OBSERVATIONS ON SOIL PKOTOZOA. 



By D. ward cutler. 

 (Rolhamsted Exferimental Station, Harfenden.) 



Introduction. 



The conclusion drawn by Russell and Hutchinson that the protozoa 

 resident in the soil are possibly detrimental to bacterial activity, and 

 that the beneficial results which are brought about by partial sterilisation 

 may in part be due to the killing of these organisms, has caused a great 

 interest to be taken in the soil protozoa, and as a consequence, a good 

 deal of literature has been produced by various observers. Much criticism, 

 however, has been directed against this hypothesis, some workers denying 

 that the protozoa have any reducing effect on bacterial numbers, others 

 asserting that these organisms are present in the soil normally as cysts, 

 and not in the active condition. 



The method of investigation, in the majority of cases, involved in- 

 oculating some medium with soil or suspension of soil, and incubating 

 for various periods of time. By this means it can be demonstrated that 

 numerous protozoa exist in the soil, but little or no idea is given as to 

 whether they are present as cysts or active forms — obviously a point of 

 great importance in its bearing upon partial sterilisation. 



Martin and Lewin(i), however, showed that there was undoubtedly 

 a tropluc fauna in the soil, but they were unable to arrive at any definite 

 conclusion as to the numbers per gram of these forms. Goodey(2), on 

 the other hand, concludes in the case of ciliates that cysts only are present. 



A systematic account of the v\ ork on soil protozoa is given by Kope- 

 loff and Coleman(3). 



There is great need therefore of a method for isolating the protozoa 

 directly from the soil within a short period of taking the sample : but it 

 should not involve the use of the incubator or any apparatus likely to 

 induce excystation of those forms which were present in the cystic state. 



The present investigation deals with two problems requiring solution 

 before a suitable method can be devised for directly counting the pro- 

 tozoa. Firstly, an efficient and direct method of counting the number 



