Improving the Reliability of Soil Potassium Testing and Recommendations

This project is in its final stages and the Potassium (K) Fellow is completing data management and statistical analyses. The study has two major objectives.

The first objective 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 corn and soybean production). Chemical and mineralogical analyses were completed for 23 soils. Soil-test K concentrations ranged from 60 to 580 mg K/kg when samples were dried at 40 degrees Celsius prior to analysis. When samples were kept moist, values ranged from 44 to 466 mg K/kg, indicating that drying samples prior to analysis tended to increase soil-test K concentrations. Non-exchangeable K, the K in the interlayers of phyllosilicate minerals, ranged from 153 to 3,510 mg K/kg or 107 to 1,860 mg K/kg, depending on the analytical method used. The moist soil test K values correlated better with crop yield response to K. The short-term fixation of the soils was tested, and found to range from K-fixing to K-releasing. Minerals present across the samples were smectite (7 to 65%), vermiculite (2 to 39%), mica (8 to 27%), and kaolinite (20 to 70%).

The second objective is to study how K additions, crop removal, and leaching of K from crop residue influences changes in the ratio between soil K and non-exchangeable K for different soil sampling dates under field conditions. 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 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 for most trials. Non-exchangeable K increased or stayed nearly the same from fall to spring but decreased in the spring in soils from northern Iowa, which are poorly to very poorly drained and formed on glacial till. The Iowa soils internal drainage seems to be as important or more important at determining differences than clay mineralogy. For both objectives, relationships among the measurements and other soil properties and precipitation are being examined at this time.

A final report will be completed during 2018.