Soil Fertility and Its Preservation 



45 



Early experiments seemed to show that the plants 

 took no nitrogen from the air, that all they got came 

 from the humus (decaying vegetable matter) in the 

 soil. But when plants of the legume family were tested, 

 it was discovered that they would grow about as well 

 in ground containing no nitrogen as in rich soil. (The 

 legumes produce their seeds in pods like bean or pea 

 pods.) In 1888, the scientist Helrigel found that upon 

 the roots of such plants 

 live multitudes of bac- 

 teria which enable the 

 plant to secure its nitro- 

 gen from the air (Figs. 

 27 and 28). These 

 " nitrogen-fixing bac- 

 teria " take the nitrogen 

 gas from the air that is 

 in the soil and unite it 

 with oxygen and other 

 elements. Then the 

 plant can use it as food 

 material. The process 

 of making nitrogen 

 unite with other ele- 

 ments is called " fixation 

 of nitrogen." 



If the student will 

 pull up and examine the 

 roots of clover, peas, 

 beans, alfalfa, peanuts, 

 or any other plant be- 



Why 

 plants 

 of the 

 legume 

 family can 

 grow in 

 poor soil 



Nitrogen' 



fixing 



bacteria 



The 

 nodules 



U.S.D.A. 



FIG. 28. A young red clover plant, an- 

 other legume, showing nodules on roots. 



