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Tillage and agricultural sustainability
Soil& Tillage Research, 20 ( 1991 ) 133-146
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Elsevier Science Publishers B.V., Amsterdam
Introduction Tillage and agricultural sustainability R. Lal Department of Agronomy, The Ohio State University, Colttmbus, OH 43210, USA (Accepted 14 February 1991 )
ABSTraCT Lal, R.. 1991. Tillage and agricultural sustainability. Soil Tillage Res., 20:133-146. Agricultural sustainability implies an increasing trend in per capita productivity to meet the present needs without jeopardizing the future potential. Soil tillage, soil surface management to alle~"iate soilrelated constraints to crop production, is a basic and an important input with short- and long-term effects on sustainabilit.v. An important effect of soil tillage on sustainability is through its impact on the environment e.g. soil degradation, water quality, emission of greenhouse gases from soil-related processes, etc. The need to attain agricultural sustainability is particularly urgent in several tropical eco-regions and soils of low-carrying capacity in the tropics. Soil tillage influences agricultural sustainability through its effects on soil processes, soil properties, and crop growth. However, there is no one blueprint of a universally applicable sustainable tillage system. Appropriate tillage systems are soil- and crop-specific and their adaptation is governed by both biophysical and socio-economic factors. In addition to increasing crop yields, tillage methods must also facilitate soil and water conservation, improve root system development, maintain a favorable level of soil organic matter content, and reverse degradative trends in the soil's life-support processes. Important components or sub-systems of conservation-effective tillage systems include mulch farming, no-till or reduced tillage systems, use of cover crops and planted fallows, agroforestry, raised beds or ridge-tillage, and soil inversion or deep plowing. The ecological limits for the applicability of these components or sub-systems differ widely. The efforts of a multi-disciplinary team (comprising soil scientists, agricultural engineers, agronomists, economists and social scientists) are needed to develop site-specific tillage methods to achieve both short- and long-term goals of agricultural sustainability.
INTRODUCTION
Tillage, or soil surface management to prepare a desired seedbed, is a major input in agricultural production. It is a labor-intensive activity in low-resource agriculture of small land-holders, and a capital- and energy-intensive activity in large-scale mechanized farming. Judiciously used, tillage can be a powerful tool to alleviate some soil-related constraints to crop production, e.g. compaction, crusting, low infiltration, poor drainage, unfavorable soil 0167-1987/91/$03.50
© 1991 - - Elsevier Science Publishers B.V.
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moisture and temperature regimes, disposal of undesirable biomass and wastes, and pest management. Improperly used, tillage can lead to deterioration of soil structure, reduced infiltration, accelerated runoff and erosion, water pollution, and degradation of soil and environment. Sustainability implies using resources for meeting present needs without jeopardizing future potential, increasing per capita productivity without causing degradation of soil and water resources, enhancing profitability without reducing total production, and increasing yields while reducing excessive dependence on off-farm inputs. Through their effects on most of the indicators of sustainability, tillage and soil surface management play a crucial role in sustainable management of soil and water resources and in meeting production goals. The choice of tillage methods depends on several factors (Fig. 1). Soil properties play an important role in determining intensity, frequency, and type of tillage required. Soils prone to erosion by wind or water should be properly managed by a conservation tillage system• On the other hand, soils prone to compaction can be tilled by a plow-based method provided that time and energy requirements and economics permit the use of such a technology. Soil moisture regime and internal drainage conditions are also important factors. Soils with slow internal drainage usually do not respond favorably to reduced tillage methods• Terrain characteristics and landscape influence the choice of equipment and degree of mechanization• Steep and unstable terrain cannot be cultivated by tractorized equipment• Important among climatic parameters are soil temperature regime, rainfall (precipitation) characteristics, and the growing season duration. In regions with a short growing season, an appropriate tillage method is the one that would permit an early sowing• Suit-
Biophysical Affecting Tillage
I • • • • •
Physical properties Chemical characteristics Clay mineralogy Horizonation Rooting deplh
.,o 0e • Topography • Drainage • S l o p e characteristics
Factors Choice
of
Methods
IC ,el • • • • •
Rainfall Radiation Temperature
Evaporative demand Growing season
Fig. 1. Factors affecting choice of tillage methods.
Ic o01 • • • * • •
G r o v ~ h duration R o o t i n g characteristics W a t e r requirement N u t r i e n t demand C a n o p y cover Biomass production
Energy Needs • Total e n e r g y requirement
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TILLAGE AND AGRICULTURAL SUSTAINABILITY
able tillage methods are those that increase the root zone temperature in spring in northern latitudes, and decrease it at the onset of monsoons in tropical climates• Crop management, cropping systems, root system characteristics, nutrient and water requirements, and growth duration are also important factors to be considered. Furthermore, root crops require different methods of seedbed preparation than grain crops• Time available for tillage operations to be performed, the energy requirements, logistic support, and a range of complex socio-economic factors affect the choice of tillage methods. The objective of this report is to provide a conceptual framework regarding the role of soil tillage in agricultural sustainability. This paper is written to provide continuity to the themes of invited papers presented at the 12th Conference of ISTRO. Papers published in this special issue address tillage requirements for soils of different geographical areas (Asia, South America, North America, Australia, West Africa) and eco-regions. Special topics (e.g. soil compaction, modeling, cover crops) are also discussed. An attempt is also made to address socio-economic considerations in soil conservation and in adoption of tillage methods• SOIL TILLAGE AND SUSTAINABILITY
Soil tillage affects sustainability through its effects on soil processes, soil properties, and crop growth (Fig. 2). Tillage may enhance or curtail these
Tillage Effects o n Agricultural Sustainability
,L Soil Processes
Soil Properties
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•
Fig. 2. Tillage effects on agricultural sustainability.
