The Geological Basis of Farming in Africa

Abstract

Soil fertility is largely a function of climate, parent material and management of the soils. Parent materials are derived from various rock types. Most of the rocks in Africa are of Precambrian age, more than 544 million years old. The oldest rocks in Africa, Archean granites and granite gneisses as well as old volcanic rocks, called greenstones, are more than 3.2 billion years old. They form the stable granite–greenstone nuclei of the continent. Folded Precambrian metamorphic belts, the so-called mobile belts, accreted around these nuclei. While the granite and granite gneiss-dominated areas are mineralogically and chemically more homogenous, the areas underlain by deeply eroded Precambrian fold belts vary widely in composition. The African post-Precambrian (<544 million years) history is marked by the development of sedimentary basins with varying sedimentary rocks at the perimeter of the igneous and metamorphic stable continent. Extensive parts of the interior of Africa are covered by Mesozoic to Cenozoic sandstone-dominated basins. Soil rejuvenating volcanic rocks extruded from lower parts of the earth crust and mantle mainly during Mesozoic and Cenozoic times in linear zones. Massive volcanic outpourings took place since about 30 million years, in Tertiary Rift environments. Soils derived from these rock types vary. Soils derived from silica oversaturated igneous and metamorphic rocks as well as sandstone-dominated lithologies form sandy soils with limiting, inherently low nutrient concentrations and low water-holding capacities. In contrast, soils derived from volcanic, silica-saturated or silica-undersaturated igneous rocks, young or old, contain higher concentrations of total Ca, Mg, P and micronutrients. Under suitable climatic conditions they weather into more clay-rich, fertile soils with high water-holding capacities. Examples of strong inherent soil differences related to the underlying parent materials are given from Zimbabwe and Uganda. Inherent differences in soil properties can spatially vary over short distances. The main nutrient constraints of African soils are N and P deficiencies. Most soils are also low in organic matter. There are many soil fertility restoration management strategies, making use of local organic and imported and local inorganic nutrient resources, such as phosphate rocks and fertilizers. Africa is endowed with large deposits of naturally occurring agrominerals and fertilizer raw materials. Natural gas, the principal feedstock for industrial N fertilizer production, is found in coastal zones of Africa, mainly at the northern and western sides of the continent. Extensive sedimentary phosphate rock resources occur along the west coast of Africa and in North Africa. Igneous phosphate rocks are found mainly in eastern and Central Africa and along linear zones in West and southwest Africa. Large gypsum deposits occur in Mesozoic strata in coastal sedimentary basins. Calcium–magnesium carbonates, in various forms, occur in almost every country of the continent.