The Denitrifying Bacteria 189 



large amounts of organic food for their development, 

 and are, therefore, most injurious when developing in 

 the presence of large quantities of fresh vegetable or 

 animal matter. Hence, denitrification may also occur 

 at times when abundant; green-manuring crops are 

 turned under. It is not advisable, therefore, to top- 

 dress the soil with nitrate of soda shortly after large 

 applications of manure or of other drganic material. 



After the organic matter has partly undergone decay, 

 the denitrifying bacteria no longer find in the residues 

 an acceptable source of food for their rapid develop- 

 ment. For this reason, well-composted manure is not 

 liable to cause denitrification in the soil, even when 

 applied in large amounts. The same applies also to 

 vigorous crops of green-manures which cease to be a 

 source of danger after they have undergone partial decay. 



Denitrification is favored by the exclusion of air. 

 The denitrifying bacteria, it seems, require a certain 

 amount of oxygen for their growth. When it is absent 

 they take it out of the nitrates. This explains the re- 

 duction of nitrates in the deeper layers of the soil, or 

 in water-logged surface soil. It accounts also for the 

 greater tendency to denitrification in heavy, compact 

 soils as compared with the more open, sandy loams. 



Drainage, liming, and thorough tillage greatly lessen 

 the danger from denitrification by improving the cir- 

 culation of air in the soil. Even when air is excluded, the 

 presence of nitrate and of easily decomposable organic 

 matter is a prerequisite for the rapid growth of the 

 denitrifying bacteria. Soils poor in humus, therefore, 

 are not liable to cause the reduction of nitrate. 



