome elimination. Generating symmetric hybrids that contain the entire genomes of both the species, however, may not be necessary. Therefore, the development of asymmetric hybrids resulting from chromosome elimination has been seen as a blessing in disguise. Such hybrids could be more useful in achieving the objective of transfer of one or a few genes, provided, of course, that the specific chromosome(s) needed are retained. In the event of complete elimination of chromosomes, resort to backfusions could prove helpful. Asymmetric hybrids also offer the advantage of being less problematic than symmetric hybrids in undergoing shoot morphogenesis (Hoffmann and Adachi, 1981). Pental and Cocking (1985) proposed that triploid plants could be synthesized for production of alien addition and substitution lines by fusing protoplasts from spore tetrads of the alien species with the cultivated variety. Pirrie and Power (1986) synthesized triploids through such a procedure using tetrad protoplasts of Nicotiana glurinosa with somatic cell protoplasts of N. tabacum. Pental et al. (1988) described a selection method for producing gameto-somatic hybrids in large numbers. Cybrids, produced by fusion of enucleated protoplasts with normal protoplasts, offer the opportunity of obtaining cytoplasmic hybrids containing cytoplasms of both the parents but the nucleus of only one. Cybrids have opened up the possibility of transferring cytoplasmically based traits such as male sterility from one species into the nuclear background of another. Enucleated protoplasts can be obtained by high doses of X-irradiation, which inactivates the nucleus, or, if one needs to be careful to avoid possible mutations, through fractionation of protoplasts into subprotoplasts (nucleus with a little cytoplasm) and cytoplasts, which are enucleated protoplasts. Whereas cytoplasts could be useful for transfer of cytoplasmic traits, subprotoplasts may serve as efficient vehicles for nuclear transfer. Zelcer et al. (1978) claimed to have transferred cytoplasmic male sterility using cybrids. From what has emerged so far from several studies on the segregation of cytoplasmic traits in somatic hybrids (Fluhr, 1983), it seems that cytoplasmic mixing does not automatically involve complementation andor recombination of cytoplasmic genomes. In fact, the studies indicate that only one of the two plastomes remains functional. Mitochondria of

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both the parents not only remain functional, however, but there is some evidence of their recombination, leading to speculations on the possibility of obtaining even greater vigor in hybrids (Bingham, 1983). There are several reports of introduction of isolated nuclei, chloroplasts, mitochondria, algae, yeast, bacteria, etc. into plant protoplasts. However, this forced relationship has so far remained ineffective. Methods have been developed for isolation of whole mitotic chromosomes from protoplasts, and their incorporation into protoplasts has also been demonstrated (Griesbach el al., 1982; Malmberg and Griesbach, 1983). However, functional expression of these transferred chromosmes was not observed. Limited gene transfer through protoplast fusion involving sexually isolated species was demonstrated by Dudits et al. (1980). The green phenotype of carrot (Daucas carota) was restored when protoplasts of a nuclear albino mutant of carrot were fused with X-irradiated protoplasts of Petroselinum hortense. Likewise, Gupta et al. (1982) transformed nitrate reductase-deficient cells of Nicotiana by fusing them with X-irradiated protoplasts of Physalis. Interestingly, Jinks et al. (1981) demonstrated limited sexual gene transfer between Nicotiana genotypes, using irradiated pollen. Similarly, it has been possible to effect limited gene transfer using irradiated pollen in wheat (Snape et al., 1983) and also in barley (Powell et al., 1983). Protoplasts, however, offer the opportunity of effecting limited gene transfer even across the barriers of sexual incompatibility.

A. AREASOF POTENTIAL USEFULNESS

IN

RICEIMPROVEMENT

Transfemng cytoplasmic male sterility in varieties with good combining ability, transferring disease resistance to and tolerance for various environmental stresses from wild species, and obtaining greater vigor in hybrid rice through mitochondria1 recombination are the main areas of potential usefulness of protoplast technology in rice improvement. Most of these objectives are too difficult or even impossible to accomplish through conventional techniques. Others, such as transfer of male sterility, involve a cumbersome procedure of backcrossing. Production of cybrids could be of great help in accomplishing the task in less time. For transfer of alien genes, somatic hybridization could be attempted in such situations where incompatibility barriers are severe to an extent of disallowing the possibility of raising hybrids through embryo rescue measures. In such situations, maybe one needs to resort to only limited gene transfer using X-irradiated protoplasts of the alien species and normal protoplasts of the cultivated variety. Alternatively, one could produce alien addition or substitution lines from triploid plants raised through gametosomatic hybrids.

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B. ISOLATION, CULTURE, AND REGENERATION OF PROTOPLASTS In rice, protoplasts have been isolated from seminal roots (Maeda, 1971), somatic calli (Wakasa, 1973; Tseng et al., 1975; Ham, 1973), leaf blades (Tseng and Shiao, 1976), and leaf sheaths (Deka and Sen, 1976). Protoplasts were fused using sodium nitrate and polyethylene glycol and multinucleate cells obtained (Harn, 1973). Tsai et al. (1978) standardized techniques for isolation, culture, and development of a large number of calli from protoplasts isolated from pollen-derived calli but regeneration of either shoots or roots was not possible. Deka and Sen (1976), however, claimed root regeneration from leaf sheath protoplasts of rice. During approximately the last 3 years or so, considerable progress has been made in rice protoplast culture techniques. Using a medium which contained four amino acids as the sole N source, Toriyamma and Hinata (1985) achieved high-frequency callus formation from protoplasts derived from anther callus. On transfer to the regeneration medium, green spots and root formation appeared on the calli. The authors concluded that the N source was critical for rice protoplast culture. In the same year, the first successful plant regeneration from rice protoplasts was reported almost simultaneously by two groups from Japan (Yamada et al., 1985; Fujimura et al., 1985). Yamada et a / . (1985) identified a male-sterile line, A-58MS. and two varieties, Fujiminori and Toyotama, among 26 rice genotypes as suitable for protoplast culture. One plantlet regenerated from protoplast-derived A-58 MS callus and eight from Fujiminori. Giving more details of their work, Yamada et al. (1986) attributed their success to (i) screening of a large number of rice genotypes, (ii) selection of rapidly growing cell lines with dense cytoplasm, and (iii) development of RY-2 medium for rice protoplasts, which, among other things, contains calf serum, glucose (as osmoticum), and a decreased level of ferrous and ammonium ions. Fujimura et al. (1985) achieved success of plant regeneration from rice protoplasts of two varieties, Nihonbare and Sasamishiki, using simpler media. The medium R, was used for protoplast culture and callus formation and N, for plant regeneration. Reports of successful plant regeneration from rice protoplasts have also come from France (Coulibaly and Demarly, 1986) and England (Abdullah et al., 1987). The latter report is particularly noteworthy in that the problem of plantlet regeneration from undigested cell clumps seems to have been satisfactorily solved. In the initial studies of this group, Thompson et a / . (1986) obtained sustained divisions in rice protoplasts isolated from cell suspensions using an agarose-solidified, amino-acid (AA)-based culture medium. Increased protoplast survival, division, and plating efficiency was achieved with an increase in agarose concentration from 0.6% to 1.2%. Further investigations (Thompson ef al., 1987) led to the identification of

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FIG. 7. Regeneration from rice protoplasts: (a) protoplast-derived colonies in agarose, 4 weeks in culture from protoplast isolation (dish is 5 cm in diameter); (b) protoplast-derived

embryogenic callus of Fujisaka-5 showing three typical embryoids with fused scutella (SC), coleoptiles (C), and roots (r) (Bar = 1.5 mm); ( c )regenerated plants derived from protoplasts, shown in the greenhouse. (Courtesy Dr. John Thompson, Botany Department, University of Nottingham, United Kingdom.)

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heat shock pretreatment as a useful technique. Heat-treated (45°Cfor 5 min) protoplasts exhibited doubling in the frequency of plated protoplast divisions as well as in plating efficiency. compared to untreated protoplasts. Colonies could be obtained even at a reduced protoplast density of 1.0 x 105/ml,but only from protoplasts subjected to heat shock. Plantlet regeneration from 10-20% of the protoplast-derived colonies of two japonica varieties was obtained (Abdullah ef al., 1986) after direct transfer to hormone-free N, medium (see Fig. 7). Interestingly, no differentiation was obtained from colonies transferred to hormone-free M S medium, which seems to indicate that the N source has an important role in rice protoplast regeneration. The protoplast isolation procedure included a purification step involving filtration through a 30-km mesh. This sieving method was adopted so as to ensure that any clumps of undigested cells were effectively removed. However, very low levels (average 0.05%) of contamination by intact cells was noticed. These elongated, thick-walled, and generally plasmolysed cells, which could be readily distinguished microscopically, showed no fluorescence after treatment with fluorescein diacetate. Furthermore, microscopic examinations revealed that none of the intact single cells underwent division. Toriyama et al. (1986) also used amino-acid medium that contained four amino acids as the sole source of nitrogen for raising finely dispersed cell suspension derived from anther calli. The isolated protoplasts were cultured in a medium that contained nitrate as the sole nitrogen source. Four haploid and 11 diploids plants were regenerated. Perhaps the most notable of the successful reports so far is that of the Shimamoto group from Japan (Kyozuka et al., 1987). They evolved novel nurse culture methods for culture and regeneration of protoplasts. The nurse methods involve the use of agarose bead-type culture in combination with actively growing nurse cells that are either in the liquid part of the culture or inside a culture plate insert placed in the center of the dish. The presence of nurse cells was found necessary for induction of divisions. The former method-the mixed nurse method-was found to be twice as effective as the latter. As little as 3 days nursing was capable of inducing divisions but 10 days was found to be best for obtaining high-colony formation efficiency. Protoplasts were isolated from suspension cultures as well as primary calluses of four japonica varieties. The callus-derived protoplasts showed lower plating efficiency; plant regeneration was obtained from both. As many as 300 plants are reported to have been obtained. In a subsequent study (Ogura et al., 1987) involving field evaluation of about 130 protoplast-derived plants, over 80% were reported to be normal. However, in a more recent report from another laboratory in Japan, Kanda et al. (1988) observed wide variations for agronomic traits among

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protoplast-derived plants. Interestingly, the protoplasts were derived from calli maintained on semi-solid medium for 7-10 months. The authors called attention to the problems of genetic instability that are associated with long-term cultures. OF RICE PROTOPLASTS C. GENETICMANIPULATION

During the last few years, considerable progress has been made in genetic manipulations, especially transformation in dicot plants. Some of the more recent developments include regeneration of transgenic rapeseed plants obtained by micro-injection of DNA into microspore-derived embryoids (Neuhaus et al., 1987); transformation of protoplasts of a Petunia flower color mutant by a maize gene (Meyer er al., 1987) resulting in a new flower color; and rapid production of transgenic tobacco plants by cocultivation of epidermal peels with a disarmed strain of Agrobacterium tumefaciens (Trinh et al., 1987). In comparison to dicots, not much progress has been made in genetic manipulation of cereal cells by either protoplast fusion or direct or indirect gene transfer. (See Lorz et al., 1988; Cocking and Davey, 1987). The limitations have been mainly that of low plating efficiency of cereal protoplasts and, until recently, regeneration of only callus from protoplasts. Also, gramineous species have not been found to be infected by Agrobacterium tumefaciens, which represents an effecient system for gene transfer through the Ti plasmid. However, direct gene transfer to cereal protoplasts, independently of Agrobacterium infection or the mediation of a Ti plasmid has been achieved (Lorz et al., 1985). Transformation was obtained by incubation of Triticurn monococcum protoplasts with plasmid pBL 1 1034, which was carrying chimeric genes encoding the neomycin phosphotransferase (NPT-11)gene as a selectable marker. Transformed cells were picked up in medium containing kanamycin and the callus tissue derived exhibited NPT-I1 enzyme activity, which was absent in the untreated tissue. The technique has also been extended to rice. Zimny et al. (1986) and Uchimiya et al. (1986) demonstrated transient expression of the NPTI1 gene in rice protoplast-derived tissue following PEG-induced DNA uptake of pBL 1103-4. Attempts are being made to improve the techniques of genetic transformation of cereal crops, including rice, using NPT as well as CAT (chloramphenicolacetyl transferase) as selectable genes (Lorz et al., 1986). In addition to PEG, electroporation has also been successfully used for inducing DNA uptake. Electroporation uses electrical pulses of high field strength to permeabilize cell membranes reversibly so as to facilitate the transfer of DNA into cells (Fromm et a f . , 1986). More recent developments include regeneration of somatic hybrids of

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rice and barnyard grass (Echinochloa oryzicola), from the Shimamoto group (Terada et al., 1987). Following electrofusion of the protoplasts, selection of the hybrids was based on inactivation of rice protoplasts by iodoacetamide and the inability of barnyard protoplasts to divide. Fortyfour shoots were obtained from hybrid calli of which 9 grew to plantlets. However, no hybrid plants could grow to maturity. Another recent development comes from Uchimiya (1988). About 200 plants were regenerated from cell lines in which stable maintenance of inserted DNA (through the vector pCT2T3 containing two marker genes) was demonstrated. However, expression of the foreign gene products were not evident but the presence of DNA sequences, homologous to inserted plasmid, was noted in one plant. And the latest highlight is the first report of the production of cybrids in which protoplasts of one of the parents was from a cytoplasmic male sterile line (Akagi et a / . , 1988).CMS protoplasts were inactivated by x-irradiation whereas, N-8 (fertile parent) protoplasts were inactivated by iodoacetamide. Following electrofusion and while inactivated protoplasts never divided, hybrid cells grew in clusters from which as many as 560 plants regenerated. Half of them had 24 chromosomes and the remaining had 48. Cybrid plants, having mitochondria from both parents and nucleus of only one, were identified. (See Table AIL)

VI. OVERVIEW AND STRATEGIES FOR THE FUTURE Since Ameniya and his group first successfully attempted in vitro culture of immature rice embryos some 30 years ago, rice tissue culture studies have progressed considerably. A number of new techniques of potential usefulness in rice improvement have been developed. That these techniques are not just new areas of fashionable research has been already demonstrated in certain cases through their effective utilization in rice improvement programs. Indeed, some of the techniques are becoming economically viable technologies of proven benefit; certain others appear very promising but warrant further investigation and standardization before the opportunities they offer can be fully appreciated or utilized. Embryo culture techniques for rice are now available for rescuing hybrid embryos from otherwise unsuccessful crosses. However, there is scope for further improvement of the techniques so as to allow germination of hybrid embryos of those crosses which have not proved successful so far. In those cases in which embryos may prove highly recalcitrant to embryo rescue techniques, it may be a good idea to induce a callus from such embryos and then attempt regeneration of plants. In studies involving production of durutn-based primary triticales (Raina, 1984), several

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apparently “normal” looking but immature hybrid embryos failed to germinate despite repeated in vitro manipulations. Dedifferentiation of the embryos followed by attempts at plant regeneration provided a useful alternative for realizing the hybrids (Raina, unpublished). Anther culture techniques offer distinct advantages in breeding new rice varieties. In addition to the time and resources that can be saved through anther culture breeding, the techniques would be most advantageous in the development of stable and fertile addition-substitution lines from intersubspecific and interspecific crosses, a very difficult proposition otherwise. There are several examples of development of anther culture-bred varieties and large-scale utilization of the techniques in hastening breeding programs. Considerable progress has been made in improving the efficiency of anther culture in rice. However, the techniques still suffer from drawbacks that hinder their wider utilization in breeding programs. Variation among cultivars in responding efficiently due to genetic and/or physiological reasons remains an impediment, particularly in the culture of indica rice varieties. Other than a few indica cultivars, anther culture efficiency in general is significantly better in japonica rice strains. It seems that the anther culture requirements of indica cultivars differ from those of japonicas. Further research efforts are therefore warranted for improvement of anther culture efficiency, especially in indica rice. The areas that need strengthening are

(i) Anther response should be enhanced. (ii) Induction of direct pollen plants through pollen embryogenesis should be attempted. The intervention of a callus phase involves problems of regeneration and genetic instability. (iii) The frequency of shoot regeneration from pollen calli should be improved. (iv) The occurrence of albinos, which is a serious problem in a number of varieties, needs to be cut down significantly if not eliminated completely. Among the factors that could lead to further advances in anther culture efficiency, the physiological status of the donor plants appears to be of considerable importance. Besides studies involving alterations in agronomic conditions and temperature and light regimes of the donor plants, use of gametocidal agents and such other chemicals which are likely to affect the committment of the uninucleate pollen toward the development of male gametophyte needs to be examined. Such pollen may thereby become more amenable to anther culture techniques. Already a 20-fold increase in anther yields has been reported in wheat when donor plants were sprayed, at different stages around meiosis, with a chemicalhybridizing agent solution, fenridazon-potassium (Picard et a / . , 1987).

