Applying Phosphorus and Potassium - New Research

It is commonplace in many parts of the US to fall-apply phosphorus and potassium. Growers often think this practice can build soil P and K levels while increasing yield and maintaining soil nutrient availability. New research from Ohio State University suggests this may not be the case. Researchers Pullman and Fulford conducted a 13-year study (9 years of which is referenced in the attached paper) on corn and soybeans in which three different rates of P and K fertilizer were fall-applied. A 0x crop removal rate, a 1x crop removal rate, and a 2x crop removal rate were used. At the end of 9 years, only 5/42 test sites responded positively to P and only 4/42 sites positively responded to K applications. Statistically speaking that means only 12 percent of the time fall-applied P had a positive response and only 9% of the time fall-applied K had a positive response.

In addition to poor yield response Pullman and Fulford were unable to significantly build soil test levels for P and K. This research suggests application timing is key for crop nutrient availability. 80-90 percent of soil-applied phosphorus becomes tied up following application. This research also appears to corroborate research done at the University of Illinois by Kahn which showed that applications of KCL often had detrimental effects on crop yield.

Kahn’s research also showed it was difficult to increase soil k levels. The final 4 years of the Ohio State trials will be published at the end of this year so expect an update with more information.

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