SOIL-FORMATION, WATER 23 



are opened to attack by any one of many agencies, both 

 mechanical and chemical. In feldspar, which is very 

 slightly soluble, hydration and hydrolysis develops 

 potassium hydrate, a very soluble compound and there- 

 fore readily removed. Its removal may develop a cavity, 

 and thus weaken the rock. Agriculturally, the removal 

 of the base is also significant. It is the basic element, 

 and therefore largely plant-food elements, or those 

 which condition soil-productiveness, such as potash, 

 lime and soda, which are removed by this process. 



It is because of the unequal solubility of minerals 

 that soil results from the process of solution. If all the 

 minerals of a rock were equally soluble, the rock might 

 be removed bodily from the exposed surface inward. 

 Solubility, operating differently for different minerals 

 in a rock mass, removes one, and leaves the others in a 

 less coherent mass, which we term soil. It therefore 

 happens that residual soils comprise the less soluble 

 portions of the rock from which they were formed. 



Materials in solution in water greatly affect its 

 capacity to dissolve minerals. Carbon dioxid is present 

 in the air in the pores of rocks and soils in much larger 

 proportion than in the air above the earth's surface. 

 It is particularly abundant in the surface layers, where 

 it is derived from organic decay. It is taken up by the 

 water as it passes along and becomes a means of solution. 

 The most striking example of this is in the case of lime 

 carbonate or limestone. In pure water this mineral is 

 soluble only to the extent of about one part in twenty-five 

 thousand, but in carbonated water its solubility is 

 about one part in one thousand, or twenty-five times as 



