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

Abstract

 Improving the precision in estimating the nitrogen (N) requirement for citrus trees on sandy soils is important for increasing N efficiency by the trees and minimizing potential losses of N in commercial citrus production areas. In this study, representative Florida soils were sampled from major citrus production areas and the electro-ultrafiltration (EUF) technique was used 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 Kjeldahl method. The presence of appreciable amounts of N_org in these soils indicates that these soils contain high proportions of the total soil N in easily mineralizable N_org forms. This study demonstrates that the EUF-extractable organic bound N must be considered in develo** N fertilizer recommendations for citrus.