BIOLOGICAL SOIL PROCESSES 



605 



For nitrogen fixation in soils, the three types of soil used for 

 ammonification were again studied. One gram of mannite was 

 added to 100 grams of soil in a beaker. Anaerobic conditions 

 were again obtained by the absorption of oxygen with pyrogallic 

 acid and caustic soda solution. 



The materials were incubated at about 30° C. for twenty-one 

 days and then the total nitrogen was determined by the modified 

 Gunning method, (Hibbard, 1910). Blanks were run at the 

 beginning and the difference between these blanks and the total 

 nitrogen at the end of twenty-one days gives the amount of 

 nitrogen fixed. 



Experiment VII. Nitrogen fixation in solution. Nitrogen in milligrams per 

 100 cc. of solution. 



Aerobic 



In this experiment 100 cc. of the solution contained in 250 

 cc. Erlenmeyer flasks was inoculated with 5 grams of the soils 

 of the different types. About one inch of sterile paraffin oil 

 was added in order to insure anaerobic conditions. 



All of the soils used are capable of fixing nitrogen under both 

 aerobic and anaerobic conditions. The greenhouse type of soil 

 is richer than the loam and the loam richer than the clay in 

 total nitrogen both at the beginning and at the end of the experi- 

 ment. The actual increase in nitrogen does not vary much with 

 any of the three soils. Lipman and Burgess (1915) also noticed 

 this and remarked in a conclusion that as a rule a high nitrogen 

 content in the soil seems to mitigate against a vigorous nitrogen 

 fixation. The nitrogen fixation seems to proceed as readily with 

 or without the presence of air in this experiment. 



