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

Site-specific nutrient management is important in smallholder farming areas of sub-Saharan Africa to enhance efficient use of scarce nutrient resources, considering that significant variability in soil fertility exists within and between farms. Over the six years of this study, multi-locational on-farm nutrient omission trials have been conducted in pilot sites in eastern and northeastern Zimbabwe to establish maize yield response to the application of nitrogen, phosphorus, and potassium (NPK), selected micronutrients (zinc, boron), manure, and lime under variable soil fertility conditions. The experiments also sought to establish nutrient-determined maize yield gaps in major maize-growing areas in Zimbabwe, and assess balanced fertilizer management strategies to optimize maize production under rainfed conditions.

In 2017, the project focused on assessing the effects of: 1) conventional tillage (CT); 2) basins-based conservation agriculture (CA) (B-CA); and 3) furrow-based CA (F-CA) on sandy soils with contrasting soil organic carbon (SOC). Farms had SOC ranging from 0.18 to 0.89% and clay content from 6 to 15%. Nutrient omission trials using N, P, K, cattle manure (M) and their combinations were set up on twenty farms, each with two fields per farm selected. Infertile fields had SOC <0.4%, were more acidic, had lower amounts of exchangeable bases (magnesium, calcium, K), available P, and total N. Grain yields increased from 0.3 t/ha for unfertilized control to 4.1 t/ha for the NPKS+M treatment. Yield response to N was consistently larger than P or K, irrespective of soil fertility status. Response to N increased with the increase in soil fertility, suggesting higher N use efficiency for soils with higher SOC. Except for NPKSM, no significant yield differences were observed under the residual and additive plots for treatments, when N was added each year. At productivity levels of <4 t/ha, there was no yield gain in applying both P and K for consecutive years, suggesting that nutrient investments by resource-constrained farmers for Year 2 could target only N application. Maize grain yields were significantly higher under B-CA compared to F-CA and CT, suggesting a positive effect due to moisture conservation and concentration of nutrients. The consistently large yields with the NPKSM treatment highlight the importance of integrated nutrient management combining mineral and organic sources of nutrients in agro-ecologies receiving unreliable rainfall. Nitrogen and P remain the most limiting nutrients, and complementary organic nutrient management approaches should be employed to increase soil carbon and sustain soil productivity.