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TOWARD SUSTAINABILITY: A Plan for Collaborative Research on Agriculture and Natural Resource Management APPENDIX C Soil Research for Agricultural Sustainability in the Tropics Rattan Lal To date, much of the increase in food production in developing countries has been achieved by bringing new land under production, expanding irrigated land area, and applying Green Revolution technologies. These means have been used to the limit as unprecedented demographic pressure has generated rapidly growing demand for agricultural products. Reserves of potentially arable prime agricultural land are limited and unevenly distributed. The population in large areas of Africa, Asia, and South America already exceeds the carrying capacity of the land. Land is indeed a scarce resource; globally, arable land per capita will progressively decline from about 0.3 hectare (ha) currently to 0.23 ha in 2000, 0.15 ha in 2050, and 0.14 ha by the year 2150. The potentially arable land that exists, moreover, is located in regions with weak logistics, poor accessibility, and very poor infrastructure. Densely populated Asia, with up to 75 percent of the world's population, has little additional arable land to convert to agricultural use (for example, Sumatra). The per capita arable land area in many Asian countries is already less than 0.1 ha. About 290 million ha of land may be suitable for agriculture in South America and 340 million ha in Africa (Buringh, 1981; Dudal, 1982). Most of these lands, however, are located in fragile and ecologically sensitive regions—tropical rain forests, acid savannahs, the drought-prone Sahel. Bringing new land under production through deforestation of tropical rain forests has severe ecological, environmental, and sociopolitical implications. Some of the potentially arable land Rattan Lal is associate professor of soil physics at the Department of Agronomy, The Ohio State University.
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TOWARD SUSTAINABILITY: A Plan for Collaborative Research on Agriculture and Natural Resource Management outside the tropical rain forest region is of marginal utility due to other constraints—the land is too steep, the region contains too little or too much water, and the soils are too shallow or show salt and nutrient imbalances. Irrigation has played a major role in increasing food production. For the decade ending in 1987, the rate of increase in irrigated land area was 1.0 percent in Asia, 1.3 percent in Central and South America, and 1.4 percent in Africa (Food and Agriculture Organization, 1986). The rate of expansion has slowed considerably as the availability of irrigable land and good quality irrigation water has become a severe constraint. Concern is growing that the impact of green revolution technologies is slowing, even in South Asia (Herdt, 1988). The influx of high-energy techniques into agricultural ecosystems has broken the yield barriers, increased output at the rate of about 2.5 percent a year, led to an overall increase in per capita food production of about 0.6 percent between 1950 and 1986, and resulted in an unprecedented boom in agricultural output in the post-World War II era. Green revolution technologies have been applied to prime agricultural land with input-responsive soils. Can this technology be applied to the impoverished soils of the humid and subhumid tropics of Africa and South America? A principal constraint may be nonavailability of essential inputs at affordable prices, the breakdown of resistance of improved cultivars to pests and pathogens, and the degradation of soil quality. Neglect, misuse, and mismanagement of soil resources are in large part responsible for the low yields, widespread poverty, and severe problems of soil and environmental degradation in tropical and subtropical regions. Consequently, the goals of a viable program in soil research for agricultural sustainability in the tropics must be to (a) maintain and enhance the biological and ecological integrity of soil resources; (b) increase agricultural production; (c) improve the income, buying capacity, and self-reliance of resource-poor farmers; (d) restore life-support processes and potential productivity of degraded ecosystems; and (e) provide support to national research institutions and development services. The next section discusses a number of issues related to soil research for sustainable agriculture. SOIL RESEARCH ISSUES The severe scarcity of arable land and mounting demographic pressures in many developing countries mean that technological innovations are needed that can bring about a quantum leap in agricultural productivity. This can only be achieved through science-based agriculture. Given resource-based agriculture with low or medium input, the minimum dietary requirement can only be met with a per capita land availability of 0.5 ha. Thus, the greatest challenge facing humanity in the twenty-first century will be to produce the basic necessities of food, feed, fiber, fuel, and raw materials
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TOWARD SUSTAINABILITY: A Plan for Collaborative Research on Agriculture and Natural Resource Management TABLE C-1 Yield in Grain Equivalents and Percentage of Cropland for Various Levels of Production Input in the World Farming System/Input Level Yield (kg/ha) Cropland (%) Average Area of Arable Land Needed (ha/capita) Shifting cultivation <100 2 2.65 Low traditional 800 28 1.20 Moderate traditional 1,200 35 0.60 Improved traditional 2,000 10 0.17 Moderate technological 3,000 10 0.11 High technological 5,000 10 0.08 Specialized technological 7,000 5 0.05 SOURCE: P. Buringh. 1981. An Assessment of Losses and Degradationof Productive Agricultural Land in the World. FAO Workshop on GroupSoils Policy. Rome, Italy: Food and Agriculture Organization of theUnited Nations. from the maximum per capita land availability of 0.14 ha or less. Technological options for sustainable management of soil and water resources in the twenty-first century must address this basic constraint. The per capita land requirement to meet basic needs depends on the inputs. The challenge is to intensify use of prime agricultural lands, with all the inputs needed to sustain productivity of soil and water resources, and to break the yield barriers. Buringh (1981) presents an optimistic scenario. He estimated various modes of agriculture and the per capita land requirement for each mode (Table C-1). The average per capita land requirement under different systems and the corresponding crop yields vary by several orders of magnitude. Two of the most encouraging aspects of this analysis are that (a) the per capita arable land area can be as low as 0.11 ha or less for a moderate level of technological inputs and (b) about 25 percent of the world's cropland is suitable for intensive use through adoption of moderate, high, or specialized technologies. There are other optimists who support Buringh's analysis and argue that the earth's natural resources have the capacity to support between 15 to 22 billion inhabitants (Calvin, 1986, cited in Hudson, 1989). Their estimates are premised on (a) total solar energy input on arable land and (b) grain production with improved technologies. They argue that food production is demand driven and that the efficiency of agricultural production systems can be drastically increased through judicious use of inputs and advances in biotechnology. During the past decade, for example, fossil-fuel input in Chinese agriculture rose 100-fold, and crop yields tripled (Lu et al., 1982). The comparatively low output of Indian agriculture can be attributed to low energy influx. India annually uses 142 kilograms (kg) of per capita coal
