Ch. 2] FORMATION OF CLAY PAN SOILS 57 



in place as a result of weathering. It is possible that the subsoil region 

 is especially conducive to clay formation, owing to favorable moisture 

 conditions. Second, clay accumulates in the subsoil as a result of 

 downward migration of colloidal clay particles from the surface soil. 



The downward migration of clay is governed by colloid chemical 

 principles, especially dispersion and flocculation. Colloidal clays such 

 as montmorillonite clays tend to form stable suspensions, provided 

 that free electrolytes (salts, acids, bases) are absent. Such clays are 

 said to be highly dispersed. Their particle sizes are very small — of the 

 order of a few hundred Angstrom units (1 Angstrom unit = 10 ~ 8 centi- 

 meters) . 



Addition of suitable amounts of electrolytes to dispersed clay sys- 

 tems will produce flocculation. When observed under the ultramicro- 

 scope, it is seen that the tiny individual clay particles unite to form 

 aggregates or flocks which may become so large that they settle read- 

 ily under the influence of gravity. The flocculation of clays by mono- 

 and divalent cations is usually reversible. Removal of excess electro- 

 lyte will restore the system to its dispersed state. 



The phenomenon of protective action of humus also must be taken 

 into consideration. Leaching a soil with dilute ammonium hydroxide 

 yields a dark-brown extract which contains colloidal humus particles. 

 This humus extract possesses the power of protective action. If a 

 small amount of colloidal humus extract is added to a dispersed clay 

 system, its flocculation value becomes higher. In other words, a 

 higher amount of electrolyte must be added to produce clay floccula- 

 tion. Conversely, the addition of colloidal humus to a flocculated clay 

 often results in dispersion of clay. The clay aggregates separate into 

 the ultimate clay particles. 



These aspects of colloidal chemistry aid in the understanding of 

 clay migration. Let us postulate a uniform parent material of medium 

 texture containing, say, 10 to 20 percent of clay, and 5 to 10 percent of 

 calcium carbonate. Loess and many alluvial deposits closely corre- 

 spond to such a hypothetical parent material. The climate is assumed 

 to be humid. 



Owing to the presence of calcium carbonate and bicarbonate, the 

 clay exists in the flocculated form. It is in a state of rest. As calcium 

 bicarbonate is being leached downward, the surface soil's electrolyte 

 concentration is reduced below the flocculation value of the clay. 

 Aided by the protective action of soil humus, the clay aggregates be- 

 gin to disperse. The fine individual particles are carried by the perco- 

 lating water to the subsoil. 



