SOIL MICROBIOLOGICAL EQUILIBRIUM 757 



available for bacterial action ; this may be due either to the removal of the fats, 

 to greater solubility of carbohydrates, nitrogen compounds, or phosphates; to 

 the killing of worms, nematodes, protozoa, algae, fungi, which are then de- 

 composed by the bacteria; or to all these combined. 



3. Microbiological balance or equilibrium. Partial sterilization produces a 

 change in the balance between the bacterial flora and the other groups of organ- 

 isms, such as the fungi and actinomyces. 



4. The protozoa are responsible for the limitation of bacteria in the soil; their 

 removal by partial sterilization leads to increased bacterial development, greater 

 decomposition of organic matter and, therefore, improved soil fertility. 



5. Toxin theory. The soil is believed to contain toxins of biological origin. 

 Partial sterilization of soil leads to their destruction, hence to improved fer- 

 tility. 



6. Destruction of fungi and bacteria which are causative agents of plant dis- 

 eases. The repeated growth, year after year, of the same crop leads to an accumu- 

 lation of fungi and insects injurious to the particular crop. Partial sterilization 

 of soil brings about the destruction of these pests. 



7. Increased nitrogen-fixation. Partial sterilization of soil is believed to render 

 a greater amount of energy available to the nitrogen-fixing bacteria. Koch, 

 however, maintains that nitrogen fixation by bacteria is decreased by partial 

 sterilization. 



Only three of these theories need be discussed at greater length. 



Protozoan theory. The "protozoan theory of soil fertility" advanced 

 by Russell and Hutchinson 63 has met with severe criticisms. Accord- 

 ing to this theory, the number of bacteria found in the soil, at any given 

 time, is not merely a function of environmental soil conditions, but 

 depends on the interrelationship between the bacteria and the pro- 

 tozoa; partial sterilization does not bring about an improvement in 

 the bacterial flora but makes the soil a better medium for the growth 

 of bacteria, by eliminating the detrimental factor. 



It was suggested 64 that the destruction of spores of disease producing 

 fungi and bacteria have more to do with the final increase in produc- 

 tiveness of heated soils than either the destruction of bacteriarloving 

 protozoa or the increase in soluble plant food. The increase in ammonia 

 formation in partially sterilized soils was believed to be due to fungi. 65 



"Russell and Hutchinson, 1909-1912 (p. 311). 



64 Bolley, H. L. Interpretation of results in experiments upon cereal cropping 

 methods after soil sterilization. Science, 33: 229-234. 1911; also 32: 529-541. 

 1910; 38: 48-50, 249-259. 1913; N. D. Agr. Exp. Sta. Bui. 107. 1913; Jachs- 

 chewski, A. On the causes which determine infertility of the soil and loss of 

 crops. Khoziastvo, Z. 1912, 1103-1108 (Intern. Inst. Agr. Bui. Bur. Agr. Inst. 

 PI. Dis., 3: 2528). 



65 Kopeloff, N., and Coleman, D. A review of investigations in soil protozoa 

 and soil sterilization. Soil Sci., 3: 197-269. 1917. 



