FERTILITY IN SOILS 



363 



soil contained a variety of them ; and some half-dozen have 

 already been picked out by Dr. Hutchinson. Ordinary soil, 

 indeed, seems to have quite a large population of protozoa 

 which have been hitherto overlooked. All such organisms 

 existing in active forms must be severe competitors with the 

 soil bacteria by reason of their large size. Some, e.g. Colpoda 

 cucullus and Amceba nitrophila y are known to devour bacteria ; 

 Colpoda seems particularly active. Dr. Hutchinson has found 



c 



J 60 



140 



120 



>a ioo 



o 



W 



>. 



o 



B 



rt 



(J 



rt 



pa 



80 



60 



40 



20 



/ 



9 



i 



/ 



/ 



Time in days 



10 



20 



30 



40 



50 



Soil + toluene + 



aqueous extract 



containing 



bacteria from 



untreated soil 



Soil + toluene 



Soil + toluene + 



5 % untreated 



soil 



Untreated soil 



60 



Fig. 4. Effect of untreated soil, and of aqueous extract containing bacteria from 

 untreated soil, on the bacterial activity in soil treated with toluene. 



these organisms in every soil so far examined : their wide 

 distribution may also be inferred from their common occurrence 

 in hay infusions. The evidence at present available goes to 

 show that these large organisms constitute the factor or one 

 of the factors — for there may be others — limiting bacterial 

 activity and therefore fertility in ordinary untreated soils. 



Further studies of these organisms in their relation to the 

 changes in soils are now in progress. Experiments are also 

 in hand to discover whether these organisms or the inhibitory 



