188 Bacteria in Relation to Country Life 



covered with a fine foam produced by the small bubbles 

 of nitrogen gas passing out of the liquid. A chemical 

 examination will show that the nitrate has all disap- 

 peared. No denitrification occurs when both the manure 

 and solution are sterilized. 



Applications of manure carry with them to the soil 

 millions of denitrifying bacteria. It has been shown 

 that the soil itself contains several species of denitrifying 

 organisms. In the presence of large quantities of manure 

 or of other organic matter, therefore, these bacteria find 

 favorable conditions for the destruction of the nitrates 

 applied with the manure, or formed from the soil- 

 humus. Large quantities of organic matter are essen- 

 tial for the rapid growth of the denitrifying bacteria. 

 When these are absent, the microorganisms fail to find 

 enough food and energy for the destruction of the ni- 

 trates. It is for this reason that, under ordinary soil 

 conditions, denitrification does not play a significant 

 part in the nitrogen-feeding of crops. 



It has been demonstrated that annual applications 

 of cow manure at the rate of sixteen tons per acre, 

 together with quantities of nitrate equivalent to 320 

 pounds per acre, fail to cause any appreciable loss of 

 nitrogen that can v be directly attributed to denitrifi- 

 cation. The destruction of nitrates by denitrifying or- 

 ganisms does not occur, therefore, in arable soils under 

 ordinary conditions of farm practice. These losses may 

 take place, however, in greenhouse or market-garden 

 soils on account of the excessive amounts of manure 

 used, and because of the activities of the denitrifying 

 bacteria in the manure and soil. These bacteria need 



