Chloride and Soybean: What to Know About Potash Fertilizer Management

By  Dan Kaiser

I have previously written about the impacts that chloride (Cl) application can have on soybean. Chloride is an essential nutrient for crops but overapplication can reduce yield for some crop species. The most common source of fertilizer chloride is potash, which is commonly applied to supply potassium (K) to crops but is roughly 50% chloride by weight. Chloride has long been known to impact soybean yield in the southern United States but has largely been ignored in the northern U.S. In Minnesota, we have conducted several research projects in the last five years focused on chloride application to soybean, so I wanted to take some time to review what we have found so far and what we are currently studying.

How big of an issue is chloride toxicity to soybean?

“Toxicity” is a strong word for the issue, but our research has demonstrated that sources of chloride fertilizer can reduce soybean yield. Studies where high rates of chloride were applied have shown yield reductions of as little as 2 bushels per acre to upwards of 15 bushels per acre. These yield reductions were caused by an application of 500 pounds of chloride per acre, which would be roughly 800 to1000 pounds of actual potash per acre. Obviously, this is an extremely high application rate of potash, but, for research purposes, it is important to use a high enough rate to tease out an effect.

In a five-year study funded by the Minnesota Soybean Research and Promotion Council, more modest rates of chloride were applied. These application rates simulated the amount that would replace one and two times the expected K₂O removal for a corn-soybean or wheat-soybean rotation, which in this case was 100 and200 pounds K₂O per acre. Four locations around Minnesota were studied: Crookston, Lamberton, Morris, and Waseca. When chloride was directly applied ahead of soybean (which usually occurs the fall before planting soybean), what we found was a 1 to 2 bushel per acre average yield reduction at all locations except for Crookston. The small yield reductions are consistent with other research I have seen in the past 15 years with periodic small, yet significant, reductions in yield with potash application. Corn and wheat were not negatively impacted in the five-year study and corn grain yield was slightly greater with chloride application at Lamberton which was not a common occurrence in this study.

What parts of Minnesota are most at risk for chloride toxicity in soybean?

Chloride toxicity in soybean can happen anywhere in Minnesota but areas in western Minnesota are more prone to issues as the chloride applied can be retained in the soil for multiple years and annual average precipitation is less than central and eastern Minnesota. Chloride is an anion that leaches with water and can be moved out of the soil profile. Trials at the West Central Research and Outreach Center at Morris did show a tendency for higher extractable chloride at the start of the studies, meaning more chloride was initially available for soybean from the soil itself. However, on an irrigated site near Becker we did find significant amounts of chloride in the irrigation water. There isn’t a simple answer to where chloride toxicity will be an issue. This is why I highly suggest soybean growers do not get too aggressive with potash application rates. What does “aggressive” mean? I would suggest not applying more than 100 pounds of actual potash before the soybean crop unless the soil test K is Very Low (then, suggested application rates should be applied).

Should you apply all of your potash in the fall?

Since I started talking about the chloride and soybean issue to consultants and growers, I have heard this question a lot. We have applied potash in the fall for most of our long-term studies and I still encounter some reductions in yield. However, a current AFREC-funded project is looking at the impact of rate and timing of potash application on corn and soybean yield. The 2022 data shows no difference in the yield of either crop based on application timing. This is an on-going study and will be repeated over the next few years. What I do know is that the amount of chloride in the plant tissue was markedly less with fall application. For some sites, there were no detectable differences in mid-season tissue chloride concentration, while at other sites the increase in tissue chloride concentration was smaller (per pound of potash applied) in the fall compared to spring. So, fall application will likely reduce the risk for chloride toxicity but will not eliminate the issue.

Are there options other than potash to supply potassium to crops?

Potassium sulfate (K₂O₄; 0-0-50-18S) is an alternative source of potassium for crops. One drawback of potassium sulfate is the higher cost compared to potash. I am working on a project looking at blending potassium sulfate with potassium chloride, but I do not have anything major to share about that project yet. I do not foresee potash being replaced as the main source of potassium for production ag cropping systems anytime soon. The message I have for most growers on the chloride toxicity to apply high rates of potash ahead of more tolerant crops like corn or small grains. In most cases, soybean has yielded the same whether I applied the potassium directly ahead of the crop or ahead of the crop preceding soybean in the rotation. In many cases, the nutrients required for soybean can be applied in the years of the rotation when soybean is not grown. Direct application of some nutrients can lead to more vegetative growth which does not necessarily help the soybean plant. This is a case where more is not necessarily better, so trying to better manage application timing is important to ensure high soybean yields.