Growth • • • • • •
Hoot development Water use efficiency Nutrient use efficiency Root:shoot ratio Harvest index Economic yield
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processes depending on antecedent conditions and the type of tillage tools used. Tillage affects agricultural sustainability through its long-term effects on the life-support processes of the soil, e.g. soil structure, soil organic matter content, rate and capacity for supplying water and nutrients to crops, soil productivity and economic profitability. An important effect of tillage on sustainability is through its impact on the environment, e.g. soil degradation, pollution of surface and ground waters. While conservation tillage can reduce soil erosion, it may increase risks of water pollution through increased use of pesticides, and surface application of fertilizer and other agricultural chemicals. In contrast, plow-based tillage methods may enhance risks of soil erosion, increase rates of mineralization of soil organic matter, and accentuate emission of radiatively active gases from soil-related processes. There is now a greater need to attain agricultural sustainability than ever before in fragile ecosystems and marginal lands of the tropics and sub-tropics. The most fragile eco-regions include humid tropics, semi-arid and arid tropics, and steeplands. Productivity and land carrying capacity of these eco-regions are low, but the demographic pressure and demands on limited reTABLE 1 Issues of agricultural sustainability in fragile lands of marginal eco-regions Region
Issues
Humid tropics
High subsistence agricultural usage of the land Reduction in fallow period Soils of low fertility and low yields due to resource-based and no-input agriculture Soil degradation due to fertility, depletion, accelerated erosion, structural deterioration and reduction in soil organic matter
Arid and semi-arid tropics
Risks of desertification due to degradation and aridization of soil and environment Perpetual drought stress High risks of crop failure Nutrient deficiency and soils of low fertility Soil compaction Low carrying capacity of land
Irrigated agriculture in dryland tropics
Water shortage Salt imbalance and salinization Poor quality irrigation water Deterioration of soil structure
Steeplands
Accelerated soil erosion, mass movement and land slides Shallow soils of low fertility Difficulties of mechanizing farm operations Energy shortage Low carrying capacity and low yields
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TILLAGEANDAGRICULTURALSUSTAINABILITY TABLE 2 Tillage and soil surface management effects on indices of agricultural sustainabflity Level of sustainability
Index of sustainability influenced by soil tillage
Plant/crop
Agronomic yield
Cropping system
Productivity
Farming system
Profit, income, resource and environment quality
Region/community
Supply, off-farm income, comparative advantage, environmental quality
Nation
Gross national product (GNP). resource sustainability, trade status
International
Per capita calorie intake
TABLE 3 Specific technologies for sustainable management of soil and resources for different ecological regions Humid
Sub-humid
Semi-arid
Soil management systems for improving water-use efficiency Mulch farming No-till Rough plowing No-till Mulch farming Tied ridges Manual clearing Contour ridges Mulch Drainage and water Agroforestry Micro-catchments management Drainage and Diggers Erosion control water management Contour bunds Water harvesting Grass hedges (Vetiver) Fallowing Early planting Salinity control Irrigation Water harvesting Soil~crop management systems for increasing nutrient-use efficiency Perennial crops Cover crops Manure/kralling Root crops Mulch farming Mulch farming Agroforestry Agroforestry Cover crops Mulch farming Mixed cropping Relay-mixed cropping Fertilizers Crop rotations N and P fertilizers In-situ burning In-situ burning Irrigation N and P fertilizers N and P fertilizers Leaching and Drainage and Drainage and water salinity control water management management
Arid Water harvesting Fallowing Early planting Grass hedges (Vetiver) Salinity Irrigation Water conservation
Manure/kralling Irrigation Water harvesting N and P fertilizers Salinity and alkalinity control
sources are high. As a consequence, resources are used to the limit, and risks of soil and environmental degradation are high. Issues of agricultural sustainability in these regions, especially those relevant to soil tillage, are listed in
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Table 1. High risks of soil degradation (due to erosion, compaction, deterioration of soil structure, and decline in soil organic matter content), low soil productivity, and low carrying capacity of land are the widespread problems of these regions. Risks of soil and environmental degradation are further accentuated by the resource-based and low-input agriculture widely practiced. Adoption of appropriate tillage systems and techniques of soil surface management can facilitate attainment of agricultural sustainability by reversing the degradative trends and restoring the productive capacity of soils. Soil tillage and conservation-effective technologies influence sustainability at different levels (Table 2 ). Soil tillage influences sustainability at crop level through its effect on agronomic yield, at cropping system level by influencing productivity, and at farming system level by enhancing profitability. Some specific tillage-based technologies for sustainable management of soil and water resources are listed in Table 3. These technologies are based on the principles of conserving soil and water resources, preventing or minimizing degradation of soil and environments, restoring degraded lands, and reducing dependence on off-farm purchased inputs while enhancing productivity and increasing profitability. The overall objective is to increase yields, generate income, and transform subsistence farming into commercial agriculture. USING SOIL TILLAGE FOR AGRICULTURAL SUSTAINABILITY
Soil tillage is a basic and an important component of agricultural production technology. In addition to preparing the desired seedbed, tillage is needed to manage crop residue, mix fertilizer in the soil, improve aeration, alleviate compaction, and optimize soil temperature and moisture regimes. The exact nature of tillage operations, however, is soil- and crop-specific (Lal, 1985b). Some specific examples of tillage and agricultural sustainability for small and medium sized farms in the tropics are listed in Tables 4 and 5, respectively. Specific components or sub-systems of tillage-based technologies in relation to sustainability for soils of tropics include the following. Mulch farming and no tillage
Crop residue mulch is an important ingredient in soil surface management. Residue mulch enhances sustainability through its effects on soil and water conservation, maintenance of soil organic matter at a favorable level, and enhancement of the activity of soil fauna. No-till or reduced tillage methods are practical means of using the principle of mulch farming. Within the ecological limits of its applicability, the no-till system can sustain high levels of
TILLAGEAND AGRICULTURALSUSTAINABILITY
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TABLE 4 Some examples of tillage-based technological packages for sustainable management of soil and water resources on small-scale farms (less than 5 ha) in the tropics Structurally active soils
Structurally inert soils
(a)
Grain crop-cover crop rotation Conservation tillage-mulch farming Strip cropping Chemical fertilizers (supplementary) Water management Irrigation
Conservation tillage and water management options will differ as follows: Contour ridges Tied ridges Periodic sub-soiling or chiseling Supplementary irrigation
(b)
Grain crop-alley cropping systems Conservation tillage Chemical fertilizers (supplementary) Water management Irrigation
(c)
Ley/mixed farming Conservation tillage Grain crop-pasture rotation Growing woody perennials to supplement food Reservoirs for runoff storage Organic manures Chemical fertilizers (supplementary) Drainage and irrigation Water harvesting
(d)
Agro-forestry systems Same as (c) but pasture replaced by shrubs and woody perennials
(e)
Smallholder plantations Cover crops (Kudzu, Centro, etc. ) Tangya system Chemical fertilizers Supplemental irrigation
