CHAPTER XX 



BACTEEIA OF THE SOIL— THE CYCLES OF THE ELEMENTS 



Importance of Bacteria in the Soil. — Were it not for 

 the presence and activities of the bacteria in the soil the 

 growth of higher plants would soon become impossible. By 

 their activity the soil bacteria are constantly making avail- 

 able for the higher plants the various substances necessary 

 for their growth. Growing crops need, among other ele- 

 ments, carbon, hydrogen, oxygen, nitrogen, sulphur, phos- 

 phorus, iron and calcium. Bacteria play only a minor role 

 in making available the carbon, hydrogen and oxygen, but 

 the other elements are practically all of them made avail- 

 able in suitable compounds by the action of the bacteria. 



Abundance and Distribution of Bacteria in the Soil. — 

 No entirely satisfactory method has been worked out for 

 determining the actual numbers of bacteria of all kinds in 

 a sample of soil. By plating and dilution methods, it is 

 possible to secure approximate counts of many kinds of 

 microorganisms, but some forms do not grow readily upon 

 ordinary culture media and may be overgrown or lost 

 entirely. The actual numbers which may be enumerated 

 vary from a few thousand to many million per gram of soil. 

 Of the microorganisms present the most abundant usually 

 are bacteria, next come molds, yeasts and the protozoa. The 

 species which are present, particularly in their comparative 

 proportion of individuals, differ greatly, depending upon 

 climatic conditions and the character of the soil. They are 

 most abundant near the surface, in general, and decrease 

 rapidly with increase in depth. In some soils which are 



238 



