SOIL MATERIAL AND ITS ORIGIN 5 



tion and lost. From these combinations the elements are liberated 

 somewhat as needed by plants. The minerals of this group are de- 

 composed by hydrochloric acid with the separation of colloidal 

 silica. 



The preceding minerals are silicates, but there are a few non- 

 silicates that should be considered in the study of soils. 



7. Calcite (CaCO 3 ). Calcite is a very common mineral exist- 

 ing as limestone and marble. Its composition is CaO, 5(i per cent, 

 and C0 2 , 44 per cent, when pure. It possesses a hardness of about 

 3, distinct cleavage and is soluble in carbonated water, one part in 

 1020 of water, forming the bicarbonate (CaH 2 (C0 3 ) 2 ). In the 

 formation of soil material from rock made up largely of calcium 

 carbonate, the insoluble impurities are left and form the soil. As a 

 general rule limestone soils are quite fertile. 



8. Dolomite (CaMg(C0 3 ) 2 ). The hardness of dolomite is 3.5. 

 It is composed of 54.35 per cent of calcium carbonate and 45.05 per 

 cent of magnesium carbonate. Dolomitic limestone is made up of 

 these minerals, though probably not always in these proportions. It 

 is slowly soluble in carbonated water, leaving the impurities to form 

 soil material. A large amount of magnesium carbonate is injurious 

 to some crops and constitutes much of the alkali in soils of humid 

 areas. 



9. Gypsum (CaS0 4 .2H 2 0). Cypsum possesses a hardness of 

 2 and the following composition : sulfur trioxid, 40.5 per cent, lime 

 32.0 and water 20.9 per cent. It is found in considerable quantities 

 in arid regions where salt lakes formerly existed, but is of compara- 

 tively little importance as a soil former, since the soil derived from 

 it has very little, value. It has, however, some value as a remedy 

 for black alkali that is so frequently found in arid and semi-arid 

 regions. 



10. Apatite (Oa-(P0 4 ). t ri). This mineral is important in 

 soils because of the phosphorus it furnishes. Fortunately it exists 

 in all rocks, though in very small amounts, and when these decom- 

 pose very little of the phosphorus is lost through solution. Hence 

 a soil will usually show a higher per cent of phosphorus than the 

 original rock. In some cases the chlorine is replaced by fluorine. 



11. Limonite (2IY,O : ,.:MI,(>) and Hematite (Fo.,0.,). Sev- 

 eral other minerals might be mentioned, among which are limonite, 

 the hydrated ferric oxide having 85. per cent of Fe,O. t and 14.4 per 

 cent of water, and hematite 30 per cent of oxygen and 70 per cent 

 of iron. One or the other of these and sometimes both arc found in 



