- Author: Chris M. Webb
Ventura County UCCE Farm Advisor, Ben Faber shares his knowledge of avocados. Today’s topic:
The most important thing you can do before planting is assess the pH of the soil before planting. Avocados are very sensitive to soil pH greater than 7. Their uptake of iron and zinc can be terribly compromised and they will suffer. Correcting the soil pH prior to planting is the easiest way to approach the problem, rather than trying to correct it later when the tree is in the ground. Otherwise, it becomes expensive and takes a long time to correct the problem without killing the tree. Elemental sulfur (not popcorn sulfur) in pellets is the easiest way to accomplish this. Watering and waiting for the sulfur to make the change and then checking to make sure the pH is really down takes about 6 months.
As for nutrients, though, the most commonly required ones are nitrogen and potassium. These can be applied as either organic (fish, soybean, manure, etc.) or synthetic forms (urea, ammonium sulfate, potassium sulfate, etc.) or a combination. The thing to remember is that the tree likes small, frequent amounts of nutrients because of its shallow root system. And because it is a subtropical plant, it goes quiescent in the winter and when the soil is cold. So nutrient applications like nitrogen are best applied when soils are warm, from spring though fall.
Newly-planted trees do not need supplemental nutrients, they are loaded up from the nursery and the young root system can easily burn until it gets established. In the first year, use the equivalent of 1 ounce per tree of nitrogen in 4- 5 applications over the spring/fall. The following year double that amount and do so for each succeeding year for the next 5 years. The more small applications you make the less total nitrogen fertilizer you will use. Use the equivalent amount of nitrogen whether it is a synthetic source or an organic source. Once the tree has started to develop a thick leaf mulch, it is possible to back off on nitrogen applications because now the mulch is contributing some of the nutrients. In many garden situations where mulch is maintained, by year 10 the nutrient status is self-sustaining and nitrogen fertilizer may not be needed at all. Just keep an eye on the leaf color to make sure it stays green, indicating adequate nitrogen.
When the avocado starts bearing fruit about year three, it may need potassium. This is not necessary in all situations throughout the state, but the harvested avocado fruit contains twice as much potassium as nitrogen and when the fruit is removed the tree can start showing potassium deficiency symptoms. This can be analyzed at a lab, but probably the best thing to do is just apply potassium sulfate at an equivalent rate to nitrogen or to use triple 15 fertilizer to meet both the nitrogen and potassium needs. Organic growers can use organic potassium sulfate or kelp. Although phosphorus is used by avocados, there are no documented cases in California where supplemental phosphorus needs to be applied.