60 THE SOIL POPULATION AND ITS DISTRIBUTION 



tinued application of ammonium sulfate without calcium carbon- 

 ate, increase the acidity of the soil to such a degree as to greatly 

 lower the numbers of bacteria. On the other hand, the applica- 

 tion of inorganic fertilizers considerably increases the microbial 

 population through both direct and indirect factors. The effect 

 may be direct, by satisfying the deficiencies of certain elements 

 required for microbial growth. It may be indirect, by affecting 

 the physical condition of the soil, by modifying the soil solution, 

 creating a greater or less solvent action, or by changing the soil 

 reaction. 



The environmental factors affecting the abundance of bacteria 

 in soil as well as their distribution at different depths are numerous, 

 most important being the factors of organic matter, moisture, 

 reaction, temperature, and air penetration. 



Influence of Organic Matter. — Additions of organic 

 materials to the soil probably exert more pronounced effects upon 

 the microbial population than any other treatment, especially 

 under humid conditions, but the microbial response is different with 

 different substances, depending upon the relative ease with which 

 they are decomposed and the types of organisms which are able 

 to attack them. The effects of introduction of plant or animal 

 residues are pronounced upon the physical condition of the soil, 

 by binding loose sandy soil more closely together, and by bringing 

 heavy clay soil to a more porous state. These effects are in general 

 desirable for the development of both plants and microorganisms. 

 Soil conditions resulting from the introduction of plant substances 

 will generally be associated with extensive development of the 

 microbial inhabitants. Soils not receiving any fresh additions of 

 organic matter become progressively depopulated, irrespective of 

 other soil conditions. 



What course the microbial changes follow will be largely deter- 

 mined by the nature of the organic materials added. Tree prod- 

 ucts are quite resistant to decomposition, and produce marked 

 effects on the organisms only over a considerable period of time. 

 The lower carbohydrates and protein substances produce effects 

 which are very quickly apparent but which persist for compara- 

 tively short periods. Owing principally to the chemical composi- 

 tion and abundance or residues which they may leave in the soil, 

 different plants affect the development of soil organisms differ- 

 ently, as will be shown in Chapter IV. 



