20 



1.1.3.3 Adsorption reactions 



1.1.3.3.1 Factors Affecting Adsorption 



In well-drained soils of humid regions, almost all of the inor- 

 ganic S occurs in the sulfate form. Because of its anionic nature and 

 the solubility of its common salts, leaching losses of sulfate can be 

 rather large. Yet plants depend on the availability of this form for 

 the S they absorb from the soil. 



Ensminger (1954) demonstrated the possibility of sulfate adsorp- 

 tion in soils from Alabama. Water or 0.1 N HC1 extracted very little 

 sulfate from these soils, whereas extractants containing a replace- 

 able anion such as phosphate (H ? P0~) or acetate (Ct^COO ) extracted 

 considerable sulfate. A 500 ppm P solution of KH 2 P0 4 was the most 

 efficient extractant used. He also showed that increasing amounts of 

 superphosphate and lime applied to a Cecil sandy clay loam (Typic 

 Hapludult) resulted in decreasing amounts of soluble sulfate. Hydra ted 

 A1„0 was found to adsorb more sulfate than a number of other soil 

 minerals and clays. Kaolinite also adsorbed significant amounts of 

 sulfate. Kamprath et al. (1956) found that 1:1 type clay minerals 

 adsorbed more sulfate than 2:1 types, and adsorption was related to pH 

 and the amounts of sulfate and phosphate in solution. 



Subsequent research by others has shown that sulfate retention in 

 soils is closely related to 1:1 type clay minerals, the presence of 

 Al and Fe hydroxides and oxyhydroxides, soil pH, and the presence of 

 competing anions, particularly phosphates (Neller, 1959; Chao, Harvard, 

 and Fang, 1962a, 1962b, 1962c; Chang and Thomas, 1963; Elkins and 

 Ensminger, 1971). Neller (1959) examined acetate-extractable sulfate 

 S in 10 soil series at different profile depths from various locations 