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TOWARD SUSTAINABILITY: A Plan for Collaborative Research on Agriculture and Natural Resource Management equivalent compared with 4,871 kg in the United Kingdom and 10,410 kg in the United States (Bureau of the Census, 1983). Given that the world as a whole does have the capacity to feed itself, what are the issues to be addressed and strategies to be adopted to achieve that goal? First, soil resources and population are unevenly distributed. Regions and countries with high demographic pressures are also characterized by low available land reserves, for example, South Asia, China, southeastern Nigeria, Rwanda, Burundi, the East African highlands, Central America, and the Caribbean. In some of those places, the daily per capita calorie intake is likely to remain below 2,500 at least through the year 2000 even with earnest efforts to improve agricultural production (Table C-2). Second, even if technical know-how exists, socioeconomic, cultural, and political considerations are often overwhelming and do not readily permit the adoption of improved science-based technologies. The potential for increased fertilizers, pesticides, improved farm implements, and other innovations is limited due to nonavailability, high cost, or both. Often the major problems are poverty and lack of resources. Subsistence farmers will only use improved inputs if they are available at affordable prices. Third, the overdependence on nonrenewable sources of energy is a global issue. All intensive systems of agricultural production are based on the use of fossil-fuel energy. In developed countries, such as Germany, the number of persons fed from 1 ha of cultivated land increased 5.6 times and the equivalent cereal yield increased 6.3 times between 1800 and 1978 (Mengel, 1990). This dramatic increase in agricultural production has been realized through the heavy use of fertilizers and other inputs. The United States invests about half of its fossil-energy input in agricultural production into supplying water (20 percent) and fertilizers (30 percent) (Pimentel, 1989). The annual amount of harvested nutrients in three major cereals (rice, wheat, and corn) is estimated at 40.1 million tons (t) of nitrogen, 15.32 million t of phosphorus, and 28.2 million t of K2O. An equivalent TABLE C-2 Food Availability: Calories Per Capita Per Day Region 1983–1985 2000 Africa (sub-Saharan) 2,050 2,190 Near East/North Africa 2,980 3,100 Asia 2,380 2,610 Latin America 2,700 2,910 Low-income countries (excluding China) 2,130 2,350 SOURCE: Food and Agriculture Organization. 1989. The State of Foodand Agriculture. Rome, Italy: Food and Agriculture Organization ofthe United Nations.
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TOWARD SUSTAINABILITY: A Plan for Collaborative Research on Agriculture and Natural Resource Management amount may be harvested in stover. The nutrients harvested must be replenished one way or another. With the current level of world yields, the annual crop uptake is estimated at 85.5 kg of nitrogen per person (Andow and Davis, 1989), which will amount to a total of 530 billion t of nitrogen uptake by crops by the year 2000. Since inputs are an inevitable consequence of ever-increasing demand for agricultural production, several strategic issues must be resolved. Can nitrogen and other essential plant nutrients (for example, phosphorous, zinc, sulphur) be synthesized from the available reserves of fossil fuels? How can alternative sources of fertilizers or power be developed to meet the energy needs of developing countries? Are organic manures a viable source of nutrients required for agriculture in developing countries? It is estimated that only 2.5 percent of nitrogen in the manure is recoverable and usable with current technology (Pimentel, 1989). Moreover, the losses of nitrogen from organic manures by volatilization (30 to 90 percent) or leaching are major sources of water and atmospheric pollution. In developing countries as a whole, only 4 percent of total commercial energy is used for agriculture, and merely 2.7 percent is in the form of fertilizer. Fertilizer use on arable land ranges from 4 to 50 kg/ha in most developing countries compared with 100 to 800 kg/ha in developed countries (Stout, 1989). Increasing use of fertilizer and other agricultural amendments is limited due to the restricted availability and high cost of nonrenewable sources of energy. Low cost and renewable hydroelectric power is available only in a few countries. Further, this premium form of energy is highly valuable and is preferably used for industrial purposes. About 70 percent of the world's nitrogen fertilizer is produced by using natural gas as the source of energy (Stout, 1989). Similar to population and land-resource availability, natural gas deposits are also unevenly distributed. Countries without natural gas have to import fertilizers. Above all, the environmental issues of intensive agriculture cannot be ignored. In addition to the dangers of agricultural chemicals, the problem of deforestation in the tropics is a major environmental issue. Bringing new land under production through deforestation of tropical rain forests, as noted, has severe ecological, environmental, and sociopolitical implications. The actual extent of deforestation in the tropics is still the subject of debate, however (Myers, 1981). In addition to loss of biodiversity and potentially valuable genetic resources, rain forest conversion presumably contributes a large proportion of total global emissions of carbon dioxide (Houghton et al., 1987; Lashoff, 1988; Tirpak, 1988), although the exact values are not known. The type, amount, and rate of gaseous emission also depend on the method of deforestation—for example, slash and burn, chain-saw clearing, bulldozers, and chemical poisoning—and on the subsequent land uses (Lal, 1987a,b). Then there is the problem of soil degradation. Currently, 5 to 7 million
