Last week, I visited the largest organic avocado farm in California. It’s those hills in the photo above — they’re covered with 1,000 acres of avocado trees. And I felt vindicated when I saw that they care for the soil under their trees in essentially the same way that I do.

But I’ll get to that in a minute. First, let’s say what our goal in growing an avocado tree is. For me, it’s a consistently fruitful tree, which means it must also be healthy. And how does soil fertility affect our achieving this goal? Avocado trees, like all plants, need certain elements to run their systems properly and produce fruit. So we add stuff to our soil when necessary.

Which nutrients avocados need

“California soils contain ample amounts of most plant nutrients,” writes Paul Moore in the 1952 Yearbook of the California Avocado Society, the Bible of avocado culture. “The function of fertilization is to supplement deficiencies in the soil, not to feed the tree directly with all its nutrient requirements. The most common deficiencies suffered by avocados are nitrogen and zinc.”

So, the soil in your yard may have all your avocado needs, but commonly in California, avocado trees need added nitrogen and perhaps zinc. How do we know if this is the case with our own particular soil?

You can get a sample of your soil (or leaves) tested by a lab such as San Diego Agricultural Laboratory.

More easily, however: “Your trees are open books. If you learn to read them you will be able to tell their nutrient needs at a glance. For example, nitrogen deficiency is indicated by pale green or yellowish green leaves. This is in contrast to the normal dark green color of leaves formed under an adequate supply of nitrogen.” (That’s Paul Moore again.)

But maybe you’d like to give it a little boost, a little extra just to make some bonus fruit? It doesn’t quite work that way.

In the book Avocado Production in California, former University of California farm advisor Gary Bender writes, “It should be remembered that there is no “magic bullet” fertilizer that will increase avocado production, unless that particular grove has a deficiency in a particular element.”

Fertilize only as needed

Extra fertilizer will not help and it might hurt. For example, studies on avocado tree responses to different levels of nitrogen fertilization have found roughly the same results for more than 50 years now.  T. W. Embleton and others wrote back in 1958 that they found that “. . . too little or too much nitrogen results in a reduction in yield, and highest production is obtained with a moderate level of nitrogen in the trees.”

More recently, a study by Mary Lu Arpaia and her colleagues, published in the 1996 CAS Yearbook, additionally found that “. . . fruit size from the high nitrogen trees tend to be smaller.” Also noticed was “a decline in post harvest quality of the fruit coming from the trees high in nitrogen.”

Therefore, to achieve our goal of a healthy tree that produces much fruit, we can periodically test our soil and leaves, but more easily we can simply keep an eye out for foliage that lacks the expected, uniform deep green color. What isn’t called for is blindly adding fertilizers as a matter of routine. Adding more than enough is not better.

Fertilizer can’t solve all problems

And if we see symptoms of a nutrient deficiency? What do we do then? Still, before jumping to the conclusion that soil fertility is the problem, assess your care of the tree, especially your watering practices. As Bender notes in Avocado Production in California, the exact same symptoms as for nitrogen deficiency “are also associated with other problems such as Phytophthora root rot and root asphyxiation due to water-logged soils. In these cases application of nitrogen will not fix the problem. If nitrogen deficiency is suspected, it should be confirmed by leaf analysis and an examination of other cultural problems in the grove.”

I’ve seen many trees that appeared sickly, yellowish, nutrient deficient, only to poke into the soil and find it soggy. That’s a watering problem, not a fertility problem.

Here’s a specific example. This tree wasn’t watered often enough when first planted, so leaves burned. Then it was watered too much too often, so now it has yellowish foliage.

Fertilizer need not apply.

(You might like to read my post, “How much and how often to water avocado trees in California,” to get a better handle on that. Also see, “Avocado leaves turning brown? Here’s why and what to do.”)

And we can’t forget about the weather. Both freezing temperatures as well as extreme heat will discolor leaves. Nor can we forget that avocado leaves don’t live forever, and when they begin to die, they turn yellow before falling off. Here’s a photo of a healthy, but naturally dying, avocado leaf:

senescent Reed avocado leaf

When I was new to growing avocados, I saw such a senescent leaf on my tree and worriedly sent a photo to an expert for advice.

Lack of fruit, as a single symptom, should not be taken as caused by nutrient deficiency, by the way. Avocado tree fruit production is always fluctuating due to natural alternate bearing, more and less conducive weather, bee activity, etc. 

After all of your care for the tree has been considered, and you’ve gotten your hands dirty feeling the soil and judging your watering practices, and the last possibility is that your soil is deficient in a nutrient, then . . .

Two main ways of supplying nutrients

. . . you can feel confident that buying a fertilizer product that will improve things. Go to a nursery and they should be able to provide you the right product. I’ve seen innumerable beautiful and fruitful avocado trees in both yard and farm settings fertilized with E.B. Stone’s Citrus and Fruit Tree Food. You might try that. Apply it by following the label directions. And do note that among its primary ingredients are chicken manure and bat guano. 

On the same day that I visited the large organic avocado farm pictured above, I also visited a smaller organic avocado farm. At the smaller operation, the farmer told us that he fertilizes his trees using Peruvian seabird guano that is injected into his irrigation system. In addition, when he planted his trees three years ago he had added a layer of wood chip mulch. “I wish I could add more mulch because I’m really convinced that it increases the soil’s cation exchange capacity,” he said.

Some mulch around his trees, but more would be better, the farmer says.

He wishes he could add more mulch? What’s stopping him? Even if he gets the mulch for free, imagine the costs of spreading such a bulky material under hundreds of trees. Injecting fertilizers into an irrigation system is far cheaper.

