STUDIES ON SOIL PROTOZOA 



may be carried out in a sterilized soil medium without causing an 

 apparent disturbance in the balance between these two classes 

 of organisms. 



TABLE IV 

 Approximale number of protozoa in reinoculated sterile soil 



A and B represent the duplicate samples of each pot. 

 + = presence of protozoa. 

 — no protozoa present. 



IV. THE BEHAVIOR OF BACTERIA IN SOILS CONTAINING PROTOZOA 

 AND FREE FROM PROTOZOA 



Methods 



In an effort to show the influence of the soil protozoa upon the 

 bacterial flora the following method was employed. Pots of 

 soil covered with, a layer of non-absorbent cotton between layers 

 of cheese cloth, to prevent reinfection, were sterilized in the 

 autoclave under fifteen pounds steam pressure for one hour. 

 Some of the pots were then inoculated with unsteriHzed soil in 

 order to introduce all of the biological factors peculiar to normal 

 soils, while the others were inoculated with a special soil con- 

 taining a varied mixture of soil bacteria but free of protozoa. 



This "protozoa-free soil" was made up of a mixed flora ob- 

 tained from several different soils by the isolation of as many 

 kinds of bacteria as could be obtained. For this purpose 

 several different soils were plated out on beef extract, casein, 

 Heyden and Ashby's agars and all of the colonies which devel- 

 oped transferred to sterile soil. Other portions of soil which had 

 been partially sterilized by heat sufficiently to kill all protozoa 