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TOWARD SUSTAINABILITY: A Plan for Collaborative Research on Agriculture and Natural Resource Management ha of arable land (0.3 to 0.5 percent) is lost every year through soil degradation. The projected loss by the year 2000 is 10 million ha annually (0.7 percent of the currently cultivated area). A high proportion of this loss occurs in ecologically sensitive regions of the tropics and subtropics, where marginal lands are being intensively cultivated. Pollution of surface and groundwater by agricultural chemicals is another major environmental hazard. A high proportion of the fertilizer that is applied is susceptible to volatilization, washed away in surface runoff or eroded soil, or leached into the ground water. There is equal concern over contamination of surface and ground waters by water soluble pesticides, such as aldicarb, ethylene dibromide, and atrazine. Although pesticide use is rapidly increasing in developing countries, drinking water supplies are scarce, rarely treated, and seldom tested for contaminants. The intensification of agriculture also involves other severe risks of environmental pollution. The so-called greenhouse effect is directly linked to agricultural activities, and soil-related processes play a major role in the emission of greenhouse gases. More organic carbon is contained in the world's soil (in the form of soil organic matter) than in the world's biota or atmosphere (Sedjo and Solomon, 1989; Stevenson, 1982). Intensive land use for seasonal crop production may lead to depletion of soil organic matter and release of carbon into the atmosphere. Burning, a basic tool in traditional agriculture, releases large quantities of greenhouse gases into the atmosphere. In addition to burning and deforestation, other agricultural practices that result in higher greenhouse emissions from tropical ecosystems include use of rice paddies (a major source of methane); intensive use of marginal lands without inputs, which leads to mining and depletion of soil organic matter; uncontrolled and excessive grazing with high stocking rates; and indiscriminate use of chemical fertilizers. In sum, the principal issues regarding soil research in the tropics with relevance to agricultural sustainability are (a) food security related to the perpetual deficit in some regions and widespread poverty and malnutrition in others; (b) land scarcity and low carrying capacity of land; (c) soil degradation due to accelerated erosion, desertification, and salinization; (d) pollution and eutrophication of natural waters; (e) heavy reliance on nonrenewable fossil fuel for certain production technologies, and (f) possible greenhouse effect due to deforestation, burning, and emission of radiatively active gases into the atmosphere by soil-related processes. SOIL-RELATED CONSTRAINTS TO AGRICULTURAL PRODUCTION Soil, the most basic of all resources, is finite on a global scale, nonrenewable in the human time frame, and extremely fragile and vulnerable to
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TOWARD SUSTAINABILITY: A Plan for Collaborative Research on Agriculture and Natural Resource Management misuse and mismanagement. This section describes soil-related constraints to intensive land use. Soil Erosion Accelerated erosion is a serious problem in several ecologically sensitive regions: the Himalayan-Tibetan ecosystem, the Andean region, the Caribbean, eastern Africa, and other densely populated regions with severe land shortage. Steeplands, which make up a large percentage of the total land area in these regions, are overexploited and grossly misused. High erosion rates are observed throughout the tropics (Table C-3 ). In South and Southeast Asia, rivers draining the Himalayan region (for example, the Ganges, Mekong, Irrawdy, and Brahmaputra) have a high sediment load ( Table C-4 ). In India, 150 million ha are subject to accelerated TABLE C-3 Selected Erosion Rates in the Tropics Region/Ecology Criteria Equivalent Field Erosion Rates (t/ha/yr) Africa Cote d'Ivoire Bare soil 138 Ethiopia Sediment load 165 Ghana Bare soil 100–313 Lesotho Sediment load 180 Nigeria Bare soil 230 Tanzania Bare soil 38–93 Asia Bangladesh 50% slope 520 India Cropland, 4–43 gullies 33–80 Java, Indonesia Imperata 345 Tropical America and the Caribbean Colombia Cropland 21.5 El Salvador Steeplands 130–260 Guatemala Steeplands cultivated in maize 200–3,600 Northeast Brazil Cropped land 115 Peru Bare soil 148 Trinidad 10–20° slope, bare 490 SOURCES: R. Lal. 1986a. Conversion of tropical rain forest: Agronomicpotential and ecological consequences. Adv. Agron. 39:173–264. Reprintedwith permission by Springer-Verlag © 1986. R. Lal. 1986b. Soil surfacemanagement in the tropics for intensive land use and high and sustainedproduction. Adv. Soil Sci. 5:1–138. Reprinted with permission bySpringer-Verlag © 1986.
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TOWARD SUSTAINABILITY: A Plan for Collaborative Research on Agriculture and Natural Resource Management TABLE C-4 Sediment Yield from Some Tropical and Subtropical Catchments Country River Sediment Yield (t/km2/year) China Dali 16,300–25,600 Indonesia Cilutung 12,000 Kenya Perkerra 19,520 New Guinea Ause 11,126 SOURCE: R. Lal. 1990c. Soil Erosion in the Tropics: Principles andManagement. New York: McGraw-Hill. Reprinted with permission. soil erosion (United Nations Environment Program, 1983). Siltation of reservoirs in northern India is about 200 percent more than was anticipated in their design (Table C-5 ; Dent, 1984). In Nepal, 63 percent of the Shivalik zone, 26 percent of the middle mountain zone, 48 percent of the transition zone, and 22 percent of the high Himalayas are subject to severe erosion. In Pakistan, the upper Indus basin is severely eroded. In China, about 46 million ha of the loess plateau are subject to severe erosion, which is raising the bed of the Yellow River by as much as 10 centimeters annually. Severe erosion is also occurring in the watersheds of the Yangtze, Huaihe, Pearl, Liaolie, and Songhua rivers (Dent, 1984). In South America, about 39 million ha or 8 percent of the Amazon basin are characterized by soils of high erodibility (Sanchez et al., 1982). In Africa, as much as 1 billion t of topsoil are lost from Ethiopian highlands each year (Brown, 1981), and the average annual rate of soil erosion from Madagascar is reported to be 25 to 40 t per ha (Finn, 1983). The Food and Agriculture Organization (1983) TABLE C-5 Annual Rates of Siltation in Selected Reservoirs in India Reservoir Assumed Rate (ha-m/100 km2) Observed Rate (ha-m/100 km2) Year of Observation Bhakra 4.29 6.00 1975 Ghod 3.61 15.51 1970 Mayurkashi 3.61 20.09 1975 Msiyhon 1.62 13.02 1971 Nizam Sagar 0.29 6.57 1967 Panchet 2.47 9.02 1974 Ramganga 4.29 17.30 1973 SOURCE: National Land Use Conservation Board. 1986. Review of Centrally-SponsoredSchemes of Soil Conservation in the Catchments of River Valley Projects.New Delhi: Government of India.