Nonetheless, beneath the tens of thousands of avocado trees at the other farm, the largest organic avocado farm in California, there is new mulch being added consistently. They make their own right there on the farm. Using massive, expensive machinery, they take green waste (pieces of plants and trees), mix in some animal manures, compost it in long heaps, and eventually spread it under their trees.

As I mentioned earlier, I felt a sense of vindication seeing such a large operation treating their trees this way because . . . look what’s under my trees.

Keeping it thick.

Even around the babies, I build it up.

Under all of my avocado trees, I keep a thick mulch of wood chips. In addition, especially under young trees, I add some compost or manure. Often, it’s chicken manure or what I call “chicken compost” from my own birds. I find that this gets young trees growing well until the wood chips break down enough to start feeding the trees more.

Here is a video showing my typical application of compost and mulch around a newly planted avocado tree.

In Avocado Production in California, Gary Bender writes, “Nitrogen is also available to plants from the breakdown of organic matter in the soil. Soil organisms convert proteins in the organic matter to ammonium compounds in a process called mineralization. California soils are historically low in organic matter, so this aspect is not too important unless organic matter is added by way of mulches or manures.” (Italics are mine.) 

The large avocado farm uses a special machine to blow out and spread the mulch and composts under trees, saving time and labor. But it remains a much more expensive way to add nutrients to the soil.

There is a bigger economic picture to see, however. The mulch not only breaks down to add nitrogen and other elements in order to act as a fertilizer, but it also functions as an herbicide by covering weeds, as well as a soil conditioner that improves water intake and holding, and a disease suppressant, etc.

How much is all of that worth beyond the fertility effects alone? We are killing many birds with one mulch stone.

(You might like to see the short article, “The Economics of Mulching.”)

Which organic matter to mulch with

What organic matter should you add as mulch? I say, vary your sources. Don’t add a single thing year after year, which can build up too much of one element and not enough of others. It can lead to the tie up of some elements or toxicity in others. Vary your sources, just as the large organic avocado farm does. (Just as is done in nature. This is nothing new.)

I add a little horse manure from my neighbor occasionally, a bit of my chickens’ manure, plus a lot of random plant materials. But more than any other single element, I mulch with lots of wood chips from tree trimmers. This stuff is bulky with big chunks, and avocado roots enjoy this characteristic for two reasons. It ensures that the mulch remains airy. You’ll find roots poking their heads up into it. The chunks of wood also feed bacteria that consume the wood’s lignin by producing an enzyme called cellulase, and this enzyme breaks down the cell walls of the fungi Phytophthora cinnamomi, the organism that causes the root rot disease in avocados.

(I recently wrote the post: “Where to get wood chips for mulch.”)

Our guide on the tour of this large avocado farm’s mulching operation was the farm’s manager along with Mary Matava, who has been working with fertilizer, mulch, and avocados in Southern California for four decades. She is an agronomist and the owner of Agri Service, the company that runs Oceanside’s El Corazon compost facility.

Matava reminded us to consider the avocado’s sensitivity to salts when mulching. Wood chips and greenwaste compost are usually low enough in salts to not worry about (although greenwaste compost can be surprisingly salty if it is made in a hot and dry location using salty irrigation water), but mushroom compost and manures should be applied with more caution as they are higher in salts.

If applying mushroom compost or manures, it is ideal to apply them in winter so that the rains can leach the salts, said Matava. About five inches of rain (or irrigation water) should suffice. Then come spring, when avocado roots again become more active, the roots won’t have to deal with salty soil as the salts have been driven below the level of the roots.

(Read an excellent article that Matava wrote: “Mulching Practices in Avocado Orchards.”)

How much mulch to add

As for how much organic matter to add to the surface under your avocado tree, that’s hard to calculate with any precision without some serious testing and quantifying of the materials. I don’t bother. I merely make sure that the soil is always covered. I add more materials whenever the layer is thinning out. This very simple approach has worked well for my avocado trees.

After a few years of constant coverage, the mulch-soil interface layer starts to look and feel like a forest soil. Intuitively, you look at the color, you smell it, you feel it, you just know it’s fertile.

Black, fragrant, sticky, able to give the tree all it needs.

And some good news: Gary Bender writes, “However, it is not necessary to apply the same amount of organic matter each year because mineralization of nitrogen, after a high release rate the first year, declines to a release rate of 5% – 6% per year from the initial application. Therefore, as the years go by, application rates of organic matter should gradually decline so as to achieve a steady release rate on nitrogen in the soil.” The organic matter sticks around, quite literally, and is able to feed your tree for longer than other fertilizer options.

Why did I feel “vindicated” upon seeing the investment into mulching that the large avocado farm had made? I was low on self-confidence, I suppose. I had been the only one I knew growing many avocado trees without adding fertilizers, with only mulching, and I wondered if I was fooling myself — despite the fact that I could see my trees growing and producing well year after year.

I’m now feeling more confident in the long-term sustainability of the way in which I fertilize my avocado trees, which is to not fertilize them at all, in the sense of adding what we usually think of as fertilizer. Rather, I merely mulch.

Mary Matava again, this time in an article she wrote for the California Avocado Society’s 2009 Yearbook, “The Benefits of Mulching Avocados”: “In fact, the feedback from growers using mulch is that they reduce or eliminate the need for inorganic nitrogen with no reduction in yield.”

You might also like to read my posts:

Growing avocados in Southern California

Growing fruit trees in Southern California

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