crop yields without resorting to lengthy fallowing for soil restoration. The data in Fig. 3 show a higher maize grain yield from no-till than from the plowed method of seedbed preparation for an Alfisol in western Nigeria. These data are based on an experiment involving continuous cultivation of maize with two crops per year for 17 consecutive years. Yield declined in both tillage systems. However, the rate of decline was 136 kg h a - l year- t in no-till compared with 170 kg h a - l year-i in the plowed treatment. Apparently, grain yields higher than those observed could have been obtained by adopting a rotation (cereal-legume) and periodic fallowing (1 year every 4-5 years)
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TABLE 5 Some examples of tillage-based technological packages for sustainable management of soil and water resources on medium sized farms (5-25 ha) in the tropics Structurally active soils
Structurally inert soils
(a )
Grain crop-cover crop rotation Conservation tillage with herbicides and periodic loosening to alleviate compaction Chemical fertilizers Planting trees or woody perennials at 1 m intervals Water management
(a)
Contour ridges Terraces and waterways Engineering structure Water management Supplemental irrigation
(b)
Grain crop-pasture rotation Water harvesting and reservoirs Conservation tillage with herbicides Tree hedges at 1 m intervals Chemical fertilizers Drainage and irrigation
(b)
Water reservoirs and engineering structures Supplementary irrigation Tied-ridge or basin tillage Water management
(c)
Plantation and cover crops Erosion control Fertilizer management Drainage and irrigation
(c)
Erosion control access on roads Fertilizer management Water harvesting Supplemental irrigation
with a restorative cover crop (Pueraria or Centrosema) (Wilson and Lal, 1986). Similar results are shown from an 8-year study conducted on another soil in western Nigeria (Fig. 4). Once again, maize grain yield was maintained at a higher level with a mulch-based no-till system than with a plowbased method of seedbed preparation. Nonetheless, yield declined with time in both cases. The rate of yield decline, however, was 0.59 t ha -1 year -1 in the no-till treatment compared with 0.71 t ha-~ year-1 in the plowed treatment. Furthermore, these tillage systems are applicable only if an adequate amount of crop residue mulch is available, if the control of weeds by herbicides is economically viable and soils are structurally active. The yield decline observed in Figs. 3 and 4 may be due to one or more of several factors, e.g. a decline in soil productivity as a result of continuous cropping without restorative fallowing. The effect of fertility decline is accentuated by monocropping.
Agroforestry Growing food crop annuals in combination with woody perennials is another innovation that can be used in association with conservation tillage to reduce risks of soil erosion, minimize off-farm inputs, and maintain a favor-
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able level of crop yields (Gholz, 1987; Steppler and Nair, 1987; Lal, 1991 ). Similar to mulch farming, however, agroforestry techniques are also soil- and crop-specific. While agroforestry may enhance or maintain acceptable levels of yields for some nitrophilic crops, it may suppress yields of leguminous crops when grown in association with leguminous shrubs (Lal, 1987), or of other crops grown in combination with highly competitive shrubs on soils of low inherent fertility (Szott, 1987). The data in Fig. 5 show that yield of maize was somewhat sustained when grown in association with Leucaena leucocephala but that of cowpea was not. The rate of yield decline was 340 kg hayear- t for maize (with a base level of 6.8 t ha- 1) compared with 96 kg ha- L year- 1 for cowpea (with a base level of 933 kg ha- ~). There is a lot of scope to develop site- and soil-specific technologies to exploit the benefits of agroforestry, especially in conjunction with locally-adapted and native tree species.
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Soil in version Easily compactable soils o f the arid and semi-arid regions require mechanical loosening to alleviate soil compaction, increase infiltration capacity, conserve water in the root zone, increase deep root system development, and decrease risks of soil erosion by wind and water. Nicou ( 1977 ) and Charreau ( 1972, 1977 ) have demonstrated from their studies in the West African Sahel that soil inversion and deep plowing may increase plant-available water reserves and increase crop yields. For soils of arid and semi-arid regions, plowing has been shown to increase porosity and root growth, and improve crop yields in structurally inactive soils (MaCartney et al., 197 l; Nicou, 1974a,b,
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Ridge-tillage Raised beds and ridge-tillage are a cultural tradition in low-input subsistence farming throughout the tropics and sub-tropics (Lal, 1990). The usefulness of ridge-tillage has been experimentally demonstrated in eastern Africa (Prentice, 1946; Pereira et al., 1967; Dagg and MaCartney, 1968; MaCartney et al., 1971; Honisch, 1974), and in western Africa (International Institute of Tropical Agriculture (IITA), 1981; Hulugalle, 1990). Tied ridges - contour ridges with cross ties to develop a series of small basins, are highly effective in soil and water conservation (Lal, 1987). In contrast to water conservation in arid and semi-arid regions, raised beds are used for draining surplus water out of the root zone in excessively wet soils (Forsythe et al., 1977; Fausey, 1984). Ridge-tillage, therefore, is aver-
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satile tool to alleviate soil-specific constraints to crop production, especially in semi-arid and arid regions. SOIL COMPACTION AND MECHANIZED AGRICULTURE
Soil compaction caused by vehicular traffic is a serious problem in mechanized farming. The problem is already severe in tropical regions whenever mechanized farming is introduced (Lal, 1985a). Soil compaction can have serious economic and environmental impacts. Economically, severe compaction can reduce crop yields and profitability. The magnitude of yield reduction, however, depends on soil types, climate, and crops grown (Hakansson, 1985; Voorhees, 1986; Raghavan et al., 1990). Accurate estimates of economic losses due to soil compaction are difficult to make owing to the confounding effects of several interacting variables. The effect of vehicular traffic on soil properties is also difficult to predict. There is a scarcity of data from long-term experiments quantifying the effects of vehicular traffic on crop yields and soil properties for different eco-regions. In addition to empirical studies, there is a need to develop reliable models to predict the effects of compaction on soil properties, and on crop growth and yields. Developing techniques to assess the susceptibility of soils to compaction using soil and climatic data, and other routinely measured variables, is a priority (Voorhees, 1987). ECONOMICS OF TILLAGE PRACTICES AND AGRICULTURAL SUSTAINABILITY
The technical feasibility of tillage systems is by itself not enough for wide adoption. Economic profitability and social acceptability are also important. Increasing per capita productivity through adoption of appropriate tillage systems is a major goal towards achieving agricultural sustainability. This implies development, validation and adoption/adaptation of appropriate tillage systems for different sized farms, and for a wide range of soils, crops and climatic environments. An important socio-economic consideration is the energy needed for tillage systems. Replacing the hoe with improved tools of soil management remains a challenge for most of the humid tropics. Why has animal traction not been adopted in several parts of Africa, even though tractorized tillage implements are out of reach for most resource-poor farmers? The answer to this and other important questions lies in socio-economic evaluation of tillage practices at the farm level. An approach that combines the principles of watershed management with that of technology transfer may be a useful tool to develop sustainable land-use systems. A multi-disciplinary team comprised of biophysical and social scientists, is needed to assess agroeconomic and socio-political acceptability of innovative tillage systems.
