2 THE PRINCIPLES OF SOIL MANAGEMENT 



mined by the character and condition of the soil. It 

 is the source of the majority of the nutritive elements, 

 it contains the water necessary for the plant and in 

 which is carried its food, it holds air in its pores, and 

 it absorbs and transmits the necessary heat. Enemies 

 of one plant may or may not be present; but, if present, 

 they may exercise a controlling influence. All the parts 

 of the soil mechanism for such it must be considered 

 are closely related to each of these essential factors, 

 and it is from this point of view of the growing plant 

 that the following treatment is developed. 



The characteristics of the soil may be viewed from 

 both the origin of the material and its properties. The 

 first of these may be termed "The Rock and Its Prod- 

 uct," and, second, in so far as they pertain to physical 

 properties, "The Soil Mass."* 



1. The Rock and Its Products 



Since all soil material forms a part of the structure 

 of the earth, its origin and derivation constitute a part 

 of the field of geology. The following discussion of 

 the rock and its products deals primarily with these 

 facts and processes. But the discussion is not taken 

 up because of its geological interest, great as that is, 

 but because of the fundamental connection these 

 have to the physical, chemical and biological proper- 

 ties of the soil which determine its ability to grow plants. 

 The kinds of minerals and rocks in which the essential 

 elements of plant-food originally occur, and the changes 



