252 CROP PRODUCTION 



bacteria multiply and through their hfe processes gather 

 the free nitrogen of the air. They store it up in the plant 

 tissues so that it becomes available for the growth of 

 the particular plants that they are living upon, and 

 through these it may later become available for the use 

 of other crops. The illustration on page 231 shows the 

 effect of the presence or absence of bacteria in the soil. 

 This relation between the germs and the leguminous 

 plants is one of the most important factors in maintain- 

 ing the fertility of the soil. By including in each system 

 of crop rotation one of these leguminous plants, which is 

 to be plowed under in whole or in part, the store both of 

 humus and nitrogen in the soil is increased to a marked 

 degree. For the nitrogen which is gathered from the 

 air is sent to all parts of the plant, so that if the green 

 crop is plowed under, most of the nitrogen remains in 

 the soil to enrich it for other crops. 



