Soil Fertility Views from the Tropics.

Soil nutrient depletion is widely acknowledged to be the number 1 biophysical constraint to increasing yields in sub-Saharan Africa. Spurred by the ongoing African Green Revolution, cereal crop yields are beginning to increase. Tropical Africa presents a different setting (mainly smallholder family farms of < 5 hectares with not enough nutrients) in comparison with large commercial farms, many with too many nutrients. Research results include that mineral fertilizers must go first while the much needed organic fertilizers have to wait until soil fertility is restored; on site soil testing to guide the different blends are needed. P is limiting in only specific regions, while K, S, Fe and Zn deficiencies are very variable. Aluminum toxicity is rare in most of tropical Africa. The issue of non-responsive soils need to be fully understood. Organic inputs are now effectively classified in terms of their quality, and together are part of an integrated soil fertility management strategy. New metrics for high P fixing soils are proposed. There is a need to find out how to utilize the 50% of the P applied that remains in the topsoil worldwide.