PART I 



SOILS. 



CHAPTER I. 

 Soils, 



To understand the origin and formation of soil, we must first 

 study the rocks from which cur soils were originally formed. 

 Rocks are composed chiefly of minerals belonging to four groups : 

 silica and silicates, carbon and carbonates. 



Silica is the most important material found in the earth's 

 foundation. It is composed of silicon and oxygen. Chemistry 

 tells us that Oxygen is the most abundant and widely distributed 

 of all the elements. Oxygen constitutes about one-half of the 

 weight of silica in its various forms sand, flint, chalk, limestone, 

 marble and clay. 



Although oxygen is found in such great abundance in the 

 atmosphere, water and soils, and is indispensable to life, yet, it 

 does not concern our study of agriculture so much as those ele- 

 ments that are often found wanting in our soils. Silicon is 

 found in all plants and may serve some purpose, but it is not 

 known to be indispensable to plant life, so we will not consider 

 it further in the study of plants. 



The study of rocks in their relation to agriculture only needs 

 to concern us to the extent of the following points : 



1. They hold in groups combinations of elements. 



2. Certain of these elements are essential to plant growth. 



3. The groups of minerals in certain rocks determine the process 

 of disintegration. 



4. The character of the resulting soils are determined by the asso- 

 ciation of minerals composing the rock. 



Silica, feldspar and mica are the constituents of granite. 

 Granite, like most rocks containing silica, has disintegrated slowly 

 under the action of atmospheric agents. 



Feldspar consists of silica, alumina, along with lime, potash 

 or soda and these elements dilate and contract unequally. Hence 

 disintegration is rapid. It is hastened by penetration of water into 



(1) 