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Media-manipulation studies must continue especially for indica rices. Use of barley starch, found to be a good replacement for agar in barley anther cultures (Sorvari, 1986), merits investigation. Together with starch medium, melibiose (an inert carbohydrate) is reported to have induced “revolutionary effects on barley anther cultures” (Sorvari and Schieder, 1987). In the starch-melibiose medium, EDAM-I1 (Enzymatically Digestable Agar-free Medium), sucrose is not necessary according to the authors. Energy and carbon could be obtained by tissue enzymatically from the starch tissue. The EDAM-11, which is composed of N, inorganic salts and organic supplements, merits further examination for rice anther culture. Furthermore, melibiose medium is reported to effect a drastic reduction in the number of albinos. Efforts also need to be made to improve techniques for large-scale anther culture, so as to minimize the involvement of labor. Also, in order to allow rice breeders to take up anther culture techniques using locally available materials and minimal resources, the requirements need to be simplified. Isolated pollen culture continues to defy in vitro manipulations, necessitating continued use of anther culture. Pollen culture has generated renewed interest because of the recent advances in plant molecular biology. Pollen culture offers attractive possibilities for transformations through micromanipulations. However, success with isolated pollen continues to be very low. The reasons for poor performance appear to be (1) the deprivation of anther-wall factor, well known for its positive role in inducing/ enhancing the nongametophytic growth in pollen, and (2) the damage the pollen seems to receive during the process of isolation. While efforts to improve the efficiency of isolated pollen culture may be continued, float anther culture, which allows automatic shedding of pollen in the liquid medium, is worthy of consideration. Some modifications may be necessary, and it may prove to be an effective system for micromanipulation stu’dies. Considerable progress has been achieved in somatic callus initiation and subsequent plant regeneration for a large number of rice varieties, both indica and japonica. During the last few years especially, several successful attempts have been made to obtain high-frequency regeneration over long durations, involving periods of a year or more. The improved methodologies have made it possible to raise large populations of regenerants for evaluation and isolation of desirable somaclonal variants. Embryo rescue of the otherwise incompatible intra- or interspecific crosses, followed by a tissue culture cycle of dedifferentiation and redifferentiation, appears to be a promising approach for achieving desirable alien gene introgression. Micropropagation techniques are available, or can be readily improvised, for quick multiplication of specific elite or important materials such as

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male sterile lines, F, hybrids, novel genotypes, and endangered wild rices. However, development of such a technology for crop cultivation (F, hybrids) purposes, does not appear feasible in terms of cost-benefits. Although the demonstration of high-frequency, long-term regeneration in certain highly embryogenic cell lines has shown heartening progress, the techniques still cannot easily be applied to mutant cell lines or other such genetically manipulated cells, which eventually must be regenerated into plants. The recent achievement of high-frequency, long-term regeneration through the use of sorbitol or mannitol needs to be examined further. Plant regeneration from cell suspensions also warrants further study. Aside from media manipulations, basic studies should especially look into the biochemical and physiological aspects of plant regeneration. The effects of the Conditioning factor merit further investigation. Of late, there has been considerable research on the isolation of celllevel mutants of agronomic importance in rice. The major advantage of cell cultures has been seen as their apparent similarity to microbial systems: the possibility of screening large populations in small areas and limited times. There are reports of isolations of cell-level mutants for high lysine and high total protein. There are also reports of the isolation of cell lines, and eventually plants, with increased tolerance to salt. However, convincing demonstrations of inheritance of the altered function have not been forthcoming, especially for traits known to be under polygenic control and known to be expressed differently under diverse environmental conditions. Whether or not selection at the cell level could be effectively and efficiently utilized in higher plants like rice, which frequently have several copies of a gene, requires a better understanding of the biochemical and molecular basis of the trait involved. Molecular techniques could also be used to investigate the mechanisms underlying the occurrence of the high frequency of variations in cell cultures. The hypothesis of the possible involvement of mobile genetic elements is, of late, gaining ground and needs to be examined. Protoplast technology has opened up an array of new opportunities for genetic manipulations in rice. Now that plant regeneration from rice protoplasts has been achieved in several laboratories, somatic cell fusion techniques for obtaining limited gene transfer and transfer of cytoplasm across barriers of sexual incompatibility are likely to become available in the not-so-distant future. Already cybrid plants carrying mitochondria from both parents but nuclei from only the fertile parent have been raised. Also, transformations of rice protoplasts have been achieved through the use of plasmid constructions carrying a selectable marker gene. The immediate task for the rice tissue culturists, therefore, is to standardize reproducible and efficient protoplast regeneration procedures. While the generally followed procedure of isolating protoplasts from selected suspension cell

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lines developed over several months, it may not seriously impede plant regeneration so far as some japonicas are concerned but it is likely to prove to be a difficult task for indica nces. Use of immature inflorescences instead of mature embryos as the callus-producingexplants may be worthy of consideration in view of the fact that the calli from the former are known to retain morphogenetic potential for a longer time. Improved protoplast culture techniques are bound to stimulate interest in molecular cloning and sequencing of rice genes and, in turn, in the study of their transformation and expression.

ACKNOWLEDGMENTS The author is grateful to V. L. Chopra. Head of the Biotechnology Centre, for his valuable suggestions, and to Sarfaraz Hadi for his assistance in the compilation of the bibliography.

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Table Al. Some Recent Anther Culture Media Developed in China and Korea, in ComparLson Wlth Chu's (1976) N6. Components KNO3 (NH4)z . so4 NH4HiP04 KH2m4 CaClz. 2H20 MgSO4 . 7Hz0 Na2-EDTA FeS04 . 7Hz0 MnSO, . 4H20 ZnS04 . 7H20 HiBOi KI C U S O ~5H10 NaMoO,. 2Hz0 CoC12 . 6HzO lnositol Glycine Thymine. HCI Pyridoxine . HCI Nicotinic acid L-Glutamine

N-6 2830 463

L-8" 3000 330

General"

N6-Y lb

3000

2830 231.5

400 400 166 185 37.3 27.8 4.4 1.5 I .6 0.8

540

400

150

166

166

185 37.3 27.8 22.3

185 37.3 27.8 4 I .5 1.6 0.8 0.025 0.25 0.025

185 37.3 27.8 4.4 1.5 1.6 0.8

10

6 I

100

2 I 0.5 0.5

2 1 0.5 0.5

2.5 5 3

2 1 0.5 0.5

265

'From Chen (1986a). bChung and Sohn (1986). Table AIL Progress in Rice Protoplast Regeneration, Somatlc Hybridization, and Transformation cell suspension Plants Genotype (explant)" or callusb raised Reference Plant regeneration

Fujiminori (ME) Nihonbare (ME) Sasanishiki (IE) Taipai-309 (SB) Tajpai-309 (ME) Fujisaka-5 (ME) Moroberekan (ME) Yamahoushi (An)

ce ce ce ce ce ce ca ce

I77 27 60

Nipponbare (ME, IE)

ce, ca

300

Iwaimochi (ME, IE)

ce, ca

300

9 50 50

10

15

Norin-I4 (ME, 1E)

ce, ca

300

Fujisaka-5 (ME, IE)

ce, ca

300

Koshihikari (ME, IE) F 77-170 (YI) Longhud (ME) Norin-8 (ME)

ce, ca ce ca ca

300 20

Somatic hybrid

Rice: c. v. Nipponbare (ME) c. v. Sasanishiki (ME) Echinorhloa oryzicola

(LS)

0. Sariva: c. v. Nipponbare (ME) c. v. Aoisora (ME) c. v. Tsukinohikari (ME)

0. officinalis (ME) 0. eichingeri (ME) 0. brachyantha (ME) 0. perrieri (ME) Cybdd

CMS line Chinsurah Boro I1 Fertile Norin-8 (?)

,

ce ce ce

44 shoots 9 plants 44 shoots 9 plants 44 shoots 9 plants

Terada er a / . (1987) Terada er a / . (1987) Terada et a / . (1987)

ce ce ce ce ce ce ce

250 250 250 250 250 250 250

Hayashi el a / . (1988) Hayashi et a/. (1988) Hayashi et a/. (1988) Hayashi et a / . (1988) Hayashi er a / . (1988) Hayashi e r a / . (1988) Hayashi et a / . (1988)

? ?

560 560

Akagi e r a / . (1988)

ce

200

Transformation

C 5924 (SR)

9

1 is

Yamada er a/. (1985,1986) Fujimura et a / . (1985) Fujimura er a/. (1985) Abdullah er a/. (1987) Abdullah er a/. (1987) Abdullah el a / . (1987) Coulibaly and Demarly (1986) Toriyama et a / . (1986) Hayashi er a / . (1986) Kyozuka el a / . (1987) Hayashi et al. (1986) Kyozuka et a/. (1987) Hayashi er a / . (1986) Kyozuka et a/. (1987) Hayashi er a / . (1986) Kyozuka er a / . (1987) Hayashi er a / . (1986) Kyozuka er a / . (1987) Ming er a / . (1987) Wan and Hsia (1987) Kanfa et a / . (1988)

Akagi er a / . (1988)

Uchimiya et a / . (1986) Uchihya (1988) "ME, mature embryo, IE. immature embryo; SB. seedling leaf base; An, anthers; Y1, young inflorescence; LS, leaf sheaths; SR,seedling roots. &, cell; ca. callus.

ADVANCES IN AGRONOMY, VOL. 42

BREEDING ANNUAL Medicago SPECIES FOR SEMIARID CONDITIONS IN SOUTHERN AUSTRALIA E. J. Crawford, A. W. H. Lake, and K. G. Boyce Department of Agriculture Adelaide, South Australia, Australia 1.

11.

111.

IV.

V.

Introduction The History of Annual Medic Pastures in Australia Plant Introduction: The Basis for Development A. Genetic Resources that Are Available 9. Criteria for Selection C. Evaluation Plant Breeding: The Creation of New Genetic Combinations A. Hybrid Annual Medic Cultivar Development: First Attempts B. General Breeding Aims in Annual Medicago C. Relationships among the Species M . rruncarula. M . littoralis. and M . tornata D. Hybrid Production, Selection, and Evaluation E. Aphid Resistance in Medicago Preservation and Commercialization A. Germplasm Conservation 9 . Computerized Documentation of the Resource C. Commercialization Scope of the Future References

I. INTRODUCTION

THEHISTORYOF ANNUALMEDICPASTURES IN AUSTRALIA Annual species of the genus Medicago, commonly known as annual medics, are an integral component of many pastures in Australia. This has occurred partly through sowing and active encouragement by farmers, and partly via natural colonization. They now constitute the principal pasture legumes over about 50 million hectares of Australia’s agricultural zones. 399 Copyright Q 1989 by Academic Press. Inc. All rights of reproduction in any form reserved.

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There are no known native Medicago in Australia, and there is no evidence to suggest that their arrival in this continent predated that of Europeans in the 18th century. Until quite recent times there have been no recorded attempts to introduce annuals of the genus, although the perennial species M . sativa (lucerne) was purposely brought into Australia for pasture by some of the earliest European settlers. However, by the early 1900s, some annual medic species were naturalized across southern Australia (Black, 1909; Kloot, 1986) and today Medicago spp. can be found in most suitable habitats south of the Tropic of Capricorn. It seems likely that apart from accidental introduction as a contaminant with other seed, a principal source of early importation (also unintended) was with domestic animals, either in hay or as burr (seed pod) material entangled in wool and hair. These medics proved to be excellent colonizers of areas disturbed either by grazing or cropping, and once in Australia they spread rapidly in parallel with animals, particularly sheep, again largely through burr entanglement in wool. Spiny-podded types of the species M. polymorpha (burr medic), M . minima (woolly burr medic), M. laciniutu (cut leaf medic), and M. pruecox (small leaf burr medic) are particularly well suited to this mode of dispersal. It is significant that these species constitute the most widely naturalized annyal medics in Australia today and are also the species that now cause quite severe problems of vegetable fault in the Australian wool clip. Historically, the principal development of annual medic pastures per se was as an adjunct to cereal cropping enterprises. From the time of their first development in the mid 1800s, southern Australia’s semiarid agricultural zones were used for cereal growing. Climatically, these zones are classified as Mediterranean-type: annual rainfall varies between 250 and 500 mm with a pronounced winter incidence. The growing season ranges from 4 to 7 months, and the summer period is characterized by an almost total lack of green forage for livestock. Winter temperatures are generally mild with occasional overnight frosts, particularly inland, and summer temperatures range to above 40°C in most years. Most of the soils in this zone are neutral to alkaline and are grossly deficient in phosphorus and nitrogen for crop production. Deficiencies of sulfur and one or more of the essential trace elements are common. After the native vegetation was cleared, cereal yields in these zones were generally slightly less than I metric ton (Mg) per hectare. However, with successive crops, nutrient depletion lead to declining yields. This is reflected in the fall of average wheat yields in Australia from about 1 Mg/ ha in the 1860s to less than 500 kg by 1900. The use of phosphatic fertilizer, fallowing, and new cultivars resulted in a recovery of yields over the next four decades, but fallowing also caused significant degradation in soil structure, and erosion by wind and water became common. In contrast,

BREEDING ANNUAL Medicugo SPECIES

40 1

the growth of a pasture ley between crops led to improved soil structure (Stephens et al., 1945). Trumble (1939) had already recognized the value of the annual medic M. tribuloides ( M . truncatula) as a pasture species. The increased use of annual medics and the other new farming practices led to the evolution of the classic ley farming system, for which southern Australia is regarded as the pioneer and world leader. In this system cereal crops are alternated with pasture leys based on annual legumes, principally annual medics, which regenerate following the softening of hard seed, which had set in previous pasture years. In the period since the introduction of this technology, cereal yields have improved by about 50% (Donald, 1%7; Webber et al., 1976). Furthermore, in this zone, which is now commonly known as the “wheat-sheep zone,” stock numbers have doubled and wool production has roughly tripled in the same period (Webber er al., 1976). Essential to the success of the system is nitrogen fixation by the pasture legume. Total fixation of up to 200 kg N per hectare under a medic pasture has been recorded over a season (P. R. Gibson, personal communication) while nitrogen addition to the topsoil averaging more than 100 kg per hectare per year has also been measured (Clarke and Russell, 1977). In addition, medics make a considerable contribution to pastures outside the wheat-sheep zone, particularly on its drier margins. Here, pastures are usually a mixture of indigenous perennial shrubs with ephemeral herbs and grasses, which grow during good seasons and remain as dry feed for droughts. Medics fall into the latter category: their excellent feed quality even when dry ensures their considerable value to this marginal pastoral zone. Medics are found over at least 20 million hectares of this zone. About 30 million hectares of the wheat-sheep zone are also currently classified as suitable for medics. The net worth of annual Medicago species to Australian agriculture is considerable. In dollar terms, Carter (1981) estimated the overall value of annual pasture legumes to Australia’s crop and livestock industries to be at least 2.5 billion dollars (U.S.) per annum. While this figure also includes a number of other annuals, principally of the TrijXum genus, it does emphasize the importance of annual medics to Australia’s agriculture. The development of medic pastures has been via two major strategies: first, the encouragement of naturalized ecotypes, mainly through the application of phosphatic fertilizers, and second, the development and sowing of improved cultivars. As naturalized ecotypes are generally neither the best-adapted nor the highest-yielding, the latter is now the most common approach. The discovery of improved types has had three identifiable, if somewhat overlapping, phases. The first phase relies on the recognition of superior local ecotypes and their propagation. Earliest cultivar development was

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based on this, with the first farmer sowings of “commercial” barrel medic (M. truncatula) (which later became known as “Hannaford”) being made in 1938 (Trumble, 1939). However, at that time genetic resources among naturalized Australian material were obviously very limited, so the second phase, that of deliberate introduction, began in the 1950s (Neal-Smith and Johns, 1967) and continues to the present day. Collection of candidate material has been centred around the Mediterranean basin and the Near East. This area is both the geographical center of origin for all species of annual Medicago (Small and Lefkovitch, 1986) and the largest area in the world with climatic zones similar to those of southern Australia (Anonymous, 1963). Such introduction and testing has proved successful and, in the period since 1960, has contributed about 80% of the new medic cultivars released (see Table 1). The third phase of improvement has been plant breeding. To date, this has been a fairly minor contributor to cultivar release. However, new constraints on medic pastures, principally in the requirement for aphid resistance, have underscored the limits of preexisting genotypes for improvement. There is little doubt that an increasing proportion of medic cultivars in the future will be the result of conscious breeding and selection programs. However, plant introduction still has a very significant role to play, a role which is outlined and discussed in Section I1 of this chapter. Section 111 deals with aspects of plant breeding for annual medic improvement, and Section IV touches on genetic resource preservation and cultivar commercialization.

II. PLANT INTRODUCTION: THE BASIS FOR DEVELOPMENT A. GENETICRESOURCESTHAT ARE AVAILABLE Heyn (1963) outlined the characteristics of 28 annual medic species, to which has recently been added M. heyniana (Greuter, 1970), but the commercially used cultivars only represent six of these species. As indicated in Table I, four commercialized cultivars have been selected from within naturalized populations well adapted to certain environments in southern Australia. Three have been produced by plant breeders, whereas 10 have resulted from direct plant exploration and introduction missions in seven Mediterranean countries. The popularity of M. truncatula (eight barrel medic cultivars) is due to its outstanding adaptation to low rainfall environments with soils high in available lime and to long-established understanding by farmers of how to manage the species for best results in terms of production and persistence.

BREEDING ANNUAL Medicago SPECIES

403

Table I Cultivars of Annual Medicago Species Commercialized in Australia ~~

Species

M . liiioralis M. M. M. M. M.

polymorphu rugosa scuiellaia iornuia iruncaiulu

Cultivars

Countries of origin

Harbinger Circle Valley, Serena Paragosa, Paraponto, Sap0 Robinson, Sava Tornafield Ascot, Borung, Cyprus, Hannaford. Jemalong, Parabinga , Paraggio, Sephi

Iran Australia (naturalized), Australia (bred) Portugal, Italy, Portugal Australia (naturalized), unknown origin Australia (bred) Australia (bred), Tunisia, Cyprus, Australia (naturalized and selected), Jordan, Italy Israel

In any annual medic cultivar development program, several selection criteria are important irrespective of climatic and edaphic constraints that may limit adaptation. These are common to both selection and breeding programs and include 1. Seed and seedling vigor 2. Seasonal herbage production and its ultimate effect on N fixation 3. Time of flowering 4. Seed production 5. Pod spininess 6. Relative levels of seedcoat impermeability 7. Tolerance to current (and prospective) pests and diseases.

