An evaluation of plant-available soil nitrogen in selected sandy soils by electro-ultrafiltration, KCl, and CaCl2 extraction methods

Improving the precision in estimating the nitrogen (N) requirement for citr us trees on sandy soils is important for increasing N efficiency by the tre es and minimizing potential losses of N in commercial citrus production are as. In this study, representative Florida soils were sampled from major cit rus production areas and the electro-ultrafiltration (EUF) technique was us ed to measure the concentrations of total EUF-extractable nitrogen (EUF-N-t ), ammonium-N (EUF-NH4+-N) and nitrate-N (EUF-NO3--N). Available organic N (N-org) was calculated as: EUF-N-t-(NH4+-N+NO3--N), The N concentrations in the EUF extraction were greater than those by the KCl or CaCl2 method. The N-org fraction, estimated by the EUF method, varied from 4.4 to 40.8 mg kg (-1) soil, equivalent to 10 to 91 kg N ha(-1) (for the top 15 cm depth soil ) and was positively correlated with the total soil N determined by the Kje ldahl method. The presence of appreciable amounts of N-org in these soils i ndicates that these soils contain high proportions of the total soil N in e asily mineralizable N-org forms. This study demonstrates that the EUF-extra ctable organic bound N must be considered in developing N fertilizer recomm endations for citrus.