TILLAGE AND AGRICULTURAL SUSTAINABILITY
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CONCLUSIONS Soil tillage plays an important role in agricultural sustainability. It influ: ences crop yields through its effects on soil properties that regulate nutrient and water supply, competition with pests, and crop-restrictive biophysical and socio-economic constraints. Appropriate tillage methods differ among soils, crops, and climatic regions, and the choice depends on a range of interacting factors. There is an urgent need to attain agricultural sustainability in fragile eco-regions and marginal lands of the tropics. These regions are characterized by low-input subsistence farming that aggravates the problem of soil and environmental degradation, and perpetuates low yields and poverty. There is no simple tillage system that can be widely used for the diverse soil and climatic conditions. However, the basic sub-systems or components of useful tillage options include mulch farming, no-till, ago-forestry, soil inversion, and ridgetillage or raised bed methods. It is, however, the specific socio-economic factors that govern the adoption of these practices into soil management technologies.
REFERENCES CATIE (Centro Agronomico Tropical de Investigacion y Ensenanza), 1987. Nitrogen Fixing Tree Association.Spec. Publ. 87-01, Turrialba, Costa Rica. Charreau, C., 1972. Probl~mespos~s par l'utilisation agricole des sols tropicaux par des cultures annuelles. Agron. Trop., 27: 901-929. Charreau, C., 1977. Controversial points in dryland farming practices in semi-arid West Africa. In: G.H. Cannell (Editor), Symposiumon Rainfed AgriculturalSemi-arid Regions.University of California, Riverside, pp. 313-360. Chopart, J.L., 1981. Le travail du sol au Srnrgal. Institut Srnrgalais de Recherches Agricoles (ISRA), Bambey,Senegal, 36 pp. Chopart, J.L., Kalms,J.M., Marquette, J. and Nicou, R., 1981. Comparison de diffrrentes techniques de travail du sol en trois 6cologies de I'Afrique de l'Ouest. Institut de Recherches Agronomique Tropicales et des Cultures Vivriers, Montpellier, France, 65 pp. Dagg, M. and McCartney,J.C., 1968. The agronomicefficiencyof N.I.A.E. mechanizedtie ridge system of cultivation. Exp. Agric., 4: 279-294. Fausey, N.R., 1984. Drainage-tillage interactions in a Clermont soil. Trans. ASAE, 27: 403406. Forsythe, W.M., Victor, A. and Gomez, M., 1977. Flooding tolerance and surface drainage requirements ofPhaseolus vulgaris L. In: R. Lal and D.J. Greenland (Editors), Soil Physical Properties and Crop Production in the Tropics. John Wiley,Chichester, pp. 205-214. Gholz, H.L. (Editor), 1987. Agroforestry: Realities, Possibilities and Potentials. Martinus Nijhoff, Dordrecht. Netherlands. Hakansson, 1., 1985. Swedish experiments on sub-soil compaction by vehicles with high axle load. Soil Use Manage., 1:113-116. Honisch, O., 1974. Water conservation in three grain crops in the Zambezi Valley. Exp. Agric., 10: 1-8.
Hulugalle, N.R., 1990. Alleviation of soil constraints to crop growth in the upland alfisols and
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associated soil groups of the West African Sudan savanna by tied ridges. Soil Tillage Res., 18: 231-247. International Institute of Tropical Agriculture, 1981. Role of tied ridges in maize production in Upper Volta. Research Highlight, 1981. IITA, Ibadan, Nigeria, pp. 7-10. Lal, R., 1985a. Mechanized tillage systems effects on properties of a tropical Alfisol in watersheds cropped to maize. Soil Tillage Res., 6:149-162. Lal, R., 1985b. A soil suitability guide for different tillage systems in the tropics. Soil Tillage Res., 5: 179-196. Lal, R., 1987. Managing soils of sub-Saharan Africa. Science, 236: 1069-1076. Lal, R., 1989. Agroforestry systems and soil surface management of a tropical Alfisol. I. Soil Moisture and Crop Yields. Agrofor. Syst., 8: 7-29. Lal, R., 1990. Ridge-tillage. Soil Tillage Res., 18:107-111. Lal, R., 1991. Myths and scientific realities ofagroforestry as a strategy for sustainable management for soils in the tropics. Adv. Soil Sci., 15: 91-137. MaCartney, J.C., Nothwood, P.J., Dagg, M. and Dawson, R., 1971. The effect of different cultivation techniques on soil moisture conservation and the establishment and yield of maize at Kongwa, central Tanzania. Trop. Agric., 48:9-17. Nicou, R., 1974a. Contribution on the study and improvement of the porosity of sand and sandy-clay soils in the dry tropical zone. Agricultural consequences. Agron. Trop., 29:110127. Nicou, R., 1974b. The problem of caking with drying out of sandy and sandy-clay soils in the arid tropical zone. Agron. Trop., 30: 325-343. Nicou, R., 1977. Le travail du sol dans les terres exond6es du S6n6gal: Motivations, contraintes. Centre Nacional de Recherche Agronomique (CNRA), Bambey, Senegal, mimeo., 50 pp. Nicou, R., 1979. Tillage in the West African tropical zone. Proceedings of the 8th Conference of the International Soil Tillage Research Organization, Hohenheim, pp. 429-434. Pereira, H.C., Hosegood, P.H. and Dagg, M., 1967. Effects of tied ridges, terraces and grass leys on a lateritic soil in Kenya. Exp. Agric., 3: 89-98. Prentice, A.N., 1946. Tie-ridging, with special reference to semi-arid areas. E. Afr. Agric. J., 12: 101-108. Raghavan, G.S.V., Alvo, P. and McKyes, E., 1990. Soil compaction in agriculture: A view toward managing the problem. Adv. Soil Sci., 11: 1-36. Steppler, H.A. and Nair, P.K.R., 1987. Agroforestry: a decade of development. International Council for Research on Agroforestry (ICRAF), Nairobi, Kenya. Szott, L.T., 1987. Improving the productivity of shifting cultivation in the Amazon Basin of Peru through the use of leguminous vegetation. Ph.D. Dissertation, North Carolina State University, Raleigh. Wilson, G.F. and Lal, R., 1986. New concepts for post-clearing land management in the tropics. In: R. Lal, P.A. Sanchez and R.W. Cummings, Jr. (Editors), Land Clearing and Development in the Tropics. A.A. Balkema, Rotterdam, pp. 371-382. Voorhees, W.D., 1987. Assessment of soil susceptibility to compaction using soil and climatic databases. Soil Tillage Res., 10: 29-38. Voorhees, W.D., 1986. Deep compaction: a different phenomenon. Crops Soils Mag., 1986:10ll.
