104 



ELEMENTARY SCIENCE 



Farmers realized that clover improves the fertility of 

 soil long before they found why this is so; later it was 

 found that clover enriches the soil in these nitrogen com- 

 pounds that are used by plants. The reason why clover 

 and other of its plant relatives produce this result is that 

 "nitrogen-fixing" bacteria live and thrive in abundance 



on the roots of these plants. 

 They live in little swellings 

 of the roots called nodules 

 or tubercles; these nodules 

 are produced by the activ- 

 ity of the bacteria which 

 live inside the roots (see 

 Fig. 45). There is air, as 

 you know, hi the soil, and 

 the bacteria in these nod- 

 ules absorb nitrogen from 

 this air, and combine it 

 with other substances. The 

 clover hay is cut, and the 

 roots left to decay in the 

 soil, thus making it much 

 more fertile. Note that 

 clover and bacteria are both necessary for this process. 

 When the nitrogen-fixing bacteria are not abundant, the 

 clover crop is much poorer than when they are. Some- 

 times fields are artificially "inoculated" with the desired 

 bacteria. Soy-beans, cow-peas, svveet clover, and alfalfa 

 are some of the relatives of clover which also bear nitro- 

 gen-fixing bacteria and root-tubercles abundantly. These 

 plants all belong to the same family, the legume or pea 

 family. 



FIG. 45. Root-tubercles. 



