CHEMICAL CONSTITUENTS OF THE SOIL 155 



bonate, which dissolves in water. On exposure to the air, the 

 ferrous carbonate is oxidized and is precipitated as insoluble 

 hydrated ferric oxide. This action does not take place in soils 

 containing carbonate of lime. A layer of hard pan, consisting of 

 rock grains cemented by limonite, is often formed below poorly 

 drained soils. 



Pyrite FeS 2 has a light yellow color and is often called fool's 

 gold. It is easily oxidized to sulphates by atmospheric agencies. 

 It is sometimes formed in badly aerated soils. 



Phosphate Minerals. Phosphates do not, as a rule, occur in 

 large quantities in the soil, but are important on account of their 

 indispensability to plant life. The important phosphate minerals 

 are: Apatite, or crystallized calcium phosphate Ca 3 (PO 4 ) 2 ; phos- 

 phate rock, or amorphous calcium phosphate ; vivianite, which is 

 hydrated phosphate of iron; wavellite, or hydrated phosphate of 

 alumina. The three phosphates named last probably occur in 

 soils. A large number of mineral phosphates are known. Dr. 

 F. W. Clarke, of the U. S. Geological Survey, assumes that all 

 the phosphoric acid of igneous rocks is present as apatite. As 

 stated above, phosphates are also found in the soil as basic com- 

 pounds of iron and aluminium. Organic phosphorus compounds 

 are also present in the soil. 



Soluble Salts. Sulphate of soda, or glauber's salt, sulphate of 

 magnesia, chloride of soda, carbonate of soda and nitrate of soda 

 may be found as constituents of soils in arid sections, and, if 

 present in excessive quantity, are detrimental to vegetation and 

 give rise to alkali soils. 



Investigations of the Mineral Constituents of Soils. Studies of 

 the mineral constituents of the soil 1 are made by means of micro- 

 scopic examination with the aid of polarized light, stains, and other 

 tests. Comparatively few mineralogical studies of the soil have 

 been made. They are sufficient, however, to show that there are 

 considerable differences in the mineral content of soils, particu- 

 larly of those widely different origin. 



Chemical examination also throws some light upon the mineral 

 1 Bull. 91, Bureau of Soils. 



