Fertilizer Impacts on Soils and Crops of Sub-Saharan Africa

Successful agricultural development has resulted in substantial alleviation of poverty and food security in Asia and Latin America since the 1960s. Much of this success can be attributed to the introduction of high-yielding varieties of crops, especially wheat and rice, which have addressed the constraints faced by farmers using traditional varieties. In Sub-Saharan Africa (SSA), however, productivity levels have remained stagnant despite the introduction of new crop germplasm. In recent years, scientists have recognized that low soil fertility is the primary constraint blocking agricultural development in SSA. Major findings from this study may be summed up in five key points. Declining fertility and SOM in SSA are a result primarily of agriculture-induced degradative processes that can be reversed using high levels of nutrient inputs as part of "agro-ecological" farming systems to recapitalize the soil. Fertilizer is recommended for recapitalization because nutrients available from organic sources in low-fertility African ecosystems are not adequate. The primary positive impact of fertilizers is to increase the biological base of the plant/soil system resulting in increased crop yields. Fertilizers and organic matter are complements rather than substitutes - both are recommended to recapitalize SSA soils. Fertilizer can increase crop yields and residues, but maximum levels of residues should be returned to the soil. Because of the very high quantities of residue or manure required to reverse declines in SOM and inadequate supplies of these materials, integrated "eco-intensive" systems are recommended to create an aggrading system, including mulch or conservation tillage and agroforestry/cover crops.