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Interpretation of Curves Describing Soil Potassium Exchange¹

ABSTRACT

Curves describing the release of K from soil samples in NaTPB solutions depict a linear relationship between the amount of K replaced and the logarithm of the extraction period. Such curves are quite different from the diffusion-controlled curves observed with mica particles and have yet to be explained. Reports of a similar relationship being observed with soil organic N release that also obeyed a second-order reaction led to an unsuccessful application of second-order kinetics to the release of K from 16 soils. On the other hand, linear K-releases versus log t curves were obtained by combining K-release curves for mica particles of different size and by placing mixtures of mica particles in the same NaTPB solution. Therefore, a soil was fractionated, and the K-release curves for the individual size-fractions were determined. The curves for the soil fractions were nonlinear and more like mica curves, whereas various combinations of these curves yielded linear K-release versus log t relationships like those observed with soils. Thus, the curves for soil K-release merely represent the combined effects of various particle sizes in the soil and not a distinctive mode of release.

¹ Journal Paper no. J-6706 of the Iowa Agr. & Home Econ. Exp. Sta., Ames, Iowa. Project no. 1722. This work was supported in part by the US Atomic Energy Commission under contract AT(11-1)-1680.

² Former Postdoctoral Associate (current address, US Soils Laboratory, Beltsville, Md.), Graduate Assistant, Former Research Associate (current address, IRI Research Institute Inc., a/c Corporacion Venezolana de Guayana, Venezuela), and Professor of Soils, respectively.

Received for publication January 26, 1971. Accepted for publication April 1, 1971.