158 DENITRIFICATION 



and cow manure are practically valueless for the nutrition 

 of plants, and that their addition to the soil is dangerous 

 on account of the consequent destruction of nitrates 

 which follows such practice. It must be noted that in 

 all the experiments very large amounts of fresh dung 

 were used, far in excess of what would be used in farm 

 practice. Wagner attributed the bad effect of dung and 

 straw chiefly to the large numbers of denitrifying bacteria 

 which they contain. No doubt these materials add 

 denitrifying bacteria to the soil, but the loss ~of nitrogen 

 observed is due more particularly to the large quantity 

 of easily assimilated organic matter which the dung 

 contains, for without this material denitrification cannot 

 proceed. Well rotted dung or farmyard manure which 

 has been ploughed into the land some time is found to 

 have little or no destructive effect upon nitrates added 

 subsequently, and does not reduce the yield of crops in 

 the same way as fresh dung does : the numbers of 

 denitrifying organisms are not reduced by rotting of the 

 manure, but the easily oxidized organic matter which 

 they need for their special denitrifying effect is greatly 

 diminished by the putrefactive changes and decay which 

 takes place in an aerated heap or in the soil. 



The reduction of crop under the conditions of these 

 pot experiments, where large amounts of fresh dung or 

 straw are used, is due in part to the destruction of 

 nitrates and loss of gaseous nitrogen ; but some of it is 

 due to another cause, namely, the assimilation of nitrates 

 by certain species of bacteria added by the dung and 

 straw, or stimulated by these materials into active 

 growth, and there occurs a locking up of nitrogen in 

 an organic form in the bodies of these organisms. 

 Berthelot and others have shown that the organic 



