366 MICROBIOLOGY OF SOIL 



isms are : ( i ) A high degree of moisture, closely approximating saturation ; 

 (2) an abundant supply of organic matter; (3) moderate temperature. 

 The thermal death point of active forms has been found by Goodey 

 to be 40 to 50, and for the cyst forms of the same organisms about 

 72. The optimum temperature for most forms is about 22. 

 Encystment of protozoa occurs within wide limits in an alkaline medium 

 containing up to .18 per cent NaOH, and in the presence of an acidity 

 represented by .09 per cent HC1. 



Protozoa are found in many greenhouse soils, due no doubt to the 

 fact that they contain a high degree .of moisture and organic matter. 

 However, in dealing with field soils some investigators have failed to 

 isolate active forms of protozoa, whereas others record the presence of 

 large numbers of these organisms. Their distribution appears to 

 parallel that of bacteria, namely, the greatest number of protozoa occurs 

 within the upper 100 mm. (4 inches) of soil, with a decrease down to 

 300 mm. (12 inches), which represents the lower limit of their activity. 



As regards the occurrence of the various groups of soil protozoa, 

 flagellates are found to be dominant over ciliates and amoebae. G. P. 

 Koch has found that the development of soil protozoa in artificial 

 culture solutions varies (i) with the kind of media employed; (2) the 

 quantity of soil used for inoculation; (3) drying of the soil; (4) different 

 kinds of soil and different soils of the same kind; (5) the temperature 

 of incubation. 



While it is generally accepted that protozoa feed upon bacteria, 

 until the relation that obtains between the various types of protozoa 

 and the different species of soil bacteria has been more fully investigated 

 the direct effect of protozoa upon bacteria must remain, to a degree, 

 indeterminate. 



Soil sterilization has had a practical application in eliminating 

 various diseases in greenhouses and infested fields. Partial steriliza- 

 tion as employed by Russell and Hutchinson while not so drastic, 

 involves serious changes in the soil, which might be considered in much 

 the same light as the phenomena attending complete sterilization 

 by means of heat and antiseptics. It is an established fact that 

 sterilization is responsible for increased plant growth, and to explain 

 this phenomenon the following theories have been advanced: 



i. R. Koch's theory of direct stimulation to plant growth a 

 physiological effect of the sterilizing agency. 



