As post-harvest soil test and fertilization season begins, gowers and ag retailers need to be aware that Iowa State University recently announced changes to potassium (K) fertility guidelines based on moist soil measurements. These may be viewed online at:

While it has been known for decades that drying can change the chemistry of soil, altering the amount of extractable potassium (K), it's just been in the last two years that Solum ( entered the market with a commercially viable process measuring soil in its field moist state.

The new potassium (K) recommendations document from Iowa State notes, "Research has indicated that the moist K test is more reliable than the test based on dried samples and is a better predictor of crop K fertilizaton need."

The new test categories are as follows:

Soil Test K Classes Very Low Low Optimum High Very High
Field Moist/Slurry 0-50 ppm 51-85 ppm 86-120 ppm 121-155 gram 156+ ppm
Crop K Fertilizer Recommendation in lb K2O5/acre
Corn 130 90 45 0 0
Soybean 120 90 75 0 0
Corn-Soybean 220 165 120 (60) 0

Speaking at a mid-year conference, Iowa State Professor of Soil science and Nutrient Management, Dr. Antonio Mallarino, reminded the audience that soil test interpretation classes should be understood in terms of probability of response. “The amounts of K extracted can differ greatly between moist and dried samples, and the differences change greatly across soil series, the soil-test K levels, and soil conditions related to drainage and moisture content cycles.” While other tests are still available for potassium, he noted that they encourage the use of the moist test for K because it has more predictive value and a more consistent meaning across different soils and conditions.