Soil Protozoa and Soil Bacteria. 



81 



to finding out how numerous they are and how their activity varies with the 

 varying conditions obtaining in the soil. 



Considerable difficulties arise in attempting to enumerate the protozoan 

 fauna, and the line of attack adopted in our laboratory is to study the 

 natural history of the trophic forms in the soil. But the attempt at 

 •enumeration has been made recently by Cuningham* in Lohnis's laboratory, 

 with the following; results : 



4. Tentative minimum estimates made by a dilution method show that the 

 trophic forms are to be numbered at least in thousands per gramme of soil. 



( Bacteria commonly occur at the rate of 4 to 10 millions per gramme of soil). 



5. A protozoan fauna introduced under suitable precautions into partially 

 sterilised soil effected a considerable reduction in bacterial numbers. 



This is as far as the protozoological investigations have gone at present. 

 Cuningham's experiments are being repeated in our laboratory. For the rest, 

 the work is not in a sufficiently advanced state to justify any conclusions as 

 to the part played by the protozoa in the soil, but it has definitely revealed 

 the presence of a trophic fauna and shows that the forms are of considerable 

 interest.f 



We can now turn to the criticism urged by Goodey which in his view is 

 sufficiently cogent to upset these conclusions. 



Goodey inoculated cultures of various Colpoda (C. cucullus, G. maupasii, 

 C. steinii), a Vorticella ( V. microstoma), and an unidentified amoeba and a 

 flagellate, into partially sterilised soils free from protozoa, and made periodical 

 counts of the numbers of bacteria. The numbers fell off, but not to any 

 greater extent than in similar soils to which no additions of protozoa were 

 made. He therefore concludes that ciliates, amoebae, and flagellates cannot be 

 included in the biological factor limiting the number of bacteria in soil. 



Two objections can be urged against Goodey 's experiments. 



1. The organisms inoculated into the soil are in the main those which 

 figure largely in cultures made by adding soil to hay infusions. It has already 

 been shown, however (v. supra), that the culture fauna is distinct from the 

 trophic soil fauna. There is therefore no evidence that the normal soil fauna 

 was put back into the partially sterilised soil : on the contrary it apparently 

 was not. Nor is there evidence, except perhaps in one case, that the added 

 organisms survived at all. 



2. The difficulty of securing an adequate control is very great and does not 

 appear to have been overcome. When a soil is partially sterilised either by 



* ' Centr. Bakt. Par.,' August 1914, and ' Journ. Agric. Sci.,' vol. 7, pp. 49-74 (1915). 

 t See for example, the paper by Thornton and Smith, " On Certain Soil Flagellates," 

 •'Roy. Soc. Proc.,' B, vol. 88, pp. 151-165 (1914). 



