CARBON AND NITROGEN CYCLES IN THE SOIL 201 



Generally, there is a gain of nitrogen ; losses are, however, 

 often recorded (248, 151, etc.), whilst a certain loss of nitrate 

 invariably occurs (p. 266). A. Koch (151^) added successive 

 small doses of dextrose to 500 grms. of loam, mixed with 

 sand and spread on plates to secure copious aeration, kept 

 uniformly moist and at 20 C. Nitrogen fixation began very 

 soon and reached its maximum after eighteen weeks, when 

 losses set in ; the results are given in Table LIV. 



TABLE LIV. NITROGEN FIXED IN SOIL BY BACTERIAL ACTION IN PRESENCE 

 OF DEXTROSE. KOCH (1516). 



For each grm. of dextrose supplied in the small doses 

 about 8 milligrams of nitrogen were fixed during the first 

 eight weeks ; but only 4 or 5 milligrams later on. In larger 

 dozes the sugar was less effective, only 5 to 6 milligrams of 

 nitrogen being fixed per grm. of sugar at first and 3 milligrams 

 later. 



Pot experiments showed that the nitrogen thus added to 

 the soil became available for plant food. Dextrose and sucrose 

 first depressed the crop, then caused an increase, and finally 

 left the soil richer in nitrogen at the end of the experiment 

 than at the beginning (Table LV.). 



But if the soil temperature fell too low nitrogen fixation 

 ceased : it was not observed at 7 C. although it appeared to 

 go on at 15 C. The optimum temperature lies between 25 

 and 30 C. 



Pfeiffer and Blank (224^), however, were unable to obtain 

 any beneficial results from sugar. The Rothamsted trials 



