EFFECT OF RATES AND METHODS OF UREA-N APPLICATION AND PRESUBMERGENCE PERIODS ON AMMONIA VOLATILIZATION LOSSES FROM RICE FIELDS IN A SODIC SOIL

In a field experiment with transplanted rice (Oryza sativa L.) during the wet season of 1990 at the Central Soil Salinity Research Institute, Karnal, India, the effects of four rates of N application (0, 80, 120 and 160 kg N/ha), two methods of urea application (before puddling, M(1), and after puddling, M(2)) and two presubmergence periods (none, S-0, and 1 week, S-1) were evaluated on ammonia volatilization losses in a sodic soil (pH 9.0, exchangeable sodium 29%). Cumulative ammonia volatilization losses were 1.43, 18.00, 22.54 and 28.29 kg/ha following applications of 0, 80, 120 and 160 kg N/ha respectively; 17.08 and 18.01 kg/ha for treatments M(1) and M(2) respectively; and 18.63 and 16.34 kg/ha for treatments S-0 and S-1 respectively. Flood-water NH4+ and pH increased during days 1-4 and then there was a sharp decline on days 4-9, followed by a gradual decline. The highest flood-water NH4+ and pH values were found for the treatments which received urea at the rate of 160 kg N/ha, after puddling (M(2)) and with no presubmergence period (S-0).