Nutrients in the West African Sudano-Sahelian zone: Losses, transfers and role of external inputs

Crop growth on acid sandy soils of the Sudano-Sahelian zone is primarily limited by the low amounts of organic matter and available mineral nutrients in the topsoil. The shortening of fallow periods with population growth, the exploitation of fire wood, spatial nutrient transfers by wind and water and net nutrient exports with crop yields in the order of 15 kg N, 2 kg P and 15 kg K ha<sup(<minus>1)> yr<sup(<minus>1)> for traditional fields planted to pearl millet (Pennisetum glaucum) in the southern Sahel have exhausted these resources. Large productivity declines in the prevailing agro-pastoral systems are the consequence. Data are presented to show how nutrient exports from individual fields are either losses for the ecosystem or sources of nutrients transferred within the ecosystem. To balance losses of nutrients in these integrated systems and to obtain sustainable increases in production, external inputs of nutrients are necessary. These may be introduced either via mineral fertilisers applied to croplands or via externally produced supplements fed to livestock. Data from regional field trials showed increases in total dry matter (TDM) of cereals with broadcast annual P application at 13 kg ha<sup(<minus>1)> ranging across three years from 19 to 88 percent for rock phosphate and from 34 to 102 percent for single superphosphate. A low-external input approach seemed more advantageous to farmers. The placement of NPK fertiliser at 4 kg P ha<sup(<minus>1)> with the seed at or shortly after planting caused average TDM increases of 70 percent for millet, sorghum, maize, cowpea and groundnut and showed an up to threefold higher phosphorus use efficiency than broadcast On the other hand, late dry season supplementation of grazing steers with millet bran not only decreased animal weight losses but also increased N and P concentrations in the faeces. The larger nutrient concentrations in the manure increased millet grain yield by 28 percent and stover yield by 21 percent on a field manured at a rate of 3 t DM ha<sup(<minus>1)>.