Question: Should I fertilize my citrus trees? If so, what fertilizer should I use and when should I use it?

Answer:

The University of California Agriculture and Natural Resources recommends regular fertilization with nitrogen. First application should be in January or February, just prior to bloom. Additional applications should be made in early May and in June. Late season fertilization should be avoided as it may affect fruit quality, delay fruit coloring, and make the rind rough.

Nitrogen is the chief nutrient required by citrus and should be applied each year. Phosphorus and potassium do not need to be applied as often since they are held in the soil much longer than nitrogen. A mature citrus tree should receive enough fertilizer each year to supply one pound of actual nitrogen. This would require about 2 pounds of urea (46%N) or 5 pounds of ammonium sulfate (21%N) or 8 pounds of blood meal (13%N). A small or dwarf tree would need less nitrogen. Split the total amount of fertilizer into three lots and apply one each in February, early May, and June.

To fertilize a tree, measure the amount of fertilizer needed and scatter it evenly on the soil below the tree’s canopy but not near the trunk. If a chemical fertilizer is used, water thoroughly after application to dissolve and carry it into the root zone. If an organic fertilizer is used, rake it lightly into the surface of the soil then irrigate. Nutrients in a chemical fertilizer are available immediately for roots to absorb. The nutrients in an organic fertilizer are released slowly as the material decomposes.

Citrus grown in containers with potting mix do best with a small amount of fertilizer applied at frequent intervals. Trees in containers are watered more often and this leeches nutrients from the soil mix. To promote good growth a fertilizer formulated for citrus can be applied according to label directions.

Citrus occasionally suffers from micronutrient deficiencies such as zinc or iron. Symptoms appear first on young leaves which have areas of light green to yellow tissue between veins that are dark green initially. These deficiencies can usually be corrected by applying a foliar application spray of a liquid chelated micronutrient fertilizer as new growth emerges in the spring. You can also apply micronutrients in the sulfated form such as zinc sulfate or iron sulfate to the soil. A micronutrient deficiency may occur when the soil remains too wet or if the soil is too alkaline with a pH above 7.5. An alkaline soil can be made more acidic by applying a small amount of soil sulfur.