Recovery of 15N‐Labeled Fertilizers in Field Experiments

Urea and oxamide, each labeled with 15 N, were compared as fertilizers in two field experiments in adjacent locations in successive years with ‘Sudax SX11’ Sorghum‐sudan hybrid ( Sorghum sudanense ) as the test crop. Four cuttings were harvested during the first experiment and three during the second. Patterns of uptake of fertilizer N were in accord with the characteristics of the two carriers. In the first harvests in both experiments the amounts of N taken up from urea were markedly greater than from oxamide; but by the third harvests, yield responses and N uptake from oxamide were greater than from urea. Total recoveries of fertilizer N from the two carriers during the entire growing season were similar as greater recoveries of urea‐N in the early summer were compensated for by greater recoveries of oxamide‐N in the late summer. Of the N added in urea in the first experiment, 51% was recovered in the crops and 28% was still in the soil (0–25cm) at the end of the growing season. Corresponding figures for oxamide were 52% in the crops and 31% in the soil. In the second experiment, when fertilizer applications and planting operations were delayed until more favorable growing weather, 93% and 99% of the urea‐ and oxamide‐N, respectively, were estimated as recovered in the crops, and measurements of the amounts of fertilizer remaining in the soil were not attempted.