Mining of Nutrients in African Soils Due to Agricultural Intensification - Principles of Sustainable Soil Management in Agroecosystems - page 403

Bosch et al. 1998; Henao and Banaante 1999, 2006; Cobo et al. 2010). This chapter
examines the causes and extent of long-term nutrient mining in African cropping
systems, focusing particularly on the impacts on the soils. The conclusion considers
ways to arrest the decline in soil fertility while increasing crop yields.

15.1.1 S oilS in A fricA
The initial soil fertility levels in the soils of Africa, as elsewhere, are largely deter-
mined by the soil organic matter (SOM) content, which in turn depends strongly on
the type of climax vegetation. The climax vegetation depends on the climate, which
in Africa ranges from arid, with a length of growing period of 0 days, to humid,
which has a length of growing period of >270 days. Vegetation types vary accord-
ingly from desert to tropical forests. Sanchez et al. (1982) suggested that some soil
orders in the tropics (Ultisols, Alfisols, and Mollisols) have similar levels of organic
matter as the same soil orders in temperate regions. However, Vlek and Koch (1992)
point out that published studies for Africa indicate that median SOM contents vary
greatly between regions, with 4% in East Africa, 0.5% in West Africa, and a range
from 0.12% to 0.17% in the Sahelian zone. They concluded that overall, the natural
endowment of African soils appears limited compared to other parts of the world
(Table 15.1).
Eswaran et al. (1997) estimate that with the low inherent fertility of African soils
and subsequent degradation of the land, only 16% of the land can be classified as
having soil of high quality, whereas 13% has soil of medium quality ( Table 15.2 ). A
total of 9 million km 2 of high- and medium-quality arable land supports 400 mil-
lion people, making up 45% of the total population, and this area is the main focus

TABLE 15.1
Distribution of Major Soil Orders in
Tropical Africa

Area

Soil Order (Million ha) Percentage

Oxisol 550 22

Aridisols 840 34

Alfisols 550 23

Ultisols 100 4

Inceptisols 70 3

Entisols 300 12

Vertisols 40 2

Total 2450 100

Source: Vlek, P.L.G., and H. Koch, Göttinger
Beiträge zur Land—und Forstwirtschaft in
den Tropen und Subtropen, 71, 139-160,
1992.