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If you are located in Florida, as I am, the average yearly rainfall is usually enough to keep mature orange trees healthy. However, because Florida soil tends to be sandy and does not retain water very well, and in addition, the rainy period is seasonal, mainly during the summer months, into the fall, you might need to supplement with watering during dry periods. During these drought periods, water your orange tree once a week, especially while fruit is forming on the tree and give it at least 25 gallons of water.
Orange trees are typically fertilized more frequently when they are young and growing. If your tree is already full grown/mature, it should not require as much fertilizer, but this would depend on your specific location and the consistency and pH of your soil. The following chart will be helpful re: fertilizing your tree and how often, depending on its age: http://manatee.ifas.ufl.edu/lawn_and_garden/master-gardener/pdfs/articles/JMfertilizingcitrus.pdf
Use an 8-8-8 NPK fertilizer (contains macronutrients: nitrogen, phosphorus and potassium) that also contains necessary micronutrients like copper, boron, manganese and magnesium. Fertilize trees that are five years and older, three times a year, in early Spring, early Summer and late Fall. Apply the fertilizer at a rate of 1 pound for each year of age, to a maximum of 10 pounds at each application; also, apply the fertilizer according to the location of the tree's drip line. Spread the fertilizer one foot for every year of age, up to ten feet. In addition to a standard NPK fertilizer, orange trees also should receive one application every year of chelated iron and one application of nutritional spray on the leaves.You will be able to find these supplies at your local garden nurseries.
This guide will also be helpful in the care of your tree: http://www.gardenguides.com/93585-feed-mulch-florida-orange-trees.html
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