Background

Concerns are growing about long-term sustainability of agriculture. Fertile land and sufficient water are vital for sustaining agriculture and livelihoods. In Africa productivity of land, however, has been decreasing with the increasing intensification of agriculture due to land degradation. The major causes of land degradation are unsustainable agricultural practices like farming on steep slopes without sufficient use of soil and water conservation measures, monocropping, excessive tillage, or declining use of fallow without appropriate replenishment of soil nutrients, burning of crop residues, conversion of forests, woodlands and bushlands to permanent agriculture, or their excessive exploitation through fuel wood and timber harvesting, overgrazing of rangelands, and lack of proper soil organic matter management.

Land degradation occurs in different forms on various land use types:

On cropland, soil erosion occurs through: water and wind; chemical degradation – mainly fertility decline – due to nutrient mining and salinity; physical soil degradation due to compaction, sealing and crusting; biological degradation due to insufficient vegetation cover, decline in soil organic matter; and water degradation mainly caused by increased surface runoff (polluting surface water) and declining water availability due to high evaporation.

On grazing land, biological degradation occurs through loss of protective vegetation cover and valuable species. As a result, alien and ‘undesirable’ species settle in the soil. Physical degradation of soil results in widespread and severe water runoff and erosion. In terms of affected area, it is estimated that overgrazing has been the most important contributor to degradation, followed by poor agricultural practices and then by overexploitation.

On forest land, biological degradation occurs through deforestation; removal of valuable species through logging; replacement of natural forests with mono-cropped plantations or other land uses (which do not protect the land), which have negative consequences including biodiversity loss and soil and water degradation.

Therefore, attempts at reducing hunger on the African continent must begin by addressing its severely depleted soils; intensifying and diversifying land use in combination with application of sustainable soil fertility management practices based on erosion control, soil protection, soil organic matter management, reduced soil cultivation, and appropriate use of soil amendments, fertilizers and irrigation. Use of appropriate seeds, and improved access to inputs and food markets also contributes to significantly improve productivity and sustain overall food production in Africa.

Chapter 2 "soils" outlines soil fertility management practices that can contribute significantly in improving productivity of African soils. It shall help farmers to manage their soils, water and farm-own, as well as foreign soil fertility resources in a sustainable way.

To begin with, the module introduces readers to perceive soil in its diversity as the basic resource to plant production and to recognize different soil types and their characteristics.