Proper Fertilization of Citrus Trees Is a Job for Nitrogen

Question: When do I begin fertilizing my citrus trees, and how do I go about it?

D.L., Villa Park.

Answer: To encourage flowering and fruit set on citrus, nitrogen is the primary nutrient that needs to be replaced. Other nutrients are only necessary if there is a deficiency.

Fully grown mandarins and oranges need 1 1/2 pounds of actual nitrogen per year; lemons 2 to 3 pounds per year, and grapefruit 1 1/2 pounds per year.

To calculate the amount of nitrogen in a bag of fertilizer, look at the label. The first number in the parentheses after the fertilizer’s name is the percentage of actual nitrogen in the fertilizer. For example, ammonium sulfate is 21-0-0 and has 21% nitrogen. Therefore, 5 pounds of ammonium sulfate gives your plant 1.05 pounds of actual nitrogen.

If you multiply the total pounds of fertilizer in the bag by the percentage of total nitrogen stated on the label, you can determine the actual nitrogen in a given weight. For example, a 10-pound bag of 12-6-6 would contain 1 1/5 pounds of actual nitrogen. A variety of nitrogen-containing products can be used, including commercial-grade fertilizers such as ammonium nitrate, ammonium sulfate, or urea.

Unlike complete fertilizers, the primary element these provide is nitrogen, and they are the least expensive. You can also use complete fertilizers listed as citrus food or citrus and avocado food.

The goal is to fertilize before and during periods in which growth occurs. Peggy Mauk, a UC Cooperative Extension Farm advisor in Riverside County, suggests an initial fertilizer application in late December or January.

Rather than give the total yearly nitrogen at one time, it is preferable to fertilize in smaller amounts three to four times per year, such as early spring, early summer and late summer.

For citrus that is 2 to 3 years old, the University of California recommends fertilizer applications three to four times a year with 2 tablespoons of nitrogen spread over the root area and 1 to 2 feet outside the drip line. Wash in with 1 inch of water. Double the amount of fertilizer the third year.

Studies have shown that you should avoid applying heavy concentrations of nitrogen fertilizer on oranges and grapefruit in summer and fall, as it can cause thicker rind, lower juice content and re-greening of Valencia oranges.

If you are growing citrus in containers that are well drained, remember that nutrients are leached or washed away quickly and will need to be replaced more frequently. Use a slow-release complete fertilizer, rather than a sole nitrogen source, especially in very sandy soils.

If you have a citrus tree growing in the lawn, be sure to fertilize it separately from the lawn, as grass will compete for water and nutrients. It might also be beneficial to apply compost occasionally to improve the structure of your soil.

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Have a problem in your yard? University of California Cooperative Extension (UCCE) Master Gardeners are here to help. These trained and certified horticultural volunteers are dedicated to extending research-based, scientifically accurate information to the public about home horticulture and pest management. They are involved with a variety of outreach programs, including the UCCE Master Garden hotline, which provides answers to specific questions. You can reach the hotline at (714) 708-1646 or send e-mail to ucmastergardeners @yahoo.com. Calls and e-mail are picked up daily and are generally returned within two to three days.