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Elsevier Science Publishers B.V., Amsterdam
Introduction Tillage and agricultural sustainability R. Lal Department of Agronomy, The Ohio State University, Colttmbus, OH 43210, USA (Accepted 14 February 1991 )
ABSTraCT Lal, R.. 1991. Tillage and agricultural sustainability. Soil Tillage Res., 20:133-146. Agricultural sustainability implies an increasing trend in per capita productivity to meet the present needs without jeopardizing the future potential. Soil tillage, soil surface management to alle~"iate soilrelated constraints to crop production, is a basic and an important input with short- and long-term effects on sustainabilit.v. An important effect of soil tillage on sustainability is through its impact on the environment e.g. soil degradation, water quality, emission of greenhouse gases from soil-related processes, etc. The need to attain agricultural sustainability is particularly urgent in several tropical eco-regions and soils of low-carrying capacity in the tropics. Soil tillage influences agricultural sustainability through its effects on soil processes, soil properties, and crop growth. However, there is no one blueprint of a universally applicable sustainable tillage system. Appropriate tillage systems are soil- and crop-specific and their adaptation is governed by both biophysical and socio-economic factors. In addition to increasing crop yields, tillage methods must also facilitate soil and water conservation, improve root system development, maintain a favorable level of soil organic matter content, and reverse degradative trends in the soil's life-support processes. Important components or sub-systems of conservation-effective tillage systems include mulch farming, no-till or reduced tillage systems, use of cover crops and planted fallows, agroforestry, raised beds or ridge-tillage, and soil inversion or deep plowing. The ecological limits for the applicability of these components or sub-systems differ widely. The efforts of a multi-disciplinary team (comprising soil scientists, agricultural engineers, agronomists, economists and social scientists) are needed to develop site-specific tillage methods to achieve both short- and long-term goals of agricultural sustainability.
INTRODUCTION
Tillage, or soil surface management to prepare a desired seedbed, is a major input in agricultural production. It is a labor-intensive activity in low-resource agriculture of small land-holders, and a capital- and energy-intensive activity in large-scale mechanized farming. Judiciously used, tillage can be a powerful tool to alleviate some soil-related constraints to crop production, e.g. compaction, crusting, low infiltration, poor drainage, unfavorable soil 0167-1987/91/$03.50
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moisture and temperature regimes, disposal of undesirable biomass and wastes, and pest management. Improperly used, tillage can lead to deterioration of soil structure, reduced infiltration, accelerated runoff and erosion, water pollution, and degradation of soil and environment. Sustainability implies using resources for meeting present needs without jeopardizing future potential, increasing per capita productivity without causing degradation of soil and water resources, enhancing profitability without reducing total production, and increasing yields while reducing excessive dependence on off-farm inputs. Through their effects on most of the indicators of sustainability, tillage and soil surface management play a crucial role in sustainable management of soil and water resources and in meeting production goals. The choice of tillage methods depends on several factors (Fig. 1). Soil properties play an important role in determining intensity, frequency, and type of tillage required. Soils prone to erosion by wind or water should be properly managed by a conservation tillage system• On the other hand, soils prone to compaction can be tilled by a plow-based method provided that time and energy requirements and economics permit the use of such a technology. Soil moisture regime and internal drainage conditions are also important factors. Soils with slow internal drainage usually do not respond favorably to reduced tillage methods• Terrain characteristics and landscape influence the choice of equipment and degree of mechanization• Steep and unstable terrain cannot be cultivated by tractorized equipment• Important among climatic parameters are soil temperature regime, rainfall (precipitation) characteristics, and the growing season duration. In regions with a short growing season, an appropriate tillage method is the one that would permit an early sowing• Suit-
Biophysical Affecting Tillage
I • • • • •
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.,o 0e • Topography • Drainage • S l o p e characteristics
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Methods
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Fig. 1. Factors affecting choice of tillage methods.
Ic o01 • • • * • •
G r o v ~ h duration R o o t i n g characteristics W a t e r requirement N u t r i e n t demand C a n o p y cover Biomass production
Energy Needs • Total e n e r g y requirement
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able tillage methods are those that increase the root zone temperature in spring in northern latitudes, and decrease it at the onset of monsoons in tropical climates• Crop management, cropping systems, root system characteristics, nutrient and water requirements, and growth duration are also important factors to be considered. Furthermore, root crops require different methods of seedbed preparation than grain crops• Time available for tillage operations to be performed, the energy requirements, logistic support, and a range of complex socio-economic factors affect the choice of tillage methods. The objective of this report is to provide a conceptual framework regarding the role of soil tillage in agricultural sustainability. This paper is written to provide continuity to the themes of invited papers presented at the 12th Conference of ISTRO. Papers published in this special issue address tillage requirements for soils of different geographical areas (Asia, South America, North America, Australia, West Africa) and eco-regions. Special topics (e.g. soil compaction, modeling, cover crops) are also discussed. An attempt is also made to address socio-economic considerations in soil conservation and in adoption of tillage methods• SOIL TILLAGE AND SUSTAINABILITY
Soil tillage affects sustainability through its effects on soil processes, soil properties, and crop growth (Fig. 2). Tillage may enhance or curtail these
Tillage Effects o n Agricultural Sustainability
,L Soil Processes
Soil Properties
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•
Fig. 2. Tillage effects on agricultural sustainability.
