CHAPTER XLIII 



BACTERIA OF THE SOIL 



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Bacteria of many kinds — both harmful and beneficial — live and grow 

 in the soil. They are often incredibly numerous, sometimes numbering 

 as high as a billion individuals in a gram of fertile garden soil. Some 

 of the relations of bacteria to decay and disease were described in the 

 preceding chapter. We shall now consider the importance of soil bac- 

 teria to cultivated plants and soil formation. Many soil bacteria are 

 important in the maintenance and improvement of fertility; some have 

 quite the opposite effect. 



Distribution. Most soil bacteria occur in the upper two to ten inches 

 of the soil where the supply of organic compounds. is usually greater. 

 In the surface inch of soil, bacteria are not so numerous because of the 

 destructive effects of light and of frequent desiccation. Relatively few 

 bacteria are found below a depth of two or three feet in humid regions. 

 In well-aerated soils, such as those of irrigated semi-deserts, bacteria 

 grow at greater depths. They are probably always present as far down 

 as the roots of plants extend. 



Nitrogen sources of green plants. Since nitrogen makes up such a large 

 part of the atmosphere of the earth, it was early assumed that green 

 plants obtain nitrogen from the air. The noted chemist, Liebig, was 

 convinced that this must be so, for he beliex ed that the ammonia of 

 the air was a sufficient source of nitrogen for plants. It required a long 

 time and many experiments to show that nitrates and ammonium salts 

 in the soil are the usual sources of nitrogen in plants. Although this fact 

 was surmised in 1838 by Boussingault of France, and in 1847 by Lawes 

 and Gilbert of the Rothamsted Experimental Farms in England, nearly 

 a half century elapsed before final proof was secured. The various 

 methods by which atmospheric nitrogen is combined in compounds 

 which green plants can utilize began to be understood just before the 

 beginning of the present century.^ 



^ For a discussion of the experiments of some of the most important contributors to our 

 knowledge of how green plants obtain nitrogen, consult any good text on soil microbiology. 



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