THE NATURE OF SOIL 41 



the soil. This nitrogen enters into their structure, 

 and is returned to the soil when the plants decay, 

 but not in the same form. It enters the plant as a 

 salt of nitrogen a nitrate; it returns to the soil 

 in combination with many other substances, and is 

 called by the chemist "organic nitrogen." The 

 important point about this is that plants cannot 

 use organic nitrogen, because it will not dissolve 

 in water, and all the food that plants get from the 

 soil must be taken in liquid form. It must first be 

 separated from its partners in the compound, and 

 then changed into a nitrate before the soil water 

 can dissolve it, and the roots of plants absorb it. 



The work of transforming valueless organic 

 nitrogen into valuable nitrates, which are plant 

 food, is performed by our tiny helpers, the "nitro- 

 gen-fixing germs." They are found in all fertile 

 soil in inconceivable numbers, busily engaged in 

 making plant food out of all vegetation that is re- 

 turned to the soil, provided the conditions are 

 right. One essential condition is that they have 

 plenty of food. All these ferments may be con- 

 sidered very minute plants; they must have food 

 like other plants. One food of the nitrogen fixing 

 germs is phosphoric acid, which is also one of the 

 most important foods of ordinary farm crops. If 

 a soil has very little phosphoric acid in it, the 

 transformation of humus into plant food is apt to 

 take place very slowly. The principal food of the 

 germs, however, is humus itself. This they can 

 use only after the leaves, stems, or other vegetation 

 has been thoroughly incorporated with the soil and 

 is rotted. 



These minute plants need moisture and a 

 medium temperature in order to thrive and do 

 their work, as the yeast ferment needs moisture and 



