- 1). Find out or estimate the age of your citrus tree. The age determines how much fertilizer the tree receives yearly. Citruses begin fruit production between 3 and 6 years of age. A citrus tree in Arizona needs between .12 lb. of nitrogen at planting and 1.5 lbs. as adult tree over 6 years old. At a year old, the tree needs .25 lb. of nitrogen. Add .25 lb. of nitrogen per year of age until 6. This works for all citrus trees growing in Arizona except grapefruit, which needs just .75 lb. of nitrogen per year after 5 years old.
- 2). Choose your fertilizer and calculate the quantity of fertilizer. Multiply the percent of nitrogen in the bag by the weight of the bag. For example, a 10 lb. bag of 10-10-10 fertilizer has 1 lb. of nitrogen. This means that 5 lbs. of 10-10-10 fertilizer has .5 lb. of nitrogen. So, you need 2.5 lbs. for a 1-year-old tree and 5 pounds for tree 2 years old, and so on. The percent of nitrogen is located on the bag's tag and is the first number in the three-number sequence.
- 3). Fertilize the tree in January-February, April-May and August-September with one-third of the amount needed for the age of the citrus by sprinkling the fertilizer under the tree. Spread the fertilizer with a rake as necessary. Water the tree thoroughly.
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