Phosphorus is one of the nutrients targeted in the nutrient reduction strategy recently proposed by the Illinois EPA. Phosphorus loss isn’t so much about amount, timing and form, but about soil and water runoff. If you keep the soil and water in the field in place, you pretty much keep phosphorus for the crop.

Phosphorus is a unique nutrient in that it is fixed in the soil and is nonmobile. The soil naturally has abundant calcium, iron and aluminum ions, which bind to phosphate and form relatively insoluble minerals or complexes that attach to soil particles. When this occurs, the phosphorus is considered fixed or tied up. In this regard, phosphorus does not behave like nitrate, which also has a negative charge but does not form insoluble complexes. And nitrate can leach down into tile line or ground water or denitrify and be lost as nitrous oxide gas.

Because of fixation, phosphorus doesn’t leach or volatilize like nitrate. However its availability is limited at the root interface during the season, and particularly in the spring when soils are cool, but improves as soil temperatures increase and there is more no mineralization.

Phosphorus availability is limited in the spring when soils are cool. That is one of the reasons farmers use starter fertilizer. And starter fertilizers contain varying amounts of phosphorus, depending on the analysis. Starter fertilizers work because nutrients are banded in or near the row in the spring when root growth is limited and soils are less active.

One of the characteristics of phosphorus (and potassium somewhat) are that they move through the soil primarily by diffusion. Diffusion is a process whereby molecules move from areas of higher concentration to areas of lower concentration to establish an equilibrium. Because of fixation, phosphorus doesn’t move by mass flow or with movement of soil water through the soil.

Diffusion in the soil is a thermodynamic process. Put simply, at higher temperatures the diffusing molecule will have more kinetic energy and will diffuse over a greater distance than at lower temperatures. So warmer temperatures means more kinetic energy and the greater the diffusion distance.

Since phosphorus movement occurs primarily by diffusion, which is directly impacted by temperature—the lower the soil temperature, the less the diffusion. Root activity is also low at cooler temperatures, so the temperature effects of diffusion and root activity—and subsequent phosphorus supply to the plant—are intertwined. Thus, concentrations of readily available phosphorus supplied by moderate to high soil test levels or in fertilizer bands often can provide an adequate supply of phosphorus when soil temperatures are cool.

Agronomist Dr. Daniel Davidson posts blogs on agronomy-related topics. Feel free to contact him at djdavidson@agwrite.com or ring 309-533-8085.