5 

 Although plants take up S in the inorganic, sulfate form, only 

 to 14% of the total soil S in the surface horizons of humid-region 

 soils is present as the sulfate ion (Ensminger, 1954; Williams and 

 Steinbergs, 1959; Neller, 1959; Freney, 1961; Tabatabai and Bremner, 

 1972a; Bonner, 1973). The inorganic sulfate and sulfide minerals are 

 associated with alkaline or acid-sulfate soils. Sulfate minerals that 

 have been identified in alkaline soils include gypsum (CaSO^ ■ 2H 2 0) , 

 hemihydrate (CaSO • 1/2H-0) , mirabilite (Na 2 S0 4 • 10H 2 0) , thenardite 

 (Na SO.) , epsomite (MgSO^ • 7H 2 0) , hexahydrite (MgSO, • 611,0), and 

 bloedite (Na o Mg(S0.) o ■ 4H 0) (Doner and Lynn, 1977). These are all 

 very soluble minerals and are quickly removed from soils where precipi- 

 tation is high enough to cause deep leaching. These minerals may be 

 used as soil amendments in humid regions but do not persist in the soil. 



Acid-sulfate soils or "cat-clays" develop when tidal flats or 

 interdistributary basins of river deltas are drained. Reduced forms of 

 S such as amorphous mackinawite (FeS) , pyrite (FeS 9 ) , or greigite 



(Fe S,) are oxidized bv Th iobacillus sp . The soil becomes extremely 

 3 4 ' 



acidic (pH 3.0 co 3.5), and crystalline jarosite [KFe 3 (OH) 6 (S0 4 ) 2 ] can 

 develop within 1 to 2 years after draining. If carbonates are present 

 in the sediments, the acidity is neutralized and gypsum forms. Few of 

 these soils are arable and where they are used for farming, S as a 

 plant nutrient is certainly not limiting. The presence cf 0.05% water 

 soluble sulfates has been suggested as an indicator of an acid sulfate 

 soil (Doner and Lynn, 1977). 

 1.1.2 Soil Organic Matter 



Most of the total S in the surface horizons of humid regions is 

 organic (Eaton, 1922; Evans and Rost, 1545; Williams and Steinbergs, 