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TOWARD SUSTAINABILITY: A Plan for Collaborative Research on Agriculture and Natural Resource Management estimates that 87 percent of the Near East and Africa north of the equator are subject to accelerated erosion. Wind erosion is equally severe in arid and semiarid regions (for example, the West African Sahel, western India, and Pakistan). In southern Tunisia, Floret and Le Floch (1973) and Le Houerou (1977a,b) observed that wind erosion rates of 10 millimeters of topsoil removed per year are common. Wind-blown dust from the Sahara causes air pollution and “sand rains” in the Caribbean (Rapp, 1974) and in northern Europe (Le Houerou, 1977a,b). It is estimated that between 25 and 37 million t of African soil are blown across the Atlantic Ocean annually (Prospero and Carlson, 1972). The global area subject to desertification is estimated to be 37.7 million square kilometers (km2)—16.6 million km2 of the world's arid regions, 17.1 million km2 of the semiarid regions, and 4.0 million km2 of the subhumid regions (Mabbutt, 1978). The global loss to desertification is estimated at 6 million ha annually, and the rural population severely affected by desertification is about 135 million (United Nations Environment Program, 1984). In several countries strong evidence exists of severe loss in soil productivity due to accelerated erosion. Instances of permanent soil productivity loss due to human-induced water erosion have been reported in several countries of Asia and Africa (Dregne, in press). Loss in agricultural production depends on soil properties, crops, management, and climate. In some shallow soils, the loss can be 50 percent or more (Lal, 1986b). Structural Deterioration and Soil Compaction An important process leading to soil degradation is the deterioration of the soil's structural properties and its ability to regulate water and air movement through the profile. Structural degradation, caused by a decline in soil organic matter and clay content and reduction in biotic activity, leads to crusting, compaction, reduced infiltration rate and low available water-holding capacity, increased soil detachability, and accelerated runoff and soil erosion. High erosion risk is a direct consequence of deterioration of soil structure. Soil compaction is a more severe problem in soils of the tropics than hitherto anticipated. Soils with low-activity clays have slight or negligible swell/shrink capacity. Decline in soil organic matter content, degradation of soil structure, and excessive drying accompanied by high soil temperatures generally lead to consolidation and compression by mechanisms not well understood. Over and above these factors is the compactive effect of heavy machinery. It is estimated that some 90 percent of the soil surface may be traversed by tractor wheels during, for example, the primary tillage operations (Soane et al., 1981). A smearing action of the plow sole results in pore discontinuity that inhibits water movement and root development.
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TOWARD SUSTAINABILITY: A Plan for Collaborative Research on Agriculture and Natural Resource Management Characterization of soil compaction is another problem that is particularly severe for heterogeneous, gravelly soils. The usual criteria, such as bulk density, total porosity, and penetrometer resistance, are not the best indicators of the problem that plant roots experience. Perhaps the void ratio, the specific volume, air:water permeability, or pore-size distribution and continuity may be better indices of plant response than the bulk density. The available research information on these aspects of soils in the tropics is rather scanty. Critical soil bulk density values for root penetration and crop growth are not known for major soils of the tropics. Crops susceptible to soil compaction include maize (Zea mays), upland rice (Oryza sativa), sorghum (Sorghum bicolor), groundnut (Arachis hypogea), cassava (Manihot esculenta), yam (Dioscorea rotundata) and cowpea (Vigna unguiculata). Decline in Soil Organic Matter Content Rapid decline in the soil organic matter content of cultivated soils is a direct effect of continuously high temperatures throughout the year, low input agriculture, and soil erosion. Some studies have shown that the rate of mineralization of organic matter content in tropical soils may be four times greater than in temperate soils (Jenkins and Ayanaba, 1977). Consequently, cultivated soils in the tropics may have lower levels of organic matter than similar soils in temperate latitudes. Lal and Kang (1982) reported large differences in the organic carbon status of soils from various ecological regions of Nigeria: forest (1.3 ± 0.08 percent) > derived savannah (0.89 ± 0.071 percent) > Guinea savannah (0.7 ± 0.06 percent). The organic matter content of a soil and its susceptibility to erosion are intimately linked. Although a decrease in organic matter content increases the susceptibility of the soil to erosion, water erosion also preferentially removes soil colloids, including the humified organic matter fraction (Lal, 1976). Lal (1980) reported a linear decline in soil organic matter content with accumulative soil erosion: Organic carbon (%) = 1.79 − 0.002 E, r = − 0.71, where E is the annual accumulative soil erosion in tons per hectare. A decrease in organic matter content of the soil also increases its susceptibility to formation of surface crust, which further enhances the risk of soil erosion. Soil erosion is also increased by the reduction in biotic activity of soil fauna that occurs with a decrease in soil organic matter content. In addition to decreases in structural stability, reductions in organic matter content have important implications in terms of plant-available water reserves in the soil. The favorable effects of organic matter content on the soil's water-retention capacity have been widely reported for soils of the tropics and subtropics (Lal, 1986b). In fact, organic matter content may
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TOWARD SUSTAINABILITY: A Plan for Collaborative Research on Agriculture and Natural Resource Management have more beneficial effects on the available water-holding capacity than the clay content. Also important are the nutritional implications, for example, the effects on cation exchange capacity, acidification, and plant nutrients. Fertility Depletion and Leaching Continuous and intensive cropping with low or no off-farm input, necessitated by land hunger and poverty, cause fertility depletion and low yields. Many of the soils cultivated by shifting cultivators and subsistence farmers of the tropics and subtropics are subject to fertility depletion through decline in soil organic matter, reduction in nutrient reserves by crop removal, leaching, and acidification. Leaching and acidification are serious problems in soils of tropical climates with seasonally humid (alfisols) and humid moisture regimes (ultisols and oxisols). Substantial areas of acid tropical soils occur in Sumatra, Malaysia, the Congo basin, the Amazon basin, and