SOIL PROTOZOA 337 



Cutler 92 purified cultures of an amoeba and a flagellate, so that they 

 contained only three species of common soil bacteria. These were 

 isolated free from protozoa, and suspensions were prepared of the bac- 

 teria alone and bacteria + protozoa, the latter in an encysted condition. 

 These suspensions were sprayed on by a fine nozzle upon 100 grams 

 of sterile soil in large sterile Petri dishes. The protozoa added per 1 

 gram of soil were: 25,000 for the Dimastigamoeba gruberi, 20,000 for 

 Cercomonas crassicauda and 1 1 to 13 millions of bacteria. At the end of 

 15 days, the numbers of protozoa were: 230,000 amoebae and 420,000 

 flagellates per gram. The bacteria, in the protozoa free culture, reached 

 a maximum of 214.4 millions in 6 days, then diminished to 169.2 mil- 

 lions in 15 days (21 per cent decrease); in the presence of the amoebae, 

 the maximum of bacteria (178.4 millions) was attained in 3 to 5 days, 

 then the decrease was more rapid, falling down to 72.8 millions in 15 

 days (59 per cent decrease) ; in the presence of flagellates the maximum 

 of 103 millions was reached in 7 days, dropping to 88 millions in 15 days 

 (14.5 per cent decrease). 



Goodey 93 has previously shown that amoebae of the limax group and 

 other larger forms can lead an active existence in the soil and exert a 

 depressing effect upon bacterial numbers. He suggested the probabil- 

 ity that a certain point must be reached in protozoan development be- 

 fore the depression in bacterial numbers is caused; this number seems to 

 be about 30,000 cells of Amoeba Umax per gram of soil. 



On the other hand, certain suggestions have been made that protozoa 

 can live in the absence of bacteria. Breal 94 (1896) believed that Colpoda 

 is active in the decomposition of plant constituents of the soil with the 

 production of ammonia. Other investigators 95,96 claimed that the pro- 

 tozoa play an important role in the decomposition of organic matter in 

 the soil. A number of protozoa are found 97 to be saprophytic in nature 

 and capable of obtaining their food by absorption. The same may be 

 true of the soil flagellates, as evidenced by the work of Thornton and 



92 Cutler, D. W. The action of protozoa on bacteria when inoculated into 

 sterile soil. Ann. Appl. Biol., 10: 137-141. 1923. 



93 Goodey, T. Further observations on protozoa in relation to soil bacteria. 

 Proc. Roy. Soc, 89: 297-314. 1916. 



94 Breal, E. Ann. Agron. 22: 362-375. 1896. 

 96 Muller, 1887 (p. 332), p. 15, 56, 167. 



96 Hiltner, L. Tiber neuere Ergebnisse und Probleme auf dem Gebiete der 

 landwirtschaftlichen Bakteriologie. Jahresb. Ver. angew. Bot. for 1907, 5: 

 200-222. 1908. 



97 Minchin, 1912 (p. 312). 



