Effect of Nitrogen Fertiliser Application on Maize Yield Across Agro-Ecological Regions and Soil Types in Zimbabwe: A Meta-analysis Approach

Accurate nitrogen (N) fertilisation increases crop yields and minimises environmental pollution in maize based production systems. In this study we report on a meta-analysis of the effects of N fertilisation on maize yield using observations from 53 studies conducted in Zimbabwe since the 1950s for possible refinement of fertiliser recommendations. We assessed the response of maize to: < 30; 30–100 and > 100 kgN ha −1 compared to the control using the weighted mean yield difference approach. Application of N fertiliser significantly ( p < 0.001) increased maize yields by between 0.25 and 1.6 t ha −1 compared to control. These effect sizes increased with N rate and rainfall amount and were higher in the better managed large scale sector than the smallholder sector, but highest (1.527 t ha −1 ) in loamy soils (20–35% clay) and lowest (0.750 t ha −1 ) in the sandy soils (< 20% clay). In addition, the effect sizes where mineral N fertilisers were applied were 6.3% higher than those for mineral + organic N fertiliser application due to slow N mineralisation of organic resources. In conclusion, soil hydraulic properties and associated N transformation processes greatly influenced maize yield response calling for optimised application rates and maize growers in Zimbabwe, and similar areas in Sub-Saharan Africa, can substantially increase yields by applying > 100 kgN ha −1 (> 1.5 fold of recommended blanket rate) in mineral form in loam soils under good management in areas that receive 600–1000 mm year −1 rainfall whilst rates of < 30 kgN ha −1 (< 50% of recommended rate) can be relied upon for positive yield response in small holder areas that receive < 600 mm rainfall year −1 .