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close this bookSustaining the Future: Economic, Social, and Environmental Change in Sub-Saharan Africa (UNU, 1996, 365 pages)
close this folderPart 2: Environmental issues and futures
close this folderTropical deforestation and its impact on soil, environment, and agricultural productivity
View the document(introductory text...)
View the documentIntroduction
View the documentTRF and its conversion
View the documentSoils of the TRF ecosystem
View the documentForest conversion and soil productivity
View the documentDeforestation and the emission of radiatively active gases
View the documentDeforestation and hydrological balance
View the documentSustainable use of the TRF ecosystem
View the documentResearch needs
View the documentReferences

Soils of the TRF ecosystem

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)

Soil type

Region

  America Africa Asia Total
Oxisols 332 179 14 525
Ultisols 213 69 131 413
Inceptisolsa 61 75 90 226
Entisolsb 31 91 90 212
Alfisols 18 20 15 53
Histosols   4 23 27
Spodosols 10 3 6 19
Mollisols - - 7 7
Vertisols 1 2 2 5
Aridisols - 1 1 2
Total 666 444 379 1,489

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

Region

    America Africa Asia Total
· 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
· Organic Histosols - 1 6 2
    100 100 100 100

Source: NRC (1982).

Table 9.7 Soil-related constraints on intensive land use for food crop production in the TRF ecosystem

Constraint Oxisols Ultisols Alfisols Inceptisols Mollisols Andisols
Physical
Accelerated erosion 2 2 3 2 1 1
Soil compaction & crusting 2 2 3 2 1 1
Root impedance 3 3 3 1 1 1
Moisture imbalance 2 2 3 1 1 1
Shallow depth 2 2 3 1 1 1
Nutritional            
N deficiency 3 3 2 2 1 1
P deficiency 3 3 2 1 1 1
Al & Mn toxicity 3 3 1 1 1 1
Micro-nutrient deficiency 3 3 2 1 1 1
Biological
Soil fauna 2 2 2 1 1 1
Biomass carbon 2 2 2 1 1 1

3 = severe; 2 = moderate; 1 = slight.