in the cerrados and llanos of Brazil and Colombia. Nitrogen is most readily lost. The extent of loss can be as high as 60 kg/ha annually from cropped land and 300 kg/ha annually from uncropped land (Suarez de Castro and Rodriguez, 1958). In Queensland, Australia, Martin and Cox (1956) reported leaching, with losses of 27 kg of nitrogen/ha from vertisols, in subhumid environments. Salt Imbalance Salt-affected soils, totaling about 323 million ha are widely distributed throughout the arid and semiarid regions (Table C-6; Beek et al., 1980). The problem is particularly severe in irrigated regions of China, India, Pakistan, and the Middle East. Productivity of irrigated lands in these regions is severely jeopardized by salt imbalance in the root zone (Gupta and Abrol, 1990). Soil structure is adversely affected by the predominance of sodium and application of irrigation water of poor quality (Gupta and Abrol, 1990; Mathieu, 1982). Nonavailability of good-quality irrigation water is a severe constraint to expanding irrigated agriculture in arid regions. Drought Stress Only 2.5 percent of the world's water is freshwater, and only a fraction of that is available for agricultural purposes. Total annual global precipitation is estimated at 350 × 103 km3, of which 78.6 percent (275 × 103 km3) falls over the oceans. Of the remainder, 64 percent evaporates, leaving merely 28 × 103 km3 for surface runoff or groundwater (Hall, 1989). Scarcity of freshwater is a general problem, especially in countries with arid and semiarid climate (annual rainfall of less than 700 mm). In addition to the low total amount, rains in such regions are highly irregular and seasonal.
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TOWARD SUSTAINABILITY: A Plan for Collaborative Research on Agriculture and Natural Resource Management overdependence on synthetic fertilizers and other agricultural chemicals. The energy and economic costs of such a strategy are prohibitive for the small landholders of the tropics. Agronomic experiments must assess the appropriate combination of inorganic and organic fertilizers to minimize dependence on synthetic fertilizers and enhance soil structure and physical characteristics. Techniques must be developed to reduce the rate of application of inorganic fertilizers by minimizing losses and increasing the recycling of nutrients. In this regard, it is important to quantify losses by volatilization, leaching, and erosion in relation to conservation tillage, application by split doses, fertilizer placement, and slow-release formulations. Technological options for nutrient recycling must be researched for crop residue management and mulch farming, legume-based rotations, ley farming with different stocking rates and controlled grazing, and agroforestry systems, including alley cropping. Nutrient-recycling mechanisms and effects must be assessed for different soils (for example, highly weathered oxisols and ultisols, which predominantly contain aluminum [Al+3] and manganese [Mn+3] in the subsoil horizons and are devoid of basic cations). The effects of alley-cropping systems on crop yields should be evaluated in terms of competition for nutrients, water, and light (Lal, 1989, 1990a). Advantages in substituting biological nitrogen fixation for inorganic fertilizers must be quantified. And finally, careful evaluation is needed of the economics of growing nitrogen versus buying nitrogen, especially with regard to timely availability, land scarcity, efficiency of nitrogen from biological resources, and environmental effects. Erosion Management Erosion management is crucial to the sustainable management of soil resources. Several technological options are available for erosion management. A stronger data base and appropriate criteria are needed, however, to guide the choice of appropriate options, including due consideration of soil types, land form and terrain characteristics, rainfall regime and hydrology, cropping/farming system, and socioeconomic factors. The pros and cons of measures to prevent versus control erosion should be carefully assessed. Preventive measures are those that enhance soil structure, decrease raindrop impact, improve infiltration capacity, and decrease runoff rate and amount. Use of these techniques is based on thorough knowledge of soil and crop management (for example, mulch farming through cover crops and planed fallows, multiple cropping, multistory canopy including agroforestry, and conservation tillage). Vegetative hedges are important tools in minimizing risks of soil erosion (Table C-8 ). Although general principles are known, there is a need to validate and adapt these practices under locale-specific conditions for major
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TOWARD SUSTAINABILITY: A Plan for Collaborative Research on Agriculture and Natural Resource Management TABLE C-8 Effects of Contour Hedges of Leucaena and Vetiver on Runoff and Soil Erosion from Shallow Soil Planted to Pearl Millet and Deep Soil Planted to Sorghum in Central India Treatment Grain Yield (t/ha) Runoff (%) Soil Erosion (t/ha) Pearl millet on a shallow soil Across-the-slope sowing 1.5 17.7 11.5 Contour cultivation along Leucaena keyline 1.7 11.8 6.2 Contour cultivation along vetiver keyline 2.0 9.0 3.3 Sorghum on a deep soil Across-the-slope sowing 3.4 21.5 18.4 Contour cultivation along Leucaena keyline 3.7 18.1 9.4 Contour cultivation along vetiver keyline 3.9 3.7 4.3 Cultivation along with graded bunds 3.5 17.3 14.2 SOURCE: Manoli Watershed Development Project. 1990. Pungabrao KrishiVidyepeeth: A report on research highlights of technical programme.Annual Report, Manoli Watershed Development Project. Photocopy. crops, cropping systems, soils, and ecological regions of the tropics. The adaptability of local tree species as vegetative hedges for erosion control and other uses should be evaluated. The adaptability of tillage systems to erosion control must be judged in terms of socioeconomic and cultural factors, availability and maintenance of implements, cost and availability of herbicides, and efficiency of soil and water conservation. The ecological limits of different conservation tillage methods must also be established. Residue Management A regular and sizable addition of organic material to soil is essential to maintain favorable organic matter content and to stimulate biotic activity of soil fauna, including earthworms and termites. Structural collapse of soils with predominantly low-activity clays can be avoided by maintaining high organic matter content and by enhancing the activity of soil fauna. Crop residue mulch is an important ingredient of any improved farming-cropping system. Although the beneficial effects of mulching are widely recognized, procuring a mulch material in sufficient quantity is a serious practical problem. Research on management of crop residue as a source of mulch must
