|Sustaining the Future: Economic, Social, and Environmental Change in Sub-Saharan Africa (UNU, 1996, 365 pages)|
|Part 2: Environmental issues and futures|
|Tropical deforestation and its impact on soil, environment, and agricultural productivity|
The predominant soils of the humid tropics are oxisols, ultisols, and alfisols (table 9.5). Oxisols and ultisols comprise 63 per cent of soils of the TRF (table 9.6). These soils are highly weathered, leached, devoid of basic cations, and relatively infertile. Young soils of moderate to high fertility (mollisols, inceptisols, and entisols) occupy about 15 per cent of the total land area. There are several soil-related constraints on intensive food crop production in the humid tropics. The principal constraints are listed in table 9.7. Oxisols and ultisols have low nutrient reserves and are prone to toxicity owing to high concentrations of Al and Mn. In general, these soils have high P-fixation capacity. Alfisols are relatively more fertile than oxisols and ultisols. However, alfisols have weakly developed structure and are highly prone to accelerated soil erosion. The effective rooting depth for food crops and annuals is generally 20-30 cm owing to either physical (compacted, concretionary, or gravelly subsoil) or chemical (Al or Mn toxicity, low P) limitations. Coupled with low plant-available water reserves, water deficiency can be a problem for shallow-rooted annuals. In contrast, upland crops can be subjected to periodic inundation and anaerobiosis. With proper management, however, the agricultural productivity of these soils can be greatly improved while minimizing risks of soil and environmental degradation. An impor tent strategy in enhancing the productive potential of these soils is to reduce the adverse effects of forest conversion.
Table 9.5 Geographical extent and distribution of major soils of the humid tropics (ha million)
Source: N RC (1993).
a. Inceptisols include Aquepts, Tropepts, Andepts, and Entisols.
b. Entisols include Fluvents, Psamments, and Lithic Entisols.
Table 9.6 Soils of the humid tropics (% of the total area)
|Principal feature||Soil type||
|· Acid, infertile||Oxisols and ultisols||82||56||38||63|
|· Moderately fertile, & well-drained||Alfisols, vertisols, mollisols,inceptisols, andisols, fluvents||7||12||33||15|
|· Poorly drained||Aquepts||6||12||6||8|
|· Very infertile, sandy||Psamments, spodosols||2||16||6||7|
|· Shallow||Lithic entisols||3||3||10||5|
Source: NRC (1982).
Table 9.7 Soil-related constraints on intensive land use for food crop production in the TRF ecosystem
|Soil compaction & crusting||2||2||3||2||1||1|
|Al & Mn toxicity||3||3||1||1||1||1|
3 = severe; 2 = moderate; 1 = slight.