By Daniel Kaiser

University of Minnesota soil fertility specialist

With fluctuating fertilizer prices, it is a good time to review a few principles related to phosphorus (P) availability in soils. We know that P is a critical element required for crop growth and development, and if deficient, the lack of P can significantly reduce crop yield. While application of P fertilizer is common, it is not always required to achieve maximum yield.

Phosphorus exists in the soil in many forms, which vary in crop availability. In fact, crop-available P only represents a small fraction of the total P in the soil. When crops take up P, nutrients in less available pools in the soil can move into a more available form, replacing what was taken up by the crop. Crop producers sometimes feel that it is most economical to re-supply the soil with the amount of phosphorus taken up by the crop. However, the soil supply of phosphorus is more complex, and soils testing Very High in phosphorus may be able to fully supply crops to achieve maximum yield for multiple crop years.

Should you apply crop removal rates of P?

One mistake made by crop producers is applying crop removal year after year without considering the soil test from their fields.

While applying crop removal rates is equivalent to hitting the easy button, if P is not required for the crop, the money spent on P fertilizer is likely best spent elsewhere. It can be argued that a portion of the yield produced each year is from P but where that P comes from is key. There is overwhelming evidence in the upper Midwest that the probability that P fertilizer is needed for Very High P testing soils is extremely low; almost zero. When we talk about soil test classifications, they represent the overall capacity of a soil to supply P, or other nutrients, to a given crop.

Some growers may doubt the usefulness of soil tests, but from a practical standpoint, the two suggested P soil tests for Minnesota are very good at predicting the need for P in a field.

There are more questions about the potassium (K) soil tests, which are impacted by soil moisture. Phosphorus soil tests tend to be consistent over time and are not affected by soil moisture like K soil tests. This means that P soil test results can be used with more confidence when deciding where or if P fertilizer is needed. Our corn fertilizer guidelines show the probability that applied P will increase yield, and the expected yield loss for corn if fertilizer were not applied, based on the current Minnesota P soil test classifications.

Many growers opt for removal-based P applications. While we know that it is highly economical to apply P fertilizer for Low testing soils, there is a limit to where it makes sense to apply fertilizer. Data from Minnesota, as well as Iowa, shows that application of P based on crop removal pays for itself for soils testing 20 ppm or lower according to the Bray P test. Soils testing higher than 20 ppm are unlikely to respond, and if they do, a low rate of starter P fertilizer should be enough. The 20 ppm critical level for the Bray-P1 test has been around for a while and is a big part of the reason why we have greater confidence in our soil tests for P than for K. Results from across the state evaluating the P critical level are consistent regardless of yield potential or changes in corn hybrids.

If you don’t apply P fertilizer this year, will there be issues?

Again, the data indicates that soil tests in the Very High class will not respond to P, so skipping a P fertilizer application for one year or even longer is unlikely to present a risk for yield loss. Research has found that soil tests should, on average, decrease by around 2 ppm per year without P fertilizer applied for High testing soils. The drop can be greater if soil tests are above 50 ppm, as it takes higher application rates to maintain that high of a soil test. However, maintaining soil tests at this high of a level makes no agronomic sense. Soils testing 50 ppm or higher may have enough P to supply crops for 15 to 20 years before fertilizer applications are needed. Allowing P soil tests to decline to levels where P fertilizer applications are more agronomic makes a lot of sense to reduce costs, enhance profitability and decrease the risk for P loss to the environment.

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