MICROORGANISMS AS A FACTOR IN SOIL FERTILITY 365 



made possible by the destruction of the protozoa (which have a lower 

 power of resistance to heat and antiseptics than bacteria) that normally 

 check the increase beyond a certain point. Under the conditions re- 

 corded a causal relationship obtains between an increase in numbers of 

 bacteria and the rate of ammonia production, which is considered to be 

 an index of fertility. 



This theory has been the basis of considerable investigation, much 

 of which has failed to corroborate the above conclusions. The fact 

 that there is an increase in bacterial numbers and in consequence, 

 enhanced fertility of the soil may not be due to the elimination of 

 protozoa but may rather be ascribed to such effects of the partial 

 sterilization process as (i) increase in available food for bacteria; 

 (2) rendering soil toxins insoluble; (3) destroying bacterio- toxins; 

 (4) acceleration of the biological processes. 



It has even been noted in some instances that partial sterilization 

 has been responsible for a decrease rather than increase in the produc- 

 tion of ammonia. Such considerations, among others, have been in- 

 strumental in stimulating investigation in another branch of soil fertility, 

 namely, soil protozoology. There has been difficulty in establishing 

 suitable methods and technic, as for example the development of 

 media favorable for the isolation and culture of soil protozoa, although 

 blood meal solution, hay infusion and soil extract have been used to 

 advantage. The organisms have been counted in the same manner as 

 bacteria, namely, by the dilution method, or by means of a standard 

 platinum loop. An adaptation of the apparatus used in the counting 

 of blood corpuscles has been successfully employed by Kopeloff, 

 Lint and Coleman. 



A study of the morphology and life history of soil protozoa reveals 

 the fact that encystment occurs under most conditions which are not 

 immediately favorable, as for example slight variations in moisture 

 content, or food. In point of fact this period of the protozoan life cycle 

 which is analogous to the spore-forming stage of bacteria forms the 

 basis for the question which arises as to the existence of protozoa, in 

 their trophic form, in field soils. Of the well-defined groups of pro- 

 tozoa (page 14), namely, flagellates, ciliates and amoebae, many types 

 have been described. Among those occurring frequently are: Colpoda 

 cucullus, Boda ovatus, Colpidium colpoda, Amceba terricola, Monas, etc. 

 The requirements for maximum development in the soil for these organ- 



