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

Abstract

AbstractCrop 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 nitrogen (N) 2 kg phosphorus (P) and 15 kg potassium (K) ha−1 yr−1 for traditional fields planted to pearl millet (Pennisetum glaucum L.) 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 bow nutrient exports at the level of individual fields, through grain yields and biomass removal as forage, firewood, construction material, and also through run‐off, wind and water erosion, leaching and volatilisation, are either losses for the ecosystem or sources of nutrients transferred within the ecosystem. Livestock is a vector of nutrients from rangelands to manured fields through forage intake and excretion and helps to shortcut nutrient cycles, but it is also a net consumer of organic matter and minerals. Through the effects of trampling on the soil and of grazing on the vegetation composition and production, livestock also indirectly affects the cycling and transfer of nutrients. Net nutrient inputs by heavy rains causing overland flow, dust deposition and through biological nitrogen fixation, are also unequally distributed and thus aggravate the fertility gradient from large nutrient ‘source’ areas such as rangelands to small ‘sink’ areas such as fallows, low lands, temporary lakes and river benches. At a finer resolution, shrubs and trees with their alternating periods of nutrient storage and recycling in leaves and wood, micro‐depressions, termite mounts and ant nests become localised points of nutrient concentration and high crop productivity. 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 by two different pathways, either via mineral fertilisers applied to croplands or via externally produced supplements fed to livestock. These pathways are complementary and each one has direct effects on the other. Fertiliser application to croplands affect livestock through the increase in available forage and crop residue feed, whereas feed supplementation affects crop production through higher amounts and better quality of manure. The adoption of either strategy by farmers requires a high nutrient use efficiency at low cost. Data from regional field trials revealed increases in total dry matter (TDM) of cereals with broadcast annual P application at 13 kg ha−1 ranging across three years from 19 to 88% for rockphosphate and from 34 to 102% for single superphosphate (SSP). Still, a low‐external input approach seemed more advantageous to farmers. The placement of NPK fertiliser at 4 kg P ha−1 with the seed at or shortly after planting caused average TDM increases of 70% for millet, sorghum, maize, cowpea and groundnut and showed an up to three‐fold higher phosphorus use efficiency than broadcast P. 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% and stover yield by 21% on a field manured at a rate of 3 t DM ha−1. However, even with such approaches the current trends of declining soil productivity can only be reversed, if local policy makers are willing to support agricultural change through better terms of trade for agricultural products.