Nitrogen and phosphorus nutrition of dryland grain sorghum at Katherine, Northern Territory. 4. 15-nitrogen studies on nitrogen carrier and method of application

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Abstract
The effect of nitrogen source and method of application on yield and N uptake of dryland grain sorghum was studied, using 15-nitrogen labelled fertilizers. The nitrogen sources were ammonium sulphate, ammonium nitrate and urea, and the methods were banded, mixed and split application, using a rate of 50 kg N ha-1 throughout. The experiment was conducted over two wet seasons, 1970-71 and 1971-72. Method of application increased yield and nitrogen uptake in the order banded > mixed > split. Source of nitrogen resulted in the order ammonium sulphate = ammonium nitrate > urea. In the drier season, urea behaved somewhat differently, i.e. banded urea > banded ammonium nitrate= banded ammonium sulphate. Calculation of percentage recovery in general confirmed these results. It was concluded that nitrogenous fertilizer is more effective when banded, and that urea was a less effective source of nitrogen than the two ammonium salts. The results obtained with 15-nitrogen revealed a significant treatment effect that was not apparent in non-tracer experiments namely, the poor performance of urea in the second year. Because of its greater sensitivity, and because it identifies the nitrogen derived from fertilizer, the 15-nitrogen technique has scope for use in field experiments comparing fertilizers and cultural techniques.