Differences in potassium forms between cutans and adjacent soil matrix in a Grey Clay Soil

Cutans are common fabric features in soil and represent foci of chemical and biological reactions. The influence of cutans on potassium forms and their transformations were investigated for a Western Australian grey clay soil. Cutans and matrix soil had similar clay mineral associations with kaolinite, smectite and illite being present, but had different chemical properties. The organic carbon content of cutans was higher than for matrix soil, while pH values and oxalate extractable/dithionate extractable iron (Feo/Fed) ratios were lower. Numerous SEM-EDS single point analyses of cutans and the plasma phase of the matrix soil indicated that the mean value of K concentration in cutans is greater than in matrix soil, and that the K concentration decreased with distance from cutan to matrix. Chemical extractions showed mean values of total K and latent exchangeable K were higher for cutans than for matrix soil, but both fixed K and exchangeable K values were the same for cutans and matrix soil. In a K adsorption/desorption experiment, 35% of K adsorbed by matrix soil could not be desorbed by 1 M NH4Ac. These results indicate that cutans are relatively enriched in K and may play an important role in determining available K and latent exchangeable K due to the special physical and chemical environments they provide in the soil.