NUTRITION 83 



has the power to form nitrites from ammonia, and the 

 other (Nitrobacter) forms nitrates from nitrites. Only 

 the nitrates are available to plants. A third kind of 

 nitrifying organism, Pseudomonas radicicola lives, as its 

 name implies, in roots, and chiefly in those of leguminous 

 plants, such as clover, lupine, locust, peas, beans, alfalfa, 

 and their near relatives. The presence of the bacterium 

 causes little swellings or nodules on the roots (Figs. 61 

 and 228). The ability to "fix'* atmospheric nitrogen is 

 possessed by several other species of bacteria, and also 

 by some of the molds. 



Root-nodules, caused by nitrogen-fixing organisms, oc- 

 cur also on roots of certain non-leguminous plants, includ- 

 ing Elaeagnaceae (Oleaster family), Myricaceae (Bayberry 

 family), Podocarpineae, the genus Alnus, of the Betulaceae 

 (Birch family), and Cycadaceae (Cycas family). The 

 roots of Cycas contain two kinds of nitrogen-fixing organ- 

 isms, Pseudomonas radicicola and Azotobacter (Fig. 241). 



82. Value of Leguminous Crops. Because of the 

 presence in their roots of organisms that can use the 

 free nitrogen of the air to form compounds of nitrogen, 

 leguminous crops are of inestimable value to agriculture. 

 In fact, they are absolutely necessary in order to maintain 

 the fertility of the soil. When any leguminous crops 

 are harvested, the roots are left in the soil with their 

 tubercles rich in compounds of nitrogen, and the com- 

 pounds (nitrates) are available to the next non-leguminous 

 crop, such as oats or corn. This is one of the main 

 reasons why good farmers always practice a rotation of 

 crops, alternating leguminous with non-leguminous plants, 

 for thereby the richness of the soil in available nitrogen is 

 maintained. Thus, for example, a certain field in Illinois 



