THE COLLOIDAL MATTER OF SOILS 161 



drying, whereby it slowly gives off water, becomes more 

 and more viscous, and at last may lose its gel qualities 

 and become hard and irreversible. It is evident, there- 

 fore, that wetting and drying, frost, and the like, become 

 factors in dealing with this form of colloidal matter. 



107. Common soil colloids and their generation. - 

 The common soil colloids may, for convenience, be dis- 

 cussed under two heads, organic and inorganic. Of the 

 former, the so-called humic acid stands as the example; 

 of the latter, silicic acid, ferric hydrate, and amorphous 

 zeolitic silicates are the commonest. 



Organic colloids. The humic colloids in a normal 

 fertile soil probably make up the bulk of the colloidal 

 matter. 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. Humic 

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

 be, and may or may not be thrown down by lime. The 

 adsorptive power of these colloids for water, gases, and 

 such materials as calcium, magnesium, and potash, is 



*Van Bemmelen, J. M. Die Absorption. Seite 114-115. 

 Dresden, 1910. Also, Die Absorptionsverbindungen und das 

 Absorptions vermogen 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. 



Warington, R. On the Part Taken by Oxide of Iron and 

 Alumina in the Adsorptive Action of Soils. Jour. Chem. Soc., 

 LM ier., Vol. 6, pp. 1-19. 1868. 



Cushman, A. S. The Colloid Theory of Plasticity. Trans. 

 Amor. Cer. Soc., Vol. 6, pp. 65-78. 1904. 



Ashley, H. E. The Colloid Matter of Clay and its Meas- 

 urements. U. S. Geol. Sur., Bui. 388. 1909. 



