RELEASE OF NITROGEN FROM UREAFORM FRACTIONS AS INFLUENCED BY SOIL-PH

Ureaforms are slow-release N fertilizers that have potential for minimizing problems associated with leaching of NO3- from soils and accumulations of NO3- in vegetable crops. We studied the effects of soil pH on rates of release of N from various ureaform fractions. Three fractions (soluble in cold water, insoluble in hot water, and soluble in hot water but insoluble in cold water) were separated from a N-15-labeled ureaform, and mineralization of N from these fractions and from labeled urea was studied in six soils having pH values ranging from 5.2 to 7.8. The results showed that rates of inorganic N(NH4+ and NO3-) release were extremely pH dependent; they varied >100-fold for the cold-water-insoluble fractions. The cold-water-insoluble fractions were largely responsible for slow release of N in acid soils, but these fractions and much of the cold-water-soluble fractions were essentially inert in soils above neutrality. The results suggest that successful use of ureaform fertilizers would require modifications in rates of application or composition of the ureaform to adjust for differences in soil pH.