VII.] DENITRIFICATION 127 



presence of organic matter like sugar, which is easily 

 oxidised, and the exclusion of all air, so that the soil 

 bacteria in want of oxygen are driven to take it out of 

 the nitrate which is present. In nature the soil does 

 lose nitrogen because of bacterial changes of this kind ; 

 we call the process denitrification, but the losses will 

 only be large when there is much easily oxidisable 

 organic matter present in the soil, and when the soil 

 gets short of air by waterlogging or some similar 

 accident. That the losses, however, may be consider- 

 able can be deduced from one of the experimental plots 

 on the wheat field at Rothamsted, where the large 

 quantity of 14 tons per acre of farmyard manure are 

 applied every year for the wheat crop. Despite all this 

 application of manure the yield of wheat has not 

 continued to increase ; indeed, for the last forty years 

 it has been almost stationary, except for seasonal 

 fluctuations. The amount of nitrogen, however, that is 

 obtained in the wheat crop is only about one-quarter of 

 the 200 lb. per acre that was applied in the manure. 

 Of the remaining three-quarters of the annual supply 

 of nitrogen, about one-quarter still remains in the 

 soil, as an unexhausted residue which will very slowly 

 be converted into plant food. But one-half of the 

 immense amount of manurial nitrogen applied has been 

 lost entirely ; it has been dissipated as nitrogen gas by 

 the bacteria in the soil causing what we have called 

 denitrification, and the loss has been so great because of 

 the enormous accumulation of organic matter in the 

 soil. Denitrification causing loss of soil capital must 

 be added, then, to the list of bacterial actions always 

 going on in the soil, or ready to take place whenever 

 the conditions become favourable to that particular form 

 of activity. 



So far, however, we have only been considering 



