CHAPTER XIII 

 Soil Protozoa 



The protozoa form numerically the most abundant group of the 

 animal population of the soil. It has been known since the work of 

 Ehrenberg that the soil harbors a number of different protozoa, but the 

 investigation of this subject has been greatly stimulated by the con- 

 tributions of Russell and Hutchinson, 1 who suggested that the limi- 

 tation of bacterial activities in the soil is due to a factor of biological 

 origin, probably the protozoa; this factor is less resistant than the soil 

 bacteria and is readily destroyed by the treatment of soil with heat or 

 disinfectants. This brings about a rapid development of the bacteria and 

 the liberation of the combined nitrogen in a form available for plant 

 growth, namely ammonia. The inference was that the soil protozoa, 

 using the bacteria as food, limit the bacterial activities in the soil and, as 

 a result of that, also limit soil fertility. On treating the soil with heat 

 and disinfectants, the protozoa are destroyed, while the bacteria remain 

 alive and begin to multiply very rapidly, their increased activities 

 resulting in the liberation of the nitrogen and an increase of plant growth. 



Additional plausibility was given to this theory by suggestions made 

 a few years previously and supported by a number of observations, 2 

 that a similar function is performed by protozoa during the self-purifica- 

 tion of water (i.e., the disappearance of pathogenic bacteria during 

 storage). In fact, in view of the close similarity between many of the 

 biochemical problems of water and sewage purification and those of the 

 soil, the extensive investigations that have been made in the former 

 fields are well worth careful consideration by the soil microbiologist. 3 



General ?norphology of protozoa. A detailed study of the morphology, 



1 Russell, E. J., and Hutchinson, H. B. The effect of partial sterilization of 

 soil on the production of plant food. Jour. Agr. Sci., 3: 111-144. 1909; 5: 

 152-221. 1913; Russell, E. J. Soil protozoa and soil bacteria. Proc. Roy. 

 Soc. B., 89: 76-82. 1915. 



2 Fehrs. Die Beeinflussung der Lebensdauer von Krankheitskeimen im 

 Wasser durch Protozoen. Hyg. Rundsch., 16: 113-121. 1906 



8 Buswell and Long. Activated sludge experiments. Illinois State water 

 supply, Bui. 18, 1923. 



311 



