360 SOILS: PROPERTIES AND MANAGEMENT 



subject, absorption by soils is, without doubt, largely 

 due to the presence of colloidal matter which exerts an 

 absorbent action for water, gases, solutes, and solids in 

 suspension. The colloidal matters in soils that contrib- 

 ute to their absorptive properties are the following : — 



(1) remains of plant and animal tissues ; 



(2) humous substances ; 



(3) colloidal iron oxide ; 



(4) colloidal silicic acid ; 



(5) amorphous colloidal silicates that have been formed 



through weathering. 



Van Bemmelen also credits crystalline silicates with 

 absorbent properties, although he does not consider that 

 their action is very important. Absorption is brought 

 about also by true chemical combination of soil com- 

 pounds with substances in solution, by which certain of 

 the cations or anions in solution are chemically combined 

 and remain in the soil in a very difficultly soluble condition. 

 263. Absorptive properties of colloidal matter. — 

 Among the products of rock weathering there have been 

 found in soils amorphous substances that are of the nature 

 of colloidal gels. These, with the other colloidal matter, 

 are contained in the very small particles that remain for 

 a long time in suspension when soil is stirred up in water. 

 These colloids are coagulated by many acids, and by 

 some bases and salts. This is especially true of the 

 material that is dialyzable. Some of these again go into 

 solution on being treated with water, while others remain 

 insoluble until they undergo molecular change. Many 

 colloids form hydrogels with soil water. These hydrogels 

 are not ordinary chemical compounds. Gels dry very 

 slowly. They adsorb water in varying quantities, not 



