120 TRANSFORMATION OF NITROGEN BY SOIL MICROBES 



groups is the same as the number of different cultures of nodule 

 bacteria required to inoculate these legumes. 



(1) Alfalfa, white sweet clover, yellow sweet clover, hubam, 

 bur clover, yellow trefoil, and fenugreek. 



(2) Red, white, crimson, alsike, bersem, and cow clovers. 



(3) Garden, canning, and field peas; hairy, spring, and wild 

 vetches; broad bean; lentil; sweet pea; and perennial pea. 



(4) Cowpea, peanut, Japan clover, velvet bean, lima bean, 

 partridge pea, wild indigo, acacia, dyer's greenwood, and tick 

 trefoil. 



(5) Garden, field, navy, kidney, wax, and scarlet runner 

 beans. 



(6) Lupines and serradella. 



(7) Soybean. 



(8) Hog peanut. 



(9) Lead plant. 



(10) Trailing wild bean. 



(11) Black or common locust, 



(12) Wood's clover. 



These varietal differences between the bacteria capable of forming 

 nodules upon the roots of various plants can be demonstrated by 

 direct inoculation upon the plants, and also by cultural and sero- 

 logical tests. Morphologically the organisms belonging to the 

 different races are quite alike (except for some differences in the 

 formation of flagella), but physiologically they are quite different. 

 Influence of Nitrate upon Nitrogen Fixation. — The 

 amount of nitrogen fixed by inoculated legumes is influenced to a 

 considerable extent by the amounts of nitrate in the soil on which 

 the plants are growing. Nitrate-nitrogen is readily absorbed by 

 legumes, and when present in abundance in the soil may supply 

 the entire nitrogen requirements of the plants. Since absorption 

 of nitrate dominates fixation of nitrogen, it is only when the 

 nitrate available to the plants is less than the nitrogen require- 

 ments of the plants that appreciable fixation occurs. This is 

 apparent from Figs. 58 and 59. The amounts of nitrogen in the 

 plants are not very different irrespective of the amounts of nitrate 

 supplied to them. However, the amounts of nitrogen fixed are 

 inversely proportional to the amounts of nitrate available. 



