Relation to Factor Limiting Bacterial Activity in Soil. 455 



content of the T. + 5 per cent, untreated soil is due to protozoal activity it 

 must be the 500 amoebae and about 10 ciliates per gramme which are respon- 

 sible for it. It seems to me highly improbable that this is the true explanation 

 when we consider the enormously larger numbers of amcebse and ciliates 

 present in the T. + Amoebae and T. + Ciliates soils, where the protozoa 

 obviously have not effected a limiting action on the bacterial contents of 

 their respective soils. 



It is clear, however, that some factor has been added to the toluenecl soil 

 in the 5 per cent, of untreated which acts as a check on bacterial growth. I 

 cannot find support in these results, however, for the assumption that this 

 limiting factor is the protozoa. I would suggest that the influences checking 

 the growth of bacteria are connected with some other property of the added 

 soil than its contained protozoa. 



General Discussion. 



The introduction of large numbers of bacteria into the samples of soil along 

 with the added protozoa must be a source of disturbance to the bacterial flora, 

 and for this reason the experiments dealt with above cannot be considered as 

 showing a clear issue between protozoa on the one hand and bacteria on the 

 other. 



I sought to reduce this source of error to a minimum, however, by the 

 continuation of the experiments over a long time, thus allowing the disturbed 

 bacterial floras to settle down so that any influence of the protozoa should be 

 judged after this steady point had been reached, i.e. after about 160 days in 

 both the 1846 and the floosfield soil. 



In order that the protozoa should have conditions, as near as I could bring 

 them about, favourable to excystation I partially air-dried all the soils after 

 they were inoculated. 



In this way I hoped to meet the criticism which might be brought against 

 the experiments that the protozoa had failed to function. Moreover the soils 

 were all kept under conditions of temperature, water-content and aeration 

 exactly comparable with those under which Russell and Hutchinson kept 

 their soils. If protozoa therefore could act as they supposed them to do in 

 their soils they had every chance of doing so in my soils. 



Another point calls for some comment. Martin and Lewin* have found 

 evidence of an abundant fauna of active amoebae and flagellates devouring 

 bacteria in certain soils which they have tested. They suggest that these 

 have probably some influence on bacterial numbers and thus on soil fertility. 

 Their results are very important direct evidence of the activity of protozoa in 



* "Some Notes on Soil Protozoa," 'Phil. Trans.,' B, vol. 205, pp. 77-94 (1914). 



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