STUDIES ON SOIL PROTOZOA 179 



moisture content of which is from 16 to 18 per cent) would be 

 obviously unwarranted. That a difference in the micro-fauna 

 does exist under various soil conditions is indicated by the fact 

 that Martin and Lewin have found amoebae to be the predominat- 

 ing types of protozoa in the soils they have studied, which are 

 very rich in organic matter, while the results reported liere, 

 as well as the data obtained by Cunningham on German soils, 

 indicate that the flagellates occur in greater numbers than do 

 the amoebae. It appears possible that in the rich soils and 

 green-house beds, which have been studied extensively at the 

 Rothamsted Station in connection with soil sickness, there 

 might be a phagocytic agent which is not active in ordinary soils. 

 This possibility, however, should not make us unmindful of 

 the fact that no direct evidence has as yet been produced which 

 indicates that such a factor exists in any cultivated soil. It 

 should also be remembered that the beneficial effects of partial 

 sterilization of soil — for the explanation of which the protozoan 

 theory was advanced — have been observed in all localities in 

 which the problem has been studied and in nearly all types of soil. 

 The question of the activities of the protozoa which lead an 

 active existence in soil is a problem upon which much work 

 could profitably be done. The active protozoa which occur 

 in soils in large numbers certainly have functions there, some 

 of which in fact may be very important. It is not desired to 

 give the impression that because the protozoa which have been 

 studisd do not exert a limiting action on the bacteria in soil 

 that it is thought that they do not ingest bacteria at all. Some 

 in all probabihty do not, while others (e.g., Monas) it would 

 appear undoubtedly do. Why active protozoa which feed upon 

 bacteria should not cause a measurable decrease in the number 

 of bacteria in soil is diflScult to explain. It would seem that the 

 excretory products of the protozoa which feed upon the soil 

 bacteria would increase the amount of available energy for the 

 rest of the bacteria so that a condition of metabiosis would be 

 established which might offset the antagonistic action of the 

 protozoa. This hypothesis does not appear unreasonable when 

 it is remembered that the chief limiting factor upon the bacteria 



