138 EFFECT OF HEAT ON SOIL BACTERIA 



rather than a diminishing effect upon the total number of bacteria 

 present; (b) an application of such quantities of toluol and carbon 

 bisulphid does not have an appreciable effect upon the number of 

 types of protozoa present; (c) a very marked increase in yield may 

 be noted following such an application when no evident change 

 occurs in the total number of bacteria present." 



Greig-Smith's Bacteriotoxin Theory. A widely different theory 

 from any so far considered is that advanced by Greig-Smith. He 

 considers that when disinfectants are added to the soil their action 

 is two-fold : They kill the less resistant bacteria and dissolve from 

 the surface of the soil particles a waxy covering to which he has given 

 the name "agricere." The surviving bacteria, which he assumes 

 are the beneficial ones, are then able to function much more rapidly 

 on account of the exposure of the food due to the removal of the 

 "agricere." 



Moreover, he considers that there is a toxin contained in the soil 

 which is soluble in dilute saline, partially destroyed by heating to 

 94 C., and rapidly decayed in aqueous solution; boiling water 

 converts it into a nutrient, or by destroying the toxin enables the 

 nutrients dissolved in the saline to act. Thus, heating the soil 

 destroys the bacteria toxin, which accounts for enhanced fertility. 

 Bottomley and others also claim to have found soluble bacterio- 

 toxins in soils. Russell and Hutchinson, on the other hand, obtained 

 wholly negative results, and conclude that soluble bacteriotoxins 

 are not normal constituents of soils, but must represent unusual 

 conditions wherever they occur. Not only could no experimental 

 evidence of the existence of bacteriotoxins be obtained, but Russell 

 and Thaysen showed that the assumption of toxins leads to difficul- 

 ties. It is necessary to suppose that heating fresh soil for fifteen 

 minutes is sufficient to produce toxins but not to destroy them, 

 whereas heating for sixty minutes both produces and destroys them, 

 and in the case of air-dried soils fifteen minutes' heating causes their 

 decomposition. 



REFERENCES. 



Vorhees, E. B. and Lipman, J. G.: A Review of Investigations in Soils Bacteri- 

 ology (U. S. D. A. Off. Exp. Sta. Bui. 194). 



Lohnis: Handbuch der Landwirtschaftlichen Bakteriologie. 



Kopeloff, Nicholas and Coleman, D. A.: A Review of Investigations in Soil Pro- 

 tozoa and Soil Sterilization, Soil Science, 1917, iii, 197-269. 



Johnson, James: The Influence of Heated Soils on Seed Germination and Plant 

 Growth, Soil Science, 1919, vii, 1-103. 



