Previous - Next
This is the old United Nations University website. Visit the new site at http://unu.edu
IV. Basic concepts of sustainable land use in the humid tropics
A. Requirements of a sustainable improved
agricultural system
B. Components of a sustainable system
C. Systems approach
Traditional farming, labor-intensive and resource-based, is compatible with an ecological environment as long as the pressure of the human and animal population is low, and yield expectations are not high. The system is workable if: (i) a favorable level of soil structure is main tained; (ii) soil erosion is controlled; (iii) nutrients are effectively recycled through the addition of ash, and seasonal crops grown in association with deep-rooted perennials; (iv) pests are kept under check through biodiversity; and (v) food security is guaranteed by mixed and multiple cropping. Most of these requirements in the past have been met through low-cropping intensity based on lengthy forest/bush following. Until the 1950s, population densities were generally low, and food needs and basic necessities were met through the cultivation of food staples grown in association with trees, other woody perennials, and some animals. Farm size was limited to what could be managed comfortably by family labor. The success of the system was attributed to a limited number of people living on land sufficient to provide a subsistence way of life. The system fails because of the high pressure of the human and animal population, a lack of sufficient land for lengthy fallow, the non-availability of off-farm purchased inputs, rapid soil degradation, low yields and dwindling returns, and the high aspirations of modern living that subsistence agriculture cannot fulfill.
A. Requirements of a sustainable improved agricultural system
1.
High energy flux
2. Less drudgery and more dignity
3.
Effective water management
4.
Soil restorative
5.
Ecologically compatible
Creative and sustainable agricultural technologies must promote symbiosis between humans and the environment. Sustainable technologies must conform to and address the specific ecological constraints and long-range consequences of the simplification of the complex, diverse, and unperturbed natural ecosystems. Sustainable development must address the issue of assuring food supply, enhancing the soils productivity, preserving environmental quality, and transforming subsistence farming into economically viable commercial agriculture. The latter is possible only if the size of farm-holdings is economically viable, and agri-based supporting industries are developed to absorb labor displaced by transforming labor-intensive farming into science-based commercial enterprise. For an improved and new agricultural system to be sustainable, it must have the following characteristics:
The overall energy input and output must be regulated to enhance the output/input ratio, and attain positive thermodynamic energy balance with respect to biological yield vs. production inputs. Both energy efficiency and flux must be increased. A low-output subsistence system can be highly efficient in terms of energy use, but is unsustainable because of low productivity. The energy efficiency of a high-input system can, however, be improved by reducing nutrient losses by effectively containing leaching and erosion, and enhancing nutrient capital through judicious inputs of chemical fertilizers and organic amendments. Nitrogen deficiency is a major constraint in most soils of the tropics and subtropics. Locale-specific research is needed to ensure an adequate supply of nitrogen for the desired level of high yields through the input of synthetic chemical fertilizers supplemented by alternate sources of nitrogen (e.g., symbiotic nitrogen fixation through legume-based rotations and agroforestry systems, organic manures and composts, azolla). Similarly, the productivity of soils notably deficient in available P can be enhanced only through substantial and regular additions of phosphatic fertilizers (Sanchez and Salinas, 1981). The efficiency of fertilizer use can no doubt be increased by use of new cultivators, and through mycorrhizal infection. The use of synthetic fertilizers being inevitable, the strategy is to decrease their rate of application through better systems of soil and crop management. Hoping to increase and sustain agricultural production by adding chemicals alone, without improved and efficient systems of soil and crop management, is bound to cause frustrations and disappointments. It is the judicious combination of both management and inputs that is crucial to a sustainable development of agriculture. With the worlds highest rate of population observed in many countries of the humid tropics, low-input systems alone are obsolete, environmentally incompatible, soil degradative, and responsible for perpetuating low standards of living.
2. Less drudgery and more dignity
Agriculture must be made a respectable, dignified, and comfortable profession devoid of drudgery and undue misery. The approach adopted should be dynamic to replace labor-intensive systems with labor efficient technologies. The new technologies may be based on animal power or motorized farm operations with varying degrees of mechanization. The scientific community, including social scientists and anthropologists, must address the issue of human values and dignity. Tilling land with obsolete manually powered tools, controlling weeds with a back-breakhig hoe, threshing grain by beating with a stick or trampling by human and animal is not an inspiring profession. Concern about over-dependence on nonrenewable and rapidly dwindling fossil fuels is genuine but is not a justification for condemning resource-poor farmers to sub-human operations.
A suitable combination of soil and crop management techniques for in situ conservation of soil water is needed to enhance production and regulate environmental quality. For these regions, small-scale irrigation in the valleys can help to grow short-season crops during the dry season or prolong the growing season.
