E. J. Kuss?:ll and H. B. Hutchinson 



141 



(2) Protozoa. Soil was inoculated into a yteriliscd 2 per cent, 

 infusion of hay, or, in other experiments, into a sterilised mixture of 

 2 per cent, hay infusion and.l"5 per cent, agar which was then poured 

 into Petri dishes. After a time large organisms were picked off from 

 the untreated soil cultures, including amoebae and ciliata. Some of 

 these were kindly examined by Professor S. J. Hickson and found to be 

 mainly Golpoda cucullus. The toluened soil cultures only contained very 

 small ciliated infusoria, the heated soil cultures contained none. The 

 extract of untreated soil generally contained small protozoa. From 

 the fact that Golpoda is a common hay infusion form we may infer 

 that it is widely distributed and capable of living and multiplying in 

 the soil. Its main food seems to be bacteria, and its action must 

 therefore be to keep down the number of bacteria and consequently 

 the amount of decomposition they effect. We may therefore conclude 

 that organisms of this class constitute a factor limiting bacterial activity 

 and fertility in ordinary soil. 



Even if certain protozoa and organisms like the algae have no 

 direct effect on bacteria they must be severe competitors in the 

 struggle for existence in so far as they are actually living in the soil. 

 The effect of this large organism is well shown in the following 

 experiment on the rate of decomposition of peptone by the extract 

 of toluened soil. Addition of an equal volume of the extract of 

 untreated soil reduced the rate of decomposition considerably and 

 addition of a mixed culture of the large organisms obtained from 

 untreated soil brought it down still more. The sterilised extract of 

 untreated soil had no effect. 



Table 14. Effect of large organisms on the rate of decomposition 

 of peptone by soil bacteria. 



