Published online by Cambridge University Press: 27 March 2009
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,, and after puddling, M2) and two presubmergence periods (none, S0, and 1 week, S1,) 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/a respectively; 17·08 and 18·01 kg/ha for treatments M1, and M2 respectively; and 18·63 and 16·34 kg/ha for treatments S0 and S1 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 (M2) and with no presubmergence period (S0).