Comparative fertilization effects on maize productivity under conservation and conventional tillage on sandy soils in a smallholder cropping system in Zimbabwe

Components of conservation agriculture (CA) are being widely promoted in southern Africa as one of the strategies to increase food security and mitigate rural poverty, despite there being scarce empirical evidence on their efficacy on degraded soils. On-farm trials were established in Eastern Zimbabwe to assess the effects of tillage systems on maize grain yields under rain-fed conditions across a soil organic matter gradient. The study compared the effects of a) conventional tillage (CT), b) basins-based CA (B-CA), and c) furrow-based CA (F-CA) on sandy soils with contrasting soil organic carbon (SOC). Fields had SOC ranging from 0.18 to 0.89% and clay content from 60 to 150 g kg−1. A nutrient omission trial using nitrogen (N), phosphorus (P), potassium (K), cattle manure (M) and their combinations was set up on twenty farms each with two fields selected by the host farmer as either poor or rich in soil fertility in Study 1, and ten farms in Study 2, each with two fields selected as in Study 1. Fields selected by farmers as poor fields had SOC <0.4%, were more acidic, had lower amounts of exchangeable bases (Mg, Ca, K), available P and total N. These lab-based results corroborated well with farmers' local rating of soil fertility. For Study 1, nutrient management significantly increased maize yields across the three years (P < 0.001) but there were no significant tillage effects observed. Grain yields increased from 0.3 Mg ha−1 for unfertilized control to 4.1 Mg ha−1 for the NPKSM treatment. Yield response to N was consistently larger than for P or K, irrespective of soil fertility status. Response to N increased with 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 Mg ha−1, 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. For Study 2, maize grain yields were significantly higher under B-CA compared with both F-CA and CT in the second year (P < 0.01). The consistently larger NPKSM yields highlight the importance of integrated nutrient management, combining mineral and organic sources of nutrients to ensure maize productivity on poor soils in agro-ecologies receiving unreliable rainfall.