- Author: Ben Faber
I was recently in an avocado orchard and saw the rounded fruit and small leaves typical of zinc deficient trees. I asked the grower if there were recent leaf analysis of the orchard, and so we looked at them. The leaves were running at 20 ppm which is low. Sufficiency runs at 50 ppm. The recommendation was to apply zinc sulfate to the soil. The recommendation included, though 200 pounds of phosphorus per acre. Phosphorus and zinc are antagonistic, meaning applying one can limit uptake of the other. In applying phosphorus at such a high rate was probably preventing uptake of zinc. It is also antagonistic to copper, iron and manganese, so all of these micronutrients can be limited by phosphorus applications.
There have only been two documented cases of phosphorus deficiency in fruit trees, walnuts on a volcanic soil in Lake County and oranges on decomposed granite in San Diego. It is an essential element, yes, but applying it when there is sufficiency in the leaves can lead to other problems which can be hard to correct. Generally speaking, phosphorus does not need to be applied to fruit trees in California. In other states that have peat soils, high carbonates or highly weathered soils, phosphorus application is a normal practice, but here make sure you need it before applying it.