NITROGEN AVAILABILITY TO CITRUS SEEDLINGS FROM UREA AND FROM MINERALIZATION OF CITRUS LEAF OR COMPOST

A pot experiment was conducted using a Candler fine sand (hyperthermic , uncoated, Typic Quartzipsamments) amended with either citrus leaves or compost, to measure the nitrogen (N) mineralization and its availab ility to two citrus rootstock seedlings. A rapid increase in NH4-N con centration was evident in the soil amended with citrus leaves as compa red to compost during the initial 14 to 20 d. Subsequently, the concen tration of NH4-N decreased in the citrus leaves amended soil. The extr actable NO3-N concentration was greater in the soil amended with citru s leaves as compared to compost, throughout the 270 d duration of the study. The N concentrations and N uptake by Cleopatra mandarin (CM) an d Swingle citrumelo (SC) seedlings grown in citrus leaf amended soil w ere very similar to those in urea amended soil. Therefore, mineralizat ion of N from dry ground citrus leaves was quite rapid. The N concentr ations in both rootstock seedlings were much lower in the compost amen ded and unamended soils as compared to those in either citrus leaves o r urea amended soils. Rapid mineralization of N from citrus leaves add ed to sandy soil, resulting in an increased availability of N, suggest ed that the contribution of N from shed leaf mineralization must be co nsidered while developing N rate recommendations for improving N use e fficiency.