280 AZOF I CAT ION 



15 C., and 24 C., respectively. Traaen, using a loam soil with a 

 maximum water-holding capacity of 27.4 per cent., obtained nearly 

 as great a fixation at 13 C. as at 25 C. when the optimum moisture 

 content was maintained. This is seen from the following: 



Nitrogen fixed in 100 gm. of soil. 



Temperature. 



13 C. . 

 25 C. . 



A temperature, favorable even though not ideal for nitrogen- 

 fixation, would occur in soils under natural conditions. The 

 temperature of soil in Utah during the months if September averaged 

 14 C., with a minimum of 10 C. and a maximum of 17 C. During 

 June, July and August the mean temperatures would be much 

 higher. 



The mean daily temperatures of the soil for Bismarck, North 

 Dakota; Key West, Florida; and New Brunswick, New Jersey; for 

 the months of June, July, August and September were 18 C., 28 C., 

 and 24.5 C., respectively. From this it is evident that during a 

 considerable period of each year an arable soil has a temperature high 

 enough for moderately rapid nitrogen-fixation. 



Although it is generally maintained that there is no nitrogen- 

 fixation in soils during the winter months, cold or even freezing does 

 not injure the organism; for the cooling of a soil, even to the freezing 

 point, increases its nitrogen-fixing powers. This is probably due to 

 the suppression of competing species and to the establishment of a 

 new flora. The same is true when the soil is heated, as may be seen 

 from the results given below. 



Temperature Nitrogen fixed, 



deg. C. per cent. 



Normal 5.11 



50 9.00 



55 14.14 



60 16.38 



65 14.42 



70 13.02 



75 11.34 



80 12.66 



85 10.36 



This soil had been autoclaved and then inoculated with a soil 

 extract which had been heated to the temperature indicated. The 

 stimulation could not, therefore, have been due to the heat rendering 

 more of the plant-food in the soil available. The results indicate 

 that many of the organisms which take part in nitrogen-fixation 

 are highly resistant to heat. It is significant that the greatest stimu- 

 lation is exerted in a soil which had been inoculated with solutions 



