POTASSIUM EXCHANGE AND FIXATION IN SOME SOUTHERN ONTARIO SOILS

Long-term (8 yr) K+ fertilization experiments were started in 1983 on Jeddo clay loam, Smithville silt loam, Oneida loam, and Vineland sandy loam in the Niagara region of southern Ontario to study K+ exchange and fixation in relation to K+ uptake by grapes. Potassium fertilization at rates of 0, 100, 200, 400, and 600 kg K+ ha−1 was discontinued after 4 yr to record the build-up of soil K+ with fertilization and its decline after fertilization ceased. This paper reports the results on K+ exchange and fixation during the 4-yr K+ fertilization period. The clay mineralogy of the soils was similar with their clays containing mainly mica and smaller amounts of vermiculite, chlorite, quartz, and feldspars. There was a highly significant linear relationship between rates of applied K+ and K+ fractions of exchangeable cations for every soil in every year of K+ application. Exchangeable K+ concentrations in the coarse-textured Vineland soil increased sharply from the first to the second year of K+ application. However, there was no consistent increase of exchangeable K+ concentrations with time in the finer-textured soils for the first 2-4 years of K+ application. This was attributed mainly to K+ fixation. This interpretation was supported by chemical and X-ray evidence that showed reduced K+ fixation capacity and transformation of vermiculite to a mica-like mineral at the high rate of K+ application. Potassium exchange equilibria and standard Gibbs free energies for the exchange of (Ca2+ + Mg2+) by K+ were calculated using the theory of multiple ion exchange equilibria. Standard Gibbs free energy values for this exchange became more negative with increase in clay and vermiculite contents of the soils which was attributed to the strong adsorption of K+ in vermiculite and vermiculite-like minerals that were concentrated in the clay fraction. Clay content is therefore the dominant factor determining K+ exchangeability in the soils of the Niagara region of southern Ontario. Key words: soil mineralogy, vermiculite contents, potassium exchange equilibria, standard Gibbs free energy