220 NITRIFICATION 



Under these conditions, of thirty-seven distinct organisms 

 tested, nineteen rednced the nitrate to nitrite, one of them 

 producing nitrogen gas also, three brought about some sKght 

 reduction, and fifteen were without action on the nitrate. 

 Eeduction to a nitrite was the most general reaction, but 

 other organisms have been found capable of carrying the 

 reduction further to nitric or nitrous oxide, or even to nitrogen 

 gas. 



It has been supposed that considerable losses of nitrogen 

 are likely to accrue from this cause Avhenever nitrate of soda is 

 used as a manure in conjunction with organic materials like 

 dung. But, notwithstanding the presence of denitrifying 

 bacteria in the soil, the conditions under which they become 

 active— absence of air, a high temperature and the presence 

 of large quantities of soluble organic matter — are so rarely 

 realised that denitrification probably plays no large part 

 in practice. For example, on the Rothamsted mangel plots, 

 where large quantities of nitrate of soda are used in conjunction 

 with dung applied every year, the recovery in the crop of the 

 nitrogen supplied in the nitrate compares favoural^ly with the 

 proportion recovered when nitrate of soda alone is used (see 

 pp. 113-4). In other words, the nitrate of soda produces 

 almost as large an increase when added to a dunged as to an 

 unmanured plot, hence very little of its nitrogen can have 

 been wastefully liberated as gas. 



Latterly the term denitrification has been used in a Avider 

 sense for all bacterial decomposition of organic bodies containing 

 nitrogen, which result in the loss of nitrogen as free gas. Such 

 actions must be always going on in soil, and serve to account 

 for the fact that there seems to be a limit to the accumula- 

 tion of nitrogen in soils, because the destructive changes 

 proceed with greater rapidity as the amount of organic 

 matter in the soil increases and provides a richer medium for 

 the development of these bacteria. For example, it is found 

 that the amount of nitrogen accumulated in the soil of the Park, 

 which has been in grass from time immemorial, shows no 



