240 



SOILS AND PLANT LIFE 



180. How Legumes add Nitrogen to the Soil. — Legumes 

 and soil fertility are inseparable. Farmers have kno-mi for 

 ages that legumes enriched the soil, but it is only in recent 

 years that the reasons for this have been discovered. 



On the roots of practicall}' all legumes are found peculiar, 

 whitish knot-hke bodies, which were long a puzzle to 



scientists, since they 

 seemed to have no pur- 

 pose nor function, yet 

 were known to be 

 necessary to the best 

 growth of the plants. 

 At length, however, 

 the puzzle was solved. 

 These bodies, which 

 are called nodules, and 

 which vary in size from 

 that of a pin point to 

 that of a pea, are the 

 homes of friendly bac- 

 teria, which gather ni- 

 trogen from the soil 

 air and convert- it into 

 plant food. A part of 

 this food is used by the 

 growng legume, but 

 much of it is returned to the soil and used by the crop 

 which is gro^vn upon the land after the clover is plowed 

 under. 



It sometimes happens that these nodules do not form 

 on the roots of legumes. This may be due to the absence 

 of the necessary bacteria in the soil ; and unless they are 

 introduced, neither the clover nor the succeeding crops 

 will thrive as they should. 



Fig. 



U. S. De-pt. 0/ AgriCTiUuTe. 

 103. — Nodules on roots of red 

 clover. 



