Abstract
The effects of different amounts of nitrogen fertilizer supplied in the irrigation water on nitrogen uptake, orange yield and quality of Shamouti oranges was studied in a field experiment in a mature orange grove. Nitrogen was supplied as ammonium nitrate in three concentrations (15, 35 and 60 ppm) resulting in resp. 100, 170 and 310 kg N/ha. Significant effects of nitrogen were found during four years of fruit yields and on the number of fruits per tree. The average fruit weight was not affected. The high nitrogen rates caused thicker peels and later development of orange colour. The leaf nitrate content was closely related to the fruit yield. Soil solution samples taken with ceramic cups gave a good indication of the soil nitrate content. It was shown that the spring flush of leaves is based mainly on tree nitrogen reserves, and not on nitrate uptake from the soil.
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