Classified Soils in the Tropics According to the USDA Soil Taxonomy, Oxisols are the most abundant soils in the humid and perhumid tropics covering about 35 percent of the land area. Ultisols are the second most abundant, covering an estimated 28 percent of the region. About half of the Ultisols and 60 percent of the Oxisols are located in humid and perhumid tropical Africa and Asia. In tropical Africa, they are abundant in the eastern Congo basin bordering the lake region; in the forested zones of Sierra Leone; in Ivory Coast; in parts of Liberia; and in the forested coastal strip from Ivory Coast to Cameroon.
The Alfisols, which have high to moderate fertility, cover a smaller area of the humid tropics. In west Africa they are found in Ivory Coast, Ghana, Togo, Benin, Nigeria and Cameroon. They are, however, the most abundant soils in Africa's subhumid and semi-arid zones, covering about one third of these regions. The Alfisols are widely distributed in the subhumid and semi-arid tropical regions of Africa, including large areas in western, eastern, central, and southeastern Africa.
Table. Geographical distribution of soils in the humid and semi-arid tropics
(millions of hectares).
(millions of hectares).
Soil order
|
Tropical Africa
|
Tropical Asia
|
Tropical America
|
Total
|
Percent
|
| Humid Tropics1) | |||||
| Oxisols |
179
|
14
|
332
|
525
|
35
|
| Ultisols |
69
|
131
|
213
|
413
|
28
|
| Alfisols |
21
|
15
|
18
|
54
|
4
|
| Others |
176
|
219
|
103
|
498
|
33
|
| Total |
445
|
379
|
666
|
1490
|
100
|
| Semi-arid Tropics2) | |||||
| Alfisols |
466
|
121
|
107
|
694
|
33
|
| Ultisols |
24
|
20
|
8
|
52
|
1
|
| Others |
972
|
178
|
198
|
1348
|
66
|
| Total |
1462
|
319
|
313
|
2094
|
100
|
1) Data from NAP (1982).
2) Data adapted from Kampen and Burford (1980). Part of the subhumid tropics is included.
