Abstract
The effects of soluble sources of N, ammonium sulphate (AS), urea ammonium phosphate (UAP) and urea, and controlled release N sources, sulphur-coated urea (SCU-8·9, SCU-1·1 and SCU-0·9) of differing N release rates, applied at 0, 50, 100 and 200 kg/ha on an acidic soil, were compared in terms of P, K, Ca and Mg uptake. P uptake usually increased with increasing N rate and SCU-1·1 and SCU-0·9 gave larger uptakes of N and P than the soluble sources. When the N rate of AS was increased from 100 to 200kg/ha, P uptake was not increased, possibly because of the adverse effect of increasing exchangeable Al. The same change in N rate of AS and UAP tended to depress K, Ca and Mg uptakes. On increasing the rate from 100 to 200 kg/ha, SCU-1·1 and SCU-0·9 increased K, Ca and Mg uptake. The factors responsible for the differences in nutrient uptake due to N sources were probably the difference in nitrification rates and consequent changes in soil pH and exchangeable Al, soil NH4-N/NO4-N ratio, and the maintenance of soil NO3-N levels during crop growth.
