FERTILITY AND SOIL LIFE 103 



food, so it appears that green plants depend upon the work 

 of soil bacteria for materials out of which they make food. 

 Thus we see that the changing of materials from the in- 

 organic to the organic condition and back again occurs 

 in a sort of cycle. At the beginning of this cycle we have 

 the work of green plants. At the end of it we have the 

 work of bacteria and other micro-organisms, which change 

 the complex dead organic substances into simpler sub- 

 stances which green plants can use. Green plants cannot 

 use these materials until they have been so changed. 



Importance of Nitrogen to Plants. Bacteria increase 

 the amount of nitrogen in the soil which is available for the 

 use of green plants, and this is very important. By avail- 

 able nitrogen we mean nitrogen in such condition that 

 plants can use it. Nitrogen is necessary to plant life. 

 Four-fifths of the air is nitrogen, as you have learned. In 

 the air then there seems to be an inexhaustible supply. 

 But green plants cannot use this atmospheric nitrogen at all; 

 they cannot use it any more than we can use sea-water to 

 drink. Shipwrecked sailors might have a whole ocean of 

 water about them, and yet perish of thirst for lack of 

 available water to drink. So with green plants and nitro- 

 gen; the nitrogen to be available for them must be pres- 

 ent in certain compounds (nitrates) which are very differ- 

 ent from the free nitrogen of the air. 



Certain bacteria, sometimes called the nitrogen-fixing 

 bacteria, have the power to use the nitrogen of the air, and 

 they change it into the compounds that can be used by 

 plants. This is not the only method by which such com- 

 pounds are produced in nature, but it is a very important 

 one. 



