Evaluating the Impact of Soil Fertility Heterogeneity on Maize Nutrient Requirement and Productivity in Smallholder Farming Systems

Small-holder farming systems in Sub-Saharan Africa are characterized by large variability in soil fertility at both farm and landscape levels, leading to variable crop productivity and crop response to additions of fertilizer. Consequently, large yield gaps arise from soil fertility differences between fields due to a combination of inherent and management factors. IPNI initiated a coordinated maize project in 2010, and has been collaborating with the University of Zimbabwe to assess maize production potential and develop site-specific nutrient management practices for variable soil fertility conditions in Eastern Zimbabwe. The project is implementing nutrient omission trials located at multiple sites, to identify which of the macronutrients (N, P, and/or K) are limiting maize growth, and to determine possible other constraints related to soil secondary and micronutrients and soil acidity.

Strong effects of nutrient application on maize yield were observed in the first season despite diminished responses due to long dry spells. The project has established three categories of fields that respond differently to application of nutrients: (i) degraded soil with less than 0.45% soil organic matter that showed a small yield response to fertilizer application; (ii) medium fertility fields that showed moderate responses to N, P, and K; and (iii) high fertility fields that showed strong responses to N, P and K. Yields with no fertilizer were less than 1 t/ha for all fields, while maximum yields were obtained when all macronutrients were applied. The yield with NPK ranged from 1.3 t/ha in degraded soils to 2.9 t/ha in medium fertility fields to 3.8 t/ha in the high fertility fields.

The project results highlight the strong effects of soil fertility variability on maize productivity and the need to balance N, P, and K supply to optimize maize productivity. Efforts to intensify maize production in north-east Zimbabwe using NPK fertilizers should target the fields in the medium and high fertility categories, while long-term strategies to restore productivity in degraded fields will be necessary to make fertilizer use viable. Project results will be used to map the distribution of different fields in north-east Zimbabwe and develop decision support tools for use by extension systems to help farmers use fertilizer resources efficiently. Zimbabwe-01