Soil Heterogeneity and Nutrient Response in Smallholder Maize Systems in Uganda: Implications for Balanced Fertilizer Recommendations

Low productivity in sub-Saharan Africa is closely linked to declining soil fertility and suboptimal fertilizer use. While balanced fertilization can improve yields and sustain soil fertility, evidence for fertilizer optimization in smallholder maize systems remains limited and site-specific. This study evaluated maize yield responses to different combinations of N, P, K, and secondary and micronutrients (SMN) across agroecological zones (AEZs) in Uganda. Multi-location trials were conducted on 101 farms over two seasons using a randomized complete block design. Multivariate analyses revealed substantial heterogeneity of maize yields across sites and AEZs, mostly due to soil properties. Nitrogen was the most dominant yield-limiting nutrient. The N × P interaction was not significant (p > 0.05) across AEZs, indicating that N response patterns were generally similar across the range of tested P rates; however, P effects varied among sites. Nitrogen response followed a quadratic trend, with strong yield gains (45–132%) at 30–80 kg N ha−1, but gains beyond 120–160 kg N ha−1 were not significant. Responses to K and SMN were inconsistent and site-specific. AEZ explained a substantially greater proportion of yield variation (77%) than fertilizer treatments (13%), emphasizing the importance of inherent soil fertility and agroecological context in determining maize productivity and fertilizer response. These findings suggest that maize fertilizer recommendations should prioritize N management with adequate P supply, while K and SMN applications should be guided by site-specific conditions.