The effect of urea pellet size and rate of application on ammonia volatilization and soil nitrogen dynamics

Abstract

Two incubation experiments were conducted to investigate the effect of pellet size and rate of application on NH3 volatilization from surface-applied urea and on subsequent soil N transformations. In the first experiment fertilizer grade urea was applied as prills (1–2 mm diameter) or in four commercially available granule sizes with diameters of 2–3 mm, 3–4 mm, 5–6.3 mm and 6.3–8 mm. The five urea pellet sizes were applied to the soil surface of one soil at various N rates. In the second experiment four soils were used where the different pellet sizes were applied at a single application rate. The daily rates of NH3 loss were measured over 14 days in the laboratory under simulated spring conditions using ventilated enclosures. In the first experiment the highest total NH3 loss (Amax) was 17.4% of the urea N applied. Amax increased curvilinearly with rate of N applied up to the equivalent of 200 kg N ha−1. Higher rates of application did not lead to any further increase. Increasing the rate of N applied delayed the time to maximum rate of loss (Tmax), lowered soil NO 3 - —N levels and decreased total mineral N that could be recovered at the end of the incubation. The effect of pellet size on NH3 volatilization was dependent on soil type. On 2 soils studied pellet size had no significant effect whereas on other soils large granules could decrease or increase NH3 loss. However the effects were small. There was little difference on any of the soils between prills and the three smallest granule sizes on Amax or Tmax. The greatest effect was with the largest granules on soil 2. There was little difference between granule sizes on soil N transformations. However, N transformations from prills appeared to be different to the other pellet sizes.