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Consultant shares best times to apply phosphorus, potassium

By Eddie Funderburg, Ed.D.
Senior Soils and Crops Consultant

Posted Dec. 1, 2016

Sometimes, a rancher takes a soil sample, sends it to a laboratory and the recommendation calls for phosphorus (P) and potassium (K). The rancher knows nitrogen should be applied near the time of use, but wonders if the P and K can be applied at other times of the year such as fall or winter. The primary reason is to incur some expenses in a different year.

The answer is "it depends." K can often be applied during the fall and winter for a summer crop. K does not bind chemically in the soil. It magnetically attaches to the clay and organic matter particles in the soil. K is positively charged and will attach to the negatively charged clay and organic matter particles in the soil. If there are not enough of these clay and organic matter particles in the soil, K can be leached from the soil. This usually happens in very sandy soils. However, if enough negatively charged clay and organic matter particles are in the soil, K can be safely applied in the fall and winter for next year's crop.

How many clay and organic matter particles are "enough"? On most soil test reports, there is a category called cation exchange capacity (CEC). CEC is an estimate of the soil's ability to hold and exchange positively charged particles like K. If your CEC is less than 4, there is a good chance K will leach from the soil. Therefore, we recommend you apply K near the time of plant uptake in these soils, which is typically in the spring. If the CEC is greater than 8, there is very little chance K will leach. In these soils, K can be applied at any time. In soils with a CEC between 4 and 8, there is a slim chance K can leach. Leaching in these soils will probably occur only in times of high-intensity excessive rainfall.

P acts differently in the soil than K. Instead of being magnetically attached to soil particles, P is chemically tied up by certain elements in the soil. The strength and duration of the P reaction depends on soil pH, which is a measure of how basic or acidic a soil is. Soil with a pH higher than 7.0 is considered basic. Soil with a pH of 7.0 is considered neutral. Soil with a pH less than 7.0 is considered acidic. P is most available to plants when the soil pH is between 6.0 and 7.0.

When soil is basic, P can be bound up with calcium particles. The higher the pH, the more strongly P is tied up. When the soil pH is higher than 8.0, the availability of P to plants is significantly reduced.

When soil is acidic, P can be bound up with iron and aluminum particles and become less available for plant uptake. When the soil is highly acidic, below pH 5.5, P availability to plants is reduced significantly. The lower the pH from this point, the more strongly P is tied up in the soil.

What does this mean from a practical standpoint? The best time to apply P is usually near the time of plant uptake. For spring crops, this means apply in the spring. However, P can be applied in the fall and winter and do almost as well as in spring if the soil pH is between 6.0 and 7.0. P availability to plants grown in acidic soils can be increased if lime is applied according to the soil test results.

P and K are necessary for plant growth and development. Soil analysis should be used to determine the need for and amount of these elements. If needed, apply the recommended rate at the correct time using the information contained in this article.

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