5 Factors to Consider for Potassium Fertilizer

Decisions on optimal fertilizer management can be challenging in years with low commodity prices, says Daniel Kaiser, University of Minnesota (U of M) Extension soil fertility specialist. In the article below, he gives five factors that farmers should consider as they make decisions for applying K for corn and soybean.

1. Focus on rate, not timing.

Applying the correct rate needed over one or two years in a crop rotation has been shown to be more important than the time the fertilizer is applied. Much U of M current data has demonstrated that timing of application in a multiyear cropping rotation is not important. Applying ahead of the crop that will get the greatest advantage from the K is the best way to get the most bang for your buck.

2. Focus on a proven yield, not a yield goal.

It can be difficult to determine what yield should be used for both a sufficiency-based or build and maintenance K application strategy. Using a historical yield average is the best option. A value you have proven can be produced is a smart way to ensure fertilizer is not overapplied. The soil itself is not devoid of K, so being exact on your predicted rates is not critical. Some fertilizer is always better than none in situations where a response to a nutrient is likely.

3. Stick to the same time of year when soil sampling.

Sampling fields at similar times of the year is critical to ensure you can accurately determine how soil test values for K change over time. Potassium is different from other nutrients in that the soil test value is not static in the field over the growing season. It can vary from fall to spring.

4. Apply K when and where it is needed.

Soil tests are still the best option for deciding when and where K fertilizer should be applied. For soils higher in clay – like loams and clay loam soils – the chance for a profitable response to K fertilizer is low when soil tests are around 200 parts per million K. For sandy soils that don't hold K well – such as loamy sands – high rates of K may not be needed, even though soil test values can be lower compared with higher clay soils.

5. Choose the right placement option.

Research has shown that banding K can be more effective in some circumstances. Broadcast application of K in reduced tillage situations like ridge- and strip-tillage or no-till can stratify K near the soil surface, which can lead to poor uptake in dry soils. While banding K is not always needed, identifying situations where it is beneficial can help ensure optimal productivity.

It is a good time to start reviewing fertilizer decisions as we wait for fields to dry this spring. There are situations where K may not be needed, so knowing which fields need K could save time this spring.

Support for this project was provided by the Minnesota Soybean Research and Promotion Council (MSR&PC) and the Agricultural Fertilizer Research & Education Council (AFREC).