SOIL PROTOZOA 279 



bacterial numbers rise and maintain a high level. Whenever 

 the detrimental factor is not put out of action, the protozoa are 

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

 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 amcebse 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 

 detrimental factor and the protozoa is not proof that the 

 protozoa constitute the limiting factor, but it affords sufficient 

 presumptive evidence to justify further examination. The 

 obvious test of adding cultures of protozoa to partially sterilised 

 soil was made, but no depression in bacterial numbers was 

 obtained ; instead there was sometimes a rise. But in view of 

 the history of investigations on malaria and other protozoan 

 diseases no great significance was attached to this early 

 failure. 



At, this stage the investigation was divided into two 

 parts : 



1. The study of the soil protozoa. 



2. The effects of the limiting factor on the biochemical 

 processes on the soil. 



No attempt had ever been made in any of the above 

 experiments to identify the protozoa, or even to ascertain 

 whether any particular form existed in the soil in .the trophic 

 state or as cysts. The variety of forms was considerable, and 

 it soon became evident that a definite protozoological survey of 

 the soil was required. 



This was accordingly put in hand. In order to give the 



