Labeled nitrogen fertilizer research with urea in the semi-arid tropics. II: Field studies on a vertisol

Field studies were conducted on an Indian Vertisol to determine the fate of 15N-labeled fertilizers applied to dryland sorghum in two successive rainy seasons. In the 1981 season, a post-emergence split-band (SB) urea application of 74 kg N/ha was superior to pre-emergence applications of surface-applied (S) or incorporated (I) urea at the same rate; 907 mm of rainfall fell during the growing period. Percentages of applied N recovered in the soil-plant system after harvest were 94%, 74% and 72%, respectively, for the SB, I and S application methods; 39%, 45% and 42% of the added N in the SB, I and S treatments, respectively, remained in the soil after the final harvest. Plant utilization of added urea-N was greater in 1980 when rainfall during the growing season was 212 mm less than in 1981. S or I applications of urea at 74 kg N/ha were equally efficient during 1980 with above-ground plant 15N recoveries of 48.0% and 48.6%, respectively. Residual soil N derived from fertilizer was of little value for a sorghum crop in the following rainy season and for a safflower crop in the post-rainy season in a double-cropping system