242 Bacteriological Department. [Bull. 161 



pressed as parts per million. In general, the amount of ni- 

 trites produced was in inverse proportion to the amounts of 

 ammonia produced. In general, the production of nitrites de- 

 creased as the depth of plowing increased. 



SUMMARY. 



The results of these preliminary experiments suggest the 

 following conclusions : 



1. Deep plowing (eight to ten inches) tends to increase the 

 number of soil bacteria in both sandy a^id silt soils. 



2. Deep plowing tends to increase bacterial activity. More 

 ammonia is produced. 



3. Deep plowing tends to decrease denitrification or the re- 

 duction of nitrates and the liberation of free nitrogen. 



4. The- volumetric method of quantitative bacteriological 

 soil analysis has the following possible advantages over the 

 gravimetric method: (a) It is more simple and convenient; 

 (6) there is less danger of contamination; (c) the results are 

 placed on a more accurate basis for comparison. The volu- 

 metric method can be used to advantage when comparative re- 

 sults are desired. 



5. Increased soil temperature increases bacterial activity. 



6. An excess of moisture in soil reduces the number of bac- 

 teria and is detrimental to bacterial activity. 



7. The maximum number of bacteria is found within the 

 fifth and sixth inches. Either side of this zone the numbers of 

 bacteria decrease. 



8. Due to certain conditions, different species of bacteria 

 are present in soil, at different times in predominating num- 

 bers. 



9. Bacterial life and activity seem to rise and fall with more 

 or less regularity. These periods of maximum and minimum 

 activity are to a certain extent independent of moisture and 

 temperature and are possibly due to the presence of bacterial 

 by-products. 



