132 NATURE AND PROPERTIES OF SOILS 



with a protective film. Such a case may be shown by adding 

 gelatin to a clay suspension. When a colloid such as hy- 

 drated ferric oxide is flocculated, it loses to a certain extent 

 its colloidal properties, and assumes the characteristics of 

 non-colloidal materials. 



73. Soil colloids and their generation. 1 — In soils there 

 seem to exist two very general and indefinite groups of col- 

 loidal materials, besides all gradations and variations : ( 1 ) vis- 

 cous, gelatinizing and reversible colloids, and (2) non-viscous, 

 non-gelatinizing, easily coagulable and irreversible colloidal 

 matter. 'The decaying organic materials in the soil and the 

 mineral matter contribute liberally to both groups. Both 

 of these groups, with their bewildering variations and grada- 

 tions, play important parts in the physical and chemical phe- 

 nomena of the normal soil. 



The organic colloidal matter in a soil rich in decomposing 

 tissue is obviously of great importance. Such material is very 

 heterogeneous, very complex, and constantly changing. As 

 yet very little study of the organic soil colloids has been made 

 because of the difficulties presented by the problem. Humus 

 colloids may be viscous or non-viscous, as the case may be, 

 and may or may not be thrown down by calcium hy- 

 droxide. The absorptive power of these colloids for water, 

 gases, and such materials as calcium, magnesium, and potas- 

 sium is very highly developed — as much so, probably, as that 

 of the inorganic colloids. These organic colloids are not only 

 added as a part of the original plant tissue but are also 

 formed during the tearing-down and splitting-off processes 



1 Van Bemmelen, J. M., Dis Absorption; Seite 114-115, Dresden, 1910. 

 Also, Die Absorptionsverbindungen und das Absorptsvermogen der 

 Ackererde; Landw. Ver. Stat., Band. 35, Seite 69-136, 1888 ; Way, J. T., 

 On Deposits of Soluble or Gelatinous Silica in the Lower Beds of the 

 Chalk Formation; Jour. Chem. Soc, Vol. 6, pp. 102-106, 1854. War- 

 ington, R., On the Part Taken by Oxide of Iron and Alumina in the 

 Adsorptive Action of Soils; Jour. Chem. Soc, 2d ser., Vol. 6, pp. 1-19, 

 1868. Cushman, A. S., The Colloid Theory of Plasticity; Trans. Amer. 

 Cer. Soc., Vol. 6, pp. 65-78, 1904. Ashley, H. E., The Colloid Matter 

 of Clay and its Measurements; U. S. Geol. Survey, Bui. 388, 1909. 



