SOIL MICROBIOLOGICAL EQUILIBRIUM 761 



matter is decomposed under these conditions, waxes and slimes are 

 formed which coat the soil particle tending to block up its pores, thus 

 interfering with aeration and drainage and protecting the organic 

 particles from the further action of the bacteria. Greig-Smith admitted 

 that protozoa may play a part in checking the multiplication of bacteria 

 in the soil, but they were not believed to be alone responsible for this 

 limitation. These results on the formation of a bacteriotoxin in the 

 soil were not confirmed by other investigators. 71 However, certain 

 metabolic products are formed by at least some soil microorganisms, 

 which are either toxic to themselves or to other organisms. The 

 removal, destruction or modification of these products may bring 

 about an increased activity of the microorganisms concerned. 



Treatment of soil by heat or antiseptics results in a number of very 

 complex processes, which cannot be explained by a protozoan theory, 

 toxin theory, presence of soil waxes, etc., although all of these may 

 play a certain part in limiting bacterial development. A series of 

 changes are brought about which lead to an increase in the soluble 

 soil organic matter, 72 and to a modification of the concentration of the 

 soil solution and of the soil reaction. The physical condition (per- 

 meability, capillarity, cohesion, surface tension) of the soil is also 

 modified by the treatment, particularly the colloidal properties of the 

 soil. 72a Quantities of toluol and CS2, insufficient to modify the number 

 and types of protozoa in the soil, were found 73 to exert a stimulative 

 effect upon bacterial activities in the soil. 



Destruction of selective groups of organisms. Different soil organisms 

 are not affected alike by disinfectants. 74 Some are injured but not 

 completely destroyed. The degree of injury also depends upon the 

 concentration and length of action of the disinfectant, on the soil 

 moisture and aeration conditions. The gelatin liquefying bacteria 

 (mostly spore-formers) are not affected at all or to only a very limited 

 extent, while the actinomyces may be considerably reduced ; the same 

 is true of other aerobic, especially non-spore-forming bacteria. When 



71 Hutchinson, H. B., and Thaysen, A. C. The non-persistence of bacterio- 

 toxins in soil. Jour. Agr. Sci., 9: 43-62. 1918. 



72 Pickering, 1910, (p. 748). 



72a Taylor, E. McK., and Burns, A. C. The effect of the summer fallow upon 

 soil protozoa in Egypt. Egypt. Min. Agr. Tech. Sci. Serv. Bui. 52. 1924. 



11 Gainey, P. L. The effect of toluol and carbon bisulfide upon the micro- 

 flora and fauna of the soil. Mo. Bot. Gard. Ann. Rpt., 23: 147-169. 1912. 



74 Hiltner and Stormer, 1903 (p. 12). 



