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The Relationship between Water-Soluble, Replaceable, and Fixed Fractions of Potash Additions to Soils¹

University of Tennessee Agriculture Experiment Station

ABSTRACT

Accelerated absorption of potassium was induced by four successive evaporations of constant charges of a red clay acid subsoil and a calcareous black clay loam with variant quantities of K₂SO₄. The extent and intensity of fixations were then determined by aqueous and ammonium acetate extractions.

It was pointed out that intensive aqueous extractions to equilibria may be an important treatment as precedent to the accurate determination of exchangeable K₂O.

For both systems, the absorptions increased with increased additions, but the highest rates of absorption were registered by the minimal additions. The calcareous soil absorbed more potash and held it more tenaciously than did the acid subsoil.

Practically complete recoveries of 100-mgm and 500-mgm K₂O additions were recovered from the red clay subsoil, leaving none in the "fixed" state, as the total effect of four aqueous extractions and four supplemental ammonium acetate extractions. The same technic registered extensive fixations, or non-exchangeable fractions, of the much greater absorptions that were effected by the calcareous soil.

It is postulated that the easily recovered K that entered the clay subsoil displaced the H of that system; whereas, the K that was absorbed by the black clay loam entered into calcium complexes and was there held tenaciously through the protective action of the more readily hydrolyzable excess of exchangeable Ca, and also CaCO₃.

¹ A contribution from the Dept. of Chemistry, the University of Tennessee Agr. Exper. Station, Knoxville, Tennessee.

² Associate in Soil Chemistry and Head of Department, respectively.