| Nitrogen (N) is the most limiting nutrient in irrigated rice production, and growers continue to be faced with the challenge of meeting crop N demand, particularly in organic production systems. Therefore the main objective of this study was to determine the availability of N from organic sources under continuous and non-continuously flooded conditions, and the returns on investment associated with organic fertility. Laboratory and field experiments were conducted to determine the effectiveness of the commonly used poultry litter, pelletized organic fertilizers (blood and bone meal 13-0-0, feather meal 12-0-0, poultry litter plus feather meal 6-3-2), and inorganic fertilizer applied as (NH 4)2SO4 in synchronizing the supply of mineralized N with the demand of N by rice, relative to an unfertilized control. Over two years the treatments were applied at a similar N rate and evaluated in two continuously flooded fields, and one field which was drained 24 days after sowing, with aerobic conditions maintained for one month as a form of weed control. In all fields, all fertilizers increased grain yield and N uptake relative to the zero N control. Relative to poultry litter, the pelletized fertilizers resulted in higher yields (9980 vs. 9267 kg ha-1), N uptake (140 vs. 114 kg ha-1), and N recovery efficiency (35 vs. 20%) although these effects were not always significant. (NH4) 2SO4 was only included as a treatment in 2008, and had the best yield response (12042 kg ha-1), which was significantly higher than the yield of rice amended with poultry litter and one of the pelletized fertilizers (12-0-0), but similar to the other pelletized products (13-0-0 and 6-3-2). In the field studies, N mineralization of all organic fertilizers occurred primarily during the first 53 days after planting and before panicle initiation. An anaerobic incubation confirmed field study results showing that mineralization of organic fertilizers occurred primarily during the first 36 days of incubation and that the N mineralization of pelletized fertilizers, which averaged 27% of N mineralized, was greater than for poultry litter (14% of N mineralized). In the field study all organic fertilizers were less effective when the field was drained for weed control. An economic assessment of the products shows that pelletized fertilizers are competitive with poultry litter when fields are continuously flooded, while any organic fertilizer applications to fields that will be drained for an extensive period may not be economically viable. |
