Slow-release urea fertilizers — effect on floodwater chemistry,ammonia volatilization and rice growth in an alkali soil

In experiments with transplanted rice (Oryza sativa L.) at the Central Soil Salinity Research Institute, Karnal, India, two methods of application of granular urea, wholly as basal dose U(W) or in splits U(S) were compared with deep, point placement (8 cm) of urea supergranules and broadcast application of two slow-release sources, sulphur-coated urea (SCU) and lac-coated urea (LCU). Comparisons were made in wet season 1984 and 1985 on the basis of ammoniacal N concentration and pH of floodwater, ammonia volatilization, rice yield and N uptake.In 1984 the highest peak concentrations of ammoniacal N (AN) in the floodwater, > 12g m^–3, and ammonia volatilization losses 54% of applied N were produced in U(W). Application of N in splits U(S) reduced peak AN levels 5g m^–3 and losses to 45.1%. LCU was ineffective in reducing peak AN levels ( 7.5g m^–3) or losses (43.6%). However SCU and USG were effective in reducing peak AN levels to < 2g m^–3 and N losses to 16.9 and 3.4% respectively. Total ammonia volatilization losses as well as the initial rate of loss correlated very well with the peak levels (second day) of AN, NH₃ (aq.) as well as equilibrium vapour pressure of NH₃. Floodwater pH was between 9.5 and 10.0.Split application of granular urea was generally more efficient in terms of yield and N recovery (41.4%, average of two years) as compared to whole application (29.5%). LCU was ineffective in improving grain yields or N recovery (30.9%). SCU was ineffective in improving grain yields but improved N recovery to 57.9%., USG increased grain yields only in first year by 19% over U(S) and improved N uptake to 60.5%. A negative linear relationship was established between N uptake by rice at harvest and AN levels in floodwater two days after fertilization which can be used as an index to evaluate fertilizers.