Micronutrients for better yields.

To feed the growing population in Sub Saharan Africa (SSA), yield levels currently attained in small-holder farming systems need to increase. In combination with other agronomic practices, the use of mineral fertilisers is indispensable for closing yield gaps. In SSA, fertiliser recommendations are often national or regional blanket recommendations, that do not take into account heterogeneity in soil fertility, resource availability, agro-ecological zones and/or crops. Furthermore, blanket fertiliser recommendations often only include N, P and K, whereas other secondary and micronutrients can also be yield-limiting. Of the micronutrients, Zn and B are expected to be the main yield-limiting nutrients in SSA soils. Zinc is not only essential for plant growth, but also for human health. A large part of the SSA population is estimated to be Zn deficient, which can partly be attributed to low soil Zn availability and consequently low Zn concentrations in crops that grow on them. The main objective of this thesis is to develop and evaluate models for predicting soil nutrient availability and to increase the understanding of the interactive effect of nutrient availability on yields and nutritional quality in SSA. The focus is on availability of macronutrients N, P and K as well as micronutrients Zn and B.