434 Observations on Soil Protozoa 



tions is employed varying only by small stages from one another an 

 estimate of the numbers of protozoa can be made within narrow limits. 



In Table III there are given the results obtained by the investigation 

 of 17 suspensions, differing from one another by the degree of concen- 

 tration. 



Results 1-8 inclusive, by the direct method, were all obtained by 

 counting 500 squares as described above, while the remaining results 

 were obtained by counting ten squares and taking the average for one 

 square. 



The close similarity of the results demonstrated that the direct 

 method was sufficiently accurate, and it was therefore employed for 

 the work described in the second part of this paper. In order to obtain 

 success with either method it is essential to secure uniform distribution 

 of the organisms in the fluid. Now any large particle of a soHd medium 

 added to the susjjension will render uniform distribution impossible by 

 providing a substratum on which many of the protozoa will aggregate. 



Therefore the best method of preparing the suspension is to add to 

 the fluid successive loopfuls of the culture, each loopful being thoroughly 

 emulsified against the side ofthe tube before entering the fluid. Even 

 distribution is secured by shaking or bj^ the successive use of a pipette. 



If the organisms are found clumped together in a suspension it 

 should be discarded. 



PART II. 



Factors concerned in the Relationship between the Protozoa 



AND THE Soil. 



As is well known it is practically impossible to find any evidence of 

 the presence of protozoa by direct examination of soil under the micro- 

 scope, even after the necessary addition of water is made. The dilution 

 method, nevertheless, demonstrates that these organisms are present 

 in the soil in at Iea.st tens of thousands per gram. In a few cases protozoa 

 have been observed by direct methods, but in numbers insignificant 

 compared with those which must have been actually present. 



Definite amounts of a suspension of amoebae and flagellate cysts 

 were added to equal weights of different substances, the surface areas 

 of whose particles varied one from the other, in order to test the action 

 of these substances on the organisms. 



The substances chosen were : 



(a) Coarse sand: ignited and treated with hydrochloric acid. 

 (6) Fine sand, treated as above. 



