THE SOIL AS A CULTURE MEDIUM 627 



vent employed. We speak now of colloidal systems rather than of 

 colloidal substances. Colloidal systems are not stable. Colloids are 

 also characterized by extended surfaces which allows them to absorb 

 water and dissolved substances from a solution. The attraction for 

 water is both chemical and physical. Heat of wetting is characteristic 

 of the soil colloids. An active soil colloid may be changed to an inac- 

 tive one by changing its physical condition (ignition) and also to some 

 extent by changing its chemical condition. 15 



Soil colloids consist largely of silica, alumina, iron oxides and or- 

 ganic matter. 16 The natural organic matter added to the soil consists 

 largely of colloidal substances (proteins, celluloses, starches, lignins). 

 When these substances are acted upon by microorganisms, they are 

 partly transformed, either directly or through a series of stages, into 

 crystalloids, and partly into new colloids, namely the soil organic matter 

 (p. 6S0). The clay portion of the soil is in itself colloidal in nature. 



Colloids are usually separated from crystalloids by dialysis, ultra- 

 filtration, and centrifuging. The osmotic pressure of colloidal solu- 

 tion is small, due to the large molecular weight; diffusion is, therefore, 

 also small ; the lowering of the freezing point is almost negligible. When 

 the colloids are transformed by microorganisms into crystalloids, the 

 molecular weight is decreased with a corresponding increase in osmotic 

 pressure, diffusion and lowering of freezing point. A study of the chemi- 

 cal properties of the proteins as organic colloids led to the conclusion 17 

 that the theory of amphoteric colloids is in its general features identical 

 with the theory of inorganic metal hydroxides. 



The colloidal properties of soil which are of special importance in 

 the growth of microorganisms in the soil as a culture medium are (1) 

 the absorption of substances from solution and their concentration 

 upon the surface of the colloid, including substances used as nutrients 

 by microorganisms and those which may be injurious to their activities; 

 (2 1 ability to absorb water in large amounts; (3) flocculation and de- 

 flocculation phenomena of colloids themselves and of bacterial cells con- 

 sidered as colloids; (4) the "sol" and "gel" states of the colloids; (5) 

 modification of soil conditions, such as reaction. 



15 Anderson, M. S. The heat of wetting of soil colloids. Jour. Agr. Res., 

 28: 927-936. 1924; Bouyoucos, G. J. The chief factors which influence the heat 

 of wetting of soil colloids. Soil Sci., 19: 477-483. 1925. 



16 Robinson, W. O., and Holmes, R. S. The chemical composition of soil 

 colloids. U. S. Dept. Agr. Bui. 1311. 1924. 



17 Loeb, J. Proteins and the theory of colloidal behavior. McGraw-Hill. 

 New York. 1922. 



