CHAPTER III 



THE CHEMICAL COMPOSITION OF SOILS 



78. Elements Present in Soils. Physically consid- 

 ered, a soil is composed of disintegrated rock mixed 

 with animal and vegetable matter; chemically con- 

 sidered, the rock particles consist of a large number of 

 simple and complex compounds, each compound being 

 composed of elements chemically united. Elements 

 unite to form compounds, compounds to form minerals, 

 minerals to form rocks, and disintegrated rock forms 

 soil. When rocks decompose, the disintegration, except 

 in a few cases, is never carried to the extent of liberating 

 the elements, but the process ceases when the minerals 

 have been broken up into compounds. While there are 

 present in the crust of the earth between 65 and 70 

 elements, only about 1 5 are found in animal and plant 

 bodies, and of these but 12 are known to be absolutely 

 essential. Only four of the elements which are of most 

 importance are at all liable to be deficient in soils. 

 These four elements are : nitrogen, phosphorus, potas- 

 sium, and calcium. 



79. Classification of the Elements. The elements 

 found most abundantly in soils are divided into two 

 classes : 