In southern Australia, a gene pool that contains in excess of 15,000 accessions of annual Medicago species from 57 countries allows flexibility in selection for these important attributes. Assessments are made in the initial nursery stage of evaluation at the Parafield Plant Introduction Centre in South Australia. Table I1 indicates the range in variability of five agronomic characteristics in accessions of the 13 most important species grown to date. B. CRITERIAFOR SELECTION 1 . Seedling Vigor Germination rate and seedling vigor following imbibition greatly influences plant establishment in environments where rainfall is limiting, a

404

E. J. CRAWFORD ETAL. Table I1

Ranges of Mean Variation in Five Major Characteristics of 13 Important Annual Medicago Species, Parafield, South Australia, 1968-1984

Number of Seedling Winter Pod Seed Hard seed accessions vigor“ productionb spininess (dplant) (%) Species M. uculeatu M . arubicu M. intertexta M. littorulis M. murex M. orhiculuris M. polymorphu M. rigidula

M.riigosu M. scutellutu M. tornata M. truncatula

M. tiirbinuta

380 210 297 858 1.51 804 1105 560 I22 I93 373 2686 147

4-15 3-12 4-24 1-17 5-14 1-14 2-12 3-12 5-13 2-12 4-20 2-15 4-13

3-25 6-12 4-30 1-13 5-22 2-22 4-58 1-12 7-2 1 4-16 1-32 1-30 6-18

0-19 1-16 4-20 0-20 0-19 0 0-20 0-19 0 0 0-14 0-20 0-12

2.1-39.2 3.4-25.1 0.5-72.2 0.3-35.9 0.9-27.4 0.1-73.4 1.2-88.9 0.1-33.1 0.7-33.6 3.6-32.0 0.8-68.4 0.4-68.4 13-25.2

46.8-100.0 36.2-100.0 26.4-99.4 22.6-100.0 47 .0-98.7 51.6-100.0 0.0-100.0 58.6-100.0 0.0-100.0 47.3-100.0 23.7-100.0 20.4-100.0 66.2- 100.0

lO(7.8) 10( 1 I.4) 10( 14.3) lO(9.3) 10( 10.0)

lO(8.7) lO(9.9) 10( 16.1) lO(8.4) 10(10.0)

3 0 0 0 10

2.9-25.8 2.2-29.1 3.7-34.8 2.342.5 4.0-29.6

84.5-98.6 0.0-78.0 54.6-97.6 54.2-95.0 80.7-99.0

-

-

-

Cultivars” M. littorulis cv. Harbinger M. rugosa c v . Paragosa M. scutellutu cv. Robinson

M. tornata cv. Tornafield M.truncatula cv. Jemalong Other species cv. Jemalong

-

-

“Seedling vigor is a mean estimate of leaf area 3 4 weeks after germination. ”Winter production is a mean estimate of herbage production 4 months after germination. ‘Sampled mid-autumn of year following maturation. “Seedling vigor and winter production of the cultivars are not comparable and only apply to their respective species. Ratings relative to the control cultivar Jemalong (10 units) are in brackets.

common occurrence in southern Australia. As these species have a woody pod that lies on the soil surface after maturity, the annual medic takes longer to “wet up” and initiate germination than does the seed-burying species T. subterraneum, commonly known as subterranean clover. The technique of Williams, Evans, and Ludwig (1964) is used as a relatively simple and reliable method to quantify seedling vigor and compare accessions to a standard control. In one study comparing 2266 genotypes of 20 annual Medicago species, Crawford (1975) showed that 15% of the entries exhibited better seedling vigor than their control cultivar. Seedling vigor generally increases with seed size and this relationship was utilized in

BREEDING ANNUAL Mcdicago SPECIES

405

developing the M. rugosa cultivar ‘Paraponto,’ which had twice the seed weight and 20% better seedling vigor than ‘Paragosa’ (Mackay, 1978). The largest range in absolute variability (Table 11) is in M. intertexta (4-24 units), whereas the largest relative range, viz., 17-fold, is in M. littoralis, which has correspondingly smaller seed. The largest-seeded accession of M. intertexta has seed twice the size of that from the largest M. littoralis accession.

2 . Seasonal Herbage Production The ability in medics to germinate rapidly and develop healthy, vigorous seedlings before soil temperatures become too low for maximum growth is important in enhancing good early winter herbage production for grazing purposes. Recovery after cutting or grazing is important under sward conditions. Genotypes with a prostrate habit and short internodes recover better after defoliation than do erect, stemmy ones. The outstanding winter production of some accessions of M. polymorpha referred to in Table I1 illustrates the potential for future cultivar development in that species.

3. Flowering Time The cultivars of the species referred to in Table I flower over a range of 67-116 days when sown in late April at Parafield, South Australia. When comparing this with the range in flowering time of the same species in Fig. 1, it can be seen that ample room for selection exists in the collection. In a low rainfall environment such as that of South Australia, early flowering is of paramount importance to ensure adequate seed set. From its commercial release in 1959, ‘Cyprus’ was the earliest-flowering barrel medic cultivar, with a mean flowering time of 91 days from germination. A very early-flowering cultivar of M. polymorpha, ‘Serena,’ (67 days) has since been released in 1982. Of the 13 species of which more than 100 accessions have been grown, the commercial species M. scutellata easily surpasses all others in earliness (Table 111). However, in a smaller group of M. doliara (41 accessions), 53.7% flowered earlier than ‘Cyprus.’ Although Cocks et al. (1980) indicated that M. truncatula accessions of Mediterranean basin origin flowered over a range of 66136 days in the South Australian environment, this range has now been extended to 62-148 days with the acquisition of a wider variety of accessions. However,

P

z

DAYS TO FLOWERING

<71

71

-

80

81

-

90

91

-

100

101

- 110

-~ M. aculeata -~ M. littoralis

>llo 130 152

M. polrmorpha

183

M. ruaosa

136

-~ M. scutellata

139

-~ M . tornata

m

157

-~ M. truncatula

?

148 -~ M. intertexta

cl

150

!a

M.

orbicularis -~ M. turbinata

172 159

I?. rioidula

154 163

-~ M. a r a b i c a -~ M. murex

150

Robinson Sava

Cyprus Parabinga Paraponto

Borung Circle Valley Harbinger Jemalong Paragosa Sap0 Sephi

3

s2u 3b ?

CULTIVARS Serena

>

Ascot Hannaford Paraggio Tornafield

FIG. 1. Range in flowering time of 13 important annual Medicago species, Parafield, South Australia.

BREEDING ANNUAL Medicago SPECIES

407

Table 111 Number of Accessions of the 13 Major Species (in Excess of 100 Accessions Grown) Flowering Earlier Than Cyprus Barrel-Medic Compared with the Total Number Grown at Parafield, South Australia

Species

Flowering earlier than Cyprus (no.)

M . aculeata M . arabira M . intertexta M . littoralis M . murex M . orbicularis M . polymorpha M . rigidula M . rugosa M . scutellatn M . tornata M . truncatirlo M . turbinatu

10 0 II I26 0 4 141 0 9 43 19 285 12

Totals

660

Flowering earlier than Cyprus Number grown

(%)

380 210 297 858

804 I I05 560 I22 I93 373 2686 147

2.6 0 3.7 14.7 0 0.5 12.8 0 7.4 22.3 5. I 10.6 8.2

7886

8.4

151

flowering time is still related to the duration of the wet season at the site of collection (Crawford, unpublished). The noncommercialized species illustrated in Fig. 1 have a range of about 9 weeks in flowering period, from about 6 weeks in the late-flowering species M . murex to about 15 weeks in M. orbicularis. This enhances the chances of selection for a large range of flowering times, subject to the suitability of agronomic characteristics for any one environment.

4 . Seed Production

Seed production in annual medics largely influences subsequent longterm persistence. Adequate seed production not only ensures a legumedominant pasture in the ley year of a cropping rotation, but annual medic pod and seed reserves also contribute a highly nutritious diet for oversummer grazing by livestock, particularly sheep. Although the crude protein concentrations vary according to the seed:burr ratio, Vercoe and Pearce (1960) measured concentrations of 45% (seed) and 6% (hull) in barrel medic, which has a mean value of about 30% seed in the pod. Crawford (1983) recorded large variations in seed:burr ratio both within and between annual medic species, with M. orbicularis having as much as 50% of its pod weight as seed.

408

E. J. CRAWFORD ET A L .

The species M. intertexta, M. orbicularis, M . polymorpha, M . tornata, and M . truncatula (see table 11) have both high seed and high herbage production potential. This enhances their value for future cultivar development.

5 . Pod Spininess Although pod spininess acts as a good seed dispersal mechanism, it has cost the Australian wool industry hundreds of millions of dollars as a vegetable fault in wool clips, (Lunney, 1983). Both M. minima and M . polymorpha have become very widespread in the sheep grazing regions of the continent and their value as forage and for increased soil fertility has to be offset against this cost. Current annual medic improvement programs give serious consideration to the degree of spininess in selecting new genotypes to become cultivars in the various farming systems. Short, straightspined species such as M. littorulis and M . truncatula allow less adherence to a sheep’s fleece than the previously mentioned and other naturalized, spiny-podded species such as M. luciniata and M . pruecox. In assessing relative pod spininess, both length and angle of insertion of the spine are taken into consideration and all variants are related to the control cultivar ‘Jemalong’ (M. truncatula) at a standard of 10 units. In one study of 210 genotypes of M. truncatula, which ranged in spininess from 1-18 units, 71 genotypes exhibited less pod spininess than ‘Jemalong’, (Crawford, 1983). Variability is shown both between and within species (Table II), with two species having at least some completely spineless accessions. M. rugosa and M . scutellata are spineless species and two cultivars have been developed from recent evaluation programs. 6 . Levels and Changes in Seedcoat Permeability

Seedcoat impermeability, or hardseededness, and the changes that occur with environmental change affect survival of a species through the fallowing and cropping phases of cereal rotations and also act as an escape mechanism against drought. At maturity, most annual medics attain near 100% seed coat impermeability in South Australia. In regions that are susceptible to sporadic and unreliable summer thunderstorms, it is desirable that high levels of hardseededness are maintained. This will inhibit germination before the onset of reliable winter rainfall patterns, which can be expected to prevent seedling death. Crawford (1971) postulated that seedcoat impermeability maintained until mid-April and followed by

BREEDING ANNUAL Medicago SPECIES

409

a rapid change to 30% permeable seed was optimal for satisfactory medic regeneration in the South Australian environment. The first-developed annual medic cultivars maintained about 90% impermeable seed over the first summer-autumn period following maturation, resulting in relatively low plant populations after the first rains. Selection for a greater rate of breakdown in hardseededness was a major criterion used when developing the M . truncatufa cultivar ‘Paraggio’ (Oram, 1982). As illustrated in Fig. 2, this cultivar doubles the relative amount of permeable seed available at the expected time of germination, compared with the commonly used barrel medic cultivar ‘Jemalong,’ and still maintains adequate levels of hard seed for preservation of the cultivar in various farming systems. Although South Australia has a Mediterranean-type climate, the incidence of summer rain in the form of sporadic thunderstorms is greater than that of the natural habitat of the annual Medicago species (Anonymous, 1963). The first developed cultivar of gama medic, M. rugosa ‘Paragosa,’ proved to be too soft-seeded in the summer-autumn period in the South Australian environment and high seedling populations were lost to desiccation following premature germination after summer rains. Crawford (1977a) also showed that 48% of a group of 79 M. rugosa accessions under test had 30% permeable seed by mid-April of the year after seed production. Selection within this group of accessions led to the development of ‘Paraponto’ gama medic, which has a higher level of hardseededness in the summer-autumn period but equal levels of softseededness at the time of germinating rains in early and mid-April as does ‘Paragosa’ (Crawford, 1981). The differences both within and between species are evident in the changes in seedcoat permeability illustrated in Fig. 2. Higher levels of impermeable seed are maintained under a cropping than under a permanent pasture system. Crawford and Nankivell (1984) showed that seed burial in a pasture-fallow-wheat-barley rotation protected seed from the temperature fluctuations on the soil surface experienced in a permanent pasture situation. A threefold greater seed reserve of ‘Jemalong’ barrel medic was evident after 4 years in a nongrazed, nonreseeding system of management in a normal cropping rotation, in comparison with permanent pasture.

7 . Resistance to Insect Pests and Diseases Increased importance is now being placed on the role of insect and disease effects on herbage plant species. Hence levels of resistance are important criteria in current cultivar development programs. The most

410

E. J. CRAWFORD E T A L .

100

,

Cyprus

Borung 90

Roblnson Jemalong

80 Tornafleld

Paragglo

70

60

50

40 M i d -Jan

Mld-Feb

E a r l y Mar

L a t e Mar

Mld-Aur

Rc. 2. Seasonal changes in seedcoat permeability in eight commercial annual medic cultivars at Parafield Plant Introduction Centre.

BREEDING A N N U A L Medicago SPECIES

41 1

important insect pests are spotted alfalfa aphid (Therioaphis trifolii), (SAA), blue-green aphid (Acyrthosiphon kondoi) (BGA), pea aphid (Acyrthosiphon pisum) (PA) and sitona weevil (Sitona discoideus) (SW). Older cultivars have varying levels of resistance to these pests but all are susceptible to one or more at different stages of development. Screening techniques under controlled conditions have been developed in an attempt to select, first, species, and second, accessions, within the most promising species having acceptable levels of resistance under field conditions. In one study cited by Crawford (1983) only one M. rotata, two M . rugosa, and three M . scutellata accessions showed complete resistance to adult sitona weevil, of 7000 annual medic accessions tested. A further 46 accessions of 10 species had varying degrees of resistance. Unfortunately, resistance to adult sitona weevil does not imply resistance to larvae. Hence no new cultivars have evolved from such programs. However, more success has been achieved in screening programs for aphid 'redistance. Temporarily useful resistance was found in existing cultivars of M . rugosa and M . scutellata, but the more popular cultivars of M. truncatula and M . littoralis were susceptible. Although other insect pests such as lucerne flea (Sminthurus viridis) (LF), red-legged earth mite (Halorydeus destructor) (RLEM), and native budworm (Heliothus punctigera) (NB) are significant pests in this environment, no cultivars with specific resistance to each or any have been released. Partly because of its geographic isolation and partly because of the semiarid nature of the environment, few plant diseases are of significance in annual medic pastures in South Australia. The most important is black stem (Phoma medicaginis), particularly in years of above-average rainfall extending into the warmer spring months. Some existing cultivars are susceptible and many species have exhibited symptoms to varying degrees in nursery evaluation programs. The most seriously affected stands were ungrazed areas that became bulky prior to commercial seed production or were deliberately left for hay production. Under these conditions, leaf rusts (Uromyces anthyllidis and U.striatus) have also reduced herbage and seed production.

C. EVALUATION Over the last half-century the objective of the medic improvement program has changed from seeking cultivars for areas where none existed to improvement over and above existing cultivars. Evaluation generally proceeds from the nursery row to the farmer utilization stage in steps best illustrated in the schematic diagram in Fig. 3. Myers et al. (1974) documented procedures for evaluating pasture plants

412

E. J . CRAWFORD ET AL. [INTRODUCTION

1

Row e v a l u a t i o n

>-

D [ ATABASEj

\1/ r &

\

Line s e l e c t i o n

/

1 PRIMARY TRIALS

I

Small sward p l o t s

Adaptation

Productivity Selecti2criteria

Persistance

I

1

I .

6

\

/

4 I

U t i l i z a t i o n trials

SECONDARY TRIALS

L.r/& \

Adaptation

Productivity

Persistance

Cultivar selection

I

\L REGISTRATION

RELEASE

COMMERCIALIZATION

FIG.3. Evaluation procedures for potential new cultivars.

# \

-3

-J \

BREEDING ANNUAL Medicago SPECIES

413

whose end use was precisely defined. Crawford (1983) adopted the attitude with annual medics that specific environments have specific demands and detailed three main stages as they were affected by the limitations imposed in semiarid environments, in which success is not forthcoming in each and every year of experimentation. He emphasized the importance of detailed meteorological records as an aid to interpretation of the data gathered, and these, together with differing edaphic characteristics, help to explain the adaptation of certain species to their environments. The stages used in evaluation in the South Australian program are ( I ) nursery rows; (2) swards for dry matter and seed production; and (3) swards for tolerance to grazing and potential animal production.

I . Nursery Rows The nursery stage serves four main purposes: (1) to establish uniformity of the accession; (2) to assess potential growth and development in the nursery environment; (3) to isolate segregants, or physical contaminants at collection, or heterogeneous allelomorphs: and (4)to produce seed for further evaluation. Uniformity is assessed by observation or measurement of 17 morphological and 12 agronomic characteristics (Crawford, 1983). The agronomic data act as a guide to potential production and help suggest geographical regions to which lines may be adapted as influenced by factors such as time of flowering and maturity and ability to regenerate in subsequent years. As the annual species of Medicago are self-pollinated, selections isolated in the nursery stage can be harvested separately to constitute a new line and further grown to establish homogeneity. High seed yield is not only an important agronomic characteristic per se but ensures adequate supplies of seed to extend the most promising accessions to the next stage of evaluation.

2. Swards f o r Dry Mutter and Seed Production The number of accessions that can be promoted from nursery rows to the sward evaluation stage is generally limited. It must be established, therefore, whether 1. All accessions should be grown in all major environments or only in some specific environments. 2. All accessions of the most appropriate species should be grown in the most appropriate environments.

414

E. J. CRAWFORD E T A L .

3. Only some accessions with the most appropriate characteristics should be grown in specific environments. Major environments are established in terms of climatic and edaphic limitations and the most appropriate species can then be selected from past experience. The relationship between soil type at the site of collection as recorded in passport data and the most likely area of adaptation in the proposed new environment is established. Local experience in South Australia confirms the superior adaptation of M . littoralis and M . tornata to light-textured soils, the latter being more tolerant of slight acidity (pH 5.0-6.5). Medicago truncatula is better adapted to loamy soils with higher levels of available lime (Crawford, unpublished data). Availability of resources and relative priorities largely determine the number of and extent to which accessions can be evaluated at this stage. Earlier evaluation programs, when the most appropriate species had yet to be determined, demanded a broader base on which to make judgements. When, however, a single or a reduced group of species have shown good adaptation, more accessions of fewer species became the logical approach. By this stage of the program Rhizobium requirement should have been established. Although commercial attempts at broad-spectrum bacterial culture development have been effective in the past in a relatively narrow group of associated species, Brockwell and Hely (1966) showed that the long-establised strain U45 is ineffective for N fixation on M. rugosa. This led to the development of strain W118 and, more latterly, strain CC169 specifically for ‘Paragosa’ gama medic. However, none of these strains are tolerant of low soil acidity, and it was not until Howieson and Ewing (1986) used Rhizobium meliloti isolated from acid soils (pH 5.0) in Sardinia, Italy, that species such as M. murex and M . polymorpha could be successfully grown on acidic soils in Western Australia. The observed persistence of rhizobia into the second year shows that there is potential to extend some annual medic species into soils hitherto found to be too acidic for medic. These are often in rainfall regions too low for subterranean clover. An important aspect of the development of medic cultivars in the ley farming system is the quantification of symbiotically fixed nitrogen by the medic stands and the subsequent availability of this nitrogen to cereal crops in the rotation. The measurement of N fixation has only become feasible with the advent of ”N diffusion techniques (McAuliffe et al., 1985; Bergerson and Turner, 1983), and this has now lead to a limited evaluation program for the measurement of N fixation at the post-release stage. Cultivars of Medicago are now being compared for their ability to fix nitrogen over a range of available soil nitrogen, and their potential for increasing soil fertility is being assessed. In a recent experiment performed

BREEDING ANNUAL Medicago SPECIES

415

at the Northfield Research Centre, ‘Paraggio’barrel medic fixed over 90% of its total seasonal nitrogen requirements under conditions of low available soil nitrogen (<10 ppm NO,), and further studies are being carried out to determine the threshold at which N fixation is significantly impaired by available soil nitrogen (P. R. Gibson, personal communication). Detailed studies on the rates of decomposition of incorporated medic residues and the availability of medic nitrogen to subsequent cereal crops have been undertaken in South Australia (Ladd, 1981; Ladd et al., 1981). These studies indicate that the propw-tion of nitrogen in a succession cereal crop derived from decomposing medic residue is only about 10-20%. Hence the value of good medic pastures is not so much in their capacity to deliver large amounts of nitrogen immediately to subsequent cereal crops, but rather in their ability to maintain or increase the total amount of nitrogen in the soil, thus ensuring long-term supplies of nitrogen from the decomposition of relatively stable organic residues.

