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THE FIRST BOOK OF FARMING 



small that it would take about 10,000 average-sized 

 soil bacteria or soil germs placed side by side to 

 measure one inch. 



A knowledge of three classes of these soil germs 

 is of great importance to the farmer. These three 

 classes of germs are : 



Nitrogen-fixing germs. 



Nitrifying germs. 



Denitrifying germs. 



NITROGEN-FIXING GERMS 



We learned in Chapter VIII that nitrogen is one 

 of the necessary elements of plant food, and that 

 although the air is four-fifths nitrogen, most plants 

 must take their nitrogen from the soil. There is, 

 however, a class of plants called legumes which can 

 use the nitrogen of the air. Clover, alfalfa, lucern, 

 cowpea, soy bean, snap bean, vetch and similar 

 plants are legumes. These legumes get the nitrogen 

 from the air in a very curious and interesting man- 

 ner. It is done through the aid of bacteria or 

 germs. 



Carefully dig up the roots of several legumes and 

 wash the soil from them. On the roots will be 

 found many small enlargements like root galls ; these 

 are called nodules or tubercles. On clover roots 

 these nodules are about the size of the head of a 

 pin while on the soy bean and cowpea they are 

 nearly as large as a pea (see Fig. 34). These nod- 

 ules are filled with bacteria or germs and these germs 



