Mineralogical budgeting of potassium in soil: A basis for understanding standard measures of reserve potassium

The study was conducted to investigate the relationship between some standard measures of soil reserve potassium (K) and soil mineralogy. Eight different agricultural soils from the N temperate and S boreal regions were studied and analyzed both by standard methods (exchangeable K, 2 M HCl- and aqua regia-extractable K) and by quantitative mineralogical methods based on X-ray powder diffraction analysis of spray-dried bulk soils. Linear regression and multivariate methods were used to assess the relationships between standard measures of soil reserve K and a number of soil chemical, physical, and mineralogical properties. A mineralogical budgeting approach, to estimate total K and its speciation between different mineral phases, is shown to be accurate after validation against total K analyzed geochemically. This approach enabled us to determine that both HCl- and aqua regia-extractable K were highly correlated with K in dioctahedral phyllosilicates and extracted 1%-17% and 5%-45% of total K, respectively. Neither extraction showed any obvious relationship to K in feldspar, which is frequently a larger reservoir of K in the soils examined. its speciation between different mineral phases, is shown to be accurate after validation against total K analyzed geochemically. This approach enabled us to determine that both HCl- and aqua regia-extractable K were highly correlated with K in dioctahedral phyllosilicates and extracted 1%-17% and 5%-45% of total K, respectively. Neither extraction showed any obvious relationship to K in feldspar, which is frequently a larger reservoir of K in the soils examined.