3. Grazing Tolerance and the Potential for High Animal Production Unlike most permanent pasture situations, in which animal production takes precedence over other benefits, the legume-ley-farming system demands legume dominance for maximum N fixation. However, animal grazing is highly desirable to optimize economic management strategies and can promote a lucrative bonus in the integrated system. To achieve this, the management of the pasture for plant productivity may be more important than the nutritive value of the plant per se. The grazing periods can be divided thus: (1) the active growing stage; and (2) the dry summer-autumn period. The effect of grazing and/or cutting on plant productivity and morphology can be gauged to some degree in small plots but more realistic data must be derived from larger plots of fewer numbers of accessions under conditions that more closely resemble normal farm practice. Defoliation is practiced at 4-6 weekly intervals up to the onset of flowering, and then the swards are allowed to mature under conditions conducive to maximum seed production. Regular defoliation throughout the growing season permits the ability of the plant to recover to be gauged but the extent and severity of defoliation may affect the ultimate total dry matter production. Time of germination and availability of moisture for continuing growth have great influence on the rate of dry matter production in the relatively mild autumn-winter environment of South Australia. Many pastures in such environments produce less than 15 kglhalday dry matter (DM). However, there is scope to improve this by selection. Crawford (l977b)

416

E. J. CRAWFORD ET AL.

showed that increases of up to 28 kg/DMha/day (over 98 days) were possible in M. arabica, M. polymorpha var. brevispina, and M. truncatula vars. longeaculeata and truncatula in a year of below-average rainfall at Parafield, which represented a 22% increase over and above the currently recommended cultivar, ‘Jemalong.’ The ability to recover after defoliation was measured, resulting in the first two of these species being able to produce more seed than the control. This figure can be further increased with additional nitrogen input. Seed production diminishes as grazing proceeds beyond the beginning of the flowering stage, and because maximum seed yields are desirable for persistence and to allow over-summer grazing, strict grazing control must be exercised during this period. Seed reserves are consumed by oversummer grazing, and subsequent plant populations can be drastically reduced in the second and subsequent years of regeneration if grazing pressures are not controlled over this period. Potential seed production can be determined immediately post-maturity, but a further sampling should be conducted after grazing and just prior to the anticipated germination rains for the new season. Carter (1980) showed that less than 2% of ‘Jemalong’ barrel, ‘Robinson’ snail, and ‘Harbinger’ strand medics survived ingestion by sheep when the animals were fed diets of approximately 750 g of pods per day in pens. This was similar to the findings of Vercoe and Pearce (1960) and illustrates the losses of seed reserves that can occur with overgrazing. The ultimate evaluation of a new herbage plant cultivar is its acceptance and use by farmers and graziers, and this can be encouraged by the provision of adequate quantities of reasonably priced seed. Continued utilization of annual medics will also be encouraged by the supply of new cultivars that have superior attributes bred into them. It is most probable, therefore, that products of plant breeding rather than evaluated introductions will constitute the bulk of new cultivars of annual medics in southern Australia in the future.

Ill. PLANT BREEDING: THE CREATION OF NEW GENETIC COMBINATIONS A. HYBRIDANNUALMEDICCULTIVAR DEVELOPMENT: FIRST ATTEMPTS Under Australia’s system of herbage plant registration, the first annual medic cultivar that derived from an artificial hybridization program was accepted for registration in 1969. This cultivar originated from a cross

BREEDING ANNUAL Medicago SPECIES

417

between two backcross lines of ‘Cyprus,’ one incorporating large seed size and the other absence of pod spine. The resultant selection process resulted in the large-seeded, early-flowering, spineless-podded cultivar ‘Cyfield’ (Barnard, 1972). Three other medic cultivars of known hybrid origin have been registered. These are the disk medic (M. tornara) ‘Tornafield,’also in 1969 (Barnard, 1972), the spineless burr medic (M. polymorpha) ‘Serena’ in 1976, and the barrel medic ‘Ascot’ in 1979 (Mackay, 1982). Of these, ‘Tornafield’ and ‘Serena’ have achieved some market penetration, the former being of use on duplex sand-over-clay soils with more than 350 mm annual rainfall and the latter on various neutral or slightly acid soils in the drier Wheat Belt areas of southern and southwestern Australia. It is noteworthy that the widespread adoption of ‘Serena’ did not occur until the discovery, isolation, and commercialization of new rhizobial strains suited to the M . polymorpha species. These rhizobial strains persist in slightly acid soils, where ‘Serena’ is now used (Howieson and Ewing, 1984, 1986). ‘Ascot’ has not been acceptable to the Australian farming community in general. Its pod spines are too long and protruding and as a consequence readily entangle in sheep wool, causing vegetable fault problems and price reductions. Although only four annual medics of hybrid origin have been registered to date, there are currently more than 15 cultivars registered that were derived more or less directly from overseas accessions or local Australian ecotypes. This is typical of the domestication procedure for a new agricultural plant, in which case initial development is centered on use of preexisting genotypes. The next and much more prolonged stage of cultivar development, that of genetic improvement based on hybridization and recombination, is, however, well under way.

B. GENERAL BREEDING AIMS I N ANNUAL Medicago Annual medic improvement through hybridization and selection has the principal aim of creating new genotypes (and thereby phenotypes) that are superior in one or more characters to those already available in annual medic collections. As such it is based on the identification of useful characters in existing genotypes and a detailed knowledge of the agronomy of annual medic pastures, as well as the technology to bring appropriate gene sources together via intra- and interspecific hybrid combinations. The agronomic and selection requirements for annual Medicago introduced into Australia are outlined in Section II,B above. The same requirements can be applied to bred lines. However, several criteria are especially applicable to breeding work, given the detailed knowledge of

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existing genetic resources and desirable gene combinations, which are limited or nonexistent within collected material. In recent years, breeding for insect resistance has received a high priority. This follows the devastation of many annual medic pastures by SAA and BGA, which first arrived in Australia in the late 1970s. Pea aphid has had little impact on annual medic pastures to date. Most of the commonly used annual medic species are susceptible to these aphids, although some accessions resistant to one aphid or the other are found in nearly all species tested. Frequently, however, resistant genes are in accessions that have severe agronomic faults. For example, many BGA-resistant lines of M . truncatula have unacceptable pod spines and quite low herbage and seed production. Through hybridization of these aphid-resistant sources with other lines having more appropriate agronomic characters, however, segregants combining all the desirable attributes can be isolated. The success of this resistance breeding, which is detailed in Section W E , can be gauged by the fact that in 1980, the South Australian Department of Agriculture (SAGRIC)medic collection contained only about a dozen different barrel medic accessions with aphid resistance and a sufficient degree of agronomic acceptibility to warrant field testing. Today, however, SAGRIC breeders produce aphid-resistant barrel medic lines at the rate of more than 100 per year, which are then distributed for field testing by agronomists in the other medic-growing States of Australia; Queensland, New South Wales, Victoria, and Western Australia, as well as South Australia. The philosophical approach of insect resistance breeding is that resistant cultivars are to be used to reduce production losses in the presence of the pest organism. However, these cultivars may be no better in the absence of that organism, and they could even be inferior in some circumstances. The success of aphid resistance breeding has raised the possibility of similar breeding programs for other pests and pathogens. There are many pests of annual Medicago in Australia, most of which are, like SAA, BGA, and PA, not of local origin. After SAA and BGA, the most important of these pests are RLEM, LF, and SW. Screening to find suitable sources of resistance is proceeding, with indications that there are differences in the degree of damage sustained by different lines under a given population pressure of each of these pests. Breeding for yield of both seed and herbage remains a principal goal. There is strong evidence that new cultivars with higher productivity can be bred and released in the future. There are many lines contained in the SAGRIC Medicago collection that are higher yielding than current commercial cultivars, both as seedlings and throughout the growing season. None of these lines are commercially acceptable for one reason or another,

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but hybrids using these as parents have led to the recovery from earlysegregating generations of higher-yieldingtypes without the flaws of their parents. More extensive field testing of such material should lead to cultivar release in the 1990s. There is also scope for higher seed yield. Many field trials have shown that there are lines with significantly greater seed yields in swards than commercial cultivars, but the commercial acceptability of these lines is usually very low for other reasons. Hybridization programs with these genotypes are in progress to overcome various defects. Apart from susceptibility to aphids, one of the most common defects in introduced lines of annual medic is excessive pod spine. As spiny pods are a problem for wool growers, acceptable pod type is therefore an important selection criterion. Accumulated evidence suggests that although pod spinelessness is simply inherited (Simon, 1965a),spine length, shape, and angle of insertion on the pod surface are not. Further; transgressive segregation can occur, in which, for example, very spiny-podded parents throw segregants with less pod spine (Lake, unpublished). This makes the prediction of hybrid potential for cultivar production difficult, but it also means that fewer lines are now rejected as parents because of excessive pod spine.

C. RELATIONSHIPSAMONG THE SPECIES M . truncatufa, M . liftoralis, AND M . tornata For the plant breeder, the closeness of relationships between species is important from the point of view of sources of genetic variation. If hybrids between an agronomically useful species and a related species produce fertile offspring that can then be backcrossed to the useful species, there is a potential to transfer genetic characters from one species to the other, thereby expanding the available gene pool within which selection can be carried out. This tool has long been used in plant improvement, particularly for the transfer of mono- or oligogenic characters, for example, for disease or pest resistance, between species. Within the Medicago genus there are 58 recognized species and subspecies that encompass an array of types of varying degrees of relationship. The taxonomy of the genus has been reviewed by various authors (e.g., Lesins and Lesins, 1979; Lesins and Gillies, 1972; Simon, 196Sa; Heyn, 1963). The genus has several species complexes, which have indistinct interfertility barriers (Small and Lefkovitch, 1986). The best known and documented of these is the perennial species loosely referred to as the “sativa-fafcata complex.” Within this complex is a large pool of genetic

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variability, with individual interfertility varying between and within species. The complex encompasses both diploid and tetraploid forms, and the actual number and identity of species is not clearly definable. Within the annual medics, there are also several species complexes, of which the best-documented and most important is the M . truncatula-M. littoralis complex (Simon and Millington, 1967). Commercially, these two species are the most important in Australia. The value of their partial interfertility is evident in the use of aphid resistance genes from the M . truncatula species in the backcrossing program with ‘Harbinger’ ( M . littoralis) described below. Simon (1965b) also found that hybrids could be obtained, albeit infrequently, from crosses between M . littoralis and M . tornata, and in view of this and recent results (Lake, unpublished), it is logical that the concept of a “truncatula-littoralis complex” should be extended to include M. tornata. A number of attempts have been made over the past few years to produce interspecific hybrids between accessions of these three species, and hybrid seed and F, plants have been produced from all three species combinations. However, successful M . truncatula-M. tornata crosses have been very difficult to achieve, and sterility of F, plants of this combination has been virtually total. The possible use of M . littoralis as a bridging species between M . truncatula and M . tornata has been investigated, and results have been most encouraging. Hybrids between selections of an aphid-resistant ‘Harbinger’ ( M . littoralis) backcross and an aphid-susceptible M. tornata line have been produced and are about to be grown through the F,. If, as expected, aphid-resistant plants are recovered from this cross, it is likely that further backcrossing into M . tornata will see the successful transfer of these genes for aphid resistance into that species. As the aphid resistance genes in this cross originated from M . truncatula (see Section 111,E,5), they will therefore have been transferred through two species barriers. Similar interspecific crosses are being attempted using other combinations of parents and species, particularly with a view to transferring insect resistances. Red-legged earth mite resistance has been found in several M. tornata lines and this may be transferable to both M . littoralis and M . truncatula. As a special case, spineless cultivars of M . polymorpha have gained in popularity and are likely to become a significant part of the annual medic seed market in Australia. Aphid resistance is comparatively rare in the species, and despite exhaustive screening, no lines with good resistance to SAA have been found to date. The possibility of resistance transfer from another species is therefore very attractive but may also prove difficult, particularly as M . polymorpha is one of the few annual Medicago with 2n = 14 rather than 16 (Simon and Simon, 1965). However, some

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useful resistance to SAA has been found in M. rigidula (2n = 14) and other sources of resistance may yet be discovered in species more closely related to M . polymorpha and with 2n = 14. Further, although crosses with 2n = 16 species must be expected to result in, at best, excessive sterility in the F,, due to abnormal chromosome pairing at meiosis, transfer of SAA resistance genes may still be possible.

D. HYBRIDPRODUCTION, SELECTION, AND EVALUATION 1 . Hybridization Techniques

Unlike the perennial species of Medicago, which are generally crosspollinated and exhibit strong inbreeding depression, all annual species of the genus are self-pollinating, although intercrossing occasionally occurs under natural conditions (Lesins and Lesins, 1979). Therefore, for a successful cross to be produced in annual medics, self-pollination has to be prevented. As anthesis occurs while the flower is still in the bud stage, some delicate dissection of the flower bud is necessary for emasculation and cross-pollination. A binocular microscope is vital for this work, as well as a very steady hand, as excessive damage to the bud will markedly reduce the frequency of successful hybridizations. Various hybridization techniques and refinements for annual species of Medicago have been described (Lesins and Lesins, 1979; Lesins and Erac, 1968; Simon, 1965a). The techniques we have used are broadly similar, but there are several refinements that have been adopted with good results, particularly at certain times of the year. The basic technique used by the SAGRIC Medic Breeding Programme follows that of Lesins and Erac (1%8) for dissection and cross-pollination, but suction emasculation, as recommended by Simon ( 1965a), is used. Once the bud is dissected and emasculated and the stigma exposed, pollen from the desired source can be introduced to the stigma. It is likely that all annual Medicago have a hyaline membrane over the stigma, as observed in M . saliva (Armstrong and White, 1935). The rupture of this membrane is necessary to allow penetration of the developing pollen tube into the stigma. Rupture, which normally occurs when the flower is “tripped” and the stigma collides with the standard petal, may be artificially achieved by gently tapping the stigma with a pair of sharp-pointed forceps at the time of pollination or by lightly rubbing the pollen into the stigmatic surface. Excessive stigma damage should be avoided by careful treatment. A good guide is that the pollen should stick fairly readily to the stigma once the membrane is ruptured, a process that releases a viscous fluid from under the membrane surface (Lesins and Lesins, 1979).

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For cross-pollination to be ultimately successful in producing hybrid seed in annual medics, it is vital that plants are healthy and vigorous, without having too great a bulk of vegetation. Removal of runners to produce a sparse, open-canopied plant is often beneficial, particularly under conditions of low tight. Maximum percentage of pod and seed set to flowers produced occurs in spring in the field and the creation of springlike conditions will also improve the percentage of seed set after artificial hybridization. In winter, supplementary lighting is vital, particularly on overcast days, in the early morning and late evening, and in the first days after pollination. Better results can be obtained if light penetrates directly to the bud. This may be achieved if, immediately after cross pollination, the bud is placed in a cotton wool-stoppered glass vial with a drop of water in the bottom. In this way the damaged bud is kept isolated, but in an environment of low transpirational stress and adequate light. The whole plant is then placed in a controlled environment chamber, in which light can be supplemented as required and heating and cooling used to reduce temperature extremes. Day temperatures within the range of 15-23°C and night temperatures of 1CL15"C are ideal. Twin fluorescent tube lights placed above benches have been used for supplementary lighting with good results. These are also used to extend the day length up to about 14 hr when required. Pod formation on successful hybrids will begin between 2 and 5 days later. Most hybridization failures occur at this stage, due either to a lack of fertilization or to flower abortion. Sangduen et al. (1983), working with various annual and perennial Medicago, found that in interspecific crosses fertilization frequently occurred, but development following fertilization was usually very limited, leading to ultimate flower abortion. After about 2 weeks, a small but fully formed pod will be evident on successful crosses. At this stage the developing pod can be removed from the glass vial and placed in a waxed paper bag and the whole plant returned to a normal greenhouse environment. With most annual Medicago, flowering is accelerated by long days (Clarkson and Russell, 1975). This may be achieved artificially through greenhouse lighting, but excessive use of low-intensity light can lead to plants with reduced fertility. Both floret size and number are also usually reduced. A good early warning of this problem is that plants become etiolated by increased internode growth. The problem can be overcome either by raising the level of useable light or by reducing the length of time over which plants are exposed to low-level supplementary lighting. Several other plant stress factors produce similar floral symptoms with associated fertility reductions. These include high temperatures, nutrient deficiencies, and attack by various pests and pathogens. Generally, a plant that has

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racemes with many large florets is one that will be most successfully used as either a male or female parent in a cross.

