Reduction of Nitrates 327 



Denitrifying bacteria. — -These are abundant in the 

 excreta of herbivorous animals and in the straw used as 

 litter. They may, therefore, when added to the soil, 

 cause the destruction of nitrates produced there by nitri- 

 fying bacteria or added in fertilizers. Yet, even in the 

 soil, denitrification takes place only when large quan- 

 tities of organic matter are present. In other words, 

 denitrification in the soil or in the manure pile is not 

 brought about by the mere presence of denitrifying 

 bacteria. 



The destruction of nitrates by these bacteria may 

 be accomplished only when the three essential conditions 

 are supplied, namely, organic matter, nitrates and bac- 

 teria. In the manure pile, nitrates do not make their 

 appearance until the decomposition of the organic 

 matter has advanced considerably; hence only two of 

 the conditions are supplied, — bacteria and organic 

 matter. In the soil, on the contrary, the nitrates and 

 bacteria are present, but not an abundant supply of 

 readily decomposable organic matter. This will ac- 

 count for the very occasional denitrification in the 

 soil occurring only after excessive appHcations of strawy 

 manure, as pointed out in a preceding chapter. 



Experiments in denitrification. — It has been noted in 

 numerous experiments carried out in Germany and 

 elsewhere, that the denitrifying effects of manure grow 

 less significant as its decomposition goes forward. 

 Wagner found in his experiments that manure two 

 weeks old when added in weighed quantities to a given 

 amount of nitrate solution, destroyed the nitrate com- 

 pletely in 14 days. When the same quantities of manure 



