STUDIES ON SOIL PROTOZOA 39 



activities of the micro-organisms and plants. Whether toxins 

 occur in soil is a disputed question, but Whitney and Cameron 

 (1904) (1910), Schreiner and Reed (1907 a) (1907 b), and Picker- 

 ing (1910 b) have demonstrated quite satisfactorily that plant 

 toxins do exist; while Bottomley (1911) and Greig-Smith (1911) 

 have submitted data which point to the existence of bacterio- 

 toxins in the soil. 



Others explain the beneficial action of heat by the changes 

 it produces in the soil compounds. Frank (1888), Pickering 

 (1910 a), Lyon and Bizzell (1910), Stone (1912) and others have 

 demonstrated an increase in the amount of soluble plant and 

 bacterial food in partially sterilized soil. Pfeffer and Franke 

 (1896) and Kriiger and Schneidewind (1899) have shown an 

 increased assimulation by plants in heated soils. Pickering 

 (1910 b), Wilson (1914) and others have further proven that 

 heat may also produce a toxic compound in the soil, the toxicity 

 increasing with an increase in the temperature used, so that soils 

 heated to very high temperatures have a detrimental effect 

 upon plant growth. 



The effect of protozoa on the bacterial flora of the soil 



There has been only a limited amount of work done upon the 

 action of the protozoa in soil aside from the indirect evidence 

 which has been acquired in the study of partial sterilization. 

 Russell and Hutchinson (1913) have failed in their attempts to 

 reduce the number of bacteria in partially sterilized soil by the 

 addition of mass cultures of protozoa; Lipman, Blair, Owen and 

 McLean (1910) in their work on ammonification in soil were 

 unable to detect any influence upon this process due to the pro- 

 tozoa; while Grieg-Smith (1912) obtained entirely negative re- 

 sults in his efforts to show that protozoa are detrimental to the 

 bacterial flora of the soil. Cunningham (1914), on the other 

 hand, claims to have demonstrated that protozoa do limit the 

 number of bacteria in soil. 