2. Field Evaluation of Hybrid Selections Lines bred by SAGRIC Medic Breeding Programme are distributed for field test to about eight principal evaluating agronomists based in five Australian states, from Queensland to Western Australia. The range of environments in which these agronomists work varies from a subtropical, predominantly summer-rainfall zone, to the classic Mediterranean climatic zones, in which there is a strong incidence of cool-season rainfall and a pronounced summer drought. The farming systems vary from permanent pasture to pasture-crop rotations in which the pasture is used both as a “break” between cereal crops and as a source of nitrogen for the crop, as well as for animal feed in the pasture year. Therefore, field evaluation of hybrid selections and lines varies dramatically according to the constraints of the local environment and management practices, and there are few rules that cover the entire range of annual medic evaluation procedures. However, there are common threads. Lines distributed are now generally resistant in the greenhouse to a greater or lesser degree to BGA and SAA. Field results are used to assess whether that resistance level is adequate under the normal range of local environmental conditions. Initial field evaluation of all hybrid selections is by necessity somewhat subjective because of the large number of hybrid selections involved and the fact that these, unlike most accessions, are both heterozygous and heterogeneous. Emphasis is placed on a high yield of herbage and seed, as well as on other readily identifiable traits such as flowering date and morphological characters associated with tolerance to grazing. High herbage yield is generally regarded as important because of its association with animal production and nitrogen fixation. While this assumption is oversimplified, the zones in which medics are grown are generally characterized by rainfall-limited herbage production and a growing season of 4-7 months. Under these circumstances, the most limiting factor to stock carrying capacity is total forage production during the growing season. High seed yield and its relationship to persistence is a principal selection criterion. Many problems associated with poor medic pastures can be attributed directly to low medic density, which in turn traces to low hard seed reserves and previous poor seed yields in pasture leys. Further, early growing season yield is linearly related to seedling density, and this generally corresponds to the early winter period, when many farmers are

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frequently short of feed. Also, dense pastures reach a peak of production per unit area earlier in the growing season and may be less affected by growing seasons shortened through drought or other factors. Rossiter (1966), working with subterranean clover, found that the most competitively successful lines were those having the highest seed yield when grown in a pure sward. The situation is likely to be similar with annual medics, thereby further emphasizing the importance of high seed yield as a selection criterion. Hybrid selections pose special problems. Testing based on lines that are hybrid bulks or early segregating generation single- plant selections may be misleading because of the high degree of variation within the line itself and the possibility of phenotypic mixture effects. However, delaying selection of single plants until the F, generation or beyond results in the carrying forward of many plants that an adequate system of early testing and selection could have discarded. A compromise system that is currently in use is based on field testing F, bulks of selected F, single plants and then reselecting single plants from within the most promising F,-F, lines. In general, hybrid populations are seed-increased in the F, and screened for aphid resistance in the F, and F, with only resistant plants being retained; single plants are selected from the aphid-resistant F, bulks. As plants are all greenhouse grown to this stage, care is taken to ensure fairly random selection, thereby preventing bias towards greenhouse-adapted types. However, selection for pod spine characteristics that will minimize wool vegetable fault problems can be and is successfully practiced at this stage as necessary.

E. APHIDRESISTANCE IN Medicago

1 . The Development of the National Annual Medic Improvement Programme In 1977, when the three aphid species SAA, BGA, and PA invaded Australia, many medic pastures were already in a weakened state, due in part to a series of droughts and to increasing cropping frequency earlier in the decade. All of the commonly used cultivars available were found to be very susceptible to one or more of these aphids. Insecticide provided the only effective control measure, but few farmers bothered to spray, as this was both time-consuming and sometimes only temporarily effective. The result was that infested paddocks usually had very low or zero seed set and medic content and quality of the subsequent pasture was further eroded. By the early 1980s, medic-dominant ley pastures, although highly

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desirable for the long-term stability of these farming systems, were very much the exception rather than the rule. In response, major funding inputs were used to build the SAGRIC Medic Breeding Programme into a nationally orientated program able to identify aphid resistant lines from the National Medicago Collection kept by SAGRIC and to produce new ones through hybridization. From this program, selected lines could then be seed-increased and distributed to pasture agronomists for field test in Queensland, New South Wales, Victoria, South Australia, and Western Australia, as part of what has become known as the National Annual Medic Improvement Programme (NAMIP). While this arrangement is a loose and informal one, it has led to a streamlining of annual medic improvement for southern Australia. All breeding and most early selection work, which is expensive in terms of labor, technology, and infrastructure, is concentrated in one organization and in one area, so there is a minimum of fragmentation or duplication of effort. At the same time, field testing is carried out by workers throughout southern Australia, thereby maximizing the chance of selecting new cultivars that are either widely adapted or alternatively locally and specifically adapted to a definable area of soil, climate, and rainfall. 2. Aphid Resistance Breeding and Selection in Medicago Annual medics are by definition annual relatives of lucerne ( M . sativa), and aphid damage problems have been associated with that species for many years, particularly in the United States. The invasion of the United States in the mid 1950s by SAA (Dickson et a f . , 1955) caused catastrophic losses of plant and yield reduction within infested lucerne stands. These losses prompted a rapid response and lucerne plants with resistance to SAA were discovered within 2 years (Stanford, 1955; Reynolds and Anderson, 1955). By 1971, there were more than 25 SAA-resistant cultivars in the United States (Hanson and Davis, 1972), and by 1983 none of over 60 semidormant to highly winter-active lucerne cultivars and brands listed as available in California were rated as less than moderately resistant to SAA, (Marble and Peterson, 1983). Pea aphid resistance breeding has been carried out for nearly 50 years in the United States, again with good success, so most cultivars released today have at least moderate resistance to this pest. Blue-green aphid was only discovered in the United States in the early 1970s and breeding for resistance to this pest is relatively recent. However, the first resistant cultivar, CUF 101, was released in 1977 (Lehman et a f . , 1983), and by 1983 there were 10 commercially available BGA-resistant cultivars in the United States (Marble and Peterson, 1983).

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The successful experience of aphid resistance breeding in lucerne gave impetus to the search for similar resistance in annual medics, following the advent of these aphid species in Australia. Mass screening procedures based on North American technology (e.g., Harvey and Hackerott, 1956), adapted according to local experience, were initiated. Early results showed a considerable variation of relative susceptibility and resistance in different lines and in the frequency of lines with useful resistance in different species. For example, most lines and accessions from the species M . scutellata (snail medic) are resistant to both SAA and BGA, whereas good resistance to SAA in M. polymorpha is very rare (Cocks et al., 1980; Lake et al., unpublished). Of the cultivars commercially available in 1979, only the gama medics (M.rugosa) ‘Paragosa’ and ‘Paraponto’ and the snail medic ‘Robinson’ (which was also known as ‘Commercial snail’) had sufficient resistance to both SAA and BGA to warrant their recommendation as aphid-resistant alternatives to susceptible cultivars. However, all of these cultivars are best described as “special purpose,” in that their rate of seed softening and hence regeneration in years after seed set is variable. 3. Aphid Resistance Screening Techniques

As with M. sativa, resistance to all three aphids appears to be independently determined in annual species; that is, a plant resistant to any one aphid is not necessarily resistant to any other aphid. Therefore, lines have to be separately and individually screened for resistance to each aphid. Within all the species tested there is variation in the degree of resistance to each aphid. Furthermore, changes in resistance with plant growth stage occasionally occur. For example, ‘Paragosa’ gama and ‘Robinson’ snail medics tend to become more susceptible to SAA with increasing ontogenetic development (Mathison et al., 1978). Hence, screening at several growth stages is also needed. Initial screening is carried out in the following way. Plants are grown in wooden flats in the greenhouse on a sandy loam of pH 743 and adequate nutrient status. Two comparison rows, one susceptible and the other resistant, are included in each flat. Aphids of one or other species are added to the plants at a given stage of growth. In order to ensure aphid species purity, and to allow year-around screening, each species is individually cultured under strictly controlled conditions. Aphid numbers build up rapidly on susceptible plants but fail to build up on any plant with better than moderate resistance. This buildup, as well as the degree of plant damage relative to control plants, is monitored until the susceptible control is either severely damaged or dead, which with seedlings usually takes

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about 2 weeks. Final ratings of degree of resistance are then assigned, and these are a guide to plant and aphid interaction and are specific for the aphid type, plant genotype, plant growth stage, and prevailing environmental conditions (Table IV). Various lines may be chosen for further testing, both to confirm earlier results and to determine the degree of resistance expression and its stability in a range of environments. Aphid resistance in some lucerne plants is environmentally sensitive. Such factors as changes in temperature and deficiency of certain nutrients have led to measureable variation in plant resistance (Hackerott and Harvey, 1959; McMurtry, 1962; Isaak et al., 1963, 1965; Kindler and Staples, 1970a,b). Similar variation in resistance expression was therefore expected within annual species, and evidence of this has been observed in some lines, in which comparative resistance has been variable at different times of the year (Lake et al., unpublished). Although low nutrient status can generally be readily and cheaply overcome in the field through chemical fertilizers, changes in temperature cannot. Therefore, repeat testing of selected lines is usually carried out on all growth stages and at several times of the year, thereby encompassing a range of temperature regimes. 4 . New Aphid-Resistant Cultivars

In terms of economic significance to medic pastures in Australia, SAA, BGA, and PA are of differing importance, a factor that affects breeding and selection criteria. A cool season aphid, BGA is prominent in most susceptible medic pastures in most years throughout Australia's medic belt. It can be found at any time during the growing season, but usually builds up rapidly in spring, and unless controlled at this time will markedly reduce seed set on the maturing medic stand, sometimes to zero. The Table IV The Categorization of Medic-Aphid Interactions

Rating

Resistant (R)

Moderate resistance (MR)

Low resistance (LR)

Adult aphid survival Aphid multiplication rates Plant damage

Very low Zero None

Environmental sensitivity"

None

Low Very low Generally undetectable Low

Moderate Low Readily measurable High

Susceptible

6) High High Severe Moderate

"The degree to which the interactions described are affected by changes in the environment.

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spotted alfalfa aphid is a predominantly warm-season aphid, and as such is generally only a problem on annual medics in autumn if the seasonal break is unusually early and is followed by an extendedrperiod of mild weather. This is most likely in the warmer temperate areas of medic use in northern New South Wales and Queensland. However, if suitable weather does occur in other areas, plant losses from susceptible stands can be very high. PA is mostly found on medic stands in the late spring. However, its multiplication rate is the lowest of the three aphids and buildup is rarely rapid enough to cause signifcant damage before the medic plant is mature and has senesced. On this basis, a prime requirement for any new cultivar is at least moderate resistance (see table IV) to BGA and at least some resistance (low resistance) to SAA, whereas some resistance to P.A. is also desirable but of much lower priority. Screening for resistance in M. truncatula has revealed very few introduced lines (2%) with moderate resistance or better to BGA. Of these, only about a dozen have low resistance or better to SAA. Most of these came from the SAGRIC annual medic collection on hand in 1978 and were discovered and undergoing field tests by 1980. Several lines emerged from this program as agronomically acceptable and generally superior under aphid attack in the field when compared to older aphid-susceptible cultivars. The first of this group of lines registered and released to commerce was the cultivar ‘Paraggio’ (Oram, 1982), which was followed by ‘Sephi’ (Oram, 1985) and ‘Parabinga’. These cultivars are generally regarded as only interim replacements for older aphid-susceptible cultivars, in that they all have some agronomic deficiencies. For example, ‘Paraggio’ is rated as only having low resistance to SAA, whereas both ‘Sephi’ and ‘Parabinga’ are more spiny than would normally be acceptable in the marketplace. Furthermore, there are large areas of the traditional medic zones in which these cultivars have significantly poorer production than do aphidsusceptible cultivars under aphid-free conditions. However, as indicated by aphid screening data already obtained, there is only a low chance that new lines from collections will be discovered that will cover these gaps, and it is at this point that the plant breeder is called upon to create recombinants that are useful in this regard. 5 . Backcrossing Aphid Resistance Genes into Selected Cultivars

One potential solution to the problems of aphid susceptibility in an older but agronomically superior cultivar of annual medic is to produce an aphidresistant version of that cultivar by backcrossing resistance genes into it. Although backcrossing is generally regarded as a conservative breeding

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procedure, there is no doubt that in the current situation with annual medics, it does offer one very sigmfkant advantage. A completely new cultivar would need to be field-tested extensively before release, and with a selfregenerating annual pasture species this would mean field trials over at least 4 years. Further, the lines tested would need to be uniform and pure breeding, and this would require selection over a minimum of 4 years from the time of hybrid production and prior to field testing. Backcrossing overcomes the need for extensive field testing if the performance of the recurrent parent is already well established, so although the breeding and selection phase will not be substantially different, a backcrossed annual medic cultivar could be released up to 4 or 5 years before a line derived from a single cross. There are several prerequisites for a backcrossing program that aims to incorporate aphid resistence into an annual. First, the genetic control of that resistance needs to be simple, preferably monogenic and dominant. Second, the ability to produce hybrids at any time of the year simply and easily is essential, so that selected plants can be effectively crossed prior to senescence. Third, a quick, accurate, nondestructive, and year-around resistance screening method is needed, so that resistant progeny can be readily identified at an early stage and then backcrossed to the recurrent parent. Information on the genetic basis of aphid resistance in Medicago is very limited, mainly because interest has been centred on M. sativa and its relatives. These are mostly cross-fertilizing tetraploids (there are some diploid wild lucernes) with a high level of inbreeding suppression. Enforced selfing generally produces inviable progeny after two-three generations. This makes controlled crosses difficult to produce and segregation ratios difficult to interpret. However, there is evidence of at least some cases in which aphid resistance is fairly simply inherited in lucerne. Jones, et al. (1950) found resistance to PA in one cross to be determined by one dominant and one recessive gene. The data of Hackerott et al. (1958) indicate that in some aphid-resistant plants, resistance to SAA is likely to be conferred by only individual genes, or at most by a few major genes, as the polycross progeny of plants selected for SAA resistance from lines with a very low frequency of resistant plants often had a very much higher frequency of resistant individuals. If resistance were polygenically determined, selection response, particularly within a tetraploid, would be much slower than that reported. A rapid response was also noted in the selection for aphid resistance within the susceptible Australian lucerne cultivar ‘Hunter River,’ which resulted in the release of ‘Hunterfield’(Oram, 1983). Aphid resistance in other species has also on occasion been traced to single dominant genes (e.g., Joppa et al., 1980; Eenink et al., 1982; Merkle and Starks, 1985; Tyler et al., 1986). A single dominant gene that confers

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resistance to SAA and a similar single gene for BGA resistance has been discovered in a line of M. truncatula (Lake, unpublished), and the line has been used for a backcrossing program with M. littoralis ‘Harbinger.’ This initially required an interspecific cross, which was produced in the early 1980s. Backcrossing was started using SAA- and BGA-resistant progeny of this cross. Because it was an interspecific cross, some abnormal phenotypes were evident, particularly in the original cross and the first backcross, which both had a reduced seed set in the F, generation and a higher than normal number of albino and partial albino seedlings in the F, progeny and beyond. Furthermore, the aphid resistance genes did not segregate normally, all of which closely parallels the experience of Simon and Millington (1967) with interspecific crosses between M. littoralis and M . truncatula. In this program, three backcrosses to ‘Harbinger’ were produced. Both BC, and BC3individuals are similar to the recurrent parent, although several apparently monogenic differences are evident in some progeny groups. A monogenic leaf marker from the donor parent (Lake, 1985) may be used to distinguish the finally released aphid-resistant cultivar from its susceptible recurrent parent in the field. The number of backcrosses necessary to restore recurrent parental phenotype with aphid resistance added is not likely to be the same for different combinations of donor and recurrent parents. In the case of the ‘Harbinger’ replacement program, in which the donor was of a different species, three backcrosses were needed, but if the donor parent is of the same species two backcrosses may be sufficient. A parallel program using the barrel medic cultivar ‘Borung’ as the recurrent parent to an aphidresistant donor is likely to see a cultivar derived from the second backcross rather than the third. The other two backcrossing programs currently underway aim at replacing the cultivars, ‘Cyprus’ and ‘Jemalong’ with closely matched but aphid-resistant versions. ‘Harbinger,’ ‘Borung,’ ‘Jemalong,’ and ‘Cyprus’ all have specific niches in which currently available aphid-resistant cultivars do not perform adequately. ‘Harbinger’ is the best annual medic cultivar on mallee sands and loams. ‘Borung’ is outstanding on heavier soils that are prone to periods of waterlogging. ‘Cyprus’ is both earlyflowering and well adapted to some heavier soils in the drier margins of the southern Australian agricultural zone. ‘Jemalong’ was Australia’s most popular and widely sown annual medic cultivar prior to the arrival of aphids and is recognized as the most generally adapted cultivar of medic released to date. It is therefore safe to conclude that aphid-resistant versions of all four of these cultivars will be readily accepted in the marketplace, and this is

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the philosophy behind the various backcrossing programs, which should see four cultivar releases completed by 1990.

IV. PRESERVATION AND COMMERCIALIZATION A. GERMPLASM CONSERVATION The success of any plant breeding and selection program is dependent on a wide base of genetic diversity. This is particularly true for annual medic species, which are grown over such a broad range of ecological and edaphic conditions in Australia. Foresight shown by conservation-conscious experts actively engaged in gerrnplasrn utilization has resulted in an Australia- wide scheme of eight Genetic Resource Centres responsible for the conservation of crop and forage species throughout Australia. Within this system the South Australian Department of Agriculture is responsible for the maintenance of Medicago species, the current collection of which is in excess of 15,000 accessions. This collection is recognized by the International Board for Plant Genetic Resources (IBPGR) as a world base collection and is used extensively by plant breeders, agronomists, and research officers of local and international organizations. Well-documented principles are adopted, with emphasis on temperature control and maintenance of low levels of relative humidity. As most annual Medicago species attain near 100% seedcoat impermeability at maturity in the relatively low humidity months of November-March at the Parafield Plant Introduction Centre, the need for further prestorage drying is minimized. First-generation pod material is stored in impervious plastic vials for long-term conservation. As seed has not been threshed from the pod, no mechanical scarification of the seed coat has taken place and it is hypothesized that this seed will remain impermeable and viable for at least 40 years. Once it is threshed, however, its “shelf life” is drastically reduced, and this reduction varies between species. Minus 20°C and plus 2°C storage facilities are currently in use as base and active collections, respectively, and an 850-Liter cryogenic storage tank will be commissioned in the near future. Representative seed samples are stored in laminated aluminium foil packets hermetically sealed and indexed for ease of retrieval. Germination is monitored periodically according to known traits of the respective species and accessions rejuvenated as required. Following determination of seed yield characteristics on material harvested from the nursery stage as previously described, an 8-g sample of

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seed is prepared for the base collection and a minimum of 10 g for the active collection. Up to 75 g can be contained within one laminated aluminium foil pack, larger quantities being contained in plastic jars hermetically sealed within cryovac sleeves. Surplus unthreshed pod residue from this stage of the evaluation procedure is currently stored at ambient temperature but will eventually be stored at 2°C. Hanson (1985) clearly outlines the steps to be taken in preparing, storing, monitoring, and distributing germplasm, and if these procedures are adopted with good-quality seed, long-term viability can be expected with Medicago species. B.

