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Soil testing is usually an adequate and reliable means of measuring soil fertility, and is generally used as a predictor of potential issues in future crops. In recent years, grain testing has emerged as a helpful trend for growers looking to determine if nutrient deficiencies — specifically potassium — were a cause of lower-than-expected yields.

According to Nathan Slaton, University of Arkansas professor of soil fertility, farmers who are looking to identify the causes of decreased yields might want to consider sending soybean grain to a testing lab.

“It is like doing an autopsy for the soybean crop,” says Slaton. “Many farmers and agronomists conduct end-of-season cornstalk tests for corn to determine how their nitrogen program worked, and it’s important that similar tests are done for soybeans.”

Linking soybean potassium levels with yields

Slaton based his theories on testing soybean grains for potassium deficiency on the idea that soybean grains have been tested for other nutrient deficiencies such as sulfur, manganese, zinc, boron, copper and molybdenum.

In his research, he looked at published and unpublished data from potassium fertilizer yield response studies across North America, including studies from Iowa, Arkansas, Canada, Tennessee, Indiana, Missouri and Virginia. The observations included yields, soil testing levels and grain concentrations with and without potassium fertilizer.

Overall, he found that soybean grains with potassium levels less than 1.65 percent or 16.5 grams potassium per kilogram of grain would improve yields 77 percent of the time when potassium fertilizer is applied.

These results indicate that farmers can test soybean grain for potassium levels (similarly to how they have used grain testing for other nutrient deficiencies) to help determine if low potassium levels were a contributor to reduced soybean yields.

Using grain tests to improve yields

Outside of the research lab, these tests can be used to help farmers determine if low potassium levels were a primary cause of lower yields.

Randall Warden, CEO and agronomist at A&L Great Lakes Laboratories, recommends farmers submit a 4-8 ounce representative grain sample to their regional testing lab. The analysis typically costs $20-25 per sample with results normally available the next business day. After receiving the test results, farmers should analyze their soil fertility levels. If low potassium levels did in fact contribute to the lower than expected yields, Slaton says farmers can then take action to improve nutrient levels by applying more potassium fertilizer to build up potassium soil levels.

As part of a comprehensive nutrient management plan, farmers should consider taking regularly scheduled soil samples to analyze soil fertility levels. Record yields over the past several years have led to increased nutrient removalacross the Midwest, meaning growers in these areas should be especially mindful of their fields’ nutrient balance levels. They can also use the eKonomics Nutrient ROI Calculator 2.0 to estimate their nutrient needs and apply potassium fertilizer as needed to raise soil fertility levels to maximize their return on investment.