21 

 in Florida. The sulfate-S contents of the surface horizons of most of 

 these soils ranged from to 4.5 ppm. Ultisols contained considerable 

 amounts of sulfate-S. There was a highly significant increase (r = 

 0.82) in sulfate content with an increase in the clay-size fraction of 

 the soil. 



In contrast to cation adsorption, data from Chao et al. (1962b) 

 indicated that sulfate retentive soils did not possess adsorption 

 maxima or definite anion exchange capacities. They suggested that 

 perhaps some other mechanism of sulfate retention is functional. No 

 other mechanism has been identified except the proposed possibility of 

 the precipitation of sparingly soluble aluminum sulfate minerals 

 (Adams and Rawajfih, 1977; Adams and Hajek, 1978). Chao et al. 

 (1962c) also found that the removal of organic matter, free aluminum 

 oxides, and free iron oxides, considerably reduced sulfate adsorption 

 in soils from Oregon. They also showed that Al-saturated clays 

 adsorbed much more sulfate than H-saturated clays. The amounts of 

 sulfate retained by reference clays were in the order: 



kaolinite > illite > bentonite. 

 These results are consistent with the observations of Berg and Thomas 

 (1959). 



Three possible mechanisms of sulfate adsorption were proposed: 



1. Anion exchange due to positive charges developed on hydrous 

 Fe and/or Al oxide or on the crystal edges of clays, espe- 

 cially kaolinite, at low pH values. 



2. Sulfate retention by hydroxy-Al complexes through coordina- 

 tion. 



3. "Salt Adsorption" resulting from attraction between the 

 surface of soil colloids and the salt. 



4. Amphoteric properties of soil organic matter which develop 

 positive charges under certain specific conditions. 



