Soil-Crop-Climate Interactions and Yield Gap Decomposition in the Sudan Savanna of North West Nigeria: Original Modelling and Field Evidence

This dissertation provides an original analysis of soil-crop-climate interactions and uses yield gap decomposition to identify the primary constraints on rainfed sorghum and millet production in the Sudan savanna of North West Nigeria, encompassing Kano, Katsina, Sokoto, and Zamfara States. The yield gap between attainable and actual farmer yields in North West Nigeria exceeds 60 percent for both sorghum and millet, but the relative contributions of soil constraints, climate variability, and management deficits to this gap have not been quantified, preventing prioritised intervention design. This study employs the APSIM crop simulation model combined with original field experimental data from 25 managed farm sites across the four states over three seasons. Soil data for model parameterisation were collected from 60 characterised profiles covering the major soil mapping units. Daily climate data were obtained from NIMET and validated against gridded satellite-derived products. Yield gap analysis followed the Tanaka-Yokozawa framework, partitioning the observed yield gap into portions attributable to climatic stress (rainfall variability), soil nutrient limitations (identified by nutrient omission trials), soil physical constraints (identified by subsoiling and mulching treatments), and management inadequacies. Findings revealed that soil nutrient limitation (principally phosphorus and nitrogen) accounted for 38 percent of the yield gap, rainfall variability for 29 percent, and management inadequacies for 26 percent. Soil physical constraints contributed 7 percent and were concentrated in compacted pan soils. The APSIM model was successfully calibrated and validated with Nash-Sutcliffe efficiency above 0.78 for both crops. The dissertation provides an original APSIM parameterisation database for North West Nigerian Sudan savanna soils and a prioritised intervention pathway for yield gap closure. Keywords: yield gap decomposition, APSIM model, Sudan savanna, soil-crop-climate interaction, North West Nigeria.