NITRIFICATION 139 



their combined action can be detected, the nitrites 

 which are formed being immediately oxidized as fast as 

 they are produced ; the two organisms appear to act 

 symbiotically and simultaneously. 



Certain conditions are essential for the nitrification 

 process besides the presence of ammonium salts and the 

 specific nitrifying bacteria. The chief of these are an 

 adequate temperature and suitable degree of moisture, 

 abundant free oxygen and carbon dioxide, a basic 

 substance, such as calcium or magnesium carbonate, 

 darkness, and the absence of soluble organic matter and 

 free ammonia. At freezing point and during winter, 

 when the temperature is below 5 or 6 C., nitrification 

 is checked, but it begins in early spring and continues 

 more and more actively during the warm summer 

 months. At 45 C. the nitrite organisms are destroyed 

 in five minutes, but the nitrate bacteria are not killed 

 below about 55 C/C the optimum temperature for both 

 is near 37 C. Excessive dry ness retards nitrification; 

 the best results, according to Coleman's experiments, are 

 obtained in the case of loamy soils, when the latter 

 contain about 1 6 per cent, of water ; when i o per cent, 

 is present the process is very slow, with 26 per cent, it 

 is stopped or greatly checked. The nitrifying organisms 

 are strongly aerobic, and a good supply of oxygen is 

 essential for the maintenance of their vital functions. 

 On this account the nitrification is most energetic in 

 well-drained, well-aerated porous soils ; in clays and 

 water-logged areas the organisms sometimes die out 

 altogether. 



Experiments with pure cultures have shown that 

 a basic substance, such as carbonate of sodium, 

 magnesium or calcium, is necessary for the oxidation 



