THE GERM LIFE OF THE SOIL 



133 



ification and nitrification are to proceed rapidly, is an ade- 

 quate supply of air in the soil and this can only be secured 

 by thorough tillage. This is illustrated by an experiment 

 in which columns of soil eight inches in diameter and eight 

 inches high were removed from a field of clay loam and car- 

 ried to the greenhouse without disturbing the structure of 

 the soil as it existed in the field. At the same time vessels 

 of similar size were filled with soil dug from a spot near by. 

 These represented unaerated and aerated soils respectively, 

 because one had been undisturbed, while the .other had 

 been thoroughly exposed to the air. Both were kept at the 

 same temperature and moisture content in the greenhouse 

 but no plants were grown in them. The production of 

 nitrates was as follows : 



Table 30. 



Formation of Nitrates in Unaerated and in 

 Aerated Soil 



170. Effect of temperature on nitrate formation. — There 

 is a considerable range of temperature through which the 

 process of nitrate formation proceeds with more or less 

 intensity. Freezing stops the fermentation, but does not 

 kill the bacteria, whose activity is resumed when the tempera- 

 ture rises to about 40° F. and increases until a temperature 

 approaching 75° to 85° F. is reached, after which the in- 

 tensity gradually diminishes. At 110° F. and above, there 

 is little formation of nitrates. 



