Most phosphate goes on in the fall as dry material and either as MAP (monoammonium phosphate) or DAP (diammonium phosphate)—often blended with potash (potassium chloride). However, how often do you consider how much of that phosphorus is available the next season?

A rule of thumb that I follow is that about 20 percent of the phosphorus you apply in a given year is available that season. The rest is fixed in the soil and eventually becomes available to crops in later years. If you have a high enough soil test level for phosphorus, and you frequently apply phosphate to match removal rates, there will be enough phosphorus becoming available to meet crop demands—and particularly when soil test levels are high. However, when soil test levels are moderate or below, phosphorus could be limiting.

Phosphorus also is limiting in the spring when soils are cool and inactive. Phosphorus availability is reduced at the root interface in the spring when soils are cool, but improves as soil temperatures increase. That is one of the reasons farmers use starter fertilizer, such as 10-34-0. Corn seems to be more sensitive than soybeans to this phenomenon, but that is because soybeans are typically planted later than corn. Now with soybeans being planted nearly at the same time as corn, soybean seedling growth may be limited by a lack of phosphorus, and no one realizes it yet.

Starter fertilizers work because nutrients are banded at a high concentration in or near the row in the spring when soil is cool, root growth is limited and soil microbes are less active. As the soil warms up more phosphorus becomes available. When phosphorus is applied in fertilizer bands, either as starter or deep banded (think strip till), there is an adequate supply of phosphorus immediately available to the plant when temperatures are cool.

One of the issues in the spring is less nutrient availability, particularly phosphorus, which moves primarily by diffusion from areas of high concentration to areas of low concentration. As roots remove phosphorus, the concentration around root hairs declines and phosphorus moves in. As root uptake is reduced at cooler soil temperatures, there is less phosphorus diffusion towards the root. Thus, concentrations of readily available phosphorus, often supplied in ample amounts in soils with high phosphorus concentrations, could be limiting in soils testing low in phosphorus.

If you’re planting soybeans early, before May, perhaps it is time to rethink how you apply phosphorus and recognize that maybe banding or starter will be a good solution.

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