26 



Experimental 

 Mineral ZPC 



Al (OH) 3 5.1 



(gibbsite) 



a-AlO(OH) 7.7 



Y-AIO(OH) 7.5 



a-Fe o 9.04 



Y-FeO(OH) 7.4 



ct-FeO(OH) 6.7 



Soils containing large quantities of these oxides and hydroxides 

 and a pH below the ZPC of the dominant minerals would be expected to 

 retain large amounts of sulfate by nonspecific adsorption. Gillman 

 (1974) found that phosphate-extractable sulfate increased with pro- 

 file depth in an Australian rain-forest soil (Rodic Hapludult) . He 

 showed that the ZPC also increased with depth from a ZPC of 4.5 at the 

 10 to 20-cm depth to 5.8 at the 210 to 240-cm depth. 

 1.1.3.4 Leaching 



Many humid-region soils have argillic horizons which contain 

 large amounts of hydrated oxides of Al and Fe and 1:1 type clay miner- 

 als. These materials may accumulate sulfate because of adsorption as 

 explained in the previous section. In coarse-textured, sandy soils 

 such as the Entisols, Spodosols, and the surface horizons of Ultisols 

 in Florida, very little sulfate adsorption takes place (Ensminger, 

 1954; Neller, 1959; Jordan, 1964). Mineralized sulfate or sulfate 

 applied in fertilizer may be readily lost by leaching under the high 

 rainfall conditions which exist during most of the growing season in 

 Florida. 



