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

Strategic targeting of nutrient resources by smallholder farmers on different field types for optimized resource use efficiencies has remained poor, resulting in sub-economic returns to investments. This study investigated efficient strategies for use of nutrient resources to narrow the exploitable yield gap in maize production on heterogeneous coarse-textured soils under rainfed conditions in eastern Zimbabwe. A nutrient omission study was implemented between 2011 and 2013 across six on-farm sites with soil organic carbon (SOC) ranging from 0.33 to 0.89% and clay content ranging between 4 to 19%. Treatments used were: i) unfertilized control, ii) NK, iii) NPS, iv) PKS and v) NPKS. The nutrients PKS were basally applied at 40, 60 and 40 kg/ha, respectively, during Year 1 and repeated at a half rate (50% of year one) during Year 2. The rate of N application was adjusted as a function of seasonal rainfall, with a mandatory initial application of 20 kg/ha N at planting and two subsequent splits of 50 kg/ha N, if soil moisture permitted. Rainfall water productivity (kg grain/mm, RWP) was used as a proxy for water use efficiency for the different nutrient management strategies. During both seasons, only 70 kg/ha N could be applied across all sites as moisture limitations forced withholding of the second N topdressing targeted at maize anthesis.

Maize productivity was significantly influenced by nutrient management across sites for both seasons. During the first season, maize yields across sites ranged from 0.25 to 0.8 t/ha for the unfertilized control and 2.1 to 3.75 t/ha for the NPKS treatment. The corresponding RWP ranged from 0.38 to 1.13 kg grain/mm for the control and 3.15 to 7.66 kg grain/mm for the NPKS treatment. For the second season, maize yields for the control treatment ranged from 0.2 to 1.2 t/ha, while those for the NPKS treatments ranged from 2.4 to 3.6 t/ha. The N, P and K response factors were 0.56, 0.45 and 0.09, respectively, indicating poor response to K and highest response to N application. These results suggest that balanced nutrient management has an overriding effect on maize grain and water productivity, but only guaranteed when soil organic carbon content exceeded 0.4%. Nitrogen and P remain the most limiting nutrients for maize production. Complementary organic nutrient management approaches should be employed to increase SOC and sustain soil productivity. Variable N application strategies must be an integral component of farmer management if losses related to fertilizer investment are to be minimized under the risky rainfed production systems.