THE SOIL. 37 



Oxygen occurs in the soil in a free state, and also 

 combined with nearly all of the other elements. 

 Combined with silicon it forms quartz, of which 

 sand is made and which is so abundant all over the 

 earth. Quartz is so abundant that it is estimated 

 to compose nearly half the rocks of the earth. 



Hydrogen seldom, or never, occurs in a free 

 state in the soil; but, combined with oxygen, forms 

 the familiar compound water, which is absolutely 

 essential at all times, and serves plant growth in 

 many ways. 



Nitrogen, although so abundant in the air, occurs 

 in very small quantities in the crust of the earth. 

 It is a very essential soil ingredient when combined. 

 It is brought and fixed in the soil through decay- 

 ing organic matter, and by germs on the roots of 

 plants of the pea family. It also occurs combined 

 with sodium and potassium as nitrates, and is often 

 applied in a pure state to the ground as a fertilizer. 

 Nitrogen has very little attraction for the other 

 elements and compounds, but the small germs 

 growing on the roots of clover, peas, and beans 

 have the power of extracting it from the air and 

 fixing it as a compound in the soil. There it does 

 a vast amount of good in replenishing the soil with 

 new plant food. 



Carbon occurs in the soil mostly as a part of 

 organic matter. It exists in the air combined with 

 oxygen as the well known gas, carbon dioxide. 

 In this form it is taken up by the leaves of plants, 