COMPUTERIZED

DOCUMENTATION OF THE RESOURCE

The magnitude of the collection emphasizes the value of computers in information storage and retrieval. Because the accessions were collected from diverse environments, detailed passport data can be invaluable to plant breeders in selecting potential genotypes with special adaptation traits. If these data are matched with performance data encompassing many morphological and agronomic characteristics measured in the nursery, selections of the most appropriate genotypes can be made and subjected to advanced testing, e.g., for insect pest and disease resistance; the resultant data being stored for use by agronomists and plant breeders of the future. The effective utilization of genebanks depends heavily on ready access to a mass of diversified information as well as a source of high-quality, viable seed of reliable integrity. The compatibility of systems incorporating genebank and plant breeding information both within and between responsible organizations enhances profitabililty in terms of ultimate cultivar development in the quickest possible time. Blixt (1984) highlighted a number of character descriptors of importance in collating genebank and plant breeding information, many of the principles of which are incorporated in the South Australian system.

C.

COMMERCIALIZATION

The results of any cultivar development program are limited if the appropriate facilities are not available to ensure recognition by the commercial seed industry and ultimate acceptance and incorporation in agricultural systems by farmers and graziers. To foster the orderly introduction of a new cultivar into commerce and to monitor its success in its initial years of commercialization, the Aus-

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tralian Agricultural Council ratified the establishment of State Herbage Plant Liaison Committees with representation from State Departments of Agriculture, CSIRO, other appropriate agricultural research organizations, and grower, processor, and merchant representatives of the seed industry of Australia. In conjunction with the Committees, the Council established a voluntary herbage plant cultivar registration scheme in which information on new cultivars is presented to a registrar and published in an official register (Barnard, 1972; Mackay, 1982; Oram, 1983, et seq.). This Committee hears evidence from an originator who wishes to register a new cultivar, which must be different from, and possess some character of merit in comparison with, previously registered cultivars. It must also have a reasonable degree of uniformity or genetic stability and its name must conform to the requirements of the International Code of Nomenclature of Cultivated Plants, 1961 edition. Twelve months notice of intention to release the proposed new cultivar is needed by the Committee, during which time further data are prepared to support its release. Once it is released, breeder’s seed is grown under contract by a private seed producer for the production of pre-basic and ultimately basic seed before release to commerce. Production from this grade of seed results in commercially available supplies of certified seed for the rural community. The ultimate success of a cultivar is measured in terms of its acceptance and adoption by the local rural community and its impact on established international sales for use in regions climatically and edaphically analogous to southern Australia.

V. SCOPE OF THE FUTURE Although the value of annual medics was only discovered in the late 1930s, the intervening years have seen considerable advances in terms of plant improvement. This has been especially so since the mid 1960s, and this improvement is, if anything, likely to accelerate over the foreseeable future. It was not until the collection missions of Crawford in 1967 and Mathison in 1974 that the extent of the variability within annual Medicago began to be fully recognized (Crawford, 1975). These and subsequent collecting missions have led to the accumulation of a broad range of genetic variants (now exceeding 15,000 accessions) on which future breeding and selection programmes can be based. In any plant improvement program, new cultivars are aimed at either increasing productivity or at reducing the inputs necessary to achieve the same production levels. With annual Medicago, new cultivars should achieve both aims. Ways in which existing cultivars may be improved

434

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upon as listed in the text include in particular greater pest and pathogen resistance and higher herbage and seed yield. However, agriculture is not static, and other areas for advance are likely to become prominent in the future. Foreseeable needs for cultivars include (1) tolerance of low nutrient availability, for example, of phosphate; (2) tolerance of acid soils, which requires both medic and rhizobial selection; (3) more efficient and higher levels of nitrogen fixation; (4) better matching of hard-seed breakdown patterns with new rotation systems; and ( 5 ) reduction of the genetic vulnerability of annual medics through cultivar release from a broader range of Medicago species. To date the medic ley farming systems have not spread much beyond southern Australia. However, these systems have considerable potential in other regions of the world with similar climates, particularly in western Asia and North Africa. As in Australia 50 years ago, the use of fallow between crops is prevalent in these areas and severe land degradation and erosion is common. Replacement of the fallow with a medic ley will substantially reduce this degradation, but attempts at technology transfer to these areas have been variable (Boyce et al., 1985). A major reason for poor results is the lack of suitably adapted cultivars for these new areas. Therefore, future improvement of annual Medicago is not likely to be confined to Australia, but should spread to other Mediterranean climatic zones in which the adoption of ley farming systems can lead to large increases in both land productivity and stability. In this, the Australian experience of such systems should prove invaluable.

REFERENCES Anonymous. 1963. “Bioclimatic Map of the Mediterranean Zone,” pp.2145. UNESCOFAO, Paris and Rome. Armstrong, J. M.,and White, W. J. 1935. Factors influencing seed setting in alfalfa. J . Agric. Sci. 25, 161-179. Barnard, C. 1972. “Register of Australian Herbage Plant Cultivars.” 9. Annual Medics, pp. 193-207. C.S.I.R.O., Canberra. Bergerson, F. J., and Turner, G. L. 1983. An evaluation of ”N methods for estimating nitrogen fixation in a subterranean clover-perennial ryegrass sward. Aust. J. Agric. Res. 34, 391401. Black, J. M. 1909. “The Naturalized Flora of South Australia,’’ p.55. Govt. Printer, Adelaide. Blixt, S. 1984. Application of computers to genebanks and breeding programmes. I n “Crop Breeding : A Contemporary Basis” (P. B. Vose and S. G. Blixt, eds.). Pergarnon, New York. Boyce, K. G., Webber, G. D., and Lake, A. W. H. 1985. Transfer of the South Australian dryland farming technology to countries in West Asia and North Africa. Proc. Int. Grass!. Congr. 15th. Kyoto pp. I 194-1 1%. Brockwell, J., and Hely, F. W. 1966. Symbiotic characteristics of Rhizobium meliloti: An

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appraisal of the systematic treatment of nodulation and nitrogen fixation interactions between hosts and rhizobia of diverse origins. Aust. J. Agric. Res. 17, 885-99. Carter, E. D. 1980. The survival of medic seeds folowing ingestion of intact pods by sheep. Aust. Agron. Conf., lst, Gatton, Queensl. p.178. Carter, E. D. 1981. Seed and seedling dynamics of annual medic pastures in South Australia. Proc. Int. Grass/. Congr., 14th, Lexington, Ky. pp.447-450. Clarke, A. L., and Russell, J. S. 1977. Crop sequential practices. In “Soil Factors in Crop Production in a Semi-Arid Environment” (J. S. Russell and E. L. Graecen, eds.), pp.279300. Univ. of Queensland Press. Clarkson, N. M., and Russell, J. S. 1975. Flowering responses to vernalization and photoperiod in annual medics (Medicago spp.) Aust. J . Agric. Res. 26, 831-838. Cocks, P. S., Mathison, M. J., and Crawford, E. J. 1980. From field plants to pasture cultivars: Annual medics and subterranean clover in southern Australia. In “Advances in Legume Science” (R. J. Summertield and A. H. Bunting, eds.), pp.569-5%. Royal Botanic Gardens, Kew, England. Crawford, E. J. 1971. Seasonal changes in seedcoat permeability of 155 lines of Medicago truncatula Gaertn., under natural field conditions. Aust. Seeds Res. Conf., Canberra 1, 14-15.

Crawford, E. J. 1975. Data bank and genetic conservation in annual species of the genus Medicago L. C.S.I.R.0 Aust. Plont Introd. Rev. 10, 11-21. Crawford. E. J. 1977a. Changes in seedcoat permeability in annual species of Medicago with special reference to the variability in M . rugosu. Desr. Aust. Seeds Res. Conf., Canberra 2, 18-21. Crawford. E. J. 1977b. Agronomic assessment of the annual sub. species of Medicago L. Proc. Int. Grassl. Congr.. 13rh, Leipzig pp.273-275. Crawford, E. J. 1981. Development of gama medic (Medicago rugosu Desr.) as an annual leguminous species for dryland farming systems in southern Australia. Proc. Int. Grassl. Congr., 14th. Lexington, K y . pp.224-226. Crawford, E. J. 1983. Selecting cultivars from naturally occurring genotypes: Evaluating annual Medicago species. In “Genetic Resources of Forage Plants” (J. G. McIvor and R. A. Bray, eds.), pp.103-115. C.S.I.R.O. Melbourne. Crawford, E. J., and Nankivell, B. G. 1984. The effect of rotations on annual medic (Medicago L.) species seed and seedling populations. Aust. Seeds Res. Conf., Lawes, Queens/. pp.155-164. Dickson, R. C., Laird, E. F., Jr. and Pesho, G. R. 1955. Spotted alfalfa aphid, (Yellow clover aphid) on alfalfa. Hilgardia 24, 93-1 18. Donald, C. M. 1%7. Innovation in agriculture. I n “Agriculture in the Australian Economy” (D. B. Williams, ed.), pp.57-86. Sydney Univ. Press, Sidney. Eenink, A. H., Dieleman, F. L., and Groenwold, R. 1982 Resistance of lettuce (Lactuca) to the leaf aphid Nasonova ribis nigri. 2. Inheritance of the resistance. Euphytica 31, 301-304. Greuter, W. 1970. Contributes floristica anestro-aegaea 15. CundoNea 25, 189-192. Hackerott, H. L., and Harvey, T. L. 1959. Effect of temperature on spotted alfalfa aphid reaction to resistance in alfalfa. J . Econ. Entornol. 52, 949-953. Hackerott, H. L., Harvey, T. L., Sorensen, E. L., and Painter, R. H. 1958. Varietal differences in survival of alfalfa seedlings infested with spotted alfalfa aphids. Agron. J. 50, 139-142. Hagon, M. W. 1971. The action of temperature fluctuations on hard seeds of subterranean clover. Aust. J . Exp. Agric. Anim. Husb. 11, 440-443. Hanson, C. H., and Davis, R. L. 1972. I n “Alfalfa Science and Technology” (C. H. Hanson, ed.), pp. 35-50. American Society of Agronomy, Madison, Wisconsin.

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Hanson, J. 1985. “Procedures for Handling Seeds in Genebanks,” pp. 1-115. IBPGR, Rome. Harvey, T. L., and Hackerott, H. L. 1956. Apparent resistance to the spotted alfalfa aphid selected from the seedlings of susceptible alfalfa varieties. J. Econ. Entomol. 49, 289291. Heyn, C. C. 1963. The annual species of Medicago. Scripta Hierosol. 12, 1-154. Howieson, J. G., and Ewing, M. A. 1984. Soil acidity and legume nodulation. West. Aidst. J. Agric. 25, 125-7. Howieson, J. G., and Ewing, M. A. 1986. Acid tolerance in the Rhizobium meliloti-Medicago symbiosis. Aust. J . Agric. Res. 37, 55-64. Isaak, A., Sorensen, E. L., and Ortman, E. E. 1963. Influence of temperature and humidity on resistance in alfalfa to the spotted alfalfa aphid and pea aphid. J. Econ. Entomol. 56, 53-57. Isaak, A., Sorensen, E. L., and Painter, R. H. 1965. Stability of resistance to pea,aphid and spotted alfalfa aphid in several alfalfa clones under various temperature regimes. J. Econ. Entomol. 58, 140-143. Jones, L. G., Briggs, F. N., and Blanchard, R. A. 1950. Inheritance of resistance to the pea aphid in alfalfa hybrids. Hilgardia 20, 9-17. Joppa, L. R., Timian. R. G., and Williams, N. D. 1980. Inheritance ofresistance togreenbug toxicity in an amphiploid of Trificum turgidumlT. touschii. Crop Sci. 20, 343-344. Kindler, S. D., and Staples, R. 1970a. Nutrients and the reaction of two alfalfa clones to the spotted alfalfa aphid. J . Econ. Entomol. 63, 938-940. Kindler, S. D., and Staples, R. 1970b. The influence of fluctuating and constant temperatures, photoperiod and soil moisture on the resistance of alfalfa to the spotted alfalfa aphid. J . Econ. Entomol. 63, 1198-1201. Kloot, P. M. 1986. Check list of introduced species naturalised in South Australia. South Australian Department of Agriculture, Technical Paper No. 14, p. 65. Ladd, J. N. 1981. The use of “N in following organic matter turnover, with specific reference to rotational systems. PIant Soil 58, 40141 I . Ladd, J . N., Oades, J. M., and Amato, M. 1981. Distribution and recovery of nitrogen from legume residues decomposing in soils sown to wheat in the field. Soil Biol. Eiochem. 13, 251-256. Lake, A. W. H. 1985. Genetic control of markers in the annual medic cv. Sephi. Proc. Aust. Agron. Conf., 3rd. Hobart p. 204. Lehman, W. F., Nielson, M. W., Marble, V. L., and Stanford, E. H . 1983. Registration of C U F 101 alfalfa (Reg. No. 119). Crop Sci. 23, 398. Lesins, K., and Erac, A. 1968. Relationship of taxa in the genus Medicago as revealed by hybridization. I. M. striata x M. littoralis. Can. J . Genet. Cytol. 10, 263-275. Lesins. K., and Gillies, C. B. 1972. Taxonomy and cytogentics of Medicago. I n “Alfalfa Science and Technology” (C. H. Hanson, ed.) pp. 53-86. American Society of Agronomy, Madison, Wisconsin. Lesins, K. A., and Lesins, I. 1979. “Genus Medicago (Leguminosae)-A Taxogenetic Study.” Dr. W. Junk, The Hague. Lunney, H. W. M. 1983. Vegetable fault in Australian wool: Classification, consequences, and economic loss. J . Ausr. Znst. Agric. Sci. 49, 207-2 11. McAuliffe, C., Chamblee, D. S., Uribe-Arango, H., and Woodhouse, W. W. Jr. 1958. Influence of inorganic nitrogen on nitrogen fixation by legumes as revealed by “N. Agron. J . 50, 334-337. Mackay, J. H. E. 1978. Medicago rugosa Desr. (gama medic) cv. Paraponto. J . Aust. Insr. Agric. Sci. 44, 223-4. Mackay, J. H. E. 1982. “Register of Australian herbage plant cultivars; supplement to the 1972 Edition.” 9. Annual Medics, pp. 82-101. C.S.I.R.O., Canberra.

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McMurtry, J. A. 1962. Resistance of alfalfa to spotted alfalfa aphid in relation to environmental factors. Hilgardia 32, 501-539. Marble, V. L., and Peterson, G. 1983. Alfalfa variety and brand characteristics for California. Proc. Calif. AIfaIfa Symp., 13th, Univ. Calif.#Holtville pp. 7-8. Mathison, M. J., Kobelt, E. T., and Baldwin, G. 1978. A guide to cultivars. Medic and subclover susceptibility to SAA and BGA. Dept. of Agriculture and Fisheries, South Australia, Fact Sheet No. 28/78. Merkle. 0. G., and Starks, K. J. 1985. Resistance of wheat to the yellow sugarcane aphid (Hom0ptera:Aphididae). J. Econ. Entomol. 78, 127-128. Myers, L. F., Lovett, J. V., and Walker, M. H. 1974. Screening of pasture plants: A proposal for standardizing procedures. 1. Aust. Inst. Agric. Sci. 40, 283-289. Neal-Smith, C. A., and Johns, D. E. 1967. Australian plant exploration 1947-1967. Plant Introd. Rev. 4, 1-6. Oram, R. N. 1982. Register of Australian herbage plant cultivars: Annual medics - ‘Paraggio.’ J. Aust. Inst. Agric. Sci. 48, 239-40. Oram, R. N. 1983. Register of Australian herbage plant cultivars: Lucerne-’Hunterfield.’ J. Aust. Inst. Agric. Sci. 49, 249-250. Oram, R. N. 1985. Register of Australian herbage plant cultivars: Annual medics-‘Sephi.’ J. Aust. Inst. Agric. Sci. 51, 83-84. Reynolds, H. T., and Anderson. L. D. 1955. Control of the spotted alfalfa aphid on alfalfa in southern California. J. Econ. Entomol. 48, 671-5. Rossiter, R. C. 1966. The success or failure of strains of Trifolium subterraneum L. in a Mediterranean environment. Aust. J. Agric. Res. 17, 425-446. Sangduen, N., Sorensen, E. L., and Liang, G. H. 1983. Pollen germination and pollen tube growth following self pollination and intra- and inter-specific pollination of Medicago species. Euphytica 32, 527-534. Simon, J. P. 1965a. Inheritance of three marker characters in Medicago truncatula Gaertn. ( = M . tribuloides Desr.). Aust. J . Agric. Res. 16, 31-36. Simon. J. P. 1965b. Relationship in annual species of Medicago 11. Interspecific crosses between M. tornata (L.) Mill. and M . littoralis Rhode. Aust. J . Agric. Res. 16, 51-60. Simon, J. P., and Millington, A. J . 1967. Relationship in annual species of Medicago Ill. The complex M. littoralis Rhode M . truncatula Gaertn. Aust. J . Bot. 15, 35-73. Simon, J. P., and Simon, A. 1965. Relationship in annual species of Medicago. I. Number and morphology of chromosomes. Aust. J . Agric. Rrs. 16, 37-50. Small, E., and Lefkovitch, L . P. 1986. Relationship among morphology, geography, and interfertility in Medicago. Can. J. Bot. 64, 45-52. Stanford, E. H. 1955. Resistant plants. Calif. Agric. 9, 5 . Stephens, G. G., Heniott, R. I., Downes, R. G.. Langford-Smith, T., and Acock, A. M. 1945. Soil, land use and erosion survey, Part of County Victoria, South Australia. C.S.I.R.O. Bulletin No. 18/8. Symon, D. E. 1961. A bibliography of subterranean clover - together with a descriptive introduction. Commonwealth Bureau of Pastures and Field Crops, Hurley. Berkshire NO. 1/1961. pp. 3-5. Trumble, H. C. 1939. Barrel medic (Medicago tribuloides, Desr.) as a pasture legume. J . Agric. South Aust. 42, 953-58. Tyler, J. M.,Webster, J. A., Sebesta, E. E., and Smith, E. L. 1986. Inheritance of biotype E greenbug resistance in bread wheat C1 17882 and its relationship with wheat streak mosaic virus resistance. Euphytica 35, 615-620. Vercoe, J. E., and Pearce, G. R. 1960. Digestibility of Medicago tribuloides (Barrel medic) pods. J. Aust. Inst. Agric. Sci. 26, 67-70. Webber, G. D., Cocks, P. S., and Jeffries, B. C. 1976. “Farming systems in South Australia.” Department of Agriculture and Fisheries, Adelaide, South Australia.

