Available N in the root area for citrus orchards fertilized with sulphur-coated-urea and ammonium nitro-sulphate

The leaching of N fertilizers in citrus causes important N losses, thus exe rting a direct influence on the economy of citrus production, and may have negative consequences on the environment. The use of controlled-release N f ertilizers may reduce leaching and be a solution to this problem. The amounts of available N, expressed as g of N in a soil cylinder correspo nding to the drip area of the tree and of 0-60 cm depth, were compared at d ifferent vegetative periods, in an orange orchard, for four treatments; two doses of a standard fertilizer (ammonium nitrate sulphate, ANS), applied i n the quantities of 11.55 kg N/ha (ANS-350) and 49.50 kg N/ha (ANS-2x750) a nd two doses of a slow-release fertilizer (sulphur-coated urea, SCU), appli ed in the quantities of 11.55 kg N/ha (SCU-350) and 24.25 kg N/ha (SCU-750) , in nine consecutive years. A treatment without N (no N) was included. Ava ilable total N throughout the whole of the vegetative cycle, and in partial critical periods, has been determined as another comparison term. In the s ame way, non-available N has been measured at the beginning and at the end of the experiment. SCU treatment applied in low doses and only in spring maintained high level s of available N in soil during the vegetative cycle, reducing N losses, co mpared with ANS treatments. In the whole annual cycle, the amounts of N wit h SCU-750 were 32% greater than those of ANS-(2x750) though the quantity of N applied with this treatment was double. SCU-350 provided 5% less N than ANS-(2x750), though the dose of N applied with this treatment was 4.3 times greater. In all critical periods, SCU-750 maintained quantities of N highe r than those with ANS-(2x750). For SCU-350, the amounts of N were similar t o those of ANS-(2x750), except in summer as a consequence of the second fer tilization with ANS-(2x750). Non-available N increased during the experimen t in all treatments providing N except in ANS-350. A good crop and quality of fruits were obtained with SCU treatments, which in conjuntion with the r eduction in dosage and rates justified its application.