How to Fertilize Citrus Trees

Organic fertilizers and mulches will make your trees healthier and more productive.

Organic fertilizers and mulches will make your trees healthier and more productive.

If you’ve been growing citrus in Southern Arizona for any length of time, chances are that at some point you have heard something like this. “You gotta fertilize your trees on Valentine’s Day, Labor Day, and 4th of July”. “No, it’s Martin Luther King Day, Presidents Day, and Memorial Day”! “No, that’s not right I’m pretty sure it’s Valentine’s Day, Memorial Day, and Halloween”? Honestly, I’ve heard every combination of random holidays that you can think of. The actual schedule for fertilizing citrus used to be Valentine’s Day, Labor Day, and Memorial Day, but about four years ago, the University of Arizona issued new recommendations.

You see, the old recommendations were better suited for growing citrus in California, and while the old “holiday” schedule was easy to remember (sort of), it didn’t line up correctly with our citrus growing season in Tucson. So, in 2015 the University of Arizona issued its new recommendations. With the new recommendations, lemons, limes and other sour fruit would keep a similar schedule however grapefruit, orange, and sweet fruit such as tangerine would now receive their final application in May or June rather than in September.

This new fertilizing schedule does in fact line up better with our growing seasons in Tucson, however there is still one fatal flaw with the University method, it still relies primarily on chemical synthetic fertilizers, not slow release organics. To verify this, I called the Pima County Master Gardener Extension office helpline and asked for recommendations for fertilizers for my citrus trees. The Master Gardener on the other end of the line suggested I go with Ammonium Sulfate, a common synthetic. When I inquired about using an organic option the person on the phone did say I could use organics but advised against it.

The only problem with organic fertilizers is figuring out what to do with all of these lemons!

The only problem with organic fertilizers is figuring out what to do with all of these lemons!

Which brings us back to the University of Arizona citrus fertilizing schedule. The reason the University and Pima County Master Gardeners recommend three separate applications of chemical fertilizers is because these applications line up with specific growth periods in citrus. Flower production in the early spring (Valentine’s Day), new leaf and shoot growth in the early summer (Memorial Day), and finally new shoot production and fruit development in the late summer and early fall (Labor Day). When you’re applying a poison, er I mean synthetic, fertilizer like ammonium sulfate, you need to time your applications to coincide with these specific growing periods.

However, when you use organic fertilizers, you’re not giving the tree shots of fast acting chemical pseudo-nutrients that kill off the soil food web. Instead, you’re feeding the soil food web around the tree so that the soil can take care of the tree. While it’s certainly possible to apply organic fertilizes in only three applications, as the University of Arizona suggests, it’s not really a good idea. For one, by the time soil microbes converted it into nutrients it wouldn’t be as helpful. It takes time for the fertilizer to break down and covert into nutrients. Second, even plant based organic fertilizers in large quantities when mixed with water will smell like dying animals and attract flies. It is for these reasons that organic fertilizers are best applied more frequently throughout the growing season in smaller applications, so that they have time to work and so pests don’t become a problem.

What’s important here is that we don’t throw the baby out with the bath water. There is still some useful information in the new University guide. We can still use the guide to determine the total lbs. of fertilizer required for our trees. Once we have this number, we can then determine how many lbs. of fertilizer we need to apply over the course of the growing season. however, instead of dividing this number by 3, you’ll instead want to divide by 6, 9 or even 12. Which means you’ll be applying less fertilizer more frequently.

For example, a large orange tree requires between 25-30 lbs. of 5-2-4 organic fertilizer in one year. Instead of applying 10 lbs. 3 times a year, you could apply 5 lbs. 6 times a year, or 3 lbs. 10 times a year.  You could even do 2 ½ lbs. monthly. When you apply organic fertilizers in small amounts in this way, you feed the soil food web which will provide your tree with a constant source of nutrients all season long. You also don’t run any risk of over fertilizing your tree when you use organics. Even if you applied all 30 lbs. at once you wouldn’t harm your tree. However, if you over-apply even just a small amount of ammonium sulfate, you could severely damage your tree and fruit set or even kill your tree.

You can make your own organic plant-based fertilizer for a fraction of the cost of store bought.

You can make your own organic plant-based fertilizer for a fraction of the cost of store bought.

Finally, when you switch to organics, you’ll realize that you’ll need to apply far more by weight than you do of synthetic fertilizers. It is for this reason alone that many people stick with synthetic fertilizers, they are way cheaper. However, you can make a large quantity of your own organic fertilizer using the recipe below:

  • 2 Bags Cottonseed Meal

  • 1 Bag Alfalfa Meal

  • 1 Bag Soybean Meal

  • 2 Bags Disper-Sul Soil Sulfur

All quantities in 50 lb. bags – Makes 300 lbs. 5-2-4 Organic Fertilizer.

It will cost roughly $130 to purchase these ingredients or roughly $.43/ lb; However, you’ll have enough fertilizer to fertilize a large orange tree for 10 years. Compare to the cost of even the cheapest organic fertilizers on the market and you’ll still be paying less than half. You can buy the cottonseed meal, alfalfa meal, and soybean meal at OK Feed and Supply on Dodge Blvd. The Disper-Sul elemental soil sulfur can be purchased at Sprinkler World on Ft. Lowell. You can also come to the community food bank where yo can buy any quantity of this low cost pre-mixed organic fertilizer.

I’ve created a separate fertilizer schedule that you can use with this home-made 5-2-4 fertilizer. It has rates for 3, 6, 9, and 12 applications. I’ve also included a schedule for deciduous fruit trees, which you can use the same mix on. To make your trees even happier, mulch your trees with a little bit of compost and a few inches of wood chip mulch. As the compost, mulch, and fertilizer break down, they will all work together at improving your soil and making your trees nice and healthy.