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TOWARD SUSTAINABILITY: A Plan for Collaborative Research on Agriculture and Natural Resource Management be closely linked with cropping systems, tillage methods, and planted fallows. Appropriate cultural practices must be developed and validated to ensure an adequate amount of residue mulch for soil protection and fertility enhancement. Live mulch, alley cropping, ley farming, planted fallows, and the use of industrial by-products are some of the cultural practices for procuring mulch that need to be validated. Their suitability depends on locale-specific biophysical and socioeconomic environments. Crop Management It is widely recognized that continuous ground cover is necessary to provide a buffer against sudden fluctuations in micro- and meso-climate and to prevent the degradative effects of raindrop impact or high-velocity winds. Ensuring protective ground cover requires research information on appropriate time of planting, optimum seed rate, improved cultivars and cropping systems, fertilizer use, pest controls, and other important aspects of crop management. The benefits of timely planting must be assessed against uncertain rains, unfavorable soil temperature regime, pest infestation, and unfavorable markets. Planted fallows, using both legume and grass covers, must be evaluated for their restorative effect on soil fertility and soil physical properties in comparison with natural fallows. How long does it take improved soil organic matter content to affect soil structure favorably? Fallow Management When crop residue mulch is inadequate, practical means must be developed to procure mulch through incorporation of an appropriate cover crop or planted fallow in the rotation. In addition to their capacity to supply residue mulch, planted fallows must also be evaluated for their usefulness in restoring physical and nutritional properties in comparison with long bush fallows. Information is needed on appropriate species of cover crops and methods for their management. Other researchable topics in fallow management include the timing and methods of suppression of the fallow crop, the timing and methods of sowing the food crop, methods of weed control and competition reduction between the fallow and food crop, and the timing and amount of nutrient released by the fallow crop. Hedge-Row Management and Agroforestry Agroforestry, as a special case of mixed cropping, involves growing deep-rooted perennial leguminous shrubs and trees in association with food crop annuals or livestock. This practice supposedly minimizes the soil-degradation risks of intensive use of arable land (King, 1979; Vergara,
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TOWARD SUSTAINABILITY: A Plan for Collaborative Research on Agriculture and Natural Resource Management 1982). Perennial crops that fruit annually, such as banana, may be more suitable for intercropping with annuals than plantation crops. Shade-tolerant staples, such as cocoyam, can be grown in association with plantation crops (for example, cocoa), especially in the early stages of plantation establishment or along the outer margins. Research must ascertain which species of trees and woody perennials are most appropriate and can be profitably grown in association with annuals or animals. More information is also needed on management systems for perennials and annuals that maximize their benefits and reduce competition. The amount of nutrients contributed by perennials depends on soil and crops; research must determine those amounts. In addition to nutrients, more needs to be learned about the water requirements of perennials and annuals. Allelopathic effects, if any, must be carefully assessed. Water Management Water management is critical in alleviating the adverse effects of recurring drought on crop and animal productivity. Lack of water during the growing season, like the lack of nutrients, is a major constraint in arid and semiarid climates. Agricultural productivity in several regions of the tropics and subtropics primarily depends on the amount, distribution, and reliability of rainfall. Efficient use of rainfall is crucial to sustainable productivity in rain-fed agriculture. An understanding of the rainfall characteristics of a region—the probability of the occurrence of a certain amount at a desired frequency, or the onset of assured rains at a given time —is an important requisite for developing sustainable systems of crop and soil management in that region. Controlling and managing runoff is a key management objective. In addition to conserving rainwater in the root zone by decreasing losses due to runoff and evaporation, means of supplementing irrigation must be explored to decrease the sensitivity of production to climate. Irrigation, a capital-intensive technology, has not been fully exploited in several regions. It is estimated, for example, that only 2 percent of the irrigable land in Africa is irrigated. In addition to the development of feasible large-scale irrigation schemes, high priority should be given to small-scale, labor-intensive schemes. Small-scale irrigation schemes may be more appropriate for resource-poor farmers and in regions where large rechargeable aquifers do not exist. Replacement of traditional devices (for example, shadoof, Persian wheel driven by animal) by diesel, electric, or wind-driven pump may improve the efficiency and increase the cropped area under irrigation. The technical and social issues related to water delivery, water allocation, and water-use efficiency must also be addressed, however. Each of the alternatives involves a different set of problems and pos-
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TOWARD SUSTAINABILITY: A Plan for Collaborative Research on Agriculture and Natural Resource Management sible remedies, and their socioeconomic and political dimensions must be taken into account. Watershed Management Sustainable management of soil and water resources is based on judicious and scientific management of all landscape units within a watershed. Widespread and severe problems of accelerated erosion and sedimentation, perpetually devastating floods, land degradation beyond the point of no return, eutrophication of water, and environmental pollution in general are traceable to poor planning and mismanagement of landscape units within watersheds. Scientific criteria for the choice of appropriate land uses, exploitation of water resources for irrigation and domestic purposes, and the development of infrastructure (including access roads) must be developed. Scientific use of a watershed for sustainable land and water development is more easily described than achieved, however. The problem is caused in large part by private ownership of small landholdings; farm boundaries cut across landscape units and natural waterways. The problem is aggravated by dubious land-tenure systems and ownership rights. Legislation, policies, and incentives are needed to foster cooperation among farmers and promote ecologically compatible development of natural resources. SYSTEMS APPROACH Although general principles may be the same, technological packages (systems) for sustainable management of soil and water resources are site specific and depend on farming-cropping systems, farm size, availability of essential inputs, and socioeconomic factors. Locale-specific