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A Acaricides, in seed coatings, 58 Acetylene nitrification and, 298 nitrous oxide production and, 298 nitrous oxide reductase inhibition, 295 reduction, in retorted oil shale, 254-256 Acid phosphatase activity, in lead and zinc mine soil, 262, 263 Acids, produced by phosphate-solubilizing microorganisms, 217 Activated carbon seed coatings, 60-61 Adhesives, for seed coating process, 50-52 Adsorbents, for inactivation of herbicides, 60-61 Africa noninversion minimum tillage systems in southern and eastern areas, 120-122 in western areas, 118-120 tied-ridge system in, 127-130 Agricultural efficiency, 88 Agrisilviculture, 3, 164-167 Apbacterium tumefoiens, 384 Agroforestry, 3, 164-167 Melopathy, in weed control, 20-21 Alley cropping, enhancement of conservation tillage efficacy, 165-167 AM (2-amino-4-chloro-6-methyl pyrimidine) ammonia volatilization, 285 denitrification and, 294-295, 296 urea hydrolysis and, 300 Ammonia volatilization. nitrification inhibitors and, 285-290 Ammonium fitionhelease, nitrification inhibitors and, 284-285 inhibition of nitrification, 281-282 Ammonium thiosulfate inhibition of nitrification, 303-304 nitrification of ammonium and, 303 urea hydrolysis and, 300-301 Amylase activity, in recultivated soils after topsoil mining, 273 from coal mine spoils, 241, 243 Ants, pasture seed theft and, 58

Aphid resistance, of Medicago species. See Medicago species, aphid resistance Asia. See also specific countries in Asia noninversion minimum tillage systems for, 122, 123 Asparaginase activity, in recultivated technogenic soils, 247, 249, 267-269 A X (4-amino-1,2,4-triazole) ammonia volatilization and, 286 denitrification and, 294-295, 296 mineralization of soil nitrogen and, 291-292, 293 urea hydrolysis and, 300 Australia commercially available Medicago species, 402-403 history of annual medic pastures in, 399-402 National Annual Medic Improvement Programme, development of, 424-425 B

Bacillus, 56-57 Bacillus megatherium, 207-208, 212, 223 Bacteria, phosphate-solublizing. See also

specific bacteria occurence and numbers in soil, 210-212 Barley yield, effects of tillage methods and depth of cultivation, 123 Basin listing. See Tied ridging BBF system, 132-133 Bean yield mulching and, 96 with no-till vs. conventional tillage systems, 105 Bed-furrow system, 130 Bendiowb, 58 Bentonite seed coating, 54 Benzoquinone, urea hydrolysis and, 300, 301 (S)-Benzyl-N-N-dutyIcarbamate, 70 Biological efficiency, 2, 14, 16-17 Biosupers, 220-223

439

440 Black gram yield, spacing and, 133-134 Buffel grass (Cenchrus ciliarib), 64-65 4-terf-Butylpyrocatechol, 303

INDEX

at minimum pore water pressure degree of saturation and, 331-332 mechanical stress and, 332-335 no-till systems and, 100-101 in tropics vs. temperate zones, C 113-116, 117 permeability/infiltration rate and, Caffeic acid, denitrification and, 295, 296 316-320 Calcium peroxide, in seed coating, 73 pore geometry and, 335-337 Carbon dioxide evolution, in recultivated recultivation of technogenic soils and, 243 technogenic soils, 262, 265-266 soil-water diffusivisity and, 320-321 Carbon disulfide, 291-292 sorptivity and, 321-323 Cassava (Manihoc esculenta), effect of traffic elimination and, 136-137 mulching on yield, 96-97 water retention characteristic and, 314-316 Catalase activity, in recultivated soil Compensation, 8 from coal mine spoils, 231-234, 245-249 Competition from iron mine spoils, 257-258 indexes for, 34-35 from manganese mine spoils, 260 for light, 9-10 from power plant wastes, 253 for nutrients, in pasture mixtures, 15-16 from refractory clay mine spoils, 265-266 with weeds, species variability of, 19 from sand opencast mine floor drift and Competition index, 34-35 spoils, 267-269 Complementary effect, 3 from sulfur mine soil, 263-264 Component crops, 3 Cation exchange capacity, 202 Component technology, 3 Cellulase activity, in recultivated soil after Conservation tillage topsoil mining, 273 criteria for, 89-90 Cenchrus ciliarib, 64-65 definition of, 89 Cereals. See also specflc cereal grains description of, 183-185 as mulch, 92 effectiveness of, 184 CGA-43089, 60 energy conservation and, 157-158, 159 China, rice breeding in, 357-361 enhancement of efficacy Chiseling, 90, 91, 137 by cover crops, 168-171 6-Chloropicolinic acid (dCPA), 290-291, 303 by crop rotation, 172-173 Clay, water retention characteristic and, 316 by live mulch, 171-172 CG1580 (2,4-diamino-6-trichloromethyl-sby multiple cropping, 172-173 triazine), 286, 295, 296 by summer fallowing, 173-177 Coal mine spoils, research on technogenic soils fertilizer response and, 152-155 in Federal Republic of Germany, 250-252 geographic areas using, 160 in Poland, 245-250 for leaching, 138-140 in United States, 236-245 for low-plant-available water reserve soils, Cold-shock treatment, effect on rice anthers 127-132 culture, 346-347 mulch farming and, 95-98 Compaction noninversion minimum tillage systems, aggregate size distribution and, 326-327 applicability of, 118 aggregate stability and, 327-328 no-till systems, 98-109 air permeability and, 324-325 for problem soils alleviation of, 102 with poor drainage, 132-136 conservation tillage for, 136-138 purpose of, 117-118 definition of, 312, 335 reasons for adopting, 184 gas diffusion and, 323-324, 325 research and development priorities for, mechanisms of soil structure change 182-183 during, 328-331 root growth and, 156-157

44 1

INDEX soil chemical properties and, 152-155 soil conservation and, 143-145 soil degradation, prevention of, 159 soil fertility and, 159 soil guide to, 177-182 soil moisture and, 147-151 soil nutritional properties and, 152-155 soil-related constraints alleviated by,

Cultivating, energy requirements for, 91 Cultural practices, to enhance effectiveness of conservation tillage, agroforestry and alley cropping, 164-166, 167, 168 Cybrids, 379-380, 380

D

126-127

soil structure improvements and, 140-143 for soils with crusting and compaction, 136-138

soil temperature and, 147-151 subsoiling as, 122-126 sustainable agriculture and, 92-95 systems approach and supportive cultural practices, 163-177 water conservation and, 143-145 Contour hedges, 166-167 Cover crops choice of, 171 dry matter yield of, 170 enhancement of conservation tillage efficacy, 168-171 mulch from, 93 Cowpea yield hedgerow spacing and, 166 mulching and, 96-97 seedbed preparation and, 166 tied ridging and, 127-128, 131 tillage systems and, 120-121, 138 6-CPA, 290-291, 303 Crop cultivars, ability to compete with weeds, 19 Crop density, weed suppression and, 19-20 Crop domestication stages, 5 Crop growth cycles, overlapping of, 9 Cropping intensity index, 3, 34 Cropping pattern, 3 Cropping systems, 3. See also Multiplecropping systems; Sole cropping systems biological stability of, 25-26 economic stability of, 25-26 genetic diversity in, 26-27 Crop residue, uses for, 92. See also Mulch Crop rotation for conservation tillage, 94-95 definition of, 3 enhancement of conservation tillage efficacy, 172-173 Crops, mulch-producing, 92-93 Crusting, conservation tillage for, 136-138

DCD. See Dicyandiamide Dehydrogenase activity, in recultivated soils from coal mine spoils, 236-239, 247-248, 251

from iron mine spoils, 257-259 from lead and zinc mine spoils, 262, 263 from manganese mine spoils, 260 from pipeline construction overburdens, 271 from retorted oil shale, 253, 253-256 from sulfur mine soil, 263-264 Denitrification, nitrification inhibitors and, 294-295 Dicalcium phosphate, in seed coating, 65-66

Dicanthium sericeum (Queensland bluegrass), 74

2,3-Dichlorohydroquinone, 303 Dicyandiamide (DCD) ammonia volatilization and, 286-287, 289-290

denitrification and, 295, 296 urea transformations and, 288 Diesel fuel requirements, for tillage systems, 91 Disease management of, in multiple-cropping systems, 24-25 nitrification inhibitors and, 303 protective seed coatings for, 57-58 resistance phytoalexins and, 374 rice somatic cell cultures and, 368 Disk planting, 91 Disk plowing, 91 4,6-di-tert-butyl-o-benzoquinone,303 4,6-di-tert-butylpyrocatechol,303 Dolomite mine spoils, technogenic soils from, 264 Double cropping, 3, 8, 10 Dwell (etridiazole), 295, 296 Dyrene (2,4-dichloro-6-(0-chloroanilino)-striazine), 290

442

INDEX

E Earthworm activity, 141-143 Economic buffering, 33 Ectomycorrhizal fungi, 222 Energy conservation, conservation tillage and, 157-158, 159 Energy-related inputs, minimizing, 182 Environmental index, 27 Environmental stress tolerance, rice somatic cell cultures and, 368-369 E m antidote for, 59-60 seed-applied, 70-71 Erosion control by conservation tillage systems, 143-145 by no-till systems, 143-145 no-till system, in tropics vs. temperature zones, 116 Etridiazole (Dwell), 295, 296

F

Fallowing enhancement of conservation tillage, 173-177

vs. plowing depth, effects on soil moisture storage on maize yield, 175 Fertility conservation tillage and, 159 for no-till systems, in tropics vs. tempeqte zones, 117 relationships, in intercropping systems, 16-17

Fertilizers broad-acre applications of, 44 mineral, 244 nitrogenous, recovery by crops, 88 phosphate, 201 in seed coatings, 62-63 efficacy of, 64-66 injury from, 66-69 soil response and conservation tillage, 152-155

species balance in pasture systems, 6 Fine particulate seed coatings, 72 Fluazifop-butyl, 70 Fungi, phosphate-solubilizing. See also speciflcfungl occurrence and numbers in soil, 210-212

Fungicide in seed coating. See Seed coating seed treatments, 57-58 Furrow blocking. See Tied ridging Furrow damming. See Tied ridging Furrow diking. See Tied ridging 0 Germany, Federal Republic of, enzymologic research on coal mine spoils, 250-252 8-Glucosidase, 247, 249 Grain. See also speciflc grain multiple-species systems, 6-7 tillage methods, effects of, 106 Green seedbed, 171-172 Growth regulators, as seedtreating agents, 74-75

Gum arabic, for seed coating, 51 Gypsum seed coating, 62

H

Hard-setting soils, no-till system, in tropics vs. temperature zones, 112-113 Heat treatment, effect on rice anthers culture, 347-348 Herbage yield, in recultivated soil after topsoil mining, 273 Herbicide antidotes, 59-60 Herbicides annual discharge of, 160 broad-acre applications of, 44 conservation tillage dependency on, 184 protective seed coatings for, 59-61 in seed coatings, 70-71 for weed control, 19 Hormones, as seedtreating agents, 74-75 Hydrophilic seed coatings, 71-72 Hydrophobic seed coatings. 72-73

I Income stability, monoculture vs. intercropping, 29-32 India, rice breeding in, 362 Indonesia, tillage methods and mulching effects on crops, 106-107

443

INDEX Inoculation of soils, with phosphatesolubilizing moicroorganisms, 212-216 Inorganic chemicals, to stimulate seed germination, 74 Insecticide, in seed coating. See Seed coating Insects management in multiple-cropping systems, 21-24 protective seed coatings for, 58-59 Integrated pest management, 24 Integration efficiencies, 2 Intercropping biological and economic stability of, 25-26

biological output variations in, 26-29 buffering and compensation in, 32-33 definition of, 3 grazing and, 5 insect populations and, 22-24 light interception in, 11-12 stability of, 28-29 Interplant compensation, 32 Interplanting, 3 Interseeding, 6 Intraplant compensation, 32 Invertase activity, in recultivated soil after topsoil mining, 273 from coal mine spoils, 231-232,245,245-249 from iron mine spoils, 257-258 from manganese mine spoils, 260 from refractory clay mine spoils, 265-266 from sand opencast mine floor drift and spoils, 267-269 Iron mine spoils, recultivated soils from, 257 Irrigation, 130, 131 K

2-Ketogluconic acid, 217-218 Korea, rice breeding in, 361-362 L

LAD (leaf area duration), 10 Land equivalent ratio (LER), 3. 34 Leaching conservation tillage and, 138-140 of pesticides, 161-162

Lead mine wastes, technogenic soils from, 260-263

Leaf area duration (LAD),10 Leaf dispersion, light use efficiency and, 12 Leaf inclination, light use efficiency and, 12 Legumes as mulch, 92 nitrogen provision in multiple-species systems, 16 for revegetating soil plots. 247-249 Light use efficiency, by multiple-species, 9-13

Lime application in no-till vs. conventional system, 139 in seed coating, 55-56 soil restoration and, 244 Lime mine spoils, technogenic soils from, 264 Live mulch, enhancement of conservation tillage efficacy, 171-172 Loams, water retention characteristic and, 316 M

Macronutrients. See ah0 specijk

macmnutrients in seed coatings, 62-63 Magnesium, in seed coatings, 64 Magnesium carbonate, 72 Maize cost of production, 159 yield alleviation of compaction and, 137 depth and width of cultivation strip and, 121-122 hedgerow spacing and, 166 mulching and, 96-97 with noninversion minimum tillage systems, 119-120 no-till vs. conventional tillage systems, 105 in poorly drained soil, effects of conservation tillage, 135-136 seedbed preparation and, 166 shallow plowing and, 119 in southern and eastern Africa, 120-121 in temperate region with no-till system, 108-109

tillage methods and, 118, 130 using no-till system in tropics, 99-101

444

INDEX

Maize-bean intercrop, income stability of, 29-32 Maneb (manganese ethylene bisdithiocarbamate), 290 Manganese deficiency, seed soaking and, 64 in seed coatings, 64 Manganese mine spoils, technogenic soils from, 259-260 Manihoc esculentn, effect of mulching on yield, 96-97 Mechanical stress, corresponding to minimum pore water pressure, 333-335 Medicago littoralis, relationships with other species members, 419-421 Medicago sativa, 65 Medicago species annual pastures in Australia, history of, 399-402 aphid resistance, 418-419 backcrossing of genes into selected cultivars, 428-431 breeding, selection and, 425-426 categorization of, 427 development of National Annual Medic Improvement Programme, 424-425 in new cultivars, 427-428 screening techniques, 426-427 commercialization of, 432-433 commercially available in Australia, 402-403 criteria for selection flowering time, 405-407 levels and changes in seedcoat permeability, 408-409, 410 pod spininess, 408 resistance to insect pests and diseases, 409, 411 seasonal herbage production, 405 seedling vigor, 403-405 seed production, 407-408 evaluation, 411-413 of grazing tolerance and potential for high animal production, 415-416 in nursery rows, 413 seed production, 415-416 of swards for dry matter and seed production, 413-415 future improvement of, 433-434 general breeding aims, 417-419 germplasm conservation, 431-432 hybridization field evaluation of selections, 423-424

first attempts in, 416-417 techniques for, 421-423 relationships among, 419-421 resource, computerized documentation of, 432 selection and breeding programs, 403 variations in characteristics, 404 Mediuago tornam, relationships with other species members, 419-421 Medicago tnmuatula, relationships with other species members, 419-421 2-Mercaptobenzothiale, 295, 296 Metalaxyl, 57 Methiocarb, 59 Methyl cellulose, for seed coating, 51 2-Methyl-l,4-napthoquinone,303 5-Methyltryptophan resistance, in somatic cell cultured rice, 373 Micronutrients. See also specific

micronutrients in seed coatings, 63-64 soaking of seeds in, 64 Microorganisms. See also specific

microorganisms Bacilh megatherium, 207-208 importance in plant nutrition, 199 phosphate-solu bilizing acids produced by, 217 effect of inoculation in soils, 212-216 mechanism of action, 216-220 occurrence and numbers in soil, 210-212 solubilization of phosphate in pure culture, 209-210 Millet yield, with noninversion or minimum tillage systems, 119-120 Mineral fertilizers, 244 Moisture conservation, effects of tillage methods, 130 Moldboard plowing, 90, 91 Molluscs, control by seed coatings, 58-59 Molybdenum deficiency, seed soaking and, 64 in seed coatings, 63-64 MON-4606, 60 Monoculture definition of, 3, 4 weed control and, 20-21 Mulch for different environments, 95-98 plastic sheets, soil temperature and, 151

445

INDEX procurement methods, 92-94 soil temperature and, 149-151 Multiple-cropping systems biological output variations in, 26-29 biologic and economic stability buffering and compensation, 32-33 statistical analysis of multiple-species systems, 33-35 definition of, 3 enhancement conservation tillage efficacy, 172-173 future applications for, 35-36 historical background, 4-5 income stability, 29-32 literature review, 1-2 pest management, 17 insects, 21-24 plant pathogens, 24-25 weeds, 17-21 terminology for, 2-4 Multiple-species systems grains, 6-7 historical background, 4-5 in pasture lands, 5-6 resource use efficiency, 7-8 for light, 9-13 for time and space, 8-9 for water, 13-14 root crops, 6-7 statistical analysis of, 33-35 Mutual cooperation, 7-8 Mutual inhibition, 7 Mycorrhizal fungi absorption of unavailable phosphate, 203 enzyme activity and, 204-205 extension of phosphate depletion zone, 205-207 inoculation into soil, 216-216 nutrient uptake and, 202-207 plant growth and, 202-207

