56 



Sfiidit'x an Soil ProtDzoti 



rather sensitive to the. presence nf mcinljcis of the other groups and 

 are probably to some extent suppressed by them in the cultures. The 

 ciliates arc always present in much smaller numbers. They are rarely 

 seen in dilutions exceeding 100. But the appearance of the various 

 groups in particular dilutions cannot be considered as giving any very 

 sure indication of the relative occurrence of protozoa in soils. 



To the question as to whether the protozoa lead an active life in 

 soil, it has been shown that the action of heat combined with the 

 dilution method does not give a definite answer. That question, 

 however, is answered in the affirmative by the results of experiments 

 which will now be discussed. 



(«) The Effect of Temperature. 



(i) On the number of protozoa in soils. For these experiments some 

 garden soil was passed through a 2 mm. sieve and placed in an ordinary 

 porous flowerpot. The moisture content was determined and adjusted 

 to 70 % of the water-holding capacity of the soil. It was kept at this 

 degree of moistness by watering with boiled water every day during 



Table 0. 



the course of the experiment. The total numbers of protozoa and 

 cysts growing on soil extract were determined by the method described 

 above, immediately after the first adjustment to 70 % w.h.c. The 

 pot was kept in succession for nine days at 5-7° C, for seven days 



