Soil Fertility Status in Equatorial Africa: A Comparison of the Great Rift Valley Regions and Central/Western Africa

Abstract

Soil fertility was measured in contrasting regions of sub-Saharan Africa, i.e., Tanzania, Rwanda, western D. R. Congo, Cameroon, Nigeria, Burkina Faso, Ivory Coast, and others, with special reference to geological and climatic conditions. The general properties of the soils in each region of equatorial Africa could be summarized as follows: when comparing the soils in central and western regions of equatorial Africa, the soils in Tanzania are affected more or less by the Great Rift Valley movement, including volcanic activity, and are considered relatively fertile. The base level is generally high, and soils are therefore not intensely acidic, and soil texture is intermediate. The soil organic matter (SOM) level is moderately high, partially affected by volcanic activity and relatively high elevation. Clay mineral composition also suggests that the soil in this region is somewhat less weathered and possibly supplies more mineral nutrients than soils in the other regions. A similar, but more definitive, advantage of volcanic soil could be found for the soils in the volcanic regions of the highlands of Rwanda and eastern D. R. Congo. The soils in this region are characterized by high base levels, high cation exchange capacity (CEC) values (which are influenced by the presence of both SOM and 2:1 clay minerals), intermediate to clayey soil texture, and relatively high SOM levels; the latter two are affected by parent materials and cool temperatures. In contrast to the Great Rift Valley regions, a large part of Cameroon is situated on the Cameroonian plateau, which is composed of Precambrian basement rocks under humid climates. The soils in this region are characterized by a strong acidic nature, high levels of exchangeable Al, fewer base components, moderately low SOM level, and clayey soil texture, dominated by inactive kaolin minerals. The soils in the western regions of equatorial Africa, such as the Nigeria/Benin and Burkina Faso/Ivory Coast/Liberia regions, are commonly characterized by the presence of sandy soils. The sand content usually exceeds 70 %, while clay content is less than 20 %. As a result, the base reserve is typically low, and the SOM level is less than 10–15 g C kg−1 soil.