Effect of nitrogenous fertilisers on soil inorganic nitrogen levels and uptake by wheat on very acid soils

Urea, ammonium sulfate and ammonium nitrate were compared as sources of nitrogen (N) for wheat grown on very acid soils at 2 sites in 1980, in the absence of lime or where lime at 2 t/ha was incorporated into the top 10 cm of soil. The plots were soil sampled each week for the first 5 weeks after sowing, and further samples were collected at 9 weeks. Wheat tops were sampled 4 times during the first 6 weeks after sowing. Soils and plants were analysed for ammonium-N and nitrate-N. Application of each fertiliser initially caused increased soil levels of ammonium-N which fell with time at both sites. Increases in nitrate-N were small and were usually not significant. At 1 site (Bunketch), and with ammonium sulfate as the N source when no lime was added, there was a slower rate of decline in ammonium-N than in the presence of lime. Fertiliser type did not result in any significant differences in ammonium and N concentrations in the soil, apart from the higher levels of nitrate-N in the ammonium nitrate treatments. At both sites and particularly at Perenjori both in the absence and presence of lime, nitrate-N concentrations in plants were higher for the treatments with N fertiliser than for the unfertilised controls. This suggests that the N applied as fertiliser ammonium is nitrified before it is taken up by the plants. At the first plant sampling at Perenjori and at the first 2 samplings at Bunketch, ammonium-N levels in the fertilised plants were higher than in the unfertilised plants, suggesting that ammonium-N was readily taken up by the plants. Plant nitrate levels were lower at Bunketch in the absence of lime, than where lime was added. Grain yields were significantly increased at both sites by N fertiliser application. The 3 fertilisers were equally effective and there was no significant response to lime. Both nitrate and ammonium-N appeared to be readily utilised by the plant.