Improving the Reliability of Soil Potassium Testing and Recommendations

The project is in its third year, and its goals are to combine basic research to characterize fundamental chemical processes with field research to improve use of soil testing for K and K fertilizer management for corn and soybean in selected soils of the North Central Region. Three complementary studies are in different stages of development.

Study 1 performed mineralogical and chemical analyses of 23 topsoil samples from 20 soil series from Illinois, Iowa, Minnesota, southeastern North Dakota, and Wisconsin). Soil-test K concentrations (ammonium-acetate on dried samples) ranged from 44 to 466 ppm, and there were wide ranges for drainage class, texture, organic matter, and pH. Results from Mg-glycerol saturated, X-ray clay mineralogical analyses showed smectite, vermiculite, mica, and kaolinite ranging from 7 to 65%, 2 to 39%, 8 to 27%, and 20 to 70% across the soils. Analyses for short-term K retention showed that temporarily fixed K may occupy almost zero to 40% of the cation exchange sites.

Study 2 incubated soil with and without K fertilizer under different wetting/drying cycles to study short-term reactions between soil K pools. Selected soils from Study 1 with contrasting properties have been selected for the incubations (2 to 20 week periods). Priority will be given to analyzing samples for changes in K pools potentially affecting crop availability of K and long-term fixation.

Study 3 conducted field response studies of how K additions and crop K removal influence relationships between soil K pools and temporal soil test K variation. We have evaluated crop response to K and standard dry-soil-test K at several Iowa short-term and long-term field trials during 2014 and 2015 (70 site-years) with corn-soybean rotations and continuous corn managed with no-till or chisel-plow tillage. Rainfall is measured at all sites. We selected about 250 plots to measure K removal with grain, K recycling, and to be sampled from a 15-cm depth at various times (harvest, late fall, and early spring) to be analyzed for soil moisture, soil-test K by dry and field-moist methods, and non-exchangeable K. The K recycling is being studied by sampling crop residues at harvest, late fall, and early spring. Soil samples also are being taken from a 15 to 30-cm depth at harvest time and in spring to measure potential downward movement of K. The non-exchangeable K analyses are not completed at this time, but we are seeing very wide measurement ranges within and across trials.