THE SOIL AS A CULTURE MEDIUM 621 



reaction. They may also exert a direct influence upon the distribution 

 and nature of action of microorganisms in the soil. 



The organic matter of the soil gives to it a brown to black color. 

 The higher moisture holding capacity of the soil is due to a large extent 

 to a higher content of organic matter. Carbon is present in the soil 

 chiefly in the various organic substances collectively termed "humus" 

 and in the form of carbonates. The nitrogen is present in the soil in 

 the form of complex proteins and their derivatives 3 as well as other 

 complex nitrogenous compounds largely derived from the cells of micro- 

 organisms inhabiting the soil; only about one per cent of the nitrogen 

 of the soil is present there as ammonia and nitrates. The various 

 minerals required by the microorganisms for their activities are present 

 partly in the mineral framework, partly in the organic and inorganic 

 soil colloids, partly in the soil solution, and partly in a precipitated 

 form. 



The soil moisture 4 is combined with the colloidal materials, forming 

 the liquid phase of the complex, and is also in a free state. The amount 

 of moisture that a soil can hold varies with the size of these particles 

 and will be low in the coarse sandy soils and greater in the fine clay 

 and especially in the peat soils, which consist largely of organic matter 

 in a colloidal state. The minimum and optimum amounts of moisture 

 for the activities of microorganisms will depend upon the nature of the 

 soil and its colloidal content. Using the evolution of C0 2 from glucose 

 as an index of the activities of microorganisms, Van Suchtelen 5 found 

 that when a loam soil contained only 4.4 per cent moisture, the activi- 

 ties were at a standstill. When the moisture was increased to 6 per 

 cent, 19 mgm. of C0 2 were formed, and with 15 per cent moisture, 208 

 mgm. were formed. Similar results were obtained when the number 

 of bacteria developing on the plate was used as an index of microbio- 

 logical activities. 6 Optimum moisture conditions, for the activities of 

 many soil microorganisms, are reached when about half the pore space 



3 Jodidi, S. L. The chemical nature of the organic nitrogen in the soil. Iowa 

 Agr. Exp. Sta. Res. Bui. 1. 1911; Potter, R. S., and Snyder, R. S. Jour. Agr. 

 Res., 6: 61. 1916. 



4 Soil sampling as well as moisture determination is discussed in detail by 

 E. Heine. Die Praktische Bodenuntersuchung. Borntniger, Berlin, 1911; and 

 J. Stoklasa. Methoden zur biochemischen Untersuchung des Bodens. Abder- 

 hald. Handb. biochem. Arbeitsmeth. 5, Pt. 2, 843-910. 1925. 



* Van Suchtelen, 1910 (p. 684). 

 8 Engberding, 1909 (p. 14). 



