Midseason application of organic fertilizer improves yield and nitrogen uptake in rice

Organic rice (Oryza sativa L.) systems in California utilize a 30-d drain approximately one month into the growing season to control weeds. This drain may result in N losses and reduce yields. Two field trials were initiated to evaluate whether top-dressing fertilizer N following the drain period could mitigate this problem by increasing N uptake and yields. Treatments evaluated rates and sources of commercially available organic fertilizers varying in N content and C to N ratio. The six treatments were a control with no top-dress fertilizer, and top-dress applications of 12-3-0 (applied at 22, 45, and 67 kg N ha(-1)) and 6-3-2 and 4-1-4 applied at 45 kg N ha(-1). Total plant N uptake at harvest increased linearly with increasing N rate at both locations. At both sites, grain yields increased linearly (by 15 to 22 kg ha(-1) for every kg N ha(-1) applied) up to 45 kg N ha(-1) above which yields plateaued at one site but continued to increase linearly at the other site. Application of 12-3-0 increased yields relative to the control at both locations but yields were not significantly different between the three fertilizer sources at the 45 kg N ha(-1) rate at either location. Total plant N uptake and N recovery were higher in the 12-3-0 treatment compared to the other fertilizer sources at only one location. An economic analysis indicates that at the yield responses reported here, top-dress N fertilizer applications were economical.