Protozoa in Relation to Soil Bacteria. 



313 



drawing conclusions even of first-rate importance from the results obtained 

 by their use, as has been done in the present investigation. 



Summary. 



1. A method is described whereby protozoa were successfully separated 

 from large numbers of bacteria and used for mass inoculation of a treated soil. 

 No positive evidence was obtained by the use of this method on the power of 

 the protozoa so treated and added to soil to function as a limiting factor to 

 bacterial activity. 



2. Further negative evidence on protozoal activity similar to that adduced 

 in an earlier communication is given by the use of mass inoculation of 

 protozoa into soil, whether the organisms are added in mixed culture with 

 bacteria or after separation from bacteria electrically, as in the case of ciliates, 

 or chemically, as in the case of encysted amceb?e. The curves for these 

 experiments are interpreted as showing the action of a nutrient limiting 

 factor. Suggestions are put forward which may furnish an explanation of 

 the failure of the protozoa so inoculated to function. 



3. Positive evidence in support of Russell and Hutchinson's hypothesis of 

 the limiting action of protozoa on bacterial increase is brought forward as the 

 result of experiments in which 5 per cent, of untreated soil is added to a 

 partially sterilised soil. The drop in bacterial numbers in mixtures of soils 

 of this sort after an initial rise is well known from Russell and Hutchinson's 

 results, but the further fact is revealed in the present investigations that 

 concomitantly with this bacterial decrease in numbers there is increase in 

 protozoal numbers. Another point brought out in the two experiments of this • 

 class is that a certain number of a small amoeba of the Umax group is present 

 in the soil at the time when the drop in bacterial numbers occurs. This 

 number is approximately 30,000 per gramme in each case, and the periodic 

 protozoal counts show that there has been active multiplication of this 

 amoeba leading up to the attainment of this number, in one soil from an 

 initial 600 ^amoebae per gramme and in the other from an initial 150 per 

 gramme. In the former case the 30,000 per gramme was reached in 43 days, 

 at which time the drop in bacterial numbers occurred, whereas in the 

 confirmatory experiment, which had an initial 150 amoebae per gramme, the 

 30,000 per gramme were present after 92 days, at which time also the 

 bacteria had dropped in numbers. There can be no doubt that the two 

 phenomena of protozoal increase and bacterial decrease, as shown in the two 

 experiments, are related as cause and effect. There is evidence also from 

 these experiments of the activity and multiplication of two other forms of 

 rhizopods, viz., Arnceba t'erricola a large thick pellicled form, and Gepliyrammba 



