BIOLOGY OF SOILS 159 



restricted by lack of moisture, oxygen, or any of the neces- 

 sary conditions, depends mainly upon the temperature. 

 It was found that about one part per million of nitric 

 acid was produced daily in a soil containing 18 per cent, 

 of humus and 0*3 per cent, of lime when kept at a tem- 

 perature of 25 C. The addition of calcium carbonate or 

 neutral potassium salts made very little difference, but 

 when potassium carbonate or neutral potassium salts and 

 lime were added together, the rate of nitrification was 

 greatly increased. 



Denitrification. The presence of oxygen is necessary not 

 only for the production of nitrates, but also for the con- 

 tinuance of the nitrogen in that state. If the conditions 

 prevailing in the soil are favourable to reduction, i.e., if 

 it is " not in a state of active oxidation, a reverse change 

 called denitrification takes place. The free element, and 

 sometimes also oxides of nitrogen, are evolved and serious 

 loss results. 



This destruction of nitrates has been attributed by some 

 authorities simply to the action of reducing agents, par- 

 ticularly organic matter. It has been observed to follow 

 the application of very large quantities of farmyard 

 manure, but it cannot occur to any extent when the soil 

 is thoroughly aerated. 



The reduction of nitrates is, however, generally believed 

 to be due in part, if not wholly, to the action of anaerobic 

 bacteria. The activity of the nitrifying ferments is greatly 

 reduced when the supply of oxygen is diminished, and a 

 point is soon reached at which their action ceases alto- 

 gether. The conditions are then favourable to the action 

 of anaerobic organisms which are normally present in the 

 .soil. It is known that many of these bacteria can reduce 

 nitrates to nitrites, and even to nitrogen. 



The liberation of free nitrogen and gaseous oxides may 

 be due to the direct action of the denitrifying organisms, 



