xv 



Contents 



CHAPTER XIX 

 DENITRIFICATION ................ . . 183-189 



Early idea of denitrification The cause of denitrification 

 Value of modern discoveries Modern conclusions con- 

 cerning denitrification The denitrifying bacteria. 



CHAPTER XX 



THE INCREASE OF SOIL-NITROGEN .......... 190-190 



The ammonia theory The nitrate theory The bacteria 

 theory. 



CHAPTER XXI 



THE NON-SYMBIOTIC NITROGEN-FIXING BACTERIA . . .197-205 

 Aerobic and anaerobic bacteria The aerobic nitrogen- 

 fixing bacteria Agricultural importance of the two 

 classes of bacteria Other nitrogen-fixing forms. 



CHAPTER XXII 



SYMBIOTIC FIXATION OF NITROGEN .......... 206-220 



The value of legumes The limitations of legumes 

 Legumes and fertilizers The cause of the soil-enriching 

 qualities of legumes Liebig's theory Other theories 

 The solution of the problem Nodules on the roots of 

 legumes The nature of the nodules The bacteria of 

 legumes The relations between legumes and the 

 nodule bacteria. 



CHAPTER XXIII 



SOIL-INOCULATION ................. 221-236 



Pure cultures Nitragin Agar as a food for cultures 

 Difficulty in using pure cultures Artificial cultures 

 Soil-inoculation in the United States A Unit. 



