Improving the Reliability of Soil Potassium Testing and Recommendations

The project is in its second year of activity. The objectives are to study relationships among soil-test K measured on dried and field-moist samples, non-exchangeable K, and mineralogy under controlled conditions for soil samples taken from contrasting soils of the Corn Belt and at Iowa field trials by considering crop K removal, while assessing temporal soil K variation. The project consists of three portions which are in different stages of development. The first portion was mineralogical and chemical analyses of soils of the western humid Corn-Belt. We are working with 23 topsoil samples collected in Illinois, Iowa, Minnesota, southwestern North Dakota, and Wisconsin.

Soil test K (ammonium acetate extractable) concentrations ranged from 44 to 466 ppm, with the the field-moist K test and 60 to 580 ppm with the dry K test. Results from magnesium-glycerol saturated, X-ray clay mineralogy analyses showed smectite, vermiculite, mica, and kaolinite ranging from 7 to 65%, 2 to 39%, 8 to 27%, and 20 to 70%. Preliminary short-term K retention results showed that temporarily fixed K may occupy 0 to 40% of the total cation exchange sites. Therefore, we were successful at identifying contrasting soils relevant to our K study. Analyses for non-exchangeable K and other basic properties have not been completed. The second portion was incubations of soil with and without K fertilizer under different wetting/drying cycles to study short-term reactions between soil K pools. This work will begin in 2015 after mineralogical and chemical analyses of the 23 soils are completed so a few of the most contrasting soils can be incubated. The final portion was a crop-response study examining how K additions and crop K removal influence relationships between soil K pools and temporal soil-test variation. We evaluated 35 Iowa short-term and long-term field trials during the 2014 crop year. All included several K fertilizer rates replicated 3 to 5 times. Most included corn and soybean rotation phases each year and are managed with no-till or chisel-plow tillage. Rainfall was measured at all sites; grain yield was measured from all plots; and we selected about 250 plots to also measure K removal with grain, soil-test K concentrations by dry and field-moist methods, soil moisture, and non-exchangeable K.

The non-exchangeable K analyses are not yet completed, and no strong conclusions are possible with one year of results; however we observed very wide measurement ranges within and across trials that will be useful in studying the relationships of interest.