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Can You Safely Apply Potash Ahead of Soybeans?

Dan Kaiser

University Minnesota Extension nutrient management specialist

After a few comments and questions I received, I wanted to circle back to a previous blog post on chloride and its impact on soybean yield. One area I have been researching, thanks to funding from the Minnesota Soybean Research and Promotion Council, is chloride and how much of an impact it can have on reducing soybean yield potential in northern climates. Potash is the major source of potassium (K) for crops across the country and is roughly 50% chloride by weight. I wanted to circle back to this topic after several questions related to potash and possible alternatives for soybean if K needs to be applied.

Is there an alternative to potash that can be used?

Potash is the most economical source of K for crops as it contains the highest concentration of K2O, by mass (60% K2O), of all K fertilizer sources. The next closest fertilizer source is potassium sulfate, which is 50-51% K2O by mass. Potassium sulfate is not always readily available and is subsequently going to cost more per unit of K2O. However, potassium sulfate is 17-18% sulfur (S) and can supply both nutrients to crops, reducing the need for additional sulfur to be applied. A side benefit from potassium sulfate is that it is a sulfur fertilizer source that will not acidify the soil, which can occur when ammonium sulfate and elemental sulfur are applied. While there are certain benefits to the product, supply is the most challenging issue. Other fertilizer products, like Sul-Po-Mag (also known as K-Mag) or polyhalite, have been tested but contain lower concentrations of K2O and would not be ideal when high rates of K are needed for a crop.

The university is testing higher rates of chloride than what I typically apply. Should I still be concerned?

In 2020, we found significant yield reductions when 500 pounds of chloride was applied, which roughly equates to a half ton of potash per acre, much more than a grower will apply. The reason we used this high of a rate is to increase the potential we will encounter yield reductions and to get a better idea of how large those yield reductions could be. Setting up a worse case scenario situation and analyzing multiple sites of data together helps us to detect small differences in soybean yield. I will be continuing this work in 2020 and will be comparing more rates of chloride to better identify more precise tolerances so that we can suggest rates of potash that will increase soybean yield.

What should I do if I need to apply potash to soybean?

If the soil test indicates that K concentration is low and the crop will respond to K, it should be applied regardless of the source.

Long-term research trials were established in 2017 at University of Minnesota Research and Outreach Centers (ROCs) in Crookston, Lamberton, Morris, and Waseca. Potash was applied ahead of soybean and wheat or corn, depending on the location, at one to two times the estimated K removal for the two-year rotation (100 or 200 pounds K2O). Soybean grain yield was lower at two of the locations where soil test K was 200 ppm or greater while yield was higher at two locations where soil test K was 170 ppm or less. When yield was lower, the rate of K applied was not important. What this data indicates is that in K-responsive situations, you do not have to worry about the source of K to apply. However, in a strictly soil test maintenance-based system, avoid applying any more than 100 pounds of potash per acre if you are applying ahead of the soybean crop.

Any potential reduction in soybean yield from the application of chloride in potash seems to be effectively managed through managing the rate and time of fertilizer application and does not require changing the source we apply. Over time, it may be possible that other forms of fertilizer, such as potassium sulfate, do become available. The intention of past posts on soybean and chloride was not to alarm crop producers and suggest radical changes to their fertilizer management. Instead, consider that there may be an issue and that small changes in management may be needed to reduce the likelihood of yield reductions.