Effects of Organic Resource Quality on Soil Profile N Dynamics and Maize Yields on Sandy Soils in Zimbabwe

Optimising the use efficiency of nitrogen (N) derived from different quality organic resources and mineral fertilizers on sandy soils with <100 g clay kg⁻¹ is a major challenge for smallholder farmers in Southern Africa. The dominant sandy soils have a poor capacity to store and supply crop nutrients due to low organic matter contents and inherent infertility. A study was conducted in Zimbabwe to determine the differential N supply effects of different quality and quantities of organic nutrient sources on maize productivity. Crotalaria juncea L., Calliandra calothyrsus Meissn., cattle manure, maize (Zea mays L.) stover and Pinus patula Schiede & Schltdl. & Cham. sawdust which represented high to low quality materials respectively, were each incorporated into soil at 1.2 and 4 t C ha⁻¹ at Makoholi Experiment Station (rainfall: 450–650 mm yr⁻¹) and tested against a sole mineral N fertilizer and control treatments. In a separate experiment conducted in farmers’ fields under different rainfall zones of Zimuto (450–650 mm yr⁻¹), Chinyika (650–750 mm yr⁻¹) and Chikwaka (>750 mm yr⁻¹), commonly available organic materials, including manure and composted miombo leaf litter, applied in varying amounts by farmers were evaluated. Nitrogen release patterns were consistent with differences in resource quality. At 3 weeks after incorporation into soil at the onset of the rains, C. juncea and C. calothyrsus had released as high as 24% and 13% of added N respectively, compared with no more than 5–6% for the rest of the amended treatments. Most of the N released was lost through leaching as evidenced by progressive movement of NO₃⁻-N bulges beyond maize rooting depth following major rainfall events. Maize yields were significantly related to the size of profile mineral N fluxes, with the best linear relationship (R² = 0.86) obtained with N available in the top 30 cm of soil at maize flowering. High grain yields of ~3 t ha⁻¹ were only achieved with C. juncea applied at 4 t C ha⁻¹, which also had highest NO₃⁻-N leaching losses. Conversely, the same application rate increased N immobilization by 30% and 42% under maize stover and sawdust, respectively, relative to the control. Results from farmers’ fields showed that organic resources traditionally used on smallholder farms are invariably of low quality relative to C. juncea and C. calothyrsus. However, they exhibited shorter N immobilization effects than was shown for maize stover and sawdust at Makoholi, suggesting that pre-application treatments, such as composting, employed by farmers enhance seasonal N benefits from these materials. Maize yields increased linearly with total N added in these resources in combination with N fertilizer, justifying the high organic matter loading strategy (e.g. >20 t ha⁻¹ for manure, fresh litter and composted litter) used by farmers who often achieve high crop yields on such coarse sandy soils in Zimbabwe.