Comparison of two chemical methods for extracting residual N fertilizer

Considerable effort has been spent in developing chemical indices to predict N mineralization. However, in spite of numerous studies, the relationship between the index value and plant N uptake has not been as apparent as hoped, and therefore, additional work is required to evaluate the ability of promising new indices to predict the extraction of mineralizable N from soil. The objective of the present study was to evaluate the use of phosphate borate and hot KCl to extract immobilized ¹⁵N-labeled fertilizer, applied 1 and 2 years previously. Soil samples (0-15 cm) were collected on 12 June 1989 from field soil fertilized in either 1987 or 1988 with ¹⁵N-labeled urea. In the laboratory, net N mineralization over 51 days and the amount of N extracted by the phosphate borate and hot KCl methods were determined. In the field, the amount of residual fertilizer and soil plus fixed N in soybeans (Glycine max) at the V5 growth stage were determined on 12 June 1989. The extractability ratio (ER^*) and the mineralizable extractability ratio (MER) were higher for mineralizable N and phosphate borate N for fertilizer applied in 1988 than 1987, while ER^* and MER values for the hot KCl were similar for both application dates. These results suggest that compositional changes occurred which influenced the extractability and mineralization of residual fertilizer applied 1 and 2 years previously, and that the phosphate borate was able to predict these changes while the hot KCl method was not.