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GREEN PLANTS AS FOOD MAKERS 



processes of soil making are aided by oxidation. A glance at some 

 crumbling stones will give you an example of this yellow oxide of 

 iron (rust) with which they are covered. So by slow degrees the 

 earth became covered with a coating of what we call inorganic soil. 

 Later, generation after generation of tiny plants and animals which 

 lived in the soil died, and their remains formed the first organic 

 materials of the soil. 



We shall later learn more about the bacteria or germs that live 

 in the soil (see Unit VII, Problem III). It is sufficient at this 





i WbLi 



Note the slopes that are gradually being worn down and are forming soil in this canyon, 

 desert soil is formed in this way. 



Much 



time for us to know that due to certain bacteria, dead plants and 

 animals are changed through decay into matter which can be used 

 by living plants. Living things must have nitrogen in order to 

 make living matter. This nitrogen comes partly from the de- 

 cayed material already in the soil, partly from fertilizers added by 

 man, and partly from fresh nitrogen supplies taken from the air 

 and " fixed " in a usable form by certain bacteria which live in the 

 roots of leguminous plants like the bean or clover. 



