THE SOIL 31 



Hydrogen is another colorless gas, the lightest known substance. 

 United with oxgyen it forms water, one of the most important and 

 abundant compounds in nature. Without water, plant and ani- 

 mal life cannot exist. 



Nitrogen is a colorless gas which forms four fifths of the air. 

 It is necessary to both plants and animals. 



Carbon, which exists in the earth-crust in three forms, is found 

 in plants and animals, often in large quantities. Plants, however, 

 get little carbon from the soil. They take most of their supply from 

 the carbon dioxide of the air, a gas formed by the union of oxygen 

 and carbon. 



Compounds. Few of these elements are found separate, or 

 free, in the soil. Usually they are combined to form compounds. 



Oxygen takes part in more compounds than any other ele- 

 ment; it unites readily with almost all other elements. Nitrogen, 

 on the other hand, takes part in very few compounds. We have 

 learned that nitrogen is necessary to both plants and animals, 

 but neither animals nor the higher plants have the power of using 

 free nitrogen, that is, nitrogen uncombined with other elements. 



How, then, do they get their supply? Animals get theirs from 

 plants, upon which they feed directly or indirectly. The nitrogen 

 in the soil or the air is prepared for plant use by little organisms, 

 called bac te'ri a, of which you will learn more later. 



Fertile and Sterile Soils. The particles of the earth-crust have 

 been so mixed and combined by wind and water, by plants and ani- 

 mals, and by chemical action, that practically the same elements 

 enter into the composition of all soils. A sterile soil is composed 

 of the same elements as a fertile one. The fertile soil, however, 

 contains all the elements necessary for plant food in sufficient quan- 

 tity and in available forms, that is, in forms which plants can use. 



Some elements, such as aluminium, are not used by plants for 



