The Basics About Phosphorus Fertilizers

Let me cover just a few basics on phosphorus and chemistry in general. When people begin talking about availability of phosphorus they usually refer to this diagram.

As you can see, the pH or alkalinity increases to the right following the line at the bottom of the graph. Each one unit change in pH is a 10 X change in alkalinity (to the right) or acidity (to the left). These one unit changes are in logarithms, just like they measure earthquakes.

At the center of this graph of pH is 7, also considered neutral. There is an equal amount of acidity compared to alkalinity. At pH 7 acidity and alkalinity are in perfect balance with one another. A one unit change from pH 7 to 8 means there is a 10 fold increase in alkalinity. A two unit change from pH 7 to pH 9 is a 100 fold increase in alkalinity (10 x 10). A three unit change from pH 7 to pH 10 is a 1000 fold increase in alkalinity (10 x 10 x 10) etc. The change is the same with increasing acidity; if the pH decreases from pH 7 to pH 6 (10), pH 7 to pH 5 (10 x 10), pH 7 to pH 4 (10 x 10 x 10).

The availability of phosphorus (bar gets wider), according to the graph, decreases with increasing acidity (below pH=6.0) as well as with increasing alkalinity above pH=7.5. But if you look closely, phosphorus availability does not disappear. It’s availability only decreases. Phosphorus is still available. But it’s availability is less.

When you apply phosphorus that is not “available” it does not mean this is 100%. It is not like a light switch; off and on. As soon as the pH of the soil reacts with the applied phosphorus, some of the applied phosphorus becomes available to plants and some of it does not. How much becomes available depends on the soil pH as well as the rest of the chemistry in the soil.

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To keep phosphorus available in the soil push your soil pH more acid in our soils. You can do this with compost and decomposing organics. Some fertilizers also decrease pH. One of them is MAP or monoammonium phosphate. This fertilizer pushes soil pH lower and improves a plants uptake of phosphorus.

Viragrow sells MAP for 80 cents per pound in 10 and 40 pound bags

Phosphorus applications do a great job stimulating mycorrhizae!

Viragrow Delivers!