228 MODIFICATION OF THE SOIL POPULATION 



of the soil micro-flora. Irrespective of whether soils are dried, 

 heated, treated with lime, acids, volatile antiseptics, or non-vola- 

 tile poisons, according to this theory it is the greater susceptibility 

 of the protozoa, which are injured and partially or completely 

 eliminated, which results in the accelerated development of bac- 

 teria. 



The value of this explanation is greatly decreased by reason of 

 the fact that it does not explain all of the numerous changes asso- 

 ciated with partial sterilization. The increase in the number of 

 bacteria and in the decomposition of organic materials need not be, 

 and frequently is not, correlated with the presence or absence of 

 protozoa; identical changes may occur whether protozoa are or 

 are not eliminated. In addition to bacteria, other microbes are 

 capable of decomposing organic matter in the soil and of liberating 

 the constituents in forms available for plant consumption. Soil 

 fungi, actinomyces, and algae were left out of consideration in this 

 hypothesis. A reasonable consideration of the relationship of 

 fungi to certain of the soil reactions indicates that they are respon- 

 sible for some of the effects ascribed to protozoa. It has recently 

 become apparent that the destruction of some of the bacteria in 

 soils does not necessarily result in an injury to soil processes car- 

 ried out by these bacteria. One need not deny, however, that 

 protozoa play a part of some importance in soil processes. Their 

 great abundance seems adequate evidence to indicate that they are 

 an important element of the microscopic population of the soil. 



The favorable effects of partial sterilization treatments on soil 

 fertihty are probably due to a number of factors, including (1) 

 changes in the physical and chemical characteristics of the inor- 

 ganic and organic soil material; (2) destruction of numerous 

 organisms, including fungi, protozoa and other invertebrate 

 animals, bacteria, actinomyces, and algae; (3) direct stimulating 

 effects of disinfectants, in some instances. Different treatments 

 differ in their effects because they act upon different soil constit- 

 uents, both quahtatively and quantitatively. The relative abun- 

 dance of the different organisms in soil is not determined by one 

 group of organisms destroying another group, although this may 

 take place under exceptional circumstances, but it is chiefly a 

 question of competition for food materials. Under most soil con- 

 ditions small amounts of available nutrient salts and energy 

 sources are present at any one time. The partial sterilization 



