NUMBERS OF MICROORGANISMS 45 



Numbers of protozoa in the soil. The protozoa exist in the soil both 

 in an active stage and in a cyst condition. The methods employed 

 should give more or less accurate information concerning the numbers 

 of protozoa in the soil in both of these two stages. It is sometimes 

 essential to be able to count protozoa in culture solutions. This can 

 be done in four different ways. (1) The most common method con- 

 sists in placing a drop of a known volume of the protozoan suspension 

 under the microscope and counting the number of protozoa. The use 

 of a slimy colloidal solution for the purpose of reducing the motility of 

 protozoa, as, for example, semen psylii, semen cydoniae or gum tragacanth 

 has been suggested. 100 Two per cent gelatin solution may also be 

 employed for this purpose. (2) The use of a standard loop devised for 

 bacteria may also be employed 101 - 102 for counting of protozoa. (3) The 

 agar plate method may be used. 103 None of these three methods is very 

 suitable for the determination of the number of protozoa in the soil 

 itself, although the first two can be used for the examination of soil for 

 the presence of living protozoa. For an accurate determination of the 

 total number of protozoa in the soil, including both the active forms and 

 the cysts, the (4) dilution method is most appropriate. 104 ' 105 ' 106 The 

 method consists in diluting the soil with sterile tap water, then placing 

 1 cc. portions of the various dilutions into sterile media, incubating, 

 examining the cultures at periodic intervals (5, 12 and 20 days) and 

 determining the highest dilution at which growth takes place. The 

 number of protozoa is thus found to lie between this and the next 

 higher dilution at which growth did not take place. If, for example, 

 a dilution of 1:1000 gives growth, while 1:10,000 does not give any, 

 there are between 1000 and 10,000 protozoa in a gram of soil. The 



100 Statkewitsch, P. Zur Methodik der biologischen Untersuchungen liber die 

 Protisten. Arch. Protistenk. 5: 17-39. 1905. 



101 Muller, P. T. Uber eine neue, rasch arbeitende Methode der bakteri- 

 ologischen Wasseruntersuchung und ihre Anwendung auf die Prufung von Brun- 

 nen und Filterwerken. Arch. Hyg. 75: 189-223. 1912. 



102 Koch, G. P. Soil protozoa. Jour. Agr. Res. 4: 511-559. 1915;5:477-488. 

 1915; Soil Sci. 2: 163. 1916. 



103 Killer, J. Die Zahlung der Protozoen im Boden. Centrbl. Bakt. II, 37: 

 521-534. 1913. 



104 Rahn, O. Methode zur Schatzung der Anzahl von Protozoen im Boden. 

 Centrbl. Bakt. II, 36: 419-421. 1913. 



108 Cunningham, A. Studies on soil protozoa. Jour. Agr. Sci. 7: 49-74. 1915. 

 106 Sherman, J. M. The number and growth of protozoa in soil. Centrbl. 

 Bakt. II, 41: 625-630. 1914; also 1916 (p. 47). 



