Apparent salt sorption reactions in an unfertilized acid subsoil

The equivalent retention of Ca²⁺ and SO₄/^2- from gypsom solutions in acid subsoils has recently been reported. The term salt sorption has been coined to describe this process, yet little known about the nature of the reactions involved. This study was conducted to determine if other cation-anion combinations would exhibit similar 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 nmol(c) kg^-1. The solutions were CaSO₄ MgSO₄, Na₂SO₄ CaCl₂, MgCl₂, and a deionized water control. This soil exhibit salt sorption with all six electrolyte solutions. The SO₄ salts were almost completely sorbed both cation and anion uptake on the order 13.8 nmol(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 extractions 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 SO₄/^2- could accounted for by neutral salt extraction. Neutral sail extractable Al was significantly reduced by salt treatment. It appeared that salt sorption in this soil my have resulted from several mechanisms including precipitation, specific adsorption, ionic strength induced charging amd ion pair adsorption operating concurrently.