Improving the Reliability of Soil Potassium Testing and Recommendations


25 Mar 2015

2014 Annual Interpretive Summary

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.