TEMPERATURE 279 



When too large a quantity of water is applied there is a tendency 

 to depress the total nitrogen fixed, as is illustrated by the following 

 results in which various quantities of water were applied to a soil 

 throughout the year under field conditions : 



Inches of Nitrogen fixed 



water applied in 100 grams 



during summer. soil. 



mgm. 



37.5 1.4 



25.0 2.1 



15.0 8.5 



None . 3.5 



The maximum for anaerobic conditions does not appear in these 

 results probably because the soil did not become filled with water 

 and because under field conditions the water rapidly drains away 

 or is evaporated. There would seem to be a correlation between 

 the water content of a soil as measured in terms of its water-holding 

 capacity irrespective of physical composition and its nitrogen-fixing 

 powers. This is brought out in Fig. 34 in which water requirements 

 for ammonification, nitrification, and nitrogen-fixation are compared. 



Temperature. Berthelot early recognized that the biological gain 

 of nitrogen in soils is dependent upon a suitable temperature. 

 He found nitrogen-fixation to occur best at summer temperatures 

 between 50 and 104 F. The process was immediately stopped on 

 heating to 230 F. Later Thiele maintained that although Azoto- 

 bacter possess the ability to fix small quantities of nitrogen under 

 laboratory conditions, the temperature would be unfavorable under 

 field conditions. Heinze, however, found that although the nitro- 

 gen-assimilating organisms are most active at a temperature between 

 20 C. and 30 C., they nevertheless fix appreciable quantities at 

 temperatures as low as 8 to 10 C. Still more recent work has show r n 

 the optimum temperature to be 28 C. and the limits of activity of 

 Azotobacter chroococcum to lie between 9 C. and 33 C. The actual 

 quantitative variation in nitrogen fixed is .seen from the results 

 reported by Lohnis. He inoculated 100 c.c. of a 1 per cent, mannite 

 soil extract with 10 gm. of soil and obtained the following fixation 

 at the various temperatures: 



Nitrogen. 

 Mgm. 



10 to 12 C 3.15 



20 to 22 C 4.55 



30 to 32 C 4.27 



Better fixation at a lower temperature is noted w r hen the soil is 

 incubated and the gain in nitrogen determined directly. Koch 

 obtained fixations of 3 mgm;, 11 mgm., and 15.5 mgm. of nitrogen 

 in 100 gm. of soil when incubated with a carbohydrate at 7 C., 