Growth • • • • • •
Hoot development Water use efficiency Nutrient use efficiency Root:shoot ratio Harvest index Economic yield
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processes depending on antecedent conditions and the type of tillage tools used. Tillage affects agricultural sustainability through its long-term effects on the life-support processes of the soil, e.g. soil structure, soil organic matter content, rate and capacity for supplying water and nutrients to crops, soil productivity and economic profitability. An important effect of tillage on sustainability is through its impact on the environment, e.g. soil degradation, pollution of surface and ground waters. While conservation tillage can reduce soil erosion, it may increase risks of water pollution through increased use of pesticides, and surface application of fertilizer and other agricultural chemicals. In contrast, plow-based tillage methods may enhance risks of soil erosion, increase rates of mineralization of soil organic matter, and accentuate emission of radiatively active gases from soil-related processes. There is now a greater need to attain agricultural sustainability than ever before in fragile ecosystems and marginal lands of the tropics and sub-tropics. The most fragile eco-regions include humid tropics, semi-arid and arid tropics, and steeplands. Productivity and land carrying capacity of these eco-regions are low, but the demographic pressure and demands on limited reTABLE 1 Issues of agricultural sustainability in fragile lands of marginal eco-regions Region
Issues
Humid tropics
High subsistence agricultural usage of the land Reduction in fallow period Soils of low fertility and low yields due to resource-based and no-input agriculture Soil degradation due to fertility, depletion, accelerated erosion, structural deterioration and reduction in soil organic matter
Arid and semi-arid tropics
Risks of desertification due to degradation and aridization of soil and environment Perpetual drought stress High risks of crop failure Nutrient deficiency and soils of low fertility Soil compaction Low carrying capacity of land
Irrigated agriculture in dryland tropics
Water shortage Salt imbalance and salinization Poor quality irrigation water Deterioration of soil structure
Steeplands
Accelerated soil erosion, mass movement and land slides Shallow soils of low fertility Difficulties of mechanizing farm operations Energy shortage Low carrying capacity and low yields
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TILLAGEANDAGRICULTURALSUSTAINABILITY TABLE 2 Tillage and soil surface management effects on indices of agricultural sustainabflity Level of sustainability
Index of sustainability influenced by soil tillage
Plant/crop
Agronomic yield
Cropping system
Productivity
Farming system
Profit, income, resource and environment quality
Region/community
Supply, off-farm income, comparative advantage, environmental quality
Nation
Gross national product (GNP). resource sustainability, trade status
International
Per capita calorie intake
TABLE 3 Specific technologies for sustainable management of soil and resources for different ecological regions Humid
Sub-humid
Semi-arid
Soil management systems for improving water-use efficiency Mulch farming No-till Rough plowing No-till Mulch farming Tied ridges Manual clearing Contour ridges Mulch Drainage and water Agroforestry Micro-catchments management Drainage and Diggers Erosion control water management Contour bunds Water harvesting Grass hedges (Vetiver) Fallowing Early planting Salinity control Irrigation Water harvesting Soil~crop management systems for increasing nutrient-use efficiency Perennial crops Cover crops Manure/kralling Root crops Mulch farming Mulch farming Agroforestry Agroforestry Cover crops Mulch farming Mixed cropping Relay-mixed cropping Fertilizers Crop rotations N and P fertilizers In-situ burning In-situ burning Irrigation N and P fertilizers N and P fertilizers Leaching and Drainage and Drainage and water salinity control water management management
Arid Water harvesting Fallowing Early planting Grass hedges (Vetiver) Salinity Irrigation Water conservation
Manure/kralling Irrigation Water harvesting N and P fertilizers Salinity and alkalinity control
sources are high. As a consequence, resources are used to the limit, and risks of soil and environmental degradation are high. Issues of agricultural sustainability in these regions, especially those relevant to soil tillage, are listed in
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Table 1. High risks of soil degradation (due to erosion, compaction, deterioration of soil structure, and decline in soil organic matter content), low soil productivity, and low carrying capacity of land are the widespread problems of these regions. Risks of soil and environmental degradation are further accentuated by the resource-based and low-input agriculture widely practiced. Adoption of appropriate tillage systems and techniques of soil surface management can facilitate attainment of agricultural sustainability by reversing the degradative trends and restoring the productive capacity of soils. Soil tillage and conservation-effective technologies influence sustainability at different levels (Table 2 ). Soil tillage influences sustainability at crop level through its effect on agronomic yield, at cropping system level by influencing productivity, and at farming system level by enhancing profitability. Some specific tillage-based technologies for sustainable management of soil and water resources are listed in Table 3. These technologies are based on the principles of conserving soil and water resources, preventing or minimizing degradation of soil and environments, restoring degraded lands, and reducing dependence on off-farm purchased inputs while enhancing productivity and increasing profitability. The overall objective is to increase yields, generate income, and transform subsistence farming into commercial agriculture. USING SOIL TILLAGE FOR AGRICULTURAL SUSTAINABILITY
