BACTERIA AND CROPS 341 



Relation of Bacteria to Free Nitrogen. — It has been known 

 since the time of the Romans that the growth of clover, peas, 

 beans, and other legumes causes soil to become more favorable 

 for the growth of other plants. The reason for this has been 

 discovered in late years. On the roots of the plants mentioned are 

 found little nodules or tubercles ; in each nodule exist millions of 

 bacteria, which take nitrogen from the atmosphere and fix it so 

 that it can be used by the plant ; that is, they assist in forming 

 nitrates which the plants use. Only these bacteria, of all living 

 plants, have the power to take free nitrogen from the air and make 

 it over into a form that can be absorbed by the roots. They live 

 in a symbiotic relationship with the plants on which they form 



Nodules on the roots of the soy bean. They contain the nitrogen-fixing 

 bacteria. (Fletcher's Soils.) Copyright by Doubleday, Page and Company. 



tubercles, for the legumes provide them with organic food. All 

 the compounds of nitrogen are used over and over again, first by 

 plants, then as food by animals, eventually returning to the soil 

 again, or in part being released as free nitrogen; but any new 

 supply of usable nitrogen must come by means of these nitrogen- 

 fixing bacteria. 



Rotation of Crops. — The facts mentioned above are made use 

 of by progressive farmers who wish to produce as large crops as 

 possible from a given area of ground. Plants that are hosts 

 for the nitrogen-fixing bacteria are raised early in the season. 

 Later these plants are plowed in and a second crop of a different 

 kind is planted. The latter grows quickly and luxuriantly because 

 of the nitrates left in the soil by the bacteria which lived with the 



H. NEW CIV. BIOL. — 23 



