FIXATION OF ATMOSPHERIC NITROGEN 597 



nitrogen from (NH^SCh or NaN0 3 , not only is the alkaloid content 

 lower, but the nitrogen content of the plant decreases accordingly. 



Influence of environmental conditions upon the growth of symbiotic 

 nitrogen- fixing bacteria. The limiting temperatures for the growth of 

 nodule bacteria are 3° and46°C. ; the thermal death point is at 60° to 62°; 

 the optimum varies between 18° and 26°. 154 Bact. radicicola is not in- 

 jured by diffused sunlight and can readily withstand direct sunlight. 

 Drying injures the organism, 155 but does not destroy it completely even 

 after two years. 156 The numbers of Bact. radicicola are greatly dimin- 

 ished as a result of direct and rapid drying, as determined by the plate 

 method ; however, the number of cells that actually remain alive is much 

 greater than the number germinating on the plate. 157 In the soil it may 

 persist for at least several years, even in the absence of the host plant. 158 



Importance of symbiotic-nitrogen fixation in the soil. In the case of 

 non-symbiotic nitrogen fixation, the evidence as to actual amount of 

 nitrogen fixed under field conditions is still of doubtful value ; however, 

 in the case of symbiotic fixation of nitrogen, the evidence is undisputed. 

 The amount of nitrogen added to the soil by the bacteria depends upon 

 the relative abundance of available nitrogen in the particular soil, both 

 in inorganic and organic forms. The poorer the soil is in available nitro- 

 gen (for the growth of the leguminous plants) and the richer it is in 

 lime, available phosphorus and potash, the greater will be the gain in 

 nitrogen. In addition to this, the kind of legume and seasonal condi- 

 tions affect the amount of nitrogen fixed. The maximum amount of 

 nitrogen was found to be fixed a little before, or just at blossoming time. 



"Warington 159 pointed out in 1891 that an approximate increase of 350 

 pounds of nitrogen per acre may be obtained as a result of the growth 

 of inoculated legumes (clover). Since then, extensive data have been 

 secured, all of which point to definite increases in soil nitrogen due to the 



154 Zipfel, H. Beitrage zur Morphologie und Biologie der Knollchenbakterien 

 der Leguminosen. Centrbl. Bakt. II, 32: 97-137. 1912. 



165 Chester, F. B. The effect of desiccation on root tubercle bacteria. Del. 

 Agr. Exp. Sta. Bui. 78. 1907. 



156 Ball, O. M. A contribution to the life history of Bacillus (Ps.) radicocola 

 Beij. Centrbl. Bakt. II, 23: 47-59. 1909. 



157 Duggar, B. M., and Prucha, M. J. The behavior of Pseudomonas radici- 

 cola in the soil. Centrbl. Bakt. II, 34: 67. 1912. 



158 Lipman and Fowler, 1915 (p. 129). 



169 Warington, R. The circumstances which determine the rise and fall of 

 nitrogenous matter in the soil. U. S. Dept. Agr. Off. Exp. Sta. Bui. 8, 22-4U 

 1892. 



