Abstract
Safflower, linseed, and wheat were grown under irrigation at Kimberley Research Station in the 1964 dry season following 6 and 18 month clean and weedy fallows. Four levels of nitrogen fertilizer, as ammonium sulphate and urea, were superimposed. Crop nitrogen yields were highest after 18 months clean fallow, and were highly correlated with the amount of nitrate-nitrogen that had accumulated in the soil profile during the preceding fallow period. For wheat, which was the most efficient user of mineral nitrogen, 160 lb nitrogen an acre as ammonium sulphate was required after 18 months weedy fallow to equal the crop nitrogen yield after 18 months clean fallow without nitrogen fertilizer. The mean crop nitrogen yield with urea was only 76.7 per cent of that with ammonium sulphate. The results indicate that the form, and possibly the distribution, of mineral nitrogen in the soil in the early stages of crop growth could be important factors determining the efficiency of nitrogen for optimum plant performance.
Published Version
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