NITROGEN UPTAKE AND GROWTH OF 2 CITRUS ROOTSTOCK SEEDLINGS IN A SANDYSOIL RECEIVING DIFFERENT CONTROLLED-RELEASE FERTILIZER SOURCES

Understanding the fate of different forms of nitrogen (N) fertilizers applied to soils is an important step in enhancing N use efficiency an d minimizing N losses. The growth and N uptake of two citrus rootstock s, Swingle citrumelo (SC), and Cleopatra mandarin (CM), seedlings were evaluated in a pot experiment using a Candler fine sand (hyperthermic , uncoated, Typic Quartzipsamments) without N application or with 400 mg N kg(-1) applied as urea or controlled-release fertilizers (CRF, ei ther as Meister, Osmocote, or Poly-S). Meister and Osmocote are polyol efin resin-coated urea with longevity of N I:release for 270 days (at 25 degrees C). Poly-S is a polymer and sulfur-coated urea with release duration considerably shorter than that of either Meister or Osmocote , The concentrations of 2 M KCl extractable nitrate nitrogen (NO3--N) and ammonium nitrogen (NH4+-N) in the soil sampled 180 days and 300 da ys after planting were greater in the soil with SC than with CM rootst ock seedlings. In most cases, the extractable NH4+ and NO3- concentrat ions were greater for the Osmocote treatment compared to the other N s ources. For the SC rootstock seedlings, dry weight was greater with Me ister or Poly-S compared with either Osmocote or urea. At the end of t he experiment, ranking of the various N sources, with respect to total N uptake by the seedlings, was: Meister = Osmocote > Poly-S > Urea > no N for CM rootstock, and Meister = Poly-S = Osmocote > Urea > no N f or SC rootstock. The study demonstrated that for a given rate of N app lication the total N uptake by seedlings was greater for the CRF compa red to urea treatment. This suggests that various N losses were lower from the CRF source as compared to those from soluble fertilizers.