Soil tillage is a basic and an important component of agricultural production technology. In addition to preparing the desired seedbed, tillage is needed to manage crop residue, mix fertilizer in the soil, improve aeration, alleviate compaction, and optimize soil temperature and moisture regimes. The exact nature of tillage operations, however, is soil- and crop-specific (Lal, 1985b). Some specific examples of tillage and agricultural sustainability for small and medium sized farms in the tropics are listed in Tables 4 and 5, respectively. Specific components or sub-systems of tillage-based technologies in relation to sustainability for soils of tropics include the following. Mulch farming and no tillage
Crop residue mulch is an important ingredient in soil surface management. Residue mulch enhances sustainability through its effects on soil and water conservation, maintenance of soil organic matter at a favorable level, and enhancement of the activity of soil fauna. No-till or reduced tillage methods are practical means of using the principle of mulch farming. Within the ecological limits of its applicability, the no-till system can sustain high levels of
TILLAGEAND AGRICULTURALSUSTAINABILITY
! 39
TABLE 4 Some examples of tillage-based technological packages for sustainable management of soil and water resources on small-scale farms (less than 5 ha) in the tropics Structurally active soils
Structurally inert soils
(a)
Grain crop-cover crop rotation Conservation tillage-mulch farming Strip cropping Chemical fertilizers (supplementary) Water management Irrigation
Conservation tillage and water management options will differ as follows: Contour ridges Tied ridges Periodic sub-soiling or chiseling Supplementary irrigation
(b)
Grain crop-alley cropping systems Conservation tillage Chemical fertilizers (supplementary) Water management Irrigation
(c)
Ley/mixed farming Conservation tillage Grain crop-pasture rotation Growing woody perennials to supplement food Reservoirs for runoff storage Organic manures Chemical fertilizers (supplementary) Drainage and irrigation Water harvesting
(d)
Agro-forestry systems Same as (c) but pasture replaced by shrubs and woody perennials
(e)
Smallholder plantations Cover crops (Kudzu, Centro, etc. ) Tangya system Chemical fertilizers Supplemental irrigation
crop yields without resorting to lengthy fallowing for soil restoration. The data in Fig. 3 show a higher maize grain yield from no-till than from the plowed method of seedbed preparation for an Alfisol in western Nigeria. These data are based on an experiment involving continuous cultivation of maize with two crops per year for 17 consecutive years. Yield declined in both tillage systems. However, the rate of decline was 136 kg h a - l year- t in no-till compared with 170 kg h a - l year-i in the plowed treatment. Apparently, grain yields higher than those observed could have been obtained by adopting a rotation (cereal-legume) and periodic fallowing (1 year every 4-5 years)
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TABLE 5 Some examples of tillage-based technological packages for sustainable management of soil and water resources on medium sized farms (5-25 ha) in the tropics Structurally active soils
Structurally inert soils
(a )
Grain crop-cover crop rotation Conservation tillage with herbicides and periodic loosening to alleviate compaction Chemical fertilizers Planting trees or woody perennials at 1 m intervals Water management
(a)
Contour ridges Terraces and waterways Engineering structure Water management Supplemental irrigation
(b)
Grain crop-pasture rotation Water harvesting and reservoirs Conservation tillage with herbicides Tree hedges at 1 m intervals Chemical fertilizers Drainage and irrigation
(b)
Water reservoirs and engineering structures Supplementary irrigation Tied-ridge or basin tillage Water management
(c)
Plantation and cover crops Erosion control Fertilizer management Drainage and irrigation
(c)
Erosion control access on roads Fertilizer management Water harvesting Supplemental irrigation
with a restorative cover crop (Pueraria or Centrosema) (Wilson and Lal, 1986). Similar results are shown from an 8-year study conducted on another soil in western Nigeria (Fig. 4). Once again, maize grain yield was maintained at a higher level with a mulch-based no-till system than with a plowbased method of seedbed preparation. Nonetheless, yield declined with time in both cases. The rate of yield decline, however, was 0.59 t ha -1 year -1 in the no-till treatment compared with 0.71 t ha-~ year-1 in the plowed treatment. Furthermore, these tillage systems are applicable only if an adequate amount of crop residue mulch is available, if the control of weeds by herbicides is economically viable and soils are structurally active. The yield decline observed in Figs. 3 and 4 may be due to one or more of several factors, e.g. a decline in soil productivity as a result of continuous cropping without restorative fallowing. The effect of fertility decline is accentuated by monocropping.
Agroforestry Growing food crop annuals in combination with woody perennials is another innovation that can be used in association with conservation tillage to reduce risks of soil erosion, minimize off-farm inputs, and maintain a favor-
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able level of crop yields (Gholz, 1987; Steppler and Nair, 1987; Lal, 1991 ). Similar to mulch farming, however, agroforestry techniques are also soil- and crop-specific. While agroforestry may enhance or maintain acceptable levels of yields for some nitrophilic crops, it may suppress yields of leguminous crops when grown in association with leguminous shrubs (Lal, 1987), or of other crops grown in combination with highly competitive shrubs on soils of low inherent fertility (Szott, 1987). The data in Fig. 5 show that yield of maize was somewhat sustained when grown in association with Leucaena leucocephala but that of cowpea was not. The rate of yield decline was 340 kg hayear- t for maize (with a base level of 6.8 t ha- 1) compared with 96 kg ha- L year- 1 for cowpea (with a base level of 933 kg ha- ~). There is a lot of scope to develop site- and soil-specific technologies to exploit the benefits of agroforestry, especially in conjunction with locally-adapted and native tree species.
