TRANSFORMATION OF UREA AND AMMONIUM-NITRATE IN AN ENTISOL AND A SPODOSOL UNDER CITRUS PRODUCTION

Chemical transformations of ammonium nitrate (NH4NO3) and urea-nitroge n (N), at different rates of application, were studied in a Candler (T ypic Quartzipsamment) and Wabasso (sandy, Alfic Haplaquod) sand by inc ubating fertilized surface soil (from 0 to 15 cm depth) samples at 10% moisture content (by weight) in the laboratory at 25 +/- 1 degrees C. During the 7 d incubation, the percentage of transformation of NH4-N into NO3-N was 33 to 41 and 37 to 41% in the Candler fine sand and Wab asso sand, respectively, at application rates of 1.00 g N kg(-1). In a parallel experiment, 85 to 96% of urea applied (equivalent to 0.25 to 1.00 g N kg(-1) soil) was hydrolyzed to NH4-N within 4 d in the Candl er soil, whereas it required 7 d to hydrolyze 90 to 95% of the urea ap plied in the Wabasso soil. No nitrification was evident for 30 days in the Candler fine sand which received urea application equivalent to g reater than or equal to 0.50 g N kg(-1). In the urea-amended Wabasso s and, the formation of NO, decreased as the rate of urea-N increased. P ossible loss of N from NH3 volatilization or inhibition of activity of nitrifiers due to elevated soil pH (8.7 to 9.2) during the incubation of urea amended soils may have caused very low nitrification.