Improvement of subsistence farming is realizable through increasing production, sustaining the higher level of net output, and preserving the productive potential of natural resources through restorative measures of soil and crop management. The desired high net output must be achieved with a minimum of soil degradation. The soil quality and its productive capacity must be preserved and improved by preventing soil erosion, promoting the high biological activity of soil fauna, improving soil organic matter content, and replacing the nutrients harvested in crops and animals through chemical fertilizers and organic amendments supported by effective nutrient-recycling mechanisms. The productive efficiency of a system must be evaluated in terms of its effect on the natural resources (e.g., change in soil organic matter reserves, pH, nutrient status, exchangeable cations, plant-available water capacity or effective rooting depth). Suitable farming systems are those that enhance soil quality. Fertility-mining and soil-degrading low-input systems must be stopped.
Simplified agricultural ecosystems are more productive but often susceptible to environmental stresses compared with the natural ecosystems. The objective of sustainable management is to minimize the vulnerability of these systems to the degradative effects of accelerated erosion, rapid depletion of soil organic matter and nutrient reserves, and excessive build-up of unfavorable flora and fauna. The risks of instability or fragility created through the simplification of an ecosystem must be minimized through appropriate soil and crop management.
B. Components of a sustainable system
1.
Nutrient management
2.
Erosion management
3.
Residue management
4.
Crop management
5.
Water management
The extensive agricultural systems based on fertility-restorative measures involving shifting cultivation or bush-fallow rotation are no longer economically viable or ecologically compatible. Sustainable systems are those that produce continuous economic returns without causing large or long-term changes to the environment, or being ethically or aesthetically unacceptable. Locale-specific and on-farm validation is needed in adapting the research information already known. There is also a need for creating new knowledge through biotechnology and other modern innovations. Nonetheless, several proven sub-systems or components are available and can be used as building blocks for formulating sustainable systems for a range of agro-ecosystems. Some relevant components/sub-systems are:
Nutrient management is crucial to sustained production. Highly weathered Oxisols/Ultisols and Alfisols, being inherently low in nutrient reserves, must have a regular and supplemental nutrient supply to facilitate intensive cultivation for increased food production. Intensive land use and high yields on soils of low inherent fertility can be achieved only by raising the nutrient levels through the use of inorganic fertilizers, organic amendments, and nutrient recycling. Nutrient enhancement for these soils is indispensable.
Although crop production can be increased by increasing fertilizer use, many small land holders and resource-poor farmers cannot afford the expense. Policy makers and economic planners must develop long range strategies to ensure a dependable and timely supply of chemical fertilizers at affordable prices. Over-dependence on synthetic fertilizers and other agricultural chemicals must be avoided because the energy and economic costs of using them are prohibitive for the small land holders of the tropics. A combination of inorganic and organic fertilizers is, therefore, a useful strategy to minimize dependence on synthetic fertilizers, and enhance soil structure and physical characteristics. The rate of application of inorganic fertilizers can also be reduced by minimizing losses and increasing the recycling of nutrients. Losses by volatilization, leaching? and erosion can be controlled through conservation tillage, timely application by split doses, fertilizer placement, and slow-release formulations. Technological options for nutrient recycling include crop residue management and mulch farming, legume based rotations, ley farming with low stocking rate and controlled grazing, and agroforestry systems including alley cropping. Nutrient recycling mechanisms can work only if there is something useful to recycle. The systems are ineffective in highly weathered Oxisols and Ultisols which predominantly contain Al+3 and Mn+3 in the sub-soil horizons and are devoid of basic cations. In alley-cropping systems, crop yields can be decreased due to competition for nutrients water, and light (Lal, 1989a, 1990a). There are some advantages in substituting biological nitrogen fixation for inorganic fertilizers. However, the economics of growing nitrogen versus buying nitrogen have to be carefully evaluated in terms of thee, land scarcity. and efficiency of nitrogen availability.
Soil erosion control is crucial to the sustainable management of soil resources. Several technological options are available for erosion management. The choice of an appropriate option should be carefully made, with due consideration of soil types, land form and terrain characteristics. rainfall regime and hydrology, cropping/farming system, and the socioeconomic factors.
Erosion preventive measures are those that enhance soil structure, decrease raindrop impact, improve infiltration capacity, and decrease runoff rate and amount. These techniques are judicious systems of soil and crop management (e.g.. mulch farming through cover crops and planted fallows, multiple cropping, multi-storey canopy including agro-forestry, and conservation tillage). The latter is a generic term (Lal, 1989c) and encompasses a wide range of tillage systems that provide at least 30% of ground cover through crop residue mulch. Conservation tillage systems are generally reduced tillage systems that eliminate or decrease the frequency of soil inversion (e.g., no-till, chisel-till, plow plant). The no-till system is generally suitable for structurally active soils and in regions with sufficient availability of crop residue for mulch and of herbicides for weed control. The suitability of different tillage systems depends on soil properties, climate characteristics, and a range of other factors, including socioeconomic and anthropological considerations (Lal, 1986b). Soils highly susceptible to erosion should, however, be managed by no-tillage or ridge tillage systems. Ridge tillage, a series of alternating ridges and furrows formed on the contour, is a widely adapted conservation measure (Lal, 1990a). The system is used with variations ranging from ridges with gentle gradients to facilitate water harvest to cross-ties to form a series of basins to conserve water in situ by the tied-ridge system (Lal, 1987c). The practice is useful on shallow soils to increase the effective rooting volume, on poorly drained soils to grow upland crops, on nutrient-deficient soils to heap up the fertile top soil, on steep lands to provide a system for safe disposal of excess runoff, and in all conditions to facilitate the harvest of roots and tuber crops.
