48 SOILS AND FERTILIZERS 



mass on rock ledges. In glacial soils, the rocks are 

 never angular, but are smooth because of the grinding 

 action during transportation. The area of glacial soils 

 in the northern portion of the United States is quite 

 large. These soils are, as a rule, fertile because of 

 the pulverization and mixing of a great variety of 

 rock. 



52. Chemical Action of Water. The chemical 

 action of water has been an important factor in soil 

 formation. While nearly all rocks are practically in- 

 soluble in water there is always some material dis- 

 solved, evidenced by the fact that all spring-water 

 contains dissolved mineral matter. When charged 

 with carbon dioxide and other gases, water acts as a 

 solvent upon rocks. It converts many oxides, as fer- 

 rous oxide, into hydroxides. The chemical action of 

 water may produce new compounds more soluble or 

 readily disintegrated, as deposits of clay, which have 

 been formed from feldspar rock by the chemical and 

 physical action of water. When rocks disintegrate, 

 chemical changes often occur; the addition of water 

 or hydration of the molecule, particularly of the sili- 

 cates, is one of the most important chemical changes. 

 Water takes as prominent a part in the decay of rocks 

 as in the decay of vegetable matter. Limestone is 

 quite readily disintegrated by water. Dissolved min- 

 erals produce many chemical changes in both rocks 

 and soils. The chemical action of fertilizers known 

 as fixation can take place only in the presence of 

 water. In fact, water is necessary for nearly all of the 



