Soil fertility enhancement through appropriate fertilizer management on winter cover crops in a conservation agriculture system

A study was carried out to determine the effects of oat (Avena sativa) and grazing vetch (Vicia darsycapa) winter cover crops and fertilizer application on SOM, phosphorus (P) pools, nutrient availability, nutrient uptake, maize yield and seedbanks of problematic weeds in an irrigated maize-based conservation agriculture (CA) system. A separate experiment was carried out to investigate the effects of the winter cover crops on nitrogen (N) management, N use efficiency and profitability. After four years of continuous rotation, the winter cover crops significantly (p<0.05) increased particulate SOM and hot water soluble carbon in the 0 – 5 and 5 – 20 cm soil depths. When fertilized, oat was better able to support SOM sequestration in water stable aggregates at 0 – 20 cm whilst grazing vetch was more effective at 20 – 50 cm. Where no fertilizer was invested, there were significant (p<0.01) reductions in biomass input and SOM on oat-maize and weedy fallow-maize rotations whereas vetch-maize rotations did not respond, both at 0 – 5 and 5 – 20 cm. Targeting fertilizer to the winter cover crop required less fertilizer, and yet gave a similar SOM response as targeting the fertilizer to the maize crop. In addition to increasing SOM in the surface soil (0 – 5 cm), the winter cover crops significantly (p<0.05) increased labile pools of P, including microbial P. The cover crops also significantly (p