456 Protozoa in Relation to Bacterial Activity in Soil. 



the soil, but this does not prove that the amoebae and flagellates are functioning 

 as the limiting factor in the sense in which that term is used by Russell and 

 Hutchinson. 



Before this can be shown to be true it will be necessary to correlate 

 protozoal activity with a decrease in the numbers of bacteria in a given soil 

 specially inoculated with protozoa. 



I have shown above (p. 446) that the presence of 10,000 amoebae per gramme 

 of soil is not sufficient to reduce the bacterial content of a soil to the level of 

 a similar soil containing no protozoa even though the soil be kept under 

 conditions of moisture, etc., favourable to the trophic existence of amoebae 

 and flagellates. 



Conclusions. 



The results of the experiments described above lead me to the conclusion 

 that the protozoa, including ciliates, amoebae, and flagellates, added to the 

 soil have not been able to act as a factor limiting bacterial activity in the soil. 



Inferentially, therefore, the ciliates, amoebae, and flagellates obtainable from 

 ordinary soil under cultural conditions do not function as the limiting factor. 



This is in accord with and extends the conclusion put forward in my earlier 

 paper,* viz., that the ciliated protozoa are present in soil only in an encysted 

 condition and cannot function, therefore, as the factor limiting bacterial 

 activity. 



There is evidence, however, in the case where a small quantity of untreated 

 soil is added to a partially sterilised soil that some factor comes into action 

 which keeps down the level of the bacterial content. The results obtained, 

 however, do not lend support to the hypothesis that it is the protozoa added 

 in the untreated soil which have this influence. 



It is shown in the case of Broadbalk 1865 soil, in which an abundant 

 protozoan fauna of amoebae and flagellates is present, and presumably active, 

 that the numbers of bacteria maintain a high level. This soil exhibits a 

 clear case of partial sterilisation being effected without the elimination of 

 protozoa. 



It is not within the province of this paper to attempt to rebut the very 

 weighty indirect evidence put forward by Ptussell and Hutchinson as to the 

 biological character of the detrimental factor. The results obtained, however, 

 warrant the conclusion that ciliates, amoebae, and flagellates cannot be included 

 in that biological factor. 



* ' Eoy. Soc. Proc.,' B, vol. 84, p. 165 (1911). 



