136 



BACTERIA AND SOIL FERTILITY 



In well aerated soils this does not occur as the denitrifiers ap- 

 pear to be only facultative anaerobes. 



The senior author failed to find any evidence of denitriiication 

 in a highly calcareous soil to which had been added from zero to 

 twenty-five tons per acre of manure and from 12.5 per cent to 

 22.5 per cent of moisture as may be seen from the following: 



Treatment 



Nitric Nitrogen 



Gain in total Nitrogen 





{^er cent) 



{per cent) 



12.5 per cent of water* 



100 



100 



1 5 per cent of water 



118 



108 



17.5 per cent of water 



121 



102 



20 per cent of water 



121 



104 



22.5 per cent of water 



123 



108 



Therefore, denitriiication may occur in a water-logged soil 

 containing large quantities of fresh organic matter or in tightly 

 packed soil, but it is doubtful if it is of any economic importance 

 in normal soils. 



Denitrifiers are mostly heat-loving microbes and hence have 

 an optimum temperature relationship considerably higher than 

 that which occurs in the average soil. The conditions, therefore, 

 which promote denitrification are just the opposite of those which 

 promote nitrification. Hence, the factors which the farmer has 

 learned promote plant growth retard the growth of injurious 

 bacteria. 



Functions of Denitrifier. — When acting in the farmer's soil, 

 the denitrifier is a robber bacteria lying in wait until the banquet 

 has been prepared by the honest-toiling nitrifiers. Then the de- 

 nitrifier slips in and wastes the precious plant-food. But from 

 what has been said concerning its habits, we find this microbe a 

 robber by force of circumstance imposed upon him by the far- 

 mer's injudicious handling of his soil-. However, the denitrifier 

 has a specific function in returning the huge quantities of nitro- 



* The soil contaming 12.5 per cent of water was taken as producing- loo per cent* 



