CHAPTER IX. 



SOIL AND SUBSOIL (Continued], 



ORGANISMS INFLUENCING SOIL CONDITIONS; BACTERIA, ETC. 

 MICRO-ORGANISMS OF THE SOIL. 



INTIMATELY correlated with the humus-substances of the 

 soil, as well as with its temporary contents of the carbohydrates 

 (cellulose, gums and sugars) from which humus is formed, 

 is the multitudinous flora of micro-organisms always present 

 and exercising important functions in connection with the 

 growth of the higher plants. Extended researches by Adametz, 

 Schloesing and Miintz, Miquel, Koch, Fraenkel, Winograd- 

 sky, Frank and many others, have thrown light upon the im- 

 mense numbers and great variety of minute organisms, es- 

 pecially of the bacterial group, present in soils, and upon their 

 distribution and activities in the same. It has been shown 

 that their numbers are greatest near (although usually not 

 at) the surface, decreasing rapidly downward and generally 

 disappearing wholly at depths between seven and eight feet; 

 the latter depth varying of course according to the nature 

 and porosity of the soil, and both depth and numbers being 

 greatest in summer. 



Numbers of Bacteria in Soils. Adametz found in one gram 

 of soil, 38,000 bacteria at the surface, 460,000 at ten inches 

 depth ; in a loam soil at the surface 500,000, at ten inches 464,- 

 ooo in each gram of earth. Of mould and similar fungous 

 germs there were only 40 to 50 in the same, 6 species being 

 true molds, while four were ferments, including the yeasts of 

 wine and beer. Fraenkel found in virgin land from near Pots- 

 dam, a sudden, marked decrease at depths of from three to 

 five feet; while in earth from inhabited places within the city of 

 Berlin, considerable numbers were still present at eight and 

 even ten feet, in some cases. 



In the researches lately made by Hohl at the bacteriological 



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