CURRENT STATUS

Technologies

Research on shifting cultivation has been conducted in Africa since the 1920s on replacing chitemene nutrients by sources other than burning. The work in the 1950s by Jurion and Henry (1969) in Zaire and of Nye and Greenland (1960) in Ghana are widely known. The anthropological basis of shifting cultivator cultures has been widely studied in Asia, Africa and Latin America. Long-term agronomic research has been conducted since the 1970s, primarily but not exclusively by four groups: at Yurimaguas, Peru, and Manaus, Brazil, by North Carolina State University (Sanchez et al. 1983, Sanchez and Benites 1987, Sanchez et al. 1987, Szott et al. 1991); at Ibadan, Nigeria, by IIT A (Yamoah et al. 1986, Juo and Law 1977, Kang et al. 1990); in northeast India by Ramakhrishnan and associates (Ramakrishnan 1984, Toky and Ramakrishnan 1981, Ramakrishnan 1987); and in Sumatra by AARD and associated institutions (Macintosh et al. 1981, Von Uexkull 1984, Wade et al. 1988). These efforts have provided several kinds of information:

• They have quantified the nutrient transfer process from biomass to ash and into soil and monitored the changes in soil properties upon cropping.

• Analysis of the dynamics of soil organic matter has shown that judicious management of inputs, vegetation cover and harvest residue can result in a sustained level of soil organic matter (SOM).

• They have determined that bulldozer clearing is inferior to traditional slash-and-burn in providing suitable physical and chemical soil properties for planting of food and tree crops. Detrimental effects of bulldozer clearing include topsoil carryover, soil compaction and the absence of ash as a nutrient-transfer process. Several major colonization projects are no longer based on bulldozer land clearing.

• They have determined that the shift in weed population from broadleaved species to grasses is one of the principal causes of land abandonment and often surpasses the depletion of soil fertility.

• They have determined that crop rotation allows crops to be grown continuously with judicious use of lime, fertilizer and green manures, producing sustainable yields in well-managed systems. With poor management, however, attempts at continuous production have resulted in sharp drops in productivity, soil compaction and erosion, even in flat areas.

• Low-input systems have the highest potential for sustainability in acid, low-fertility soils if they are based on the use of aluminium-tolerant germplasm of annual crops, pastures or trees. Systems based on this principle have shown sustainable production for more than 10 years at research stations, with evidence of improvement rather than degradation of physical, chemical and biological soil properties.

• Keeping the soil surface covered at all times is a key principle for sustainability in the humid tropics. Soil erosion can be controlled with the use of agroforestry systems, including alley cropping on slopes, live fences in pastures, annual crop-tree crop systems and managed forest fallows. The presence of perennial vegetation further promotes nutrient recycling by litter and root turnover. This is particularly effective in pastures and agroforestry systems.

• Current research indicates that shifting cultivation can be replaced by alternative systems that meet the food and fibre needs of the humid tropical farmer while providing for additional income by producing high-value, low-volume crops for export. With these crops (including rubber, palm oil, heart of palm, tropical fruits, pepper, medicinals) the humid tropics have a comparative advantage. Changing consumer values in the First World toward more nutritious and ecologically friendly products may increase this comparative advantage.

• Research on plantation forestry shows that many of the principles applicable to agricultural systems are also appropriate for soil conservation, fertility enhancement, weed control and crop selection in forest management.

The research synthesis shows that some alternatives are possible; there definitely is hope. But research has been conducted on an insignificant scale and primarily at research stations. Such a knowledge base needs to be expanded geographically and adapted to specific climate, soil and socio-economic constraints with different market opportunities. Research needs also to expand from on-station to on-farm testing.

Sustainable management options for acid soils of the humid tropics have been developed to fit different landscape positions, soils and levels of development of the socioeconomic infrastructure (Sanchez 1987). For instance, the principal sustainable management options and alternatives to slash-and-burn for one region, the Selva Baja of Peru, are paddy rice production of alluvial soils, low-input cropping, continuous cultivation, legume-based pastures, agroforestry, perennial crop production (rubber, oil palm) and plantation forestry. Their place in the landscape can be distinguished (fig. 1).

Nutrient recycling must be enhanced in all systems, to minimize the need for external nutrient inputs and maximize their efficiency. The management of crop and root residues is crucial in this regard (Swift 1987). Approaches proposed by TSBF on quantifying the nutrient release of organic input and managing soil organic carbon, nitrogen and phosphorus are major components of the research on low-input cropping, agroforestry and pastures. The promising results in predicting the rate of nutrients released from leguminous materials based on their polyphenolic contents (Palm and Sanchez 1991) provide for the first time an opportunity for the quantitative management of organic input in a manner comparable to the management of chemical fertilizers.

Figure 1. Management options for acid soils of the humid tropics.

However, even in situations where nutrient cycling is possible on a significant scale, it is necessary to employ supplemental fertilizer to maintain productivity. Research conducted by IFDC in Africa has shown that judicious use of fertilizers in combination with a programme of crop residue management is superior to the use of either fertilizers or nutrient cycling alone. Where phosphate rock deposits are available, it may be possible to substitute these agrominerals for commercial fertilizer phosphates.

For every hectare put into these sustainable soil management technologies by farmers, 5 to 10 hectares per year of tropical rain-forests will be saved from the shifting cultivator's axe, because of their higher productivity. Estimates at Yurimaguas, Peru, for various management options (Sanchez et al. 1990) are given in table 2. These estimates will vary with climate and soils.

Such technologies are particularly applicable to secondary forest fallows, where clearing does not contribute significantly to global warming because of the small tree biomass. The use of secondary forest fallows is of very high priority, because in many areas they are a feasible alternative to primary forest clearing. Many of the degraded or unproductive pasture or croplands resulting from poor management practices can also be reclaimed using some, but not all, of these available technologies.

Furthermore, research must also be more process oriented to better understand the 'why' questions and focus beyond the 'whats' listed in the preceding section. In particular, there is need to understand the processes that link agricultural and forestry management of these ecosystems to sustainable conservation of the surrounding environment. In addition, socioeconomic research should address 'how' farmers adopt or adjust new technologies, 'how' farmers and communities make decisions on short-term gain versus long-term resource conservation and 'how' the new technologies affect farmer decisions on forest clearance. Finally, research efforts should be more inclusive, with greater participation of NARS, NGOs and developed country institutions, effectively linked with the global climatic-change community (Bouwman 1990) and biodiversity, aiming at a common research agenda.