THE SOIL AS A CULTURE MEDIUM 633 



optimum moisture conditions tend to bring greater amounts of material 

 into solution than are found under saturated water conditions; with 

 lower temperatures, the opposite effect was observed. Below two 

 feet, the muck soils are very inactive, the ability of producing soluble 

 materials decreasing regularly from the surface to the water level, in- 

 dicating that aeration greatly influences this process. 



A definite correlation was found 38 between bacterial activities in the 

 soil and the thickness of the moisture film. The optimum thickness of 

 the film in the case of Bac. mycoides was found to be between 20 and 40 

 microns. This film was obtained in sand of 1 mm. diameter at a 

 moisture content of about 10 per cent. In arable soils with a grain 

 size not more than 0.1 mm., it would require more than 50 per cent of 

 moisture to produce the optimum film thickness. 



Lowering of the freezing point and conductivity of the soil 39 can 

 also be used as indices of changes in the composition of the soil solu- 

 tion. Pantanelli 40 suggested the use of electrolytic conductivity of 

 soils for studying the course of solubilization of soil constituents by 

 microorganisms; this was found, in most cases, to vary with the bacterial 

 content of the soil. 



It is doubtful whether the actual concentration of the soil solution 

 can be determined by the electrical bridge, since in most of these 

 measurements an excess of water is added. 



Soil reaction and microbiological activities. The nature and quantity 

 of substances present in the soil in a colloidal condition, which act 

 as buffers or are capable of combining with acids and bases, the nature 

 and amount of bases present in the soil either in an absorbed condi- 

 tion or in the form of carbonates, influence the reaction of the soil, the 

 medium in which the microorganisms live and act. Soil acidity may be 

 due either to free organic and inorganic acids, which liberate hydrogen- 

 ions, or to a non-saturation of the soil organic and inorganic complexes 

 with bases, which results in a replacement of the base by hydrogen. 41 



38 Rahn, 1913 (p. 623). 



39 Davis, R. O. E., and Bryan, H. The electrical bridge for the determination 

 of soluble salts in soils. U. S. Dept. Agr. Bur. Soils, Bui. 61. 1910; Konig, J., 

 Hasenbiiumer, J., and Glenk, K. Uber die Anwendung der Dialyse und die 

 Bestimmung der Oxydationskraft fur die Beurteilung des Bodens. Landw. Vers. 

 Sta., 80: 491-534. 1913. 



40 Pantanelli, E. Elektrolytische Bestimmung der biologischen Bodenauf- 

 schlieszung. Centrbl. Bakt. II, 42: 439-443. 1915. 



41 Gedroiz, K. K. Soils unsaturated with bases. Method of determining in 

 soils the hydrogen present in an absorbed condition. Soil requirement of lime 



