XVII 



UTILISATION OF THE SOIL 211 



seed only 

 nodules if 



are living 



which contain bacteria. These bacteria possess the 

 power of fixing free nitrogen. Plants of the Grass 

 family, turnips, and other food-plants not belonging 

 to the Pea family, do not 

 possess these nodules, and 

 cannot fix free nitrogen. 

 Clover or peas grown from 



develop the 



the bacteria 



in the soil. 



This is the reason why 

 Boussingault's experi - 

 ments andtheKothamsted 

 experiments were so con- 

 tradictory, for sometimes 

 nodules were present and 

 sometimes they were not. 

 When they were present 

 the plant could fix free 

 nitrogen, when they were 

 not it could not. 



A number of other 

 investigations were made 

 which confirmed these 

 statements, and we know now the reason why 

 clover enriches soils. It is not the clover which 

 enriches the soil, but the insignificant bacteria on 

 its roots, which can take free nitrogen from the air 



FIG. 46. Nodules on the Roots 

 of Clover. 



