INTERACTIVE MECHANISMS OF ANION ADSORPTION WITH CALCIUM LEACHING AND EXCHANGE

Anions can be adsorbed to soil colloids by specific (inner-sphere comp lexation) and nonspecific adsorption (outer-sphere complexation and di ffuse ion association). The mechanism for anion adsorption determines whether and by what reaction anions in soil solution mill enhance meta l adsorption. The objectives of this study were (i) to determine the i nfluence of the accompanying anion on Ca leaching and cation exchange in a sandy, kaolinitic Ultisol, (ii) to describe possible mechanisms f or anion effects on Ca adsorption based on diffuse double-layer theory , and (iii) to discuss the practical implications of these mechanisms on soil fertility management. The Ca forms used in this study were cal cium chloride, calcium nitrate, calcium sulfate, calcium phosphate, an d calcium carbonate. Undisturbed soil cores from a Plinthic Kandiudult were collected, and a Ca solution (5, 500, and 1500 mg L(-1)) was app lied to soil columns. The effluent was collected from each column, and cations (Ca2+, Mg2+, K+) in solution mere measured. Exchangeable base s and effective cation exchange capacity (CEC) were measured in soil h om soil cores after effluent collection. Sulfate and phosphate retarde d Ca breakthrough, compared with chloride and nitrate. Specific adsorp tion of sulfate and phosphate in the inner-sphere increased the net ne gative surface charge of the soil and, thus, increased Ca adsorption. Higher influent concentration increased the extent of ion pairing (CaS 0(4) degrees), and consequently, enhanced Ca adsorption directly. Calc ium breakthrough was most gradual in the carbonate effluent as a resul t of ion pairing and increased pH dependent charge leading to both dir ect and indirect enhancement of Ca adsorption. The Mg peaks for phosph ate and carbonate were broader and shorter than those peaks for nitrat e, chloride, and sulfate. The choice of the accompanying anion in Ca f ertilizer materials could be critical in controlling the leaching of C a from the soil surface and optimizing the exchange and subsequent lea ching of other cations.