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Relation Between Potassium Fixation and the Oxidation State of Octahedral Iron¹

ABSTRACT

Samples of the <2-µm fractions of five pure smectites and six soil clays were reduced by sodium dithionite in the presence of a potassium citrate-bicarbonate buffer. Subsequently, they were freed of excess electrolyte by dialyzing them in a deoxygenated, 10^–3 M KCl solution. Subsamples of the resulting clays were used to determine Fe²⁺/total Fe, the ratio of ferrous Fe to total Fe; m_w/m_c, the mass ratio of water to clay, at a swelling pressure of 0.3 MPa; K_T, the total K; K_E, the exchangeable potassium; and K_F, the fixed K. Identical samples of the same smectites and soil clays were subjected to the same experiments except that they were treated with sodium sulfate instead of sodium dithionite. Hence, they remained in the oxidized state. Then the differences in the experimental results for the reduced and oxidized samples were determined. For the several samples, it was found that K_F, the change in K_F, was proportional to (m_w/m_c), the change in m_w/m_c. Also, it was found that K_F was an exponential function of 1/Fe²⁺, where Fe²⁺ is the change in Fe²⁺, but that the constants in this function were different for the pure smectites than for the soil clays. From these findings it was concluded that K⁺ fixation is enhanced by the reduction of octahedral ferric Fe in the clay crystal and that the difference between the pure smectites and soil clays was attributable to nonsmectite minerals in the latter.

¹ Contribution from the Agronomy Dep., Purdue Univ. Agric. Exp. Stn., West Lafayette, IN 47907. Journal Paper no. 10 539.

² Visiting Scholar from Southwest Agricultural Univ., Chongqing, China; Professor of Soil Chemistry; and Professor of Soil Mineralogy, Agronomy Dep., Purdue Univ., West Lafayette, IN 47907, respectively.

Received for publication December 30, 1985.

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