Understanding cropping systems in the semi-arid environments of Zimbabwe: options for soil fertility management

African smallholder farmers face perennial food shortages due to low crop yields. The major cause of poor crop yields is soil fertility decline. The diversity of sites and soils between African farming systems isgreat,therefore strategies to solve soil fertility problems should suit the opportunities and problems encountered in the different climatic regions. This thesis characterizes the semi-arid regions of south-western Zimbabwe and explores some of the strategies that can be used to provide farmers with more options for soil fertility improvement.Resource flow maps were used to study the characteristics of the semi-arid farming system of Tsholotsho (Mkhubazi)in south-westernZimbabwe. The results revealed that farmers in the region face perennial cereal grain shortages, but the poorly-resourced farmers are the most affected. Nutrient management is limited to the use of limited amounts of manure by the better-resourced and medium-resourced farmers. Poorly-resourced farmers did not apply any nutrients to their crops.The use of low rates of manure and fertilizer is one option that farmers in the semi-arid regions can adopt. Farmers who had access to small amounts of manure and fertilizer were able to increase cereal yields through farmer participatory research experiments. Previously the farmers did not apply manure to crops. In 2003-2004, with good rainfall maize yields due to manure applications at 3 and 6 t ha −1 were 1.96 and 3.44 t ha −1 compared to 1.2 and 2.7 t ha −1 from plots without. Top dressing with 8.5 kg N ha −1 increased yields to 2.5 t ha −1 with 3 t ha −1 of manure, and to 4.28 t ha −1 with 6 t ha −1 of manure. In dry years manure in combination with N fertilizer increased grain yield by about 0.14 and 0.18 t ha −1 .The research results also showed that it is possible to successfully grow grain legumes under the semi-arid conditions and derive substantial residual yield benefits to sorghum grown after the legumes. New varieties of grain legumes seemed to be well adapted to dry environments.Sorghum grain yields after legumes reached1.62 t ha -1in 2003/04, more than double the yields in the sorghum after sorghum rotation, and the yields were also higher in 2004/05.The Agricultural Production SIMulator (APSIM) was used to model the legume-sorghum rotation to test it's capability in simulating cropping systems in the semi-arid southernAfrica.The model output of N and water stress factors on plant growth assisted in better understanding the water, N and plant growth interactions within a cropping season, as well as the residual benefits of legumes interacting with variable seasonal conditions. The model showed that the residual benefits of the legumes were driven by nitrogen availability more than water even under the semi-arid conditions.Further research will focus on the simulation of long-term effects of the manure/fertilizer experiments and the legume-cereal rotations. The use of farming systems models is required in order to get a better understanding of the functioning of smallholder farming systems in semi-arid regions and identify possible development pathways of the systems.