A regular and sizable addition of organic material is essential to maintain a favorable level of soil organic matter content and to stimulate the biotic activity of soil fauna (e.g., earthworms and termites). The structural collapse of soils with predominantly low-activity clays can be avoided by maintaining high organic matter content and enhancing the activity of soil fauna. Crop residue mulch is an important ingredient of any improved farming/cropping system. Frequent applications of 4 to 6 t/ha of residue mulch applied to the soil surface are beneficial for soil and water conservation, regulating soil moisture and temperature regimes, improving soil structure, enhancing the biological activity of soil fauna, and protecting soils from high-intensity rains and from ultradesiccation. Mulching also suppresses weed growth.
While the beneficial effects of mulching are widely recognized, procuring a mulch material in sufficient quantity is a serious practical problem. The management of crop residue as a source of mulch is, therefore, linked closely with cropping system, tillage methods, and planted fallows. A range of cultural practices is available to ensure an adequate amount of residue mulch for soil protection and fertility enhancement.
Live mulch, alley cropping, ley farming, planted fallows, and use of industrial by-products are some of the cultural practices specifically adopted to procure the mulch. Once again, the suitability of a practice depends on the locale-specific biophysical and socio-economic environment.
A continuous ground cover is necessary to provide a buffer against sudden fluctuations in micro- and meso-climate, and to prevent the degradative effects of raindrops or high-velocity winds. Timely planting, use of viable seed at optimum rate, improved cultivars and cropping systems, fertilizer use, and pest control are all important aspects related to crop management. The benefits of timely planting include a buffer against uncertain rains, an unfavorable soil temperature regime, pest infestation, and an unfavorable market. Planted tallow's, both legume and grass covers, are generally more effective in restoring soil fertility and improving soil physical properties than natural fallows. Soil organic matter can be increased and soil structure improved over a short period of two to three years.
There are several methods of managing cover crops. Live mulch is a system of growing grain/food crops through a low-canopy cover crop. The latter is suppressed chemically or mechanically. Mixed cropping, sequential cropping, and relay cropping are recommended to create diversity, so that many crop species of varying growth cycles, canopy structures, root systems, and water and nutrient requirements can be grown to maximize the use of a limited resource. The benefits of mixed cropping, growing more than one species simultaneously on the same field, are generally greater under adverse conditions with low inputs than in regions of few constraints with high inputs (Willey, 1979; Okigbo, 1987).
Agroforestry involves growing trees and woody shrubs in association with food crop annuals and/or animals. Alley cropping is a special system of agroforestry whereby food crop annuals are grown between rows of specially planted woody shrubs or trees. Shrubs are pruned regularly during the cropping season to prevent shading, provide mulch, and reduce water use by perennials. If properly managed, pruning from these crops can provide 30 to 50 kg/ha of N (Ghuman and Lal, 1990). When planted on the contour, hedges also decrease losses due to runoff and soil erosion (Lal, 1989a).
Mixed farming can be a stable system for small land holders, provided pastures are lightly grazed, the stocking rate is low, and animal waste is applied to the land to replenish soil fertility. Mixed farming with an excessive stocking rate and uncontrolled grazing is usually unsuccessful and degradative to soil and environment, as is the case in the African Sahel.
Water management is crucial to avoid mid-season drought, prolong the growing season, enhance rainfall effectiveness, reduce the risks of accelerated erosion, and minimize leaching losses of nutrients. The effectiveness of rainfall is drastically reduced by soil degradation, and can be enhanced by in situ conservation of water by increasing infiltration and reducing runoff, and improving soil water retention in the root zone by decreasing soil evaporation. In situ conservation of soil water is achieved by conservation tillage, mulch farming, and appropriate cropping systems. Runoff management, safe disposal of excess water and its storage in small reservoirs for supplementary irrigation and for livestock, is also an important strategy.
Although general principles may be the same, technological packages (systems) for the sustainable management of soil and water resources are site-specific and depend on farming/cropping systems, farm size, the availability of essential inputs, and socio-economic 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 need to be assessed through research. The systems research is conducted preferably on ,benchmark" soils of "ecological regions". By so doing, the agro-economic productivity of different production systems can be related to soil and climatic characteristics. 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. The alternate agronomic approach is field experimentation on a step-by-step gradual improvement level, by substituting the component that is the major constraint to crop and animal production. Identification of, the major constraint and alleviation through technological options are once again topics of on-farm research. The agronomic approach is a long-term strategy aimed at transforming low-input subsistence farming into science-based commercial agriculture. Researchable priorities in this approach involve an assessment of the components or subsystems outlined in three stages (i.e.. traditional farming, improved farming, and commercial agriculture). In addition, specific research priorities should be given to soil and crop management practices that increase the efficiency of water and fertilizer use, and restore eroded and degraded lands. There is also a need for more research into the development of high-yielding crops/cultivars through biotechnology.