626 PRINCIPLES OF SOIL MICROBIOLOGY 



Due to its large capacity for absorbing water, the soil organic matter 

 causes a swelling of the soil when wetted. The influences of successive 

 drying, moistening and freezing modify the nature of the organic mat- 

 ter and make it more available to the activities of microorganisms with 

 the production of larger amounts of C0 2 and nitrate. When a soil 

 is well cultivated, the formation of spaces allows a rapid development 

 of the fungi, actinomyces and aerobic heterotrophic bacteria. These 

 decompose the organic matter, use the available oxygen, and form 

 C0 2 as one of the chief products. The gases formed produce a certain 

 pressure which brings about a further extension of the spaces between 

 the soil particles and create a condition referred to in German as 

 "Bodengare," or puffing up of the soil similar to the rising of bread. 

 An improved tilth results. This condition is favored by fertilizing the 

 soil with stable or green manure, by liming the soil and by thorough 

 cultivation. The colloidal organic matter increases the water holding 

 capacity of the soil and helps to warm up the soil more rapidly, due to 

 a decrease in the evaporation of water, to better drainage and absorp- 

 tion of the radiant heat of the sun. The organic colloids exert a 

 definite buffer effect upon the reaction of the soil solution; 13 they also 

 regulate the composition and concentration of the solution. 



Colloidal condition of soils 1 * and microbiological activities. Colloids 

 are characterized by a group of reactions which were believed originally 

 to differentiate them sharply from crystalloids, and especially by the 

 fact that they show no marked tendency to diffuse in solutions or to 

 pass through semi-permeable membranes. With the advance of our 

 knowledge on colloids, it has been found that no sharp line of demarca- 

 tion can be drawn between them and crystalloids. Both conditions 

 depend on the method of preparation, including the nature of the sol- 



13 Oden, Sven. The application of physico-chemical methods to the study of 

 humus. Trans. Faraday Soc, 17: 288-294. 1922. 



14 The following texts give an excellent discussion of the subject of colloids. 

 Ostwald, Wo. Colloid chemistry (Tr. Fischer). 1916; Zsigmondy, R. Kol- 

 loidchemie. 5th Ed. Spamer. Leipzig. 1925; Taylor, \V. W. Chemistry of 

 colloids. 3d. Ed. New York. 1915; Freundlich, H. Kapillarchemie. 2d 

 Ed. Leipzig. 1922; Ehrenberg, P. Die Bodenkolloide. Steinkopff. Leipzig. 

 2d Ed. 1922; Bechhold, H. Colloids in biology and medicine (Tr. J. G. M. Bul- 

 lowa). Van Nostrand Co. New York. 1919; Burton, E. F. The physical 

 properties of colloidal solutions. Longmans. 2d Ed. 1921; Casuto, L. Der 

 kolloidale Zustand der Materie. Steinkopff. 1913; Vv'iegner, G. Boden und 

 Bodenbildung in kolloidchemischer Betrachtung. T. Steinkopff. 3rd Ed. 

 Dresden. 1924; Bogue, R. H. The theory and application of colloidal beha- 

 vior. 2 vols., McGraw-Hill. New York. 1924. 



