142 TRANSFORMATION OF NITROGEN BY SOIL MICROBES 



Soil reaction may be appreciably modified by active nitrifica- 

 tion, due to the fact that the process results in the oxidation of 

 ammonia to nitrate which leads to increases in the hydrogen-ion 

 concentration of soils. When the nitrate is absorbed by plants 

 or removed by drainage waters it is accompanied by certain cations 

 (Ca, Mg, Na, etc.) which leave the soil poorer in neutrahzing sub- 

 stances. The increase in acidity is particularly pronounced 

 where ammonium salts are added to soils. This change is not 

 particularly undesirable within certain limits, since it brings more 

 of relatively insoluble plant nutrients into solution as the acid is 

 formed, but where large amounts of ammoniacal substances gain 



10 20 30 40 50 60 70 80 90 100 

 Percentage of Oxygen 



Fig. 61. — Influence of aeration upon nitrate formation (from Plummer). 



entrance to soils, the increase in acidity becomes sufficiently great 

 to justify the addition of lime to overcome the acid condition. 



Influence of Soil Aeration upon Nitrate Formation. — 

 Since the processes of nitrification are autotrophic, considerable 

 quantities of carbon dioxide are required. Since the transforma- 

 tions require large amounts of oxygen they must take place under 

 aerobic conditions. Certain definite concentrations of these 

 gases, higher than those that occur in the atmosphere, are most 

 favorable to nitrification. Under soil conditions, oxygen is the 

 only one of the two gases which may not be present in sufficient 

 abundance. The decomposition processes of heterotrophic organ- 

 isms are responsible for the liberation of large amounts of carbon 

 dioxide, and the mixture of gases in the soil is always much richer 

 in carbon dioxide than the atmosphere. The rate of diffusion of 



