CHAPTER VIII. 



CHEMICAL CONSTITUENTS OF THE SOU. 



The soil is composed of disintegrated rocks, containing organic 

 matter. Its constituents are, therefore, inorganic and organic. 



The inorganic constituents consist of the original rock minerals, 

 products of their partial decomposition, and their final products 

 of decomposition. The organic constituents consist of unchanged 

 residues of plants and animals, intermediate substances formed by 

 the action of bacteria, molds, and other agencies, and the final 

 products of decomposition, namely, carbon dioxide and water. 



Primary and Secondary Minerals. By far the greater portion 

 of the crust of the earth, and of the soils thereon, is composed 

 of silica, and combinations of silica with bases, termed silicates. 

 A large number of silicates are known, many of which are very 

 complex in constitution. Igneous rocks, which are the oldest 

 rocks, are composed entirely of silica or silicates. Primary sili- 

 cates undergo chemical changes, under the action of the air, 

 water, and other natural agencies, whereby other silicates and 

 other minerals are formed. The unchanged minerals found in 

 igneous rocks are for this reason termed primary minerals, and 

 those produced from them by chemical agencies are called 

 secondary minerals. 



Soils are composed of three classes of minerals : 

 (a-) Primary minerals, the unchanged minerals of igneous 

 rocks, not easily affected by chemical reagents. . 



(b) Hydrated silicates, which are intermediate products of the 

 decomposition of the primary minerals, and more easily acted on 

 by chemical reagents and the roots of plants. 



(c) Final products of weathering. 



The relative abundance of these three classes of minerals in the 

 soil will depend on the age and nature of the rock material and 

 the nature and activity of the weathering agencies. Old soils are 

 naturally more highly decomposed than are soils of more recent 

 origin. Transported soils consist of a greater variety of minerals 



