Management of Nitrogen Fertilizer Application in Citrus Orchards

Management of nitrogen fertilizer application for citrus trees was evaluated to obtain an enlightened understanding of plant nitrogen requirements, amount of fertilization needed, appropriate fertilization timing, and nitrogen uptake and storage in these plants. The knowledge thus obtained could be effectively exploited toward formulating recommendations that ensure optimized nitrogen fertilization. Mature citrus trees reportedly contain 500 to 1000 kg N/ha, 40 to 50 percent of which belongs in the leaves and fruits while only 15 to 20 kg N is deposited in their skeletal structure. These findings and the long-term survey of citrus fertilization experiments in Iran and elsewhere indicate that annual application of 200 kg N/ha seems sufficient for stable fruit production, improved fruit yield and quality, and proper tree growth and health. Moreover, it is found that 30-50% of this amount of N application will be incorporated into the fruits while about one tenth will be deposited and stored in the tree, with the balance between nitrogen uptake and consumption by the tree being secured by N leaching from the soil profile and lost as gas into the air. The results of an experiment with labeled fertilizers (15N) showed that the highest N-uptake rate in citrus trees occurred during the period from fruit set to fruit maturity and, further, that N uptake was very low during the post-harvest period, especially during minimum tree activity (winter) and the beginning of the growing season. This indicates that nitrogen storage in older tissues plays the greatest role in the growth and development of leaves, branches, flowers, and fruits at the beginning of the growing season when N uptake from the soil is still very low (i.e., when branches and flowers begin to sprout and fruits are in the form). It has also been reported that the nitrogen present in soil organic matter ranges from 1000 to 2000 kg/ha in orchard soils containing 1 to 2% O.M. and that both the soil nitrogen content and that stored in the tree bodies play important roles in regulating the supply of nitrogen to new organs (i.e., leaves, blooms, and fruitlets), especially in the beginning of the growing season. It follows that the purpose behind N fertilization should be to ensure stable production, proper tree growth, and improved fruit yield and quality such that soil application of fertilizers before the flowering and fruit set stages have no effects on the growth of spring buds or flowering and fruit setting in the current year. Producers are, therefore, recommended to start fertilizer application with a maximum of 10-15% of the annual plant requirement during the period prior to flowering and fruit setting, gradually increase application rate (by some percentage of the annual requirement) as determined by the phenology of fruit growth, and increase it to its maximum rate by the middle of the first stage up to the early second stage of fruit growth. Then, in the middle of the second stage of fruit growth, nitrogen rate should be reduced to its minimum or stopped altogether (depending on plant variety).