and on-farm synthesis of packages is needed on the basis of the components and sub-systems described above. The agronomic productivity, economic profitability, and ecological compatibility of such packages must be assessed through appropriate research. Systems research is preferably conducted on “benchmark” soils or “ecological regions.” In that way, the agroeconomic productivity of different production systems can be related to soil and climatic characteristics. This approach will facilitate transfer of technology to similar soils and environmental conditions elsewhere. Systems research necessitates a pan-disciplinary approach involving scientists with expertise in soil science, hydrology, climatology, agricultural mechanization, agronomy, plant improvement, pest management, economics, sociology, and anthropology. Results obtained from field experimentation can be validated against predictive models. The latter may be biophysical models, economic-productivity models based on linear programming, or statistical models based on systems analysis of empirical data.
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TOWARD SUSTAINABILITY: A Plan for Collaborative Research on Agriculture and Natural Resource Management The alternative agronomic approach involves field experimentation and gradual, step-by-step improvement of the system through substitution of the component that is the major constraint to crop and animal production. The aim of on-farm research is to identify the major constraint and alleviate its effect by devising technological options. The agronomic approach is a long-term strategy aimed at transforming low-input subsistence farming into science-based agriculture. Researchable priorities in this approach involve assessment of the components or subsystems under on-farm conditions and with the active involvement of farmers. In addition, specific research priority should be given to soil and crop management practices that increase the efficiency of water and fertilizer use and restore eroded and degraded lands. CONCLUSIONS Soil research must be mission oriented. Its objective is the alleviation of production-related constraints in intensive agriculture. In that context, sustainable management of soil and water resources implies meeting current needs without jeopardizing future potential. Thus, sustainability of soil and water resources must be judged with tangible criteria—soil and water conservation, productivity, restoration of degraded soil, reduction in off-farm inputs for the same level of production and profitability, increase in labor productivity, and so forth. Major considerations in terms of research and development priorities are outlined below. Farming systems and technologies that enable people to live in comfort and in symbiosis with nature must be developed. Many misconceptions still persist about the actual potential and constraints of tropical ecosystems. Although considerable progress has been made in the recent past toward replacing myths with facts, the reasons for the lack of sustainable-yield farming systems in tropical ecosystems are not fully understood. The objective of research and development is to achieve high and sustainable yields, but with low inputs and with reduced damage to soil and environments. The goal is to achieve optimum sustainable yields with modest inputs, rather than maximum yields based on high capital and energy inputs. The first research priority should be to determine why the research information already available is inadequate. Although the innovative concepts and subsystems proposed in the literature are technically sound, their economic evaluation and social acceptability must be assessed for varying socioeconomic environments. These concepts should be evaluated as integral parts of the overall system rather than as individual components of the improved technology. Improved soil management is sustainable only within economically improved farming systems. The promising innovations and improved subsystems already developed to alleviate specific biophysical constraints related to soil and environ-
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TOWARD SUSTAINABILITY: A Plan for Collaborative Research on Agriculture and Natural Resource Management ments should be integrated into farming systems as specific case studies. Methodologies involving linear programming and systems analysis should be standardized to facilitate establishment of blueprints of farming systems for locale-specific situations on the basis of subsystems and component technology already developed. Baseline data are needed on the soil, climate, water, and vegetation resources of the tropics, and their potentials and constraints. Resource data banks must be established for major agroecological zones, and their productive potential of the zones must be defined on the basis of conceptual models. Land-use planning is the key to sustainable development of land and water resources, and it requires systematic surveys of soil, hydrology, vegetation, and terrain at a practical scale (<1:50,000). Few, if any, developing countries have programs for detailed and systematic evaluations of natural resources at this scale. The emphasis on these surveys is not for soil classification and mapping per se, but for assessing the potential of, and constraints on, natural resources and for developing management options for sustained production without causing degradation of fragile land resources. The data bank established on natural resources can be used to provide site-specific information for choosing technological options through the use of geographic information systems, global positioning systems, and digital elevation models. Rapid research progress has to be made by the year 2000, when the demands made on soil resources will be greater than ever before. The low-input systems now being recommended are already obsolete. Now is the time for researchers to provide the components or subsystems of medium-to high-input technology. Specifically, research for the twenty-first century must provide, as a top priority, basic data from well-designed, long-term field experiments on soil management that involve technologies with different levels of inputs. These experiments must address the following scientific concerns: Restoration of eroded and degraded lands. This deserves high priority, particularly to reduce the need to clear and develop new land. Methods should be developed to restore and rehabilitate eroded and degraded lands. Land-evaluation criteria should indicate the time when soil should be taken out of production and put under a restorative and ameliorative phase. Knowing the critical limits of soil properties for different levels of management is crucial in this endeavor. Soil compaction. The problem of soil compaction will become severe with intensive land use and increasing mechanization. There is a need to develop routine methods for characterizing soil compaction. No standardized procedures are available for minimizing soil compaction by motorized farm equipment or for restoring production on soils hitherto compacted.