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Soil in version Easily compactable soils o f the arid and semi-arid regions require mechanical loosening to alleviate soil compaction, increase infiltration capacity, conserve water in the root zone, increase deep root system development, and decrease risks of soil erosion by wind and water. Nicou ( 1977 ) and Charreau ( 1972, 1977 ) have demonstrated from their studies in the West African Sahel that soil inversion and deep plowing may increase plant-available water reserves and increase crop yields. For soils of arid and semi-arid regions, plowing has been shown to increase porosity and root growth, and improve crop yields in structurally inactive soils (MaCartney et al., 197 l; Nicou, 1974a,b,
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Ridge-tillage Raised beds and ridge-tillage are a cultural tradition in low-input subsistence farming throughout the tropics and sub-tropics (Lal, 1990). The usefulness of ridge-tillage has been experimentally demonstrated in eastern Africa (Prentice, 1946; Pereira et al., 1967; Dagg and MaCartney, 1968; MaCartney et al., 1971; Honisch, 1974), and in western Africa (International Institute of Tropical Agriculture (IITA), 1981; Hulugalle, 1990). Tied ridges - contour ridges with cross ties to develop a series of small basins, are highly effective in soil and water conservation (Lal, 1987). In contrast to water conservation in arid and semi-arid regions, raised beds are used for draining surplus water out of the root zone in excessively wet soils (Forsythe et al., 1977; Fausey, 1984). Ridge-tillage, therefore, is aver-
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satile tool to alleviate soil-specific constraints to crop production, especially in semi-arid and arid regions. SOIL COMPACTION AND MECHANIZED AGRICULTURE
Soil compaction caused by vehicular traffic is a serious problem in mechanized farming. The problem is already severe in tropical regions whenever mechanized farming is introduced (Lal, 1985a). Soil compaction can have serious economic and environmental impacts. Economically, severe compaction can reduce crop yields and profitability. The magnitude of yield reduction, however, depends on soil types, climate, and crops grown (Hakansson, 1985; Voorhees, 1986; Raghavan et al., 1990). Accurate estimates of economic losses due to soil compaction are difficult to make owing to the confounding effects of several interacting variables. The effect of vehicular traffic on soil properties is also difficult to predict. There is a scarcity of data from long-term experiments quantifying the effects of vehicular traffic on crop yields and soil properties for different eco-regions. In addition to empirical studies, there is a need to develop reliable models to predict the effects of compaction on soil properties, and on crop growth and yields. Developing techniques to assess the susceptibility of soils to compaction using soil and climatic data, and other routinely measured variables, is a priority (Voorhees, 1987). ECONOMICS OF TILLAGE PRACTICES AND AGRICULTURAL SUSTAINABILITY
The technical feasibility of tillage systems is by itself not enough for wide adoption. Economic profitability and social acceptability are also important. Increasing per capita productivity through adoption of appropriate tillage systems is a major goal towards achieving agricultural sustainability. This implies development, validation and adoption/adaptation of appropriate tillage systems for different sized farms, and for a wide range of soils, crops and climatic environments. An important socio-economic consideration is the energy needed for tillage systems. Replacing the hoe with improved tools of soil management remains a challenge for most of the humid tropics. Why has animal traction not been adopted in several parts of Africa, even though tractorized tillage implements are out of reach for most resource-poor farmers? The answer to this and other important questions lies in socio-economic evaluation of tillage practices at the farm level. An approach that combines the principles of watershed management with that of technology transfer may be a useful tool to develop sustainable land-use systems. A multi-disciplinary team comprised of biophysical and social scientists, is needed to assess agroeconomic and socio-political acceptability of innovative tillage systems.
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CONCLUSIONS Soil tillage plays an important role in agricultural sustainability. It influ: ences crop yields through its effects on soil properties that regulate nutrient and water supply, competition with pests, and crop-restrictive biophysical and socio-economic constraints. Appropriate tillage methods differ among soils, crops, and climatic regions, and the choice depends on a range of interacting factors. There is an urgent need to attain agricultural sustainability in fragile eco-regions and marginal lands of the tropics. These regions are characterized by low-input subsistence farming that aggravates the problem of soil and environmental degradation, and perpetuates low yields and poverty. There is no simple tillage system that can be widely used for the diverse soil and climatic conditions. However, the basic sub-systems or components of useful tillage options include mulch farming, no-till, ago-forestry, soil inversion, and ridgetillage or raised bed methods. It is, however, the specific socio-economic factors that govern the adoption of these practices into soil management technologies.
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Hulugalle, N.R., 1990. Alleviation of soil constraints to crop growth in the upland alfisols and
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associated soil groups of the West African Sudan savanna by tied ridges. Soil Tillage Res., 18: 231-247. International Institute of Tropical Agriculture, 1981. Role of tied ridges in maize production in Upper Volta. Research Highlight, 1981. IITA, Ibadan, Nigeria, pp. 7-10. Lal, R., 1985a. Mechanized tillage systems effects on properties of a tropical Alfisol in watersheds cropped to maize. Soil Tillage Res., 6:149-162. Lal, R., 1985b. A soil suitability guide for different tillage systems in the tropics. Soil Tillage Res., 5: 179-196. Lal, R., 1987. Managing soils of sub-Saharan Africa. Science, 236: 1069-1076. Lal, R., 1989. Agroforestry systems and soil surface management of a tropical Alfisol. I. Soil Moisture and Crop Yields. Agrofor. Syst., 8: 7-29. Lal, R., 1990. Ridge-tillage. Soil Tillage Res., 18:107-111. Lal, R., 1991. Myths and scientific realities ofagroforestry as a strategy for sustainable management for soils in the tropics. Adv. Soil Sci., 15: 91-137. MaCartney, J.C., Nothwood, P.J., Dagg, M. and Dawson, R., 1971. The effect of different cultivation techniques on soil moisture conservation and the establishment and yield of maize at Kongwa, central Tanzania. Trop. Agric., 48:9-17. Nicou, R., 1974a. Contribution on the study and improvement of the porosity of sand and sandy-clay soils in the dry tropical zone. Agricultural consequences. Agron. Trop., 29:110127. Nicou, R., 1974b. The problem of caking with drying out of sandy and sandy-clay soils in the arid tropical zone. Agron. Trop., 30: 325-343. Nicou, R., 1977. Le travail du sol dans les terres exond6es du S6n6gal: Motivations, contraintes. Centre Nacional de Recherche Agronomique (CNRA), Bambey, Senegal, mimeo., 50 pp. Nicou, R., 1979. Tillage in the West African tropical zone. Proceedings of the 8th Conference of the International Soil Tillage Research Organization, Hohenheim, pp. 429-434. Pereira, H.C., Hosegood, P.H. and Dagg, M., 1967. Effects of tied ridges, terraces and grass leys on a lateritic soil in Kenya. Exp. Agric., 3: 89-98. Prentice, A.N., 1946. Tie-ridging, with special reference to semi-arid areas. E. Afr. Agric. J., 12: 101-108. Raghavan, G.S.V., Alvo, P. and McKyes, E., 1990. Soil compaction in agriculture: A view toward managing the problem. Adv. Soil Sci., 11: 1-36. Steppler, H.A. and Nair, P.K.R., 1987. Agroforestry: a decade of development. International Council for Research on Agroforestry (ICRAF), Nairobi, Kenya. Szott, L.T., 1987. Improving the productivity of shifting cultivation in the Amazon Basin of Peru through the use of leguminous vegetation. Ph.D. Dissertation, North Carolina State University, Raleigh. Wilson, G.F. and Lal, R., 1986. New concepts for post-clearing land management in the tropics. In: R. Lal, P.A. Sanchez and R.W. Cummings, Jr. (Editors), Land Clearing and Development in the Tropics. A.A. Balkema, Rotterdam, pp. 371-382. Voorhees, W.D., 1987. Assessment of soil susceptibility to compaction using soil and climatic databases. Soil Tillage Res., 10: 29-38. Voorhees, W.D., 1986. Deep compaction: a different phenomenon. Crops Soils Mag., 1986:10ll.