STUDIES ON SOIL PROTOZOA 41 



other hand, investigated field soils of Massachusetts and claimed 

 that the number of protozoa present would have to be increased 

 many fold in order to be considered a factor in the limitation of 

 bacterial numbers. Gainey (1912) studied the protozoa in 

 Missouri soils and likewise concluded that the number was not 

 sufficient to be a factor in soil fertiHty. Rahn (1913) by use of 

 the dilution method determined the number of protozoa in 

 Michigan soils. From the limited data submitted by him it 

 would appear that the soil contains about one thousand protozoa 

 per gram. Killer (1913) tested various methods for the deter- 

 mination of the soil protozoa but concluded that all of the 

 methods now known are of limited and doubtful value. Recently 

 Cunningham (1914) has reported studies of German soils which 

 he has demonstrated to contain quite large numbers of protozoa. 



Methods 



The dilution method, which has been employed to some 

 extent for estimating the number of protozoa in soil, has been 

 used in this work. It is obviously impossible to devise one 

 medium which will favor equally the development of all of the 

 various forms of unicellular animal organisms found in the soil. 

 In the preliminary work undertaken, various media were used 

 with the idea of finding which forms of protozoa are most abun- 

 dant in soil, and which media are best adapted to those par- 

 ticular forms. 



From observations on several media it was early observed that 

 the flagellates make up the greater portion of the protozoan 

 population of the soils which were studied. The survey of media 

 has not been very extensive, but a dilute soil extract has given 

 the most satisfactory results for determining the number of 

 flagellates. This medium is also well suited for the growth of 

 ciliates and amoebae. The use of soil extract seems appropriate 

 as it would appear that the orgam'sms favored by this medium 

 would be the ones most likely to be leading a trophic life in the 

 soil. One per cent hay extract, which has been used by most 

 investigators in the study of the soil protozoa, has not given as 