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TOWARD SUSTAINABILITY: A Plan for Collaborative Research on Agriculture and Natural Resource Management The problem should be addressed from various aspects (for example, machinery, rotations, and farming systems). Traffic-induced soil compaction warrants an interdisciplinary research approach by soil and crop scientists working together with soil engineers and machinery designers. Soil erosion. Additional basic data are needed on soil erosion and its control and on predicting water runoff and soil loss under different land uses and in different farming systems. In addition to basic factors affecting erosivity and erodibility, the numerical limits of “soil loss tolerance” should be established. This information is important for long-range planning for different land uses. Conceptual and empirical models relating crop yield and soil loss to different levels of management are needed for assessing the economic consequences of accelerated soil erosion. Unless the relationship between erosion and productivity is developed, it will be difficult to plan development strategies and choose soil management methods. Soil management. Management of soil structure is still a mystery. Why does the structure of most soils of the tropics deteriorate so rapidly, and how can it be prevented? Development of crust and surface seal is a major problem in soils containing predominantly low-activity clays, including those in the semiarid tropics. Techniques of soil surface management should address this problem. Increasing output. Increasing agricultural output per unit of input will remain a challenge for generations to come. The inputs may be natural resources or soil amendments. In the semiarid and arid tropics, increasing output per unit of water is the basic ingredient of a successful technology. In humid regions with highly leached soils, limited plant nutrients (for example, calcium, nitrogen, and phosphorus) are the major consideration in developing improved farming systems. Also to be studied are soil-water-fertility interactions for different agroclimatic regions, including leaching patterns and salt and water balance. Tillage systems. The development of appropriate tillage systems for different-sized farms and for a wide range of soils, crops, and climatic environments is an important research consideration. Given the merits of no-fill systems in controlling erosion and conserving water, their potential should be exploited. The ecological limits to the application of these no-fill techniques can be greatly extended by improving the agronomic practices associated with their implementation. Tillage systems should be geared toward alleviating specific soil-related constraints to crop production—soil temperature, soil and water conservation, soil compaction, maintenance of soil structure, and soil organic matter content. Soil dynamics. The evolution of the physical, chemical, and biological properties of soil should be studied in representative farming systems and under varied land uses in order to establish the cause-effect relationship
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TOWARD SUSTAINABILITY: A Plan for Collaborative Research on Agriculture and Natural Resource Management between land use and soil properties. The agronomic output should always be assessed in terms of the impact of land use on soil properties. Soil constraints. The soils of humid regions have special toxicity constraints. Research is needed on how to adapt crops to these nutrient constraints. In arid and semiarid environments, on the other hand, salt accumulation in the surface soil horizon is a special constraint. Basic studies of salt and water balance in different farming systems and in different ecological regions should provide the basis of management systems to overcome these problems. Nutrient recycling. To decrease inputs of chemical fertilizer, priority should be given to research on agroforestry techniques, planted fallows, and other nutrient-recycling systems, such as use of organic wastes and farm by-products. Irrigation. There is a need to develop irrigation potential fully, especially in sub-Saharan Africa. Expansion of irrigable cropped area warrants high priority. Both the technical and social issues related to water delivery and water allocation must be addressed. Each of these involves a different set of problems and possible remedies, and each has socioeconomic and political dimensions that must be taken into account. REFERENCES Andow, D. A., and D. P. Davis. 1989. Agricultural chemicals: Food and environment. Pp.192–235in Food and Natural Resources D. Pimentel and C. W. Hall eds. San Diego, Calif.: Academic Press. Beek, K. J., W. A. Blokhuis, P. M. Driessen, N. Van Breemen, R. Brinkman, and L. J. Pons. 1980. Pp. 47–72in Problem Soils: Their Reclamation and Management ILRI Publication No. 27. Wageningen, Netherlands: International Institute for Land Reclamation and Improvement. Brown, L.R. 1981. World population growth, soil erosion, and food security. Science 214:995–1002. Bureau of the Census. 1983. Statistical Abstract of the United States. 1983. 104th ed. Washington, D.C.: U.S. Government Printing Office. Buringh. P. 1981. An Assessment of Losses and Degradation of Productive Agricultural Land in the World. FAO Workshop on Group Soils Policy. Rome, Italy: Food and Agriculture Organization of the United Nations. Cervinka, V. 1989. Water use in agriculture. Pp. 142–163in Food and Natural Resources D. Pimentel and C. W. Hall eds. San Diego, Calif.: Academic Press. Dent, F. J. 1984. Land degradation: Present status, training and education needs in Asia and the Pacific. UNEP Investigations on Environmental Education and Training in Asia and the Pacific. FAO Regional Office. Bangkok, Thailand: Food and Agriculture Organization of the United Nations. Dregne, H. E. 1990. Erosion and soil productivity in Africa. J. Soil Water Conserv. 45:431–436. Dregne, H. E.In press. Erosion and soil productivity in Asia. J. Soil Water Conserv. 46. Dudal, R. 1982. Land degradation in a world perspective. J. Soil Water Conserv. 37:245–247. Food and Agriculture Organization and United Nations Environment Program. 1983. Guidelines for the Control of Soil Degradation. Rome, Italy: Food and Agriculture Organization of the United Nations.
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Representative terms from entire chapter: