Types of bonds between K and soil constituents were summarized with published data; Volk's remineralization model and a model of Gorbunov were rejected. Experimental results suggested that an equilibrium existed between exchangeable and unexchangeable K. Only soils with an inorganic colloid fraction could fix K in an unexchangeable form.Fixation capacity was correlated with clay percentage.Differences between river clay and marine clay were clarified by content of minerals low in K and by the influence of naturally occurring humic substances. Micaceous earth was especially K fixing while humic substances did not fix potassium but increased the capacity of minerals to fix K.Distortion of the humic film by oxidation or a pH shift lowered the fixation capacity of soils. Adding artificial humic substances to soil did not restore fixation capacity but lowered it by blocking access to the grid.Field trials showed that prolonged K consumption without replenishment enlarged the K-fixing capacity. On K-fixing soils, heavy K dressings had a transient effect, so a K dressing in autumn was purposeless. Fixation of K was so strong that even under heavy K dressings, there was no leaching of K.
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