Is Timing of K Application Something That Should Be Considered?

By Daniel Kaiser

While many growers opt for P and K applications in the fall, applying any fertilizer months ahead of when the crop needs it can present some risk for lessening the availability of a nutrient as it has more time to react with the soil. Past AFREC-funded research has shown that spring application of phosphorus resulted in higher corn yield compared to fall application. The next step was to look at whether the same can be said about potassium.

Past research has shown that the availability of potassium measured by soil tests does change over time where soil test values are higher in the spring and lower in the summer at points in time where crops are taking up high amounts of potassium. From the perspective of timing, I have not found that the soil test varies in June regardless of whether fertilizer K was applied in the fall or spring. In contrast, soil test phosphorus was almost always less when comparing fall versus spring fertilizer applications in high pH soils. In effect, the K should be available to the crop it is applied regardless of when the fertilizer was applied.

In total, corn and soybean yield were evaluated at nine locations each from 2022 to 2024. Soil test potassium was 150 ppm or less in all but two of the locations (these two locations were less than 200 ppm but greater than 150). In total, there were two locations where corn grain yield was significantly increased by potassium and four soybean locations where grain yield was increased. Time of fertilizer application affected corn grain yield at one of nine locations while soybean grain yield was impacted by time of fertilizer application of four of the nine locations. For corn, the spring application of potassium increased yield over fall at the one location. For soybeans, fall application out yielded spring in all the four instances where time of application significantly differed.

The decrease in soybean yield was expected as past research has shown that the chloride in KCl (potash) fertilizer can negatively affect soybean yield. One goal of this project was to see if fall application could reduce the update of chloride by the soybean plant as the chloride anion moves into the plant as it takes up water and uptake is not regulated. If chloride is present the plant will take it up. Plant tissue tests have shown significantly less chloride in the plant tissue when KCl is applied in the fall. Therefore, for those applying potash ahead of soybean avoiding spring application may reduce the risk for any potential yield loss from excess uptake of chloride by the soybean plant. I would still recommend not applying more than 100 pounds of KCl per acre ahead of the soybean crop. Since the data shows not negative impacts on corn grain yield very high applications can be split with the majority of the KCl being applied ahead of the corn crop to reduce potential yield reductions in soybean.

This study is funded by the Minnesota Agricultural Fertilizer Research and Education Council (AFREC). One additional year is planned to add three additional corn and three soybean locations in 2025.