52 



Sti(dies on. Soil Protozoa 



regard to the regularity of the results. The ordinary soil extract 

 + -1 % K2HPO4 4- protozoa-free culture was therefore used in all 

 subsequent work and the last dilution in which all four parallels gave 

 a positive result after five days' incubation at 22° C. was adopted (quite 

 arbitrarily, of course) as the protozoal content of the material examined. 

 In all cases the results are given as numbers per gram of soil. 



AVith regard to the cause of the irregularity in the development in 

 the dilutions, it is most probably to be explained on the supposition 

 that the protozoa adhere very readily to the soil particles. It is 

 exceedingly likely that the amoebae in particular may be carried over 

 from dilution to dilution in this way. 



In the last paper, 58° C. was suggested as a temperature which 

 would kill off all active soil protozoa capable of development on .soil 

 extract and so allow of a distinction being drawn between active and 

 encysted forms. This temperature has been adopted in combination 

 with the dilution method already described. Two sets of dilutions 

 are generally made, the first with the untreated soil, the second with 

 the soil after heating to 58° C. The heating is generally carried out 

 in the 100 dilution. 



Table 2. 



As a result of further work it appears probable that a temperature 

 of 58° C. kills a number of the encysted protozoa in addition to the 

 active forms. Thus, for example, it has been found that pure cultures 

 of certain flagellate and ciliate cysts do not e.\cyst after being heated 

 to 58° C. and subsequently brought into fresh media. The results of 

 some experiments on the effect of drying on the protozoa may also be 

 cited in this connection. Two samples of soil were allowed to dry at 



