xiv Contents 



CHAPTER XIX 



Pages 

 Denitripication . . . .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 . .190-196 



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 — Alinit. 



