ACTIVITIES OF SOIL PEOTOZOA 281 



direct from a culture medium into partially sterilised soils, but 

 succeeded when introduced through the medium of some 

 untreated soil. In these circumstances the protozoa lived, and 

 the numbers of bacteria were reduced. The protozoa used in 

 these investigations were amoebae of the limax type, these 

 being the forms common in the soil. 



Thus it was proved that these protozoa lead an active life 

 in the soil, and that one result of their activity is to keep down 

 the numbers of bacteria. 



The further problem was put in hand of finding out how 

 numerous are the protozoa in the soil, and how this activity 

 varies with the different conditions obtaining in the field. 

 A dilution method is used somewhat similar to that adopted 

 for enumerating the soil bacteria. The investigation is still 

 only in its early stages, but already it is clear that amoebae 

 and flagellates are present in at least tens of thousands per 

 gram of soil, while ciliates can be found only in hundreds. 

 Some of the organisms appear to be new to science, and many 

 of them are of considerable interest.* 



The other part of the investigation consists in studying the 

 effects of these detrimental organisms on the process of plant- 

 food production in the soil. For this purpose it is not necessary 

 to find what the detrimental organisms are ; it is sufficient to 

 divide the soil organisms into two groups in their relations to 

 the processes of food production a useful group, and a 

 detrimental group. The latter are, more generally speaking, 

 more readily killed than the former. Conditions that are 

 harmful to active life in the soil tend, therefore, to reduce their 

 numbers and lead ultimately to an increased activity of the 

 useful bacteria. On the other hand, conditions favourable to 

 active life tend to keep up the detrimental organisms and 

 therefore to reduce the useful bacterial activity. It is thus 

 possible to account for a number of obscure and paradoxical 

 effects that have hitherto caused considerable perplexity. It 



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

 Flagellates," Roy. Soc. Proc., B. 88 (1914), 151-165. 



