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Apparent Salt Sorption Reactions in an Unfertilized Acid Subsoil

Dep. of Agronomy, Univ. of Kentucky, Lexington, KY 40546-0091

M. E. Sumner

Dep. of Crop and Soil Sciences, Univ. of Georgia, Athens, GA 30602

*Corresponding author (rpearce{at}ca.uky.edu).

ABSTRACT

The equivalent retention of Ca²⁺ and SO^2-₄ from gypsum solutions in acid subsoils has recently been reported. The term salt sorption has been coined to describe this process, yet little is known about the nature of the reactions involved. This study was conducted to determine if other cation-anion combinations would exhibit similar behavior and to examine the extractability of the sorbed ions. An unfertilized acid Cecil (clayey, kaolinitic, thermic Typic Kanhapludult) subsoil was equilibrated using a saturated paste technique, with solutions of six different salts at a loading rate of 14.0 mmol_c kg^-1. The salt solutions used were CaSO₄, MgSO₄, Na₂SO₄, CaCl₂, MgCl₂, and a deionized water control. This soil exhibited salt sorption with all six electrolyte solutions. The SO₄ salts were almost completely sorbed with both cation and anion uptake on the order 13.8 mmol_c kg^-1. Calcium and magnesium chloride were sorbed at 11.7 mmol_c kg^-1, and NaCl was sorbed at 8.2 mmol_c kg^-1. The treated soils were air dried and extracted with successive portions of deionized water or 0.5 M NH₄NO₃. A relatively large proportion of both cations and anions were removed by repeated extraction with water, suggesting that ionic strength played an important role in the salt sorption process. Nearly all of the sorbed cations and Cl^- were recovered by neutral salt extraction, but <100% of the SO^2-₄ could be accounted for by neutral salt extraction. Neutral salt extractable Al was significantly reduced by salt treatment. It appeared that salt sorption in this soil may have resulted from several mechanisms including precipitation, specific adsorption, ionic strength induced charging, and ion pair adsorption operating concurrently.

Received for publication September 29, 1995.

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