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SCIENCE PROGRESS 



Attempts to bring the partially sterilised soil back to its 

 original condition by inoculation with organisms from the 

 untreated soil led to some remarkable results. Addition of 

 small quantities (0*5 per cent.) of untreated soil or of a filtered 

 aqueous extract of the soil containing bacteria, considerably 

 increased the amount of decomposition and the total number 

 of organisms. But addition of large quantities (5 per cent.) of 

 untreated soil gave no such increase ; instead the numbers were 

 reduced much more than if the added soil were a mere inert 

 diluting mass ; they were actually halved. The depressing 

 effect was not shown at once but only after the lapse of some 

 time. 



The result of the small inoculation is what might be ex- 

 pected from the enfeeblement of the organisms brought about 

 by partial sterilisation. But the effect of the addition of 5 per 

 cent, of untreated soil (fig. 4) can only be interpreted as show- 

 ing that something actively detrimental to bacteria has been 

 introduced. The conclusion may be drawn that the untreated 

 soil contains a factor limiting the development of bacteria, this 

 factor being put out of action by toluene or by heat. 



Since the reduction in the rate at which ammonia is pro- 

 duced is not at once operative when untreated soil is added 

 to soil treated with toluene, we may infer that the limiting 

 factor is probably biological ; this conclusion was strengthened 

 by other experiments. The factor does not appear to be 

 bacterial, since its effects do not show in the aqueous extract 

 of the soil ; and it does not come into evidence in propor- 

 tion as the bacteria develop. Search was therefore made for 

 larger organisms such as infusoria, amoebae and other protozoa. 

 None were found in the heated soil and only small ciliate 

 infusoria in the soil treated with toluene. But the untreated 



