160 SOILS: PROPERTIES AND MANAGEMENT 



Just how this phenomenon of flocculation or agglutina- 

 tion may be accounted for theoretically it is rather difficult 

 to state. The general theory is one of electrification. It 

 is found that certain colloids, when subjected to the 

 proper electric current, will migrate to either the positive 

 (anode) or the negative (cathode) pole. These particles 

 evidently carry a charge of electricity. Ferric hydrate, 

 aluminium hydrate, and basic dyes, for example, move 

 toward the cathode and carry a positive charge ; while 

 arsenious sulfide, silicic acid, gold, silver, and acid dyes 

 move toward the anode and are negative. It is assumed 

 that as long as the colloidal particles remain charged 

 they repel each other and the colloidal state persists. 

 When an electrolyte is added the ionization is supposed 

 to cause a discharge of the repellent electricity carried 

 by the colloidal particles, and flocculation or agglutina- 

 tion immediately takes place. 



Certain colloids may flocculate certain others, as the 

 gelatinization of silicic acid by ferric hydrate. At times 

 one colloid may protect another, probably by surrounding 

 it with a protective film. Such a case may be shown by 

 adding gelatin to a clay suspension. When a colloid such 

 as ferric hydrate is flocculated, it loses to a certain extent 

 its peculiar properties, and assumes the characteristics of 

 ordinary materials. It is evident, therefore, that if the 

 properties exhibited by colloidal materials become either 

 directly or indirectly detrimental to plants, their floccula- 

 tion would be beneficial. In field practice this is usually 

 accomplished by the addition of lime. The colloidal 

 material existing in a normal soil and possessing a gelat- 

 inous nature, similar in general to gelatin, is probably 

 not all flocculated by the addition of ordinary amounts 

 of electrolytes. This material may be influenced by 



