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Thirteen trials, each with 3 nitrogen (N) sources (urea, ammonium nitrate, and ammonium sulfate) and 2 N rates (25 and 75 kg N/ha), were carried out during 1987-89, to measure the rate of disappearance of ammonium-N on different soils. Six soil categories were examined, from very acid to calcareous light soils, and from medium to heavy textured soils. Plots were planted with wheat, and at the higher rate of N, fallow plots were included to distinguish plant uptake from other processes such as nitrification and immobilisation that cause the disappearance of ammonium N. Reduction in concentration of ammonium-N was rapid on high pH, light soils (2-3 weeks at Dongara 1988), and slower with decreasing soil pH (e.g. >19 weeks at Merredin 1987). Nitrate-N concentration increased on fertiliser-treated plots at all sites, indicating that nitrification was taking place. Ammonium-N decline was slower with ammonium sulfate supplied than with urea or ammonium nitrate, consistent with its greater acidifying effect in the soil. This difference did not occur on the alkaline light soils, where reduction in concentration of ammonium-N was rapid for all sources. In 1989, the rate of decline of ammonium-N was considerably slowed because the soil surface containing the ammonium-N was dried during a very dry spring with little effective rainfall in September and October. |
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