Fertilizer impacts on soils and crops of sub-Saharan Africa

This paper: (1) evaluates the potential impacts of fertilizer, both positive and negative; (2) suggests ways in which positive impacts can be maximized and negative impacts minimized; and (3) identifies national strategies that have the greatest potential to achieve positive impacts and address the constraints of farmers. Major findings are as follows: (1) Declining fertility and soil organic matter (SOM) in SSA are a result primarily of agriculture-induced degradative processes (especially soil mining, tillage, and accelerated erosion) that can be reversed using high levels of nutrient inputs as part of "agro-ecological" farming systems to recapitalize the soil. (2) Fertilizer is recommended for recapitalization because nutrients available from organic sources in low-fertility African ecosystems are not adequate. (3) The primary positive impact of fertilizers is to increase the biological base of the plant/soil system, resulting in increased crop yields. If the system is properly managed, the outcome can be a fertile and efficient cycling system for nutrients and water due to improved soil structure associated with increased levels of SOM. Since there is competition for uses of crop residues (fuel, construction, animal feed), biomass production needs to increase and alternatives found to satisfy other demands for crop residues. (4) 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 (or equivalent manure) should be returned to the soil. (5) Because of the very high quantities of residue or manure required to reverse declines in SOM and the 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. If SSA is to recapture its native levels of soil fertility, long-term fallows, which accomplished this task in the past, must be replaced with (or adapted to) appropriate integrated systems that include fertilizers or other effective input sources, as well as no-till (or mulch tillage), cover crops, rotations, and/or agroforestry practices based on sound "agro-ecological" principles. This is especially critical for smallholder farmers who make up the vast majority of agricultural producers in SSA and who face severe economic and technical constraints. (Author abstract, modified)