SOIL PROTOZOA 327 



ing factors in the development of protozoa in the soil. According to 

 Cutler, 66 there is an intimate mechanical association between the proto- 

 zoa and the soil particles which depends on a mutual surface action, and 

 the capacity of various substances, such as sand, soil and clay, for re- 

 taining these organisms is specific and constant. The difficulty of seeing 

 living protozoa in the soil is, therefore, due to the fact that the organisms 

 rigidly adhere to the soil particles, and, up to a certain limit, they can 

 be completely removed from a suspension by shaking for a few minutes 

 with soil. Only in exceptional cases can organisms be dislodged suffici- 

 ently as to be recognized under the microscope. This is probably the 

 reason why some investigators failed to observe living protozoa in the 

 soil. It is beyond any doubt that protozoa lead a trophic existence in 

 the soil; this is especially true of the small flagellates. Even if most of 

 them encyst after a continuous dry period, the first rain that brings the 

 moisture content of the soil to optimum will lead to a rapid excystation 

 of the protozoa and to a longer or shorter period of activity. 



A method was described by which it is possible to estimate the number 

 of protozoa present in the soil both in a trophic and in a cyst condition. 58 

 The total number is first determined by the dilution method. A fresh 

 portion of the soil is then treated with 2 per cent HC1 (specific gravity 

 1.15) over night, whereby all active forms are killed. A second count 

 by the dilution method gives the number of protozoa present in the soil 

 in the form of cysts. The difference between the first and second counts 

 gives the number of active protozoa. In this connection reference may 

 be made to the work of Cunningham and Lohnis, 59 who found 60°C. 

 to be the thermal death point of active protozoa (44° for flagellates, 48° 

 for amoebae and 54° for ciliates), while 72°C. was found to kill the cysts. 

 However, the temperature destructive to protozoa in the soil would be 

 different from that of culture solutions. A temperature of 58°C. was 

 used 60 for distinguishing between cysts and active forms, bat it was soon 

 found that many cysts are also killed at that temperature. Drying 

 reduced the number of protozoa, particularly ciliates and flagellates, 

 amoebae preferring a somewhat drier soil. 



The protozoa are widely distributed in the soil, comprising ciliates, 

 flagellates, amoebae and thecamoebae. Some investigators reported 



68 Cutler, D. W. A method for estimating the number of active protozoa in 

 the soil. Jour. Agr. Sci., 10: 135-143. 1920. 



69 Cunningham and Lohnis, 1914 (p. 320). 



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

 1915. 



