132 



EXPERIMENT STATION. 



[Jiin. 



upon sterilized soil and found ti decrease in the number of bac- 

 teria and in the amount of annnonia present. This would show 

 that there is some limiting factor in the original soil that limits 

 bacterial action. They claim that this limiting factor is not 

 chemical but biological. 



In the experiments wdiich we have described and in those 

 which follow we are unable to comprehend how protozoan forms 

 play any role whatsoever in the decrease of bacteria. If this is 

 true this limiting factor must be a chemical or physical property 

 of the soil, and one on which sterilization has a marked effect. 



Protozoa as a Factor in the Bacterial Floka of Soils. 

 The remaining contents of the soil culture used in inoculating 

 decoctions in the experiments of Tables II. and III. were sub- 

 jected to a careful microscopic examination for various forms of 

 protozoa. Our labors w^ere without results, however, no protozoa 

 being found ; but it is quite possible that a few might have been 

 introduced at the time of inoculation of the decoctions. To avoid 

 any possibility of introducing protozoa into decoctions the experi- 

 ments shown in Table VIII. were made. 



Table VIII. — Shoicing Comparison of N'umber of Bacteria in Decoctions 

 of Sterilized and Unsterilizcd Loam and Subsoils. {Inocidations made 

 with Water Culture of B. subtilis.) 



Soil. 



NoMBEK OF Bacteria in 1 Cubic Centi- 

 meter OF Decoction. 



Experiment I. Experiment 2 



Average; 



Sterilized loam, 

 Unsterilized loam, . 

 Sterilized subsoil, 

 Unsterilizcd subsoil, 



5,952,960 

 127,484 

 279,840 



2,060,640 



4,913,800 

 Hi, 964 

 283,380 



2,901,244 



5,423,300 

 117,324 

 281,610 



2,480.942 



The data given in the above table show that Bacillus subtilis 

 multiply in great numbers in all the decoctions. About the same 

 relative number of bacteria were found here as in the decoctions 

 sho^vn in the experiments given in Tables II. and III. A greater 

 number of Bacillus subtilis were found in the sterilized loam 

 decoctions as compared with the unsterilizcd ; also a greater 



