A soil texture label tells a farmer how much sand, silt and clay is present. It does not reveal which clay minerals make up that clay—and University of Minnesota researchers are finding that this mineralogy can influence potassium availability and fertiliser decisions.
Why clay type changes potassium behaviour
Clay particles have layered structures with negatively charged sites that hold nutrient ions. Kaolinite has low cation-exchange capacity and no interlayer space; illite has moderate capacity and spaces that can fix potassium; smectite has high capacity and interlayers that swell, shrink, trap and release potassium as moisture changes.
Researchers combined previous field trials and recent on-farm work, then grouped soils by the ratio of smectite to illite. The critical soil-test potassium level—the point associated with 95% of maximum yield—was 85 parts per million in illite-dominated soils and 106 ppm in higher-smectite soils.
A possible route to more precise recommendations
The result suggests some high-illite fields may need less applied potassium to reach maximum corn and soybean yield. Direct mineralogy testing can cost more than US$300 per sample, so the team began mapping broad mineral patterns across Minnesota as a practical alternative.
Clay mineral abundance follows large-scale soil formation and is less variable within a field than nutrient levels, making regional maps plausible. However, only about half the collected samples had been analysed when the article was published. The researchers had not yet changed official fertiliser guidelines and cautioned growers to await the completed analysis.
Read the Extension update
Potassium management: Maps showing soil clay type could help Minnesota farmers was written by Leanna Leverich and Dan Kaiser for University of Minnesota Extension.