624 



PRINCIPLES OF SOIL MICROBIOLOGY 



(2) rapidity of diffusion of nutrients. A high moisture content is 

 more favorable for bacteria, but it diminishes aeration; when the soil 

 is saturated with water, oxygen can penetrate only by solution, which 

 is too slow for the development of aerobic organisms. When the 

 moisture content of the soil is diminished, aeration increases. The 

 larger the inorganic soil particles the greater are the possibilities for 

 aeration. The smaller the inorganic soil particles, the greater is the 

 amount of moisture required for the activities of microorganisms and 

 the less is the soil aerated. 



TABLE 65 

 Comparative mineral composition of surface soil and of lithosphere 



COMPOUNDS 



Si0 2 



AI2O3 



Ti0 2 



Fe 2 3 



MnO 



Na 2 



K 2 



CaO 



MgO 



P 2 6 



S0 3 



N 2 



Organic matter 

 Carbon 



Aeration of the soil or the diffusion of the soil gases greatly influence 

 the activities of microorganisms. Oxygen is needed for oxidation proc- 

 esses, CO2 for the activities of autotrophic bacteria, and nitrogen for 

 the nitrogen-fixing organisms. 11 Oxidation favors the activities of ni- 

 trate-forming and nitrogen-fixing bacteria, fungi, actinomyces, and other 

 organisms which bring about the decomposition of organic matter. 

 The processes of decomposition may be so rapid in well aerated soils 

 that farmers must resort to rolling, marling and manuring in order to 

 maintain the proper supply of organic matter in the soil. In hepvy 

 fine-grained soils with insufficient aeration, the decomposition of or- 



11 Russell, E. J., and Appleyard, A. The composition of the soil atmosphere. 

 Jour. Agr. Sci., 7: 1-48. 1915; 8: 385^17. 1917. 



