When my avocado and citrus trees are happy, I’m happy. When I think of happy fruit trees, I think of green leaves on strong, well-placed branches, lots of fruit and no weed or insect pest bothering them. Is the tree itself really happy when it’s in this state? I’ll leave that debate to the philosophers. An important key promoting happy fruit trees is to keep them well-nourished by feeding them on a regular basis. I’m going to focus on the one nutrient that is not abundant in our local soils and yet is used in large amount by citrus and avocado trees.
Nitrogen is essential to all plants and is a major building block of chlorophyll, which gives leaves their green color and allows plants to turn the sun’s light energy into food to feed themselves. Trees that aren’t getting enough nitrogen have poor growth; small, pale leaves; and poor fruit production. There are several other diseases, pests and soil conditions that may cause these symptoms, but nitrogen deficiency is a leading cause and will certainly show up in trees that aren’t fed regularly.
Assuming you have nitrogen deficiency and not root rot or some other malady, the problem can be fairly easily corrected with several readily available fertilizers. Ammonium sulfate, calcium nitrate, urea and ammonium nitrate are a few commercial chemical fertilizers that supply nitrogen. If you want to go with an organic fertilizer blood meal, cotton seed meal, alfalfa meal and fish emulsion are good sources of nitrogen. Manures can also be good sources of nitrogen but if over-used, especially on avocados, can cause other problems because of high salt content and other soil reactions. Make sure they are well-composted before using them.
Mature, healthy avocado and citrus trees need about 1 to 2 pounds of nitrogen per year to maintain good, vigorous growth and good fruit production. This can be applied monthly during the growing season or during the trees’ growth flushes in early and late spring. If you have an irrigation system with a fertilizer injector, it’s easy to fertilize monthly. If you don’t, many people will apply a dry fertilizer equal amounts in late February or early March and again in mid-May.
The amount of fertilizer to apply will depend on the amount of nitrogen in the fertilizer you decide to use. If you’re not sure, start out with the lighter dose of 1 pound of nitrogen for a mature (6 year old-plus), standard (nondwarf) citrus or avocado tree.
Fertilizers have the three major nutrients expressed as percentages on the front of the packaging. Nitrogen is the first number, followed by phosphorous and potassium. A simple way to find out how much of a particular fertilizer to give a mature tree is to divide the number of pounds of nitrogen you want to give the tree by the percentage of nitrogen in the fertilizer you have on-hand. For example, to give a mature, 25-year-old avocado tree 1.5 pounds of nitrogen from a bag of 16-6-8 fertilizer, divide 1.5 (pounds of nitrogen) by .16 (percent of nitrogen in fertilizer) to get 9.375 pounds of fertilizer per year. As another example, if you want to use an organic form of nitrogen like blood meal 12-0-0, divide 1.5 (pounds nitrogen) by .12 (nitrogen in blood meal) to get 12.5 pounds of blood meal fertilizer per year for that tree. This amount of fertilizer should be given to the tree in two or more applications. For example you can give your tree 4.5 pounds of 12-0-0 in May, 4 pounds in July and 4 pounds in September.
This just scratches the surface of the nutritional needs of citrus and avocados trees. As with other aspects of tree care, there are lots of good, in-depth information available online, in the library, your local nursery or from your friendly and happy, local pomologist. Here is a page with some good dry, in-depth fertilizer information for avocado trees that is applicable to citrus as well.