Decisions on optimal fertilizer management can be challenging in years with low commodity prices. Work is underway to overhaul the corn and soybean potassium (K) guidelines in Minnesota. When making decisions for applying K for corn and soybean, here are a few things that you should consider:
1. Focus on rate, not timing
Applying the correct rate that is needed over one or two years in a crop rotation has been shown to be more important than the time when the fertilizer is applied. Much of our current data has demonstrated that timing of application in a multi-year 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
When making decisions about how much K to apply, it can be difficult to determine what yield should be used for both a sufficiency-based or build and maintenance strategy. Using a historical yield average is the best option. A value which you have proven can be produced is a smart way to ensure fertilizer is not over-applied.
The soil itself is not devoid of potassium, 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 and can vary from fall to spring.
4. Apply K when and where it is needed
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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 very low when soil tests are around 200 parts per million K.
For very sandy soils which do not hold K well, such as loamy sands, high rates of K may not be needed even though soil test values can be lower compared to 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 if soils become dry. While banding K is not always needed, identifying situations where it is beneficial is key to 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 potassium may not be needed, so knowing which fields need K could save time this spring.