CHAPTER III 



THE CHEMICAL COMPOSITION OF SOILS 



67. Elements Present in Soils. — Physically consid- 

 ered, a soil is composed of disintegrated rock mixed 

 with animal and vegetable matter ; chemically consid- 

 ered, the rock particles are composed of a large 

 number of simple and complex compounds, each 

 compound in turn being composed of elements chem- 

 ically united. Elements unite to form compounds, 

 compounds to form minerals, minerals to form rocks, 

 and disintegrated rocks form soil. When rocks decom- 

 pose, 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 15 are found in animal and plant bodies, and of 

 these but 12 are 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, potassium, and calcium. 



68. Classification of the Elements. — The elements 

 found most abundantly in soils are divided into two 

 classes : 



