How trapped phosphate becomes dissolved phosphate

Last fall I wrote an article about using gypsum (calcium sulfate) to trap phosphorus on the landscape. And it is true that gypsum has a lot of affinity for phosphate since it’s soluble, and calcium and phosphate bind to form tricalcium phosphate. Any free calcium in the soil solution reacts immediately with phosphate in the soil solution to form tricalcium phosphate—a slowly soluble mineral that is not readily available to plants.

After that story was posted on ILSoyAdvisor and then picked up and posted on the Plant Management Network, I received several emails from readers who had never heard of this strategy. They wondered if phosphate would ever become available again once it is tied up as a mineral. This question relates to a basic understanding of the chemistry of phosphate and its interaction in the soil.

One reader posted this question in an email, “I read your article in the latest PMN update on using gypsum to help tie up the surface P and cause less runoff. I agree that there are some areas of the country which have P levels far above what is needed for maximum crop yield and growth. However there are many other areas that do not have high P soil levels and that need to have P2O5 fertilizer applied. Growers are becoming more informed and are not making fertilizer applications when the ground is frozen or on fields that are sloped or have little residue on the soil surface and this is helping the situation. The point I am trying to get to here is that with an application of gypsum, yes that (calcium) will unite with the phophate to make it less available. But if a grower is applying P2O5 fertilizer to feed his crop, why would he want to apply gypsum to make it (phosphate) less available. In fact with the compound that is formed it is hard to say when that P will become available.”

The first question is about the fate of tricalcium phosphate and whether it will become available. The rule of thumb I was taught many years ago about applying phosphate fertilizer is that only 20 percent is available the first year and the remaining 80 percent becomes available in later years, while at the same time that applied phosphate fertilizer applied a year or two earlier is now becoming available. So in reality some of the phosphate applied that ties up with calcium continues to come available in subsequent years so there is always a soluble supply available.

So what is the mechanism that releases fixed phosphate and makes it available again? We often refer to that as chemical weathering of minerals that release individual nutrients back into the soil. This fixed phosphate pool contains inorganic phosphate compounds that are insoluble and resistant to mineralization. However both roots and soil microbes continually release organic acids back into the soil. These acids break down (chemical weather) and release phosphate back into into the soil’s active pool that feeds the solution pool of dissolved phosphate. So as phosphate molecules are being tied up with calcium, other phosphate molecules are being released.

The second question asks, “Why tie up phosphate in soils that test low in available phosphorus?” I assume he’s inferring that these soils aren’t at risk of phosphate loss. Soil test levels for P1 available phosphorus, be it 5, 15, 30 or 45 ppm P2O5, really don’t make much difference. If the phosphate is on the soil surface, either attached to soil particles or dissolved in water, this phosphate can and will move off the landscape if water and soil flows. Of course in a lowland field with little or no slope and surface drainage (excluding tile) there is little risk of phosphate loss and really no time to apply gypsum to tie it to the landscape. But then there are other reasons to use gypsum anyhow.

Agronomist Daniel Davidson, Ph.D. posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com or ring him at 402-649-5919.