INFLUENCE OF PARTIAL STERILIZATION OF SOIL 227 



microbes, among which are the protozoa. It is quite generally 

 agreed that protozoa exist in soils in the trophic state and, since 

 many of them feed largely upon bacteria, they limit bacterial 

 development to a certain extent. One may assume that some 

 of the most important soil processes bearing upon soil fertility are 

 carried out by bacteria. If it were possible to destroy the protozoa, 

 the bacteria would begin to develop rapidly, reaching much larger 

 numbers than in the original soil. If bacteria are the causative 



20 40 



Time (D a y s> 



Fig. 81. — Influence of partial sterilization treatments upon numbers of bac- 

 teria, numbers of fungi, and accumulation of soluble nitrogen (ammonia plus 

 nitrate) in an acid soil rich in organic matter (after Waksman and Starkey). 



agents of the liberation of plant nutrients, the removal of such 

 organisms as the protozoa would favor bacterial development, 

 and also the processes which are of the greatest importance to 

 soil fertility. The treatment of soil with antiseptics and heat 

 brings about the destruction of protozoa, and this change has as its 

 direct result the favorable effect upon soil fertihty. 



From such considerations it was concluded that protozoa com- 

 prise the limiting factor to the development of bacteria in all soils 

 under various conditions; by their characteristic habits of feeding 

 they hold the bacteria in restraint and consequently limit soil 

 fertility. The removal of protozoa, particularly of the amoebae 

 and ciliates, permits a much enhanced development of the rest 



