Physical Conditions Affecting Nitrification 177 



bacteria will develop only where there is a supply of 

 air and of nitrogenous material capable of being con- 

 verted into nitrate. 



It seems, therefore, that in the more open sandy 

 loams, or sandy soils, where the air circulates more 

 freely at greater depths, the development of the nitri- 

 fying bacteria is favored somewhat in the deeper layers 

 of the soil. In regard to the material capable of nitri- 

 fication, it is true that in the soil this consists of roots, 

 stubble, and other plant residues accumulated mostly 

 in the surface soil. For this reason, the nitrifying 

 bacteria will grow almost exclusively in the place where 

 these accumulations are concentrated, that is, in the 

 surface soil. Indeed, it has been shown, experimentally, 

 that samples of soil from depths greater than three 

 feet usually fail to cause nitrification in nitrifiable 

 materials. 



Soil bacteriologists are now endeavoring to compare 

 the nitrifying power of different soils under identical 

 conditions, and to study the various influences that 

 encourage or discourage the bacteriological activities 

 in the soil. The fact has been recognized for some time 

 that not only are the nitrogenous materials nitrified at 

 a different rate in different soils, but that the order of 

 nitrification may be different. For instance, it has been 

 observed that sulfate of ammonia is usually changed 

 to nitrate more quickly than dried blood or cottonseed 

 meal. Nevertheless, soils are occasionally met with in 

 which the reverse seems to be true. According to the 

 views held at present, the organic nitrogen in dried 

 blood or cottonseed meal must be changed first to am- 



