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 Elseagnaceae (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 
