194 H. JOEL CONN 



Spore-bearers, but no appreciable multiplication of spore-formers ; 

 nor was there any decrease large enough to be detected in the 

 number of actual spores. Meanwhile the odor of the soil was 

 enough to show that ammonification was vigorous. It is per- 

 fectly possible that a repetition of this test might yield different 

 results; but evidently this experiment did not furnish the right 

 conditions for the growth of the spore-forming bacteria. Also 

 it is plain that ammonification can take place without them. 



These results leave our knowledge as to the significance of 

 spore-forming soil bacteria in a rather unsettled state. It has 

 been quite generally taken for granted in the past that they 

 are active in soil and of great importance. Perhaps their strik- 

 ing appearance in plate culture has led to the assumption that 

 they could grow equally vigorously in soil. Yet they com- 

 prise but a small part of the soil flora, and even at that they do 

 not seem to be present in vegetative form under normal con- 

 ditions. Spores are generally regarded as inert. 



Never the less these spore-forming bacteria of the soil do not 

 decrease in numbers, and spores cannot live forever. Their 

 occurrence in soil cannot be due to accidental contamination, 

 or their numbers would not be so constant. If it is true, as 

 these results indicate, that they are of practically no importance 

 under normal field conditions, it becomes a matter of much 

 interest to learn under what conditions they can become active 

 and multiply. 



SUMMARY 



1. The number of spore-forming bacteria in soil is relatively 

 constant and is about the same in all the soils studied. Three 

 of the spore-forming bacteria aways present in soil — B. rnycoides, 

 B. cereus, and B. megatherium — were selected for the purpose 

 of comparison, because their colonies on gelatin plates are quite 

 readily distinguishable. The total number of these three or- 

 ganisms, as determined by means of gelatin plates, proved to 

 be between 400,000 and 1,500,000 per gram in the soils studied. 

 They always comprised less than 10 per cent and usually less 

 than 5 per cent of all the colonies developing on gelatin. 



