Potassium Extraction and Fixation Potential in Western Corn Belt Soils in Relation to Mineralogy.

The soil mineralogy can significantly affect the efficiency of K fertilization, estimates of plant-available K by soil testing, and differences between K extraction methods. The objective of this research was to study how the soil mineralogy affects K extracted by several methods and short-term estimates of K fixation capacity. Soil samples were collected from 18 selected soil series important to corn and soybean in Illinois, Iowa, Minnesota, North Dakota, and Wisconsin. For two soil series, samples were collected from contrasting K treatments of long-term experiments; a control receiving no K and an annual non-limiting K rate. The soils were characterized for particle size distribution, organic matter, extractable cations, pH, calcium carbonate, and other properties. Specific K related measurements were soil-test K by the ammonium-acetate and Mehlich-3 methods using both dried and undried soil sample handling procedure, non-exchangeable K by the NaBPh₄ method, K fixation potential, and total K. The mineralogical composition of the clay (< 2 μm) and fine silt (2 – 20 μm) fractions of each soil was determined by a semi-quantitative method based on a combination of X-ray diffraction and thermal analysis. The proportion of vermiculite and smectite in the clay fraction of the soil was quantitatively determined by using the Ca/Mg exchange method for smectite and the K/NH₄ exchange method for vermiculite. The concentration of discrete illite in the clay fraction was determined by measuring non-exchangeable K after digestion of the clay fraction with aqua regia and HF. The results were being summarized at the time this abstract was written.