594 PRINCIPLES OF SOIL MICROBIOLOGY 



should be considered as a synthesized product. The bacteria of the 

 monotrichcus type do not produce as much gum as those of the peri- 

 trichous type. 141 Different strains of the same organism vary greatly in 

 the amount of gum formed. 142 A further study of the nature of this 

 substance and its role in the fixation of nitrogen is desirable. 



Influence of reaction on the growth of Bad. radicicola and nodule forma- 

 tion. Symbiotic nitrogen-fixing bacteria have limiting reactions 

 which are not so sharply defined, however, as in the case of Azotobacter 

 and Clostridium. Maze 143 was the first to call attention to the fact 

 that there are acid resistant and acid sensitive types of nodule bacteria. 

 A sharp line of demarcation was obtained between the reaction permit- 

 ting growth and the one entirely preventing it. 144 The nodule bacteria 

 were divided, according to their sensitiveness towards acidity, into 

 five groups: 



limiting pH 



1. Alfalfa and sweet clover 4.9 



2. Garden pea, field pea and vetch 4.7 



3. Red clover and common beans 4.2 



4. Soybeans and velvet beans 3.3 



5. Lupines 3.15 



Of the nodule bacteria, the alfalfa organism is most sensitive to acid- 

 ity and the lupine organism the most resistant to acidity. It was sug- 

 gested that a correlation exists between the acid resistance of bacteria 

 and acid resistance of higher plants. The critical reaction values for 

 nodule formation are pH 4.0 and 9.0 to 10.0, with an optimum at about 

 pH 7.0; this optimum may be higher or lower depending on the legume. 

 Different biological types of alfalfa and soybean organisms may vary in 

 their limiting reactions. 145 In the case of the soybean organism one 

 strain was found to have its acid limit at pH 4.0 to 4.5 and the other 

 at 4.5 to 5.0. The optimum was found to be at pH 4.8 to 6.5, depending 

 also on strains; the alkali limit was not so sharp. 



Germination of alfalfa seed in the soil is practically the same at reac- 

 tions ranging from pH 4.5 to 7.0; it is greatly reduced, however, at pH 



141 Burrill and Hansen, 1917 (p. 126). 

 H2 Wright, 1925 (p. 127). 



143 Maze, M. Les microbes des nodosites des l£gumineuses. Ann. Inst. Past., 

 13: 145-185. 1899. 



144 Fred and Davenport, 1918 (p. 582). 



146 Stevens, 1925 (p. 135); Wright, 1925 (p. 127). 



