220 NITRIFICATION 



Under these conditions, of thirty-seven distinct organisms 

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

 producing nitrogen gas also, three brought about some slight 

 reduction, and fifteen were without action on the nitrate. 

 Reduction 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 whenever 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 mangold 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 favourably with the 

 proportion recovered when nitrate of soda alone is used (see 

 pp. 113-14). 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., 



Besides denitrification proper another type of loss of 

 nitrogen occurs. Under partial aerobic conditions the 

 bacterial decomposition of organic bodies containing nitrogen 

 may result in the loss of nitrogen as free gas. Such actions 

 probably go on in soil, and serve to account for the fact 

 that there seems to be a limit to the accumulation 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 



