The total nitrogen content of the soil was significantly increased at each of four levels of superphosphate applied to a densely sown subterranean clover pasture on a coarse-textured sand at Kojonup, W.A. During the first 4 years of the experiment nitrogen build-up was similar for all four treatments, but over the 11 year term the build-up was significantly slower at the lowest level of superphosphate. The relationship between the age of the pasture and soil nitrogen was linear at the lowest level of fertilizer but curvilinear at all other levels. The relationship between total superphosphate applied and soil nitrogen was curvilinear. There were no differential effects of the fertilizer treatments on either the bulk density or the pH of the soil. At the two lower levels of superphosphate a high percentage of both phosphorus and sulphur was retained in the major plant root zone (0–10 cm) of the soil. At the two higher levels most of the phosphorus was retained but there were heavy losses of sulphur. The results of pot trials with soil taken from the experiment at the end of the 11 year period showed that the different levels of superphosphate had induced large differences in the phosphorus status of the soil but had had much smaller effects on the sulphur status. The possible effect of this loss of sulphur on the accumulation of soil organic matter is discussed.
Nitrogen loss from sheep urine was measured on two soil types under different surface cover and moisture conditions at a location with high summer temperatures. Some of the factors influencing nitrogen loss were studied in pots and lysimeters. Grass plants utilized almost half the nitrogen applied in urine. Loss of nitrogen by volatilization and leaching was considerably less under a grass cover than on bare soil. When urine was applied during the hot summer months, there were large losses (50 per cent) of nitrogen even under a grass cover. During the summer, rewetting of urine patches to simulate rainfall increased the amount of nitrogen lost. Eighty per cent of the urine nitrogen was lost after three wettings. Frequency of wetting was more important than volume of water applied. Urine application markedly increased the pH of the soils over a long period.
Under a subterranean clover pasture on a light-textured soil at Kojonup, Western Australia, soil nitrogen accumulated at a fairly steady rate of 42 lb/acre/year over a 5 year period. The increase in soil nitrogen accounted for differences in wheat production following the clover ley whether measured as total dry weight, yield of grain, or uptake of nitrogen. The clover treatments had little effect on the percentage of nitrogen in the grain or on the baking quality of the flour. Each additional year of clover gave an additional yield of 120 lb of wheat grain per acre for the first crop. Yields for second crops were considerably lower than the first but residual effects were evident. A practical implication of the results is that short leys of 1 to 2 years' duration followed by a single cereal crop could be a suitable cropping practice for the Kojonup area.
Loss of nitrogen from maturing subterranean 'clover and soft brome plants was measured in both field and pot experiments. In subterranean clover, nitrogen loss commenced after seed setting. In soft brome grass, the loss began at flowering and continued until senescence. Nitrogen loss from herbage may be due to translocation to burrs and seeds, to leaching by rain and dew or to volatilization to the air. The practical implications of this nitrogen loss in terms of animal production are discussed.
Ammonium sulphate was applied to a grazed pasture on a duplex soil for five years at annual rates of nil (N0), 280 kg ha-1(N1) and 840 kg ha-1(N3). N0 became clover dominant and N3 became grass dominant but net increase in nitrogen in the top 10 cm of soil over five years was similar on N0, N1 and N3. Fertilizer nitrogen was rapidly lost after heavy rains in autumn. On all treatments, soil accumulated inorganic nitrogen in summer. A balance sheet for sulphur could not account for 48 per cent of sulphur in the top 10 cm on N0 (superphosphate only), 82 per cent on N1 and 90 per cent on N3. Ammonium sulphate decreased pH, exchangeable calcium, magnesium and potassium (but not sodium), and cation saturation, and increased exchangeable acidity. Apart from pH, these effects were confined to the top 10 cm of soil. The results show that in a sandy-surfaced soil ammonium sulphate is an inefficient source of nitrogen and sulphur because nitrogen and sulphur are readily lost.
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