CHAPTER III 



CHEMICAL COMPOSITION OF SOILS AND ITS RELATION 



TO PLANTS AND ANIMALS 



In the previous chapter it has been shown how the physical 

 composition of soils, their mode of formation, and their charac- 

 teristics have given rise to several classes and many types of soils. 

 In this chapter soils shall be considered from a chemical point of 

 view, and in this respect their relation to plants and animals noted. 



Fig. 14. — A few soil grains enlarged. A sand particle or a clay particle may represent 

 a combination of the elements silicon and oxygen; or a combination of potassium, alumi- 

 num, silicon and oxygen, etc. 



Soil Materials Composed of Elements. — ^The building materials 

 of soils, as has been learned, are mineral particles and organic 

 matter. These substances, like all material things, are composed 

 of chemical elements.^ Thus, when a soil is taken into a chemical 

 laboratory and analyzed, its chemical composition is obtained and 

 this composition, as of nearly all substances, may be expressed in 

 two ways; viz., as elements and as oxides of elements; that is, the 

 elements united with oxygen (Fig. 14). 



Chemical Composition of Soils. — ^In order to make simple and 



1 Chemical elements are substances that cannot be reduced to anything 

 else. For example, iron, carbon, hydrogen and oxygen are elements, since 

 they cannot be split up iato anything else. Water is not an element because 

 it can be reduced to something else; viz., two elements— hydrogen and oxygen. 



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