The kinetics of ammonia disappearance from deep-placed urea supergranules (USG) in transplanted rice: the effects of split USG application and PK fertiliser

Deep placement of urea supergranules (USG) in transplanted rice cultivation can protect nitrogen from losses. In the 20 x 20 cm plant geometry, the 4 g urea application corresponds to 116 kg N ha(-1). However, the ammoniacal nitrogen concentration at the placement sites increases as the mass of the USG increases, and with the 4 g USG it takes too long for the ammonia concentration to decrease to 10 mM, the concentration at which root uptake commences. Therefore, split applications (two applications of 2 g USG) were studied regarding the change of ammoniacal nitrogen concentration at the point of application. The first application was always at transplanting, at a depth of 10 cm; the second application was made 41 days after transplanting (DAT) (tillering stage) or 62 DAT (panicle initiation). These times correspond to the beginning or the end of ammonia disappearance (plant uptake) from the site of placement, respectively. Ammonia disappearance from the placement site is faster for the second application, and it appears that the rice roots took up ammonia at a higher concentration: 20 mM for the second application versus 10 mM for the first application. The kinetics of ammonia disappearance from a single application of 2 g granules at transplanting was also influenced by the PK application rate. The rate of ammonia disappearance, the number of tillers per hill, and grain yield all increased with PK rate. The constant which characterizes the first order rate of ammonia disappearance below 10 mM also increased with PK level, proportionally to the number of tillers.