vii.] DEN1TRIF1CATI0N 227 



soil may well be due to the reduction of nitrates to 

 gas, by the combustion of organic matter with the 

 oxygen of the nitrate, especially in ill-drained soils in 

 wet weather. Gayon and Dupetit, in 1886, isolated two 

 organisms from sewage which would reduce nitrates 

 to gas in the presence of organic matter, the action 

 being chiefly carried on when oxygen was absent ; it 

 came to a standstill when plenty of air was supplied, so 

 that the organism had no need to attack the nitrate 

 to obtain oxygen. Both in their experiments and in 

 others, the presence of an abundant supply of soluble 

 organic matter was one of the necessary conditions 

 for the destruction of the nitrates. The denitrifying 

 bacteria are widely distributed. Warington found, out 

 of thirty-seven species of bacteria examined, only 

 fifteen failed to reduce nitrate, twenty-two reduced 

 it to nitrite, and one of them liberated gas. P. F. 

 Frankland, again, found that fifteen out of thirty 

 organisms derived from dust or water would reduce 

 nitrate to nitrite. In fact, a large number of bacteria, 

 when deprived of oxygen and in contact with abundant 

 organic matter, will obtain the oxygen, which they 

 normally require for the breaking up of the organic 

 matter, at the expense of the nitrate. 



Many experiments, in which farmyard and other 

 organic manures have been employed in conjunction 

 with nitrate of soda and similar active compounds of 

 nitrogen, have shown a smaller crop for the manures 

 used together than when either was employed singly. 

 These results were particularly apparent when large 

 quantities of material like fresh horse-dung or chopped 

 straw were used in pot experiments. With well-rotted 

 dung, the effect of organic material in depressing the 

 yield which should be given by the nitrate was not 

 so great. 



