It is such a simple little letter, P. It stands for the element phosphorus. It is often misspelled as phosphorous which is an adjective, but even in technical literature it is misspelled. But that's not the end. Phosphorus is an element that takes many forms called oxidation states. When it is in the form of phosphate or phosphoric acid, it is a fertilizer – H3PO4. But when it is in the form of H3PO3 or phosphonate or phosphonite or phosphite or phosphorous acid, it does not perform like a fertilizer. It acts more like a stimulant for a plant to fight off Phytophthora or Pythium. And it works well for avocado and citrus root rots, as well as citrus brown rot.
But a grower recently told me that there is no end of confusion about these two very different forms of P.
A recent article helps to clear up some of this confusion
and more if you are still interested
Sick avocado on the left and healthy on the right
It is more than just the confusion about the effects of phosphonates, but also how to spell the words associated with the P atom. Phosphorus with an ending in “us” is the element we know as P, while Phosphorous with a “ous” ending is the adjective of P. So an acid containing Phosphorous acid is written H3PO3 while phosphoric acid is H3PO4. These are both strong acids and can hurt and cause damage if splashed on the skin. When either is reacted with calcium or potassium hydroxide, a salt is formed which is less dangerous to users, but as with any chemical can be misused.
The salt formed from Phosphorous acid is called calcium phosphite or calcium phosphonate depending on what naming system is used to describe it. Whereas when these bases are reacted with phosphoric acid, the result is calcium or potassium phosphate. These salts are relatively benign in contact with skin. Labels on containers often call phosphorous acid, “soil applied” whereas the phosphite forms are called “leaf applied”. The “soil applied” when applied to a leaf can cause damage, whereas, the leaf applied is much less likely to cause damage to both plant and applicator. It can be applied to the soil, as well. It's much safer to use the leaf applied in either application technique.
The phosphites are often registered as fertilizers, but they have little nutrient effect. Most of their effect is to boost the plant's immunity to Phytophthoras and pythiums. This is called fungistasis and the material is called a fungistat. They don't act as a fungicide when normally applied to kill these organisms.
So you can see there is a lot of confusion in the phosphorous world. Knowing the proper spelling, pronunciation and use is note only good grammar, it makes good farming.
To read more, see:
There are few documented cases of phosphorus (P) deficiency in tree crops in California.
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.