288 SOIL CONDITIONS AND PLANT GROWTH 



the bacterial numbers rise and maintain a high level. When- 

 ever the detrimental factor is not put out of action the protozoa 

 are not killed. To these rules we have found no exception. 



Table LXXVIII. 



-Numbers of Bacteria and Ammonia Production in 

 Partially Sterilised Soils. 



C = ciliates. A = amoebae. M = monads. 



Further, intermediate effects are obtained when a series of 

 organic liquids of varying degrees of toxicity is used in 

 quantities gradually increasing from small ineffective up to 

 completely effective doses. The detrimental factor is not 

 completely suppressed but sets up again after a time, so that 

 the rise in bacterial numbers is not sustained. But the 

 parallelism with ciliates and amoebae is still preserved : they 

 are completely killed when the detrimental factor is completely 

 put out of action ; they are not completely killed, but only 

 suppressed to a greater or less degree, when the detrimental 

 factor is only partly put out of action. 



Now this parallelism between the properties of the detri- 

 mental factor and the protozoa is not proof that the protozoa 

 constitute the limiting factor, but it affords sufficient presump- 

 tive evidence to justify further examination. 



Soil Protozoa. 



The preceding experiments show that protozoa commonly 

 occur in soils but they give no information as to the kinds, 

 the probable numbers, or the state in which the protozoa exist, 

 whether active or simply as cysts. 



