NITROGEN 



117 



Until that time the Chinese explanation that "beans are 



good for the soil" was as good as any. 



In the last fifty years 

 many investigators have 

 worked on the subject, 

 and it has been demon- 

 strated that when le- 

 gumes have certain bac- 

 teria present on their 

 FIG. 51. (i) wheat grown without nitro- roots they are able to 



gen, all other foods supplied. (2) Clover o- rnw ; n Qn ;i thai rln nnf 

 grown without nitrogen. (3) Clover without r W m SO11S ttiat a n l 

 nitrogen, but inoculated with legume bacteria. contain any nitrogen. 



The free nitrogen of the air in the soil has been proved to 

 be the source of their supply. If the right kind of bacteria 

 are not present, a legume cannot grow without nitrogen 

 in the soil. No other farm plants are able to obtain nitro- 

 gen in this way (Fig. 51). 

 Peas, beans, clover, alfalfa, 

 peanuts and vetches are 

 some of the legumes. Look 

 at the roots of any of these 

 plants and you will find 

 small bunches on them. 

 On clover they are a little 

 larger than a pinhead (Fig. 

 14), but on beans the nod- 

 ules are as large as small 

 sweet peas. (Fig. 52). 

 These nodules are caused 

 by a certain kind of bac- 



FIG. 52. Nodules in which the nitrogen-fix- 

 (PseudomonaS radl- ing bacteria live on the roots of a bean 



