Relation to Factor Limiting Bacterial Activity in Soil. 445 



V. microstoma soils are, during the greater part of their course, so 

 closely akin that within the limits of experimental error they may be 

 considered identical. In one of them no protozoa are present, whereas 

 in the other amoebae and flagellates are present. If the latter are the 

 limiting factor they have failed to reduce the numbers of bacteria below 

 the level of the untreated soil containing no protozoa. 



Fig. 2 shows the curves for the three samples of soil inoculated with the 



too 



32 63 S2 124 153 161 232 2S4 324 363 383 419 486 SB 



DAYS 



Fig. 2. 



three different species of Colpoda, viz., Col. cucullus, Col. steinii, and 

 Col. matcpasii, together with the curve of the untreated soil. 



There is a marked similarity between all four curves ; all of them are of 

 the same type and show no very pronounced differences. On the whole, 

 the bacterial content of the three inoculated soils has remained higher 

 than that of the untreated soil, in spite of the fact that each contained 

 many hundreds of protozoa per gramme. They thus fail to indicate any 

 action by the protozoa of a limiting character on the bacterial population of 

 the soil. 



In the Col. cucullus soil there are, roughly, about 750 Col. cucullus and 

 1000 flagellates per gramme, the latter only being found in the later 

 determinations by the agar-plate method. The Col. steinii soil contains 

 about 100 Col. steinii and 1000 flagellates per gramme, whilst the Col. 

 maupasii soil contains about 1000 Col. maupasii, 200 amoeba?, and 100 

 flagellates per gramme.* 



One would have expected that had the protozoa been capable of acting 

 as a check on the growth of bacteria in the soil, they would have brought 

 their numbers to a level well below that of the bacterial content of the 

 untreated soil. Instead of this, however, we find after 519 days all three soils 

 showing a higher bacterial content than the untreated soil. 



* The numbers given for the protozoal counts are those obtained in the last 

 determination. 



The amoebae which occur now in these soils are due most probably to infection either 

 of the soil samples during the initial air-drying or of the mass cultures. 



