52 SCIENCE AND SOIL 



more common rocks (see Table 4) the loss of potassium and sodium 

 is very great, while most of the iron and phosphorus is likely to be 

 found in the residue. 



Sulfates. Natural sulfates are confined chiefly to hydrated cal- 

 cium sulfate, CaSO 4 (H 2 O) 2 or CaO 2 S(OH) 4 , containing about 18.6 

 per cent of sulfur and more than 20 per cent of water of hydration. 

 This is the mineral called gypsum. It occurs in numerous deposits, 

 at various depths, and sometimes extends over hundreds of square 

 miles, as in northern Ohio. Under the name of land-plaster this 

 mineral has been used very extensively in places as a soil stimu- 

 lant. Traces of calcium sulfate are found in most limestones and 

 in some other rocks. 



Sulfids. The sulfids of iron are widely distributed in nature. 

 Iron disulfid, FeS 2 , is commonly known as pyrite, also called 

 " fool's gold," because of its glitter and yellow color. One form of 

 iron disulfid decomposes quite readily when exposed to air and 

 moisture, and yields ferrous sulfate, FeSO 4 , as one of the products. 



Phosphates. These occur in small amount in connection with 

 many other rocks and minerals, principally in the form of calcium 

 phosphate, Ca 3 (PO 4 ) 2 , specifically called tricalcium phosphate, 

 when mentioned in connection with the artificial dicalcium phos- 

 phate, Ca 2 H 2 (PO 4 ) 2 , or monocalcium phosphate, CaH 4 (PO 4 ) 2 . 



Granite commonly contains a trace of phosphorus, and in gneiss 

 about . i per cent of phosphorus is found as an average, correspond- 

 ing to 10 pounds of calcium phosphate (2 pounds of phosphorus) 

 in one ton of gneiss. Limestones also contain calcium phosphate, 

 as a rule. While the amount is usually less than i per cent, some 

 quite extensive deposits of phosphatic limestone exist which con- 

 tain from 10 to 30 per cent of tricalcium phosphate. In places 

 where such rocks have been long exposed near the surface, the cal- 

 cium carbonate has been largely removed by leaching, so that the 

 remaining porous rock may contain as high as 75 per cent of the 

 phosphates of calcium, iron, and aluminum, in which the calcium 

 compound, Ca 3 (PO 4 ) 2 , greatly predominates (as in the Tennessee 

 brown rock phosphate). 



There are also some natural deposits of compact calcium phos- 

 phate rock, varying in purity from about 40 to 80 per cent (as the 

 Tennessee blue rock phosphate). These deposits of phosphate 



