162 SOILS: PROPERTIES AND MANAGEMENT 



very highly developed — more so, probably, than that 

 of the inorganic colloids. These organic colloids are 

 formed during the tearing-down and splitting-off processes 

 of bacterial activity. Some of the humic materials are 

 thrown off in a sufficiently fine state of division to assume 

 the condition that has been designated as colloidal. Of 

 course the chemical forces of weathering are also operative 

 in this process of organic colloidal production. 



Mineral colloids. — The inorganic soil colloids, especially 

 ferric oxide and silicic acid, are less complex than the 

 organic and have been more thoroughly studied. Such 

 colloids are generated during the operation of the ordinary 

 forces of weathering, especially the chemical phase. For 

 example, when a feldspar undergoes decomposition, the 

 following reaction may be used to illustrate the possible 

 change that takes place : — - 



2 KAlSi 3 8 + 2 H 2 + C0 2 = H4Al 2 Si 2 9 + 4 Si0 2 + 



K 2 C0 3 



Kaolin practically always has its origin in this way, 

 together with an alkali carbonate and silica. The process 

 is essentially one of hydration and carbonation ; the C0 2 

 by reacting with the alkali permits the process to go on. 

 The silica may go in three directions, according to con- 

 ditions — to free quartz, to hydrated silicates, and to 

 colloidal silica. Similar reactions may be written for 

 iron and aluminium, but they can only show, as does 

 the above, the general trend of the change. In general it 

 can be concluded that most inorganic colloids arise from 

 ordinary chemical weathering, together with secondary 

 minerals of various kinds. Such colloids must be very 

 dilute and are difficult to study because of their reaction 

 among themselves. 



