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Release of Potassium from Nonexchangeable Forms from Size Fractions of Several Iowea Soils¹

ABSTRACT

Surface samples of 13 Iowa soils were fractionated into six size fractions. K release from nonexchangeable forms from these size fractions was measured by an extraction for 10 minutes with boiling 1.0 N NHO₃. The K release to HNO₃ per unit weight was highest in the 0.2- to 2-micron fraction for 12 of the 13 soils, was highest in the <0.2-micron fraction for one soil, and decreased with increase in particle size in the silt fractions. For each of six size fractions the release per unit weight was correlated with release from the whole soil.

The calculated percentage distribution of K release among the size fractions suggested that the clay fractions contributed about 60% and the coarser fractions contributed about 40% of the K release from the whole soil.

For eight soils the lowest total K was in the <0.2-micron fraction and the highest total K was in the 2- to 5-micron fraction.

The percentage of the total K which was soluble in HNO₃ was highest in the <0.2-micron fraction and decreased with increase in particle size. The percentage solubility of K in the clay fraction was not related to the degree of weathering of the soils, but the percentage solubility in the silt fractions was closely correlated with degree of weathering.

¹ Journal paper No. J. 1962 of the Iowa Agricultural Experiment Station, Ames, Iowa. Project No. 624.

² Formerly instructor of soils, Iowa State College, now Assistant Professor, Ohio State University.