Biogeochemistry of Phosphorus in Midwestern Mixed Clay Soils

by Cliff Ramsier, Technical Director

Much of the fertilizer world believes that corn yield is directly related to the amount of soil test phosphorus. In an environment where every dollar matters, proper understanding and use of phosphorus can improve yield and profitability.

Most of us understand that substantial quantities reside in soils (400# in sandy loam soil to more than 10,000# in clay loam soil) in each plow acre at 62/ 3 inches deep. This  translates to 2,400 to 60,000 pounds in the total soil profile. While only a very small portion of this is soluble, it does represent the amount of potentially available phosphorus once mineralized, if the soil environment is properly managed.

The three phosphorus pools in soil environments are solution, labile, and fixed or stable. The solution pool is the source for plant uptake. While this pool is only a very small fraction of the total phosphorus in soils, it is replenished as many as 500 times throughout the growing season when soil biology is functioning at optimum. The principle source for the replenishment of solution P is from the labile pool. This pool contains phosphorus in forms that are relatively easy to convert into soluble forms. These forms include organic P that is easily mineralized by enzymes, and inorganic P that is adsorbed into the surface of soil particles and as calcium phosphate, etc. Since these forms are not soluble, the organic forms must be degraded by enzymes (phosphatase) and the mineral forms must be solubilized by acid formation generated by soil organisms. Both of these processes are biological processes and, therefore, are regulated by the relative conditions for biological activity. The final pool is the hard mineral phase, which requires very strong acids for long periods of time and, therefore, this pool is resistant to mineralization. This mineral pool consists mainly of iron and aluminum phosphates which are very strong bonds.

Phosphorus arrives at the plant root via concentration gradient, which means that it moves from the point of dissolution to the root very slowly and along a tortuous path where it can easily be precipitated. This makes the presence of mycorrhizal fungal association on the root very helpful. Once the extended fungal hyphae have captured the phosphorus, the fungus transports it directly to the plant. This is another benefit of a biologically-active soil. We apply Clean Start® in the seed trench to reduce the distance the phosphorus needs to travel for uptake.

As a reference, P1 is an indicator of the P in solution and P2 indicates the portion of the labile pool that is relatively easy to solubilize to P1. These are purely chemical extractions and, therefore at best, an estimate of what will really happen during the cropping season. As has been the experience of most growers, soil test values of P1 & P2 have very little impact on crop production. On the other hand, the condition of the soil, relative to biological activity, is more closely related. Real soil quality is measured by the portion of yield that is delivered by biological activity as compared to that which is caused by adding fertilizers.

This is really a complicated system that, when fully appreciated, is both easy to manage, and an excellent example of a perfect design. It is a design that allows for plant nutrition over eons of time without causing off-site problems. As has been said before, soils are designed to last a humanity. This is simply another example.