APPARENT SALT SORPTION REACTIONS IN AN UNFERTILIZED ACID SUBSOIL

The equivalent retention of Ca2+ and SO42- from gypsum solutions in ac id subsoils has recently been reported. The term salt sorption has bee n coined to describe this process, yet little is known about the natur e 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 Ce cil (clayey, kaolinitic, thermic Typic Kanhapludult) subsoil was equil ibrated using a saturated paste technique, with solutions of six diffe rent salts at a loading rate of approximate to 14.0 mmol(c) kg(-1). Th e salt solutions used were CsSO4, MgSO4, Na2SO4, CaCl2, MgCl2, and a d eionized water control. This soil exhibited salt sorption with all six electrolyte solutions. The SO4 salts were almost;completely sorbed wi th both cation and anion uptake on the order 13.8 mmol(c) kg(-1). Calc ium and magnesium chloride were sorbed at approximate to 11.7 mmol(c) kg(-1) and NaCl was sorbed at 8.2 mmol(c) kg(-1). The treated soils we re air dried and extracted with successive portions of deionized water or 0.5 M NH4NO3. A relatively large proportion of both cations and an ions were removed by repeated extraction with water, suggesting that i onic strength played an important role in the salt sorption process. N early all of the sorbed cations and Cl- were recovered by neutral salt extraction, but <100% of he SO42- could be accounted for by neutral s alt 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 ad sorption, ionic strength induced charging, and ion pair adsorption ope rating concurrently.