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A. Sustainability and soil quality
B. Sustainability and forest conversion
C.
Attaining sustainability
D.
Objectives
Agricultural sustainability and judicious use of soil and water resources in the humid tropics are major global issues of modern times because of the interplay among human population, socio-economic and political factors, and natural resources of the fragile ecoregion. Mismanagement of soil resources and inefficient, resource-based agricultural systems are causing serious degradation of the ecoregion and perpetuating food deficit, malnutrition, and poor standard of living. In an attempt to develop the "last frontier", vast areas of tropical rainforest (TRF) are devastated annually by fire, machete, axe, chainsaw, bulldozers, and defoliants to grow food for the ever-growing population produce industrial raw material, and develop the infrastructure for new migrants into the region (NRC, 1993a). Ecologically incompatible methods of forest conversion, inappropriate land uses, and unscientific systems of soil and crop management based on fertility-mining techniques accelerate soil erosion, pollute natural waters, disrupt water and energy balances at micro- and meso-scales within the ecosystem, and disrupt cycles of elements (e.g., C, N, and S) with global ecological consequences (Post et al., 1990). A major global consequence of deforestation, burning, and conversion to unsustainable land uses is the release of staggering amounts of CO2 and other radiatively active or greenhouse gases into the atmosphere (Woodwell et al., 1983; Detwiler, 1986; Detwiler and Hall, 1988a, b; Crutzen and Andreae, 1990; Houghton, 1990a, b; Houghton and Skone, 1990; Schlesinger, 1991, 1993; Lal et al., 1994). If improved and scientific methods of forest conversion, land uses. and agricultural systems are not adopted in the near future, gross disturbances of the fragile TRF ecoregion can lead to irreversible degradation of soil and the environment (WRI, 1992 93; Oldeman, 1994).
A. Sustainability and soil quality
Sustainable agricultural systems imply successful management of resources for agriculture to satisfy changing human needs while maintaining or enhancing the quality of the environment and conserving natural resources (TAC, 1989; CGIAR, 1990b). High productivity is an important aspect of agricultural sustainability in the humid tropics. However, most of the prevalent land uses and predominant agricultural systems in the humid tropics are characterized, with a few notable exceptions of silviculture and rice-based systems, by low crop and animal productivity, rapid decline in productivity with continuous use, and high rates of soil and environmental degradation. Land-use systems based on fertility-mining practices of low input are usually unsustainable. Sustainable management of soil resources implies maintaining high productivity per unit area on a continuous basis, enhancement of soil quality, and improvement in environmental characteristics (Lal and Miller, 19931. Major attributes of sustainable land use are:
- using land resources on a long-term basis;
- meeting present needs without jeopardizing future potential;
- enhancing per capita productivity;
- maintaining/enhancing environmental quality; and
- restoring productivity and the environmental regulatory capacity of degraded and impoverished ecosystems.
The principal objective of a sustainable land-use system is to maintain a high level of productivity, maintain or improve environmental and aesthetic attributes, and enhance soil quality. Sustainability is intimately linked to soil quality (Fig. 1), which must be maintained or enhanced. Inputs in Fig. 1 refer to additions of off-farm essential nutrients and amendments, while management refers to judicious use of on farm resources to minimize waste and the risks of soil and environmental degradation.
Soil quality refers to the soils ability to produce economic goods and services, and maintain acceptable standards of environmental quality and functions within the ecosystems potential and constraints (Lal and Miller, 1993). Soil quality depends on a range of soil properties and processes. Soil properties important to its quality are soil structure, soil organic matter content, plant-available water and nutrient reserves, aeration, and rate and magnitude of nutrient cycling and transformations. Deterioration in soil quality affects the soils life-support processes. Soil processes important to soil quality include accelerated erosion, leaching soil organic matter, and fertility depletion leading to nutrient deficiencies and toxicities, and anaerobiosis.
Fig. 1 Soil quality in relation to soil properties, soil processes, and environmental quality
B. Sustainability and forest conversion
Deforestation of TRF by unsuitable methods and conversion to agricultural land use with resource-based systems can adversely affect the soil properties and processes that lead to a rapid decline in soil quality with a negative impact on sustainability and environmental quality. These adverse and negative effects have been the basis of rethinking about continuous cultivation in the humid tropics (Fearnside, 1987; Lugo, 1988; Goodland, 1991). In these cases, anthropogenic perturbations set in motion interacting soil and environmental degradative processes (Fig. 2). Soil degradative processes include: (i) decline in soil structure as related to compaction, crusting, decrease in water retention and transmission properties, poor aeration, and impeded root growth: (ii) reduction in the quality and quantity of soil organic matter and disruption in cycles of C. N. P, and S. leading to nutrient depletion and toxicity and decline in soil biomass carbon and soil biodiversity; (iii) perturbation in the hydrological cycle, in conjunction with deterioration in soil structure, leading to high losses due to runoff and accelerated erosion, high evaporation causing frequent drought stress, and increase in leaching losses of bases and other plant nutrients; and (iv) perturbation of energy balance leading to high soil and air temperatures decrease in the minimum relative humidity, and overall aridization of the micro- and meso-climates. There are, of course, some positive effects favorable to crop growth (e.g., increase in radiation and less competition for water from deep-rooted trees).
The objective of using improved technology is to minimize the magnitude and rate of soil degradation set in motion by conversion of TRE to agricultural land uses, and to enhance soil quality and resilience so that productivity can be sustained with minimal adverse impact on soils and environment. Some data with promising results from on-station experimentation in Africa, Latin America, and Asia are available on sustainable use of soil resources in the humid tropics. Given the re search information on productive potential and technical know-how available for sustainable management of these regions, it is technically feasible to manage these resources intensively for sustained agricultural production without jeopardizing their future potential (Jurion and Henry, 1969; Sanchez et al., 1982, 1983; Sanchez and Benites, 1987; Lal, 1986a, 1987c, 1989c; Juo, 1989; Alegre and Sanchez, 1991).
Fig. 2 Land degradative processes set in motion by deforestation of TRF and conversion to agricultural land use
The objective of this compendium is to collate available research information on scientifically proven methods of deforestation for conversion to agricultural land use, and on soil and crop management systems for sustained use of soil and water resources in the humid tropics. The compendium provides an overview of the improved and science-based technological options for sustainable production and cites examples of appropriate techniques along with their impact on production, soil properties, and environment. The material presented is written simply so that it can be used by practitioners, extension agents, policy makers, and others interested in developing and managing TRF ecoregions. Careful evaluation is also done to identify knowledge gaps and the need to fine-tune technology by adaptive research for representative soils and ecological conditions. Although most of the examples cited are drawn from the research done in West Africa, the compendium identifies generic technological options that can be adapted further to achieve sustainable use of soil resources in the TRF ecoregion. The focus of this compendium is on soil and water management, with full realization that other aspects (i.e., pest management, improved crops and cultivars, cattle ranching, socio-economic and policy considerations) are also important to sustainable use of these resources. Therefore, readers are referred to other literature on these important topics (Fearnside, 1983, 1986; Hecht et al., 1988: Ehui and Hertel, 1989, 1992a, b; Buschbacher, 1990: NRC. 1993a).