180 JAMES M. SHERMAN 



in the soil is the food supply. In liquid cultures, on the other 

 hand, the limiting factor is not the food supply but the accumu- 

 lation of detrimental by-products; the number of bacteria soon 

 reaches its maximum and then begins to decline gradually. 

 It can readily be seen that if predatory protozoa are added to 

 liquid cultures, in which the bacterial flora is in a comparatively 

 inactive condition due to the presence of harmful by-products, 

 a very striking reduction in bacterial numbers will be noted. 

 Whatever the effect of protozoa on bacteria in solutions may be 

 the results herein reported appear to indicate that imder ordi- 

 nary conditions they are not able to Hmit the bacterial flora 

 when acting in soil. 



Summary 



1. Determinations made by means of the dilution method 

 indicate that the normal fertile soil has a protozoan content 

 approximating 10,000 per gram. 



2. In the soils studied the flagellates were the predominating 

 type of protozoa and not the ciliates nor amoebae. 



3. Colpoda cucullus apipears to be the most widely distributed 

 cihate in soil and is occasionally found in numbers approxi- 

 mating 1,000 per gram. 



4. Certain of the soil flagellates are active in soils of normal, 

 and even subnormal, moisture contents. 



5. Tests made with the ciliates Colpoda cucullus, Balantio- 

 phorus elongatus and Oxytricha sp. show that these organisms are 

 not active under ordinary soil conditions. 



6. Colpoda cucullus is probably active whenever the moisture 

 content is much above normal, but not under ordinary conditions 

 of moisture. 



7. Active soil protozoa attain greater numbers when inoculated 

 into previously sterilized soil than in normal soil. 



8. Sterile soils when inoculated with normal soil and with 

 an artificial soil culture which is free of protozoa show a differ- 

 ence in the total number of bacteria as determined by the plate 

 culture method, due to a difference in the complexity of the two 

 flora. 



