DISTRIBUTION AND EXTRACTABILITY OF POTASSIUM IN SIZE FRACTIONS OF SANDY, FELDSPATHIC SOILS

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Abstract
Five soils were sampled or a study with the following objectives: (1) to quantify the K distribution in various fractions of a group of sandy feldspathic soils not currently under cultivation; (2) to estimate, using laboratory analyses, the capability of these soils to supply adequate K from mineral sources for plant growth, and (3) to determine whether soils series adequately group soils with respect to their K-release behavior. All soils have enough available K to supply any immediate crop growth requirements, and most are rated high or very high, based on current soil test calibrations. Even though textural and mineralogical differences exist among the soils, all have over 400 mg K/kg soil in the slowly available form, which can be released over time. The sources of slowly available K in the coarse-textured soils include mica from the clay separate and feldspars from the very fine and fine sands. The contribution of the coarse size fractions to K availability is confirmed because the clay alone accounts for only about half of the slowly available (NHO3-extractable) K. The A horizons of all soils have consistently more immediately and slowly available K than the C horizons, a characteristic possibly related to corresponding differences in organic matter content. Calculation of discriminant function allows the soils to be grouped by series based on their K-release properties. The most important factors in the grouping appear to be those describing the quantity of slowly available K and the soil clay content.

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