By Scout Nelson
Many growers apply phosphorus (P) and potassium (K) fertilizers in the fall, months before the crop needs them. However, applying nutrients too early may reduce their availability due to soil reactions. Research funded by AFREC has already shown that spring phosphorus application improves corn yield. The question is—does potassium work the same way?
Studies show soil test potassium levels change seasonally. Spring tests usually show higher potassium than summer, when crops absorb more. Interestingly, the soil potassium level in June was similar whether K was applied in fall or spring. In contrast, phosphorus levels were lower in spring if applied in the fall, especially in high pH soils. This suggests potassium remains available to crops regardless of application timing.
From 2022 to 2024, corn and soybean yields were tested at nine sites each. Soil potassium was below 150 ppm at most sites. Potassium significantly increased corn yield at two sites and soybean yield at four. Timing affected corn yield at one site, where spring application led to higher yields. For soybeans, however, fall application produced higher yields at four locations.
This yield drop in spring-applied potassium for soybeans relates to chloride. Potash (KCl) contains chloride, which soybeans absorb passively with water. “If chloride is present the plant will take it up.” Applying KCl in fall reduces chloride content in plant tissue, possibly lowering yield risks.
For soybeans, avoiding spring potash application is advised to reduce chloride uptake. Applying no more than 100 pounds of KCl per acre before soybean planting is recommended. Since corn yields were not negatively impacted, higher KCl amounts can be applied before corn and split across seasons to protect soybean yields.
Overall, potassium application timing matters more for soybeans than for corn, and fall applications appear safer and more effective for soybean health and productivity.
Photo Credits:istock-fotokostic
Categories: Minnesota, Crops, Corn, Soybeans