Improving the Reliability of Soil Potassium Testing and Recommendations


22 May 2017

2016 Annual Interpretive Summary

The goal of this project is to improve soil testing for potassium (K) and the assessment of crop K fertilization needs. There is much more uncertainty in the use of soil testing for K than for phosphorus (P). For example, we hypothesize that processes involving the so-called non-exchangeable soil K pool and the residue K pools have much more important impacts on soil-test K levels variation between two crops and crop response to K fertilization than is generally recognized.

This project has two objectives. Objective one is to study the relationships among soil-test K, non-exchangeable K, and relevant properties of soils of the western humid Corn-Belt (those with large acreages dedicated to row-crop production). Chemical analyses have been completed for 23 contrasting soils. Soil test K ranged from 60 to 580 mg K/kg when samples were dried prior to analysis. When samples were kept moist, soil test K ranged from 44 to 466 mg K/kg, indicating that drying samples prior to analysis tended to increase soil test K levels. Non-exchangeable K, the K in interlayers of phyllosilicate minerals, ranged from 153 to 3510 mg K/kg or 107 to 1.860 mg K/kg, depending on the analytical method used. The fixation behavior of soils was tested, and found to range from K-fixing to K-releasing. Minerals present across the 23 samples were smectite (7 to 65%), vermiculite (2 to 39%), mica (8 to 27%), and kaolinite (20 to 70%). Relationships among these and other tests are being examined and will be reported in the future.

Objective two is to study, under field conditions, how K additions, crop removal, and leaching of K from crop residue influences changes in the ratio between soil test K and non-exchangeable K. On average, soybean removed 68% more K than corn, averaging 74 kg K/ha. Initial residue samples taken at grain harvest averaged 81 kg K/ha for corn and 30 kg/ha for soybean. In the spring at the time of the final residue sampling, 45 kg K/ha had been recycled back into the soil from corn residue and 24 kg K/ha had been recycled back from soybean residue. Even with this recycling, soil tests from samples taken in the fall at harvest were higher than those taken in the spring. Non-exchangeable K increased or stayed nearly the same from fall to spring for all soils except those from northern Iowa. Soils from the Des Moines Lobe region, are poorly to very poorly drained soil series that formed on glacial till and non-exchangeable K in these soils decreased from fall to spring.