N l,8-Naphthalic anhydride, 59 1,4-Naphthoquinone, 303 Natural ecosystems, genetic diversity in, 26-27 Neomycin phosphotransferase, 384 New Zealand, enzymological research on soils after topsoil mining, 272, 273

Nitrapyrin (2-chloro-6-trichlormethyl pyridine) ammonia volatilization and, 286, 288 denitrification and, 294-295, 296 development of, 280 fertilizer nitrogen immobilization and, 292, 293 nitrate movement and, 305 nitrogenase activity and, 304 nitrogen transport/movement and, 282-284 nitrous oxide emissions and, 297-298, 299, 305-306 phytotoxicity of, 303 urea hydrolysis and, 300 Nitrification definition of, 279 retardation, effect on microsite chemistry of soils, 306 Nitrification inhibitors. See also spcflc

inhibitors ammonia volatilization and, 285-290, 305 ammonium fixation/release and, 284-285, 305 definition of, 279-280 denitrification and, 294-295 immobilization of soil nitrogen and, 290-294 literature on, 280 microbial immobilization and, 305 mineralization of soil nitrogen and, 290-292 nitrogen cycling in soil and, 305-306 and nitrogen transformations other than nitrification, 281 nitrogen transport/movement and, 281-283 nitrous oxide emissions and, 296-297, 305-306 plant disease and, 303 plant quality and composition and, 303 urea hydrolysis and, 300-303 Nitrite, formation from ammonium oxidation, 293-294 Nitrogen biological transformations of, 290-295 fixation, by Medicugo species, 401 in seed coatings, 62-63 transport and movement, nitrification inhibitors and, 281-284 uptake, tillage methods and, 153-155 usage in multiple-species systems, 15-16

446

INDEX

Nitrosophenols, 294 Nitrous oxide emission, via nitrification and denitrification, 295-303 Noninversion minimum tillage systems applicability of, 118 in semiarid and arid West Africa, 118-120

for south and west Asia, 122, 123 for southern and eastern Africa, 120-122

No-till systems applicability of, 98 criteria for success, 99 definition of, 90 for different environments, 98-109 geographic areas using, 160 soil suitability for, 142-143 in temperate regions, 108-109 in tropics, 99-108 in tropics vs. temperate zones advantages and disadvantages of, 110-117

Peanut yield effects of tillage methods and residue management on, 123 with noninversion or minimum tillage systems, 119-120 tillage system effects on, 106, 107 Pectinolyase, 237-238 Pelleting, 47 Perennial system, weed control in, 20 Permethrin, 58 Peroxidase, 232. 235 Peroxides, in seed coatings, 73 Pesticides leaching of, 161-162 transport to water, 161 volatilization losses of, 161 Pest management, in multiple-cropping systems, 17 weeds, 17-21 pH, of seed coating, fertilizer injury and, 68-69 Phalaris (Phahrisaquatica), 65

Phaseolus vulgaris, 96

erosion and, 116 hard-setting soils and, 112-113 soil compaction and, 113-116, 117 soil fertility and, 117 soil moisture and, 110-112 soil temperature and, 110 Nutrients. See ako specij7c nutrients usage efficiency in multiple-species systems, 14-17

0 Oil shale, retorted, technogenic soils from, 253-256 ory~a sativa, factors affecting

anther and pollen culture efficiency, 344-356 O~YZU species. anther culture of, 356 Overyielding, 3, 7, 8

Phenylmercuric acetate (PMA), denitrification and, 294-295, 296 Phenylphosphorodiamidate(PPD), ammonia volatilization and, 289 Phosphatase activity, in recultivated soils after topsoil mining, 273 from coal mine spoils, 237, 241, 243 from iron mine spoils, 258 from manganese mine spoils, 260 from retorted oil shale, 253-256 Phosphate depletion zone, extension by mycorrhizal fungi, 205-207 microbial solubilization in pure culture Fonditions, 209-210 plant-available microbially mediated increases in, 199-200

mycorrhizal effects on, 202-207 sources of, 200-202 soil, forms and transformation of, 200-201

P

Paraplowing, 90, 102, 137 Pasture species competition for nutrients, 15-16 fungicide seed treatment and, 57 multiple-species systems in, 5-6

unavailable, absorption of, by mycorrhizal fungi, 203-204 Phosphate fertilizers, as source of plantavailable phosphate, 201 Phosphobacterins development and use in USSR, 219, 222 organic phosphate mineralization and, 207-209

447

INDEX phosphate-solubilizing microorganisms and, 219 use in North America, 223 Phosphorus microbially mediated increases, future technologies and, 222-223 in seed coatings, 61-62, 63, 65 soil response to, tillage methods and, 155 usage in multiple-species systems, 15 Phytoalexins, disease resistance and, 374 Phytotoxicity, nitrification inhibitors and, 303 Pigeonpea, light interception in, 10-11 Plant growth substances, 219 Planting, energy requirements for, 91 Plant pathogens. See Disease Plowing depth sorghum yield and, 124 vs. fallowing, effects on soil moisture storage on maize yield, 175 Plow planting systems, 91 PMA (phenylmercuric acetate), denitrification and, 294-295, 296 Poland, enzymological research on recultivated soil from coal mine spoils, 245-250 from power plant wastes, 252-253 from sand opencast mine floor drift and spoils, 267-269 Pollutants, annual discharge to waterways, 88 Pollution, conservation tillage and, 160-162 Polyphenol oxidase, 232, 235 Pore water pressure, minimum degree of saturation at, 331-332 mechanical stress and, 333-335 Potassium azide denitrification and, 294-295, 2% nitrous oxide emissions and, 298-299 urea hydrolysis and, 300 Potassium ethylxanthate denitrification and, 295, 296 urea hydrolysis and, 301 Power plant wastes, technogenic soils from, 252-253 PPD (phenylphosphorodiamidate), ammonia volatilization and, 289 Proteinase, 232, 234 kudomonas, 57 Aeudomonas put&, 220 Puddling system, 107-108

Q Quadruple cropping, 3 Queensland blue grass (dicanthium sericeum), 74

R R-25788, 59-60 Raised beds, effects on black gram yield, 133-134 Ratoon cropping, 3 Relative yield total (RYT), 34 Relay cropping, 8-9 Resource use efficiency of multiple species, 7-8 for light, 9-13 for time and space, 8-9 for water, 13-14 indexes for, 34-35 Respiration, 243 Rhizobia, inoculant coatings, 55-56 Rhizoctonia, 56-57 Rice Hua-Yu No. 1 variety, 358-359 importance of, 339-340 yield with noninversion minimum tillage systems, 119-120 tillage systems and, 102-104, 106, 107 Rice tissue culture anther culture, 340 advantages of, 341-342, 386 albinos and incubation temperature and, 354-355 callusing medium and, 348-351 diploidization of haploids, 342-344 future advances in, 386-387 genotype and, 344-345 of japonica variety, 363-364 of oryza species and interspecific hybrids, 356 outside of China, 361-362 physiological status of donor plants and, 345-346 practicability of, 363-365 pre- and postinoculation treatment of anthers and, 346-348 procedure for, 358. 359 stage of anther and, 348 technique, 344

448

INDEX

definition of, 340 embryo culture, 341 techniques for, 385-386 float anther culture, 351-352 future of, overview and strategies for,

Row crops, in tropics, no-till methods for, 104-105

Ryegrass-alfalfa mixture, revegetation of bentonitic clay mine spoils and, 267 RYT (relative yield total), 34

385-389

genetic evaluation and utilization of anther-derived plants, 356-362 micropropagation techniques, 387-388 ovary culture, 365-366 pollen culture, 341-344. 387 callusing medium and, 348-351 genotype and, 344-345 induction of direct plant growth, 355-356

isolated, 352 physiological status of donor plants and, 345-346 pre- and postinoculation treatment of anthers and, 346-348 regeneration and, 352-354 stage of anther and, 348 protoplasts, 378-380, 388-389 genetic manipulations of, 384-385 improvement and, 380 isolation, culture and regeneration of, 381-384

somatic cell culture, 366-367, 387 callus induction and single-cell cultures, 369-370

significance for rice improvement, 367-369

somaclonal variation, 375-378 somatic embryogenesis and regeneration, 370-372 in vitm selection, 373-374 Ridge-furrow system, 91 Ridge planting, 91 Rock phosphate, as source of plantavailable phosphate, 201-202 Rock phosphate-sulfur mixtures, 220-223 Rodents, protective seed coatings for, 59 Romania, enzymological research on recultivated soils from iron mine spoils, 257-259 from lead and zinc mine wastes, 262-263 Root crops multiple-species systems, 6-7 tropical, mulching and, 102-103 Root growth, conservation tillage and, 156-157

Rotation. See Crop rotation

S

Sand, water retention characteristic and, 316 Seedbed preparation. See also Tillage systems cowpea yield and, 166 importance of, 90 maize yield and, 166 Seed coating advantages and disadvantages of, 75-76 benefits of, 44 coating materials, 52-53 definition of, 47 establishment ability and, 75-76 evolution of, 44-47 to facilitate planting, 53-54 future applications, 76-77 herbicide, 70-71 hydrophilic, 71-72 hydrophobic, 72-73 to increase oxygen supply, 73 with macronutrients, 62-63 with micronutrients, 63-64 nutrient types for early seedling nutrition, 61-62 efficacy of, 64-66 injury from, 67-68 with macronutrients, 62-63 performance of, factors affecting, 45-46 pH, fertilizer injury and, 68-69 problems with, 45 process, 48 adhesives for, 50-52 coating materials, 52-53 equipment for, 48-50 fluid-bed approach, 50 problems with, 49 protection of seed against damage by, 69 protective against animals, 59 against diseases, 57-58 against herbicides adsorbents, 60-61 antidotes, 59-60 against insects, 58-59 against pests, 58-59

INDEX Seed inoculation definition of, 48 processes, for rhizobia coatings, 55 rhizobia coating, 55-56 vesicular-arbuscular mycorrhizal fungi coating, 56 Seedlings, tillage methods and, 138 Seed pelleting, definition of, 47 Seeds, protection from fertilizer injury, 69 Seed soaking, definition of, 47 Seed structure, effect on tolerance to fertilizer injury, 68 Seed tablets definition of, 47 production of, 49-50 Seed treatments definition of, 47 processes of, 73-75. See also Seed coatings Septoria nodorum, 57 Sequential cropping, 3 Sewage sludge, 244 Simultaneous polyculture, 4 Soaking of seeds in growth regulator solutions, 74-75 in inorganic chemical solutions, 74 in macronutrients, 63 in micronutrients, 64 Sodium diethyldithiocarbamate, 295, 2% Sodium trithiocarbonate, urea hydrolysis and, 300, 301 Soil compaction. See Compaction Soil conservation, conservation tillage and, 143-145 Soil degradation arable land loss from, 86 prevention of conservation tillage and, 159 guidelines for water contents and mechanical stresses, 331-335 processes leading to, 86-87 Soil drainage, nitrogen response to tillage methods and, 155 Soil moisture conservation tillage and, 147-151 fertilizer seed coating injury and, 69 no-till system, in tropics vs. temperature zones, 110-112 tillage methods and, 138 Soils classification systems for, 178-180 mechanical loosening, 125-126

449

poorly drained, conservation tillage for, 132-136 problem, conservation tillage for, 126-127 technogenic. See Technogenic soils, enzymological research texture of, water retention characteristic and, 316 tillage groups, 178-179 in United Kingdom, 179-180 Soil structure changes during compaction, mechanisms of, 328-331 definition of, 311, 335 effects of compaction, on aggregate stability and, 327-328 improvement by conservation tillage, 140-143 irreversible changes in, water contents and mechanical stresses conducive to, 331-335 matrix, aggregate size distribution, 326-327 microscopic description of, 311-312 pore geometry air permeability and, 324-325 gas diffusion and, 323-324, 325 permeability/infiltration rate, 316-320 soil-water diffusivisity and, 320-321 sorptivity, 321-323 water retention characteristic and, 313-316 Soil temperature conservation tillage and, 147-151 tillage methods and, 138 in tropics vs. temperature zones, using no-till system, 110 Sole cropping. See uko Monoculture definition of, 3 stability of, 28-29 Somaclonal variation, 367 Sorghum light interception in, 10-11 protection from acetanilide herbicides, 60 yield cultural practices and, 176 effects of tillage methods and plowing depth, 124 fallowing and, 175 tied ridging and, 127, 131 and tillage systems in southern and eastern Africa, 120-121 Sowing of seed ballistics, improvement by seed coating, 54 precision of, seed coatings and, 53-54

450

INDEX

Soybeans cost of production, 159 effects of tillage methods, 138 yield mulching and, 96-97 with no-till vs. conventional tillage systems, 105 Space, efficiencies of multiple-species systems, 8-9 Spacing, effects on black gram yield, 133-134 Spatial arrangement, 4 Spraying. energy requirements for, 91 Starch graft polymers, in seed coatings, 72 StWptOmyceS, 56-57

Stress environmental, tolerance of rice somatic cell cultures and, 368-369 mechanical, corresponding to minimum pore water pressure, 333-335 Strip tillage, 90 Stylwnthes gukmensis, 62 Sub-bentonite seed coating, 54 Subsoiling, as conservation tillage, 122-126 Sulfatase activity, in recultivated soil after topsoil mining, 273 Sulfathiazole denitrification and, 294-295, 296 urea hydrolysis and, 300 Sulfur oxidation, rock phosphate-sulfur mixtures and, 220-222 in seed coating, 62 Sulfur mine spoils, technogenic soils from, 263-264

Sunflower yield with no-till vs. conventional tillage systems, 105 and tillage systems in southern and eastern Africa, 120-121 Superphosphate, in seed coating, injury from, 67-68 Sustainable agriculture conservation tillage and, 89-90 energy-related inputs and, 163 low-input, 88 T

Technogenic soils, enzymological research from bentonitic clay mine spoils, in USSR, 266-267

from coal mine spoils in Federal Republic of Germany, 250-252 in Hungary, 250 in Poland, 245-250 in United States, 236-245 in USSR, 230-236 from iron mine spoils in Romania, 257-259 in USSR, 257 from lead and zinc mine wastes in Romania, 262-263 in United Kingdom, 260-262 from lime and dolomite mine spoils, in USSR, 264 from manganese mine spoils, in USSR, 259-260

from overburdens after pipeline construction in United States, 271 in USSR, 269-270 from power plant wastes, in Poland. 252-253

from refractory clay mine spoils in USSR, 264-266 from refractory clay mine spoils, in USSR, 264-266 from retorted oil shale, in United States, 253-256

from sand opencast mine floor drift and spoils in Poland, 267-269 in USSR, 269 from soils remaining after topsoil mining, in New Zealand, 272, 273 from sulfur mine spoils, in USSR, 263-264 Temperate regions no-till systems for, 108-109 poorly drained soils in, effects of conservation tillage, 135-136 Thailand, effects of crop rotations and soil erosion. 173

Thiobacillus thiooxidans, 221 Thiobacillus thiopam, 221 Thiourea denitrification and, 295, 296 urea hydrolysis and, 300-302 Tied ridging, 127 Tillage management regions, 179 Tillage systems. See a h specific systems conventional, 91 criteria for conservation tillage system, 89-90

45 1

INDEX energy requirements for, 91 grain yields in tropics and, 100-101 method and intensity. effect on runoff and erosion, 145 sorghum yield and, 124 timing of, 91 transport of fertilizers and pesticides and, 161 types of, 90-91 yields in southern and eastern Africa and, 120-121 Time, efficiencies of multiple-species systems and, 8-9 Topsoil mining, recultivation of soils after, 272, 273

Bee species, for revegetation of soil plots, 247-249

Pichodem spp., inoculation of seeds with, 56

Urease activity, in recultivated soils after topsoil mining, 273 from coal mine spoils, 231, 232, 237, 241, 243, 245-248

from iron mine spoils, 258 from lead and zinc mine spoils, 261-263 from manganese mine spoils, 260 from refractory clay mine spoils, 265-266 from sand opencast mine floor drift and spoils, 267-269 V

Vesicular-arbuscular mycorrhizal fungi future use of, 222-223 in recultivation of soil from coal mine spoils, 241 seed inoculant coatings, 56

Pfolium subtermneum, nutrient seed coatings for, 61 Triple-cropping pattern, 3 'Ifopical regions no-till systems in, 99-108 tillage rating index, 181

W

Water conservation of, conservation tillage and, 143-145

U Union of Soviet Socialist Republics, research on recultivated soils from bentonitic clay mine spoils, 266-267 from coal mine spoils, 230-236 from iron mine spoils, 257 from lime and dolomite mine spoils, 264 from manganese mine spoils, 259-260 from overburdens after pipeline construction, 269-271 from refractory clay mine spoils, 264-266 from sand opencast mine floor drift and spoils, 269 from sulfur mine spoils, 263-264 United Kingdom, research on recultivated soils from lead and zinc mines, 260-262 United States, research on recultivated soils from coal mine spoils, 236-245 from overburdens after pipeline construction, 271 from retorted oil shale, 253-256 Urea hydrolysis, nitrification inhibitors and, 300-303

content in soil, effect during compaction, 329 enhancement of uptake by hydrophilic seed coatings, 71-72 pressure, soil pore, during compaction, 330-331

uptake by seeds, 74 use efficiency, in multiple-species systems, 13-14

Water retention characteristic (WRC), 313-316

Weeds factors influencing incidence of, 18 management, in multiple-cropping systems, 17-21 mechanical control of, soil macrostructure and, 141 role in total ecosystem, 21 Wheat yield effects of tillage methods and residue management on, 123 with no-till vs. conventional tillage systems, 105 in temperate region with no-till system, 109 tied ridging and, 131

452

INDEX

Wheel-track planting, 91 White clover (??flo/iurnrepens), 6 WRC (water retention characteristic), 313-316

X

Xanthates, urea hydrolysis and, 300, 301 Xylanase activity, in recultivated soil after topsoil mining, 273

Y

Yield. See also specific crops components, compensation among, 32-33

interspecific vs. intraspecific competition, 7-8

mulching and, 96-98 stability biological diversity and, 26 evaluation of, 25-26 tied-ridge system and, 127-130 variations in, 26-29

Zinc, in seed coatings, 63, 64 Zinc mine wastes, technogenic soils from, 260-263 Zonal tillage, 90, 133