THE COLLOIDAL MATTER OF THE SOIL 133 



incident to bacterial activity, during which, compounds are 

 thrown off in such a state of division as to assume the condi- 

 tion that has been designated as colloidal. Of course the chem- 

 ical forces of weathering are also operative in this process of 

 organic colloidal production. 



While some inorganic soil colloids, as silicic acid and hy- 

 drated ferric oxide, are rather simple chemically, most of 

 the mineral colloidal material is extremely complex. The soil, 

 especially when of a clayey nature, always contains large 

 amounts of complicated hydrated aluminum silicates of con- 

 stantly varying constitution. 1 Such material, whether simple 

 or complex, arises from ordinary weathering reactions and 

 develops in the soil as the latter is built up. A simple ex- 

 ample may be cited. When a feldspar undergoes decomposi- 

 tion the following reaction may be used to illustrate the pos- 

 sible change that takes place : 



2KAlSi 3 8 + 2H 2 + C0 2 = H 4 Al 2 Si 2 9 + 4Si0 2 + K 2 C0 3 



Orthoclase Water Carbon Kaolinite Silica Potassium 



Dioxide Carbonate 



Kaolin almost always originates in this way, an alkali car- 

 bonate and silica being formed at the same time. The proc- 

 ess is essentially one of hydration and carbonation; the car- 

 bon dioxide by reacting with the alkali permits the process to 

 go on. The silica may go to one or more of three possible 

 destinations, according to conditions, — to free quartz, to col- 

 loidal silica or to make up complex colloidal hydrated alu- 

 minum silicates. The last mentioned condition seems the most 



1 The Bureau of Soils have prepared a colloidal solution from soil 

 by passing a well shaken mixture of soil and water through a Sharpies 

 centrifuge. The colloidal matter was separated from its dispersive 

 medium by means of a porcelain filter. This ultra-clay seemed to be 

 a mixture of various colloids and consisted mainly of hydrated alu- 

 minum silicates with varying amounts of ferric hydroxide, silicic acid, 

 organic matter and possibly aluminum hydroxide. 



Moore, C. J., Fry, W. H., and Middleton, H. E., Methods for Deter- 

 mining the Amounts of Colloidal Material in Soils; Jour. Ind. and 

 Eng. Chem., Vol. 13, No. 6, pp. 527-530, June, 1921. 



