632 



PRINCIPLES OF SOIL MICROBIOLOGY 



acids by microorganisms, absorption of inorganic elements by higher 

 plants, and many other changing conditions, cause an unceasing fluctua- 

 tion in the composition and concentration of the soil solution. The 

 osmotic pressure of the soil solution varies 34 from 0.1 to 1 atmosphere 

 in most soils to 4.5 to 16.5 atmospheres in soils with low moisture 

 content. In normal soils, the concentration of the soil solution ranges 

 between 0.1 and 1 atmosphere, depending on the rainfall, fertilization 

 and plant growth. 35 



The soil solution contains calcium nitrate and bicarbonates, some 

 organic matter, Na, Mg, Si, CI, S0 4 , small amounts of K, and traces 

 of ammonia and phosphates. The following constituents were found 

 in a soil solution obtained by the soil pressure method 36 (table 66). 



It is from this solution that the microorganisms obtain a large part 

 of their food and in it they leave their waste products. The colloidal 



TABLE 66 

 Composition of soil sohition 



nature of the soil has an important bearing upon the nature of the soil 

 solution. The rates of solubility, decomposition of organic matter, 

 carbon dioxide production, nitrification, absorption of the soluble con- 

 stituents by plants, microorganisms and soil particles, all have an 

 important bearing upon the nature and concentration of the soil solu- 

 tion. At a given moisture content, the rate of formation of soluble 

 material increases with the temperature. 37 At higher temperatures, 



34 Bouyoucos, G. J., and McCool, M. M. The freezing point method as a new 

 means of measuring the concentration of the soil solution directly in the soil. 

 Mich. Agr. Exp. Sta. Tech. Bui. 24. 1915; 27, 1916; 31. 1910. 



35 Hoagland, D. R. The freezing point method as an index of variations in 

 the soil solution due to season and crop growth. Jour. Agr. Res., 12: 369-395. 

 1918. 



36 Morgan, J. F. The soil solution obtained by the oil pressure method. Soil 

 Sci., 3: 531-545. 1917. 



37 McCool, M. M., and Whiting, L. C. Some studies on the rate of formation 

 of soluble substances in several organic soils. Soil Sci., 11: 233-247. 1921. 



