FATE AND EFFICIENCY OF NITROGEN FERTILIZERS APPLIED TO WETLAND RICE .1. THE PHILIPPINES

Urea is the main form of fertilizer N applied to wetland rice. As part of an effort to evaluate the efficiency of N fertilizers, conventional urea, sulfur-coated urea (SCU), urea supergranules (USG), and sulfur-coated urea supergranules (SCUSG) were compared with ammonium sulfate on an Aquic Tropudalf at the experimental farm of the International Rice Research Institute (IRRI) in the Philippines. Two experiments were conducted in the wet season (1978,1979) and one in the dry season (1979). All fertilizers were labeled with 5% or 10% excess 15N so that the fertilizer-N balance at two or three sampling times during the growing season could be constructed and the magnitude of N loss assessed. The SCU, USG and SCUSG were applied at transplanting, and the whole dose of N was 15N-labeled. The urea and ammonium sulfate applications were split: 2/3 was broadcast and incorporated at transplanting, and 1/3 was broadcast at panicle initiation; only the intial dose was 15N-labeled. Deep-point placement (10 cm) of urea supergranules (USG) between the rice hills consistently provided the highest plant recovery of 15N in all experiments and at all harvest times; recoveries ranged from 48% to 75% with an average of approximately 58% at maturity. Among the fertilizers broadcast and incorporated before transplanting, average plant recoveries of 15N were only approximately 35% and 26% from urea and ammonium sulfate respectively. Plant recovery of 15N from the broadcast and incorporated SCV (37%) was far inferior to that from USG. SCUSG did not improve plant recovery over USG alone although sulfur coating delayed the plant uptake of 15N from the USG. The 15N not accounted for in the plant and soil was presumed lost. Loss of N from urea and ammonium sulfate was high (63%) in the dry season. Coating with sulfur gave a slight improvement, and deep placement of USG and SCUSG greatly reduced the losses. Losses of N were substantially lower in the wet season than in the dry season for broadcast and incorporated urea, SCU, and ammonium sulfate (9%-30%), whereas losses from deep-placed urea remained more or less the same as in the dry season. Net immobilization of 15N from the broadcast fertilizers in the wet season ranged from 49%-53% in the 1st experiment and from 16%-32% in the 2nd experiment, presumably because of aquatic weeds and green algae; immobilization was proportionally less at higher rates of fertilizer application. Deep placement reduced the extent of 15N immobilization in the soil plus roots to less than 21% in all experiments.