132 SOIL PHYSICS AND MANAGEMENT 



exists as long as the charge is the same. This condition is not con- 

 fined to colloidal particles alone, but to coarser material in sus- 

 pension. 



If an electrolyte is added to the solution and the ions and 

 particles carry opposite electric charges, floccules, are formed which 

 settle to the bottom. If the ions and colloidal particles have the same 

 electric charges the colloidal condition is maintained. If two col- 

 loids of opposite charges are brought together, mutual precipitation 

 will take place, and if they are the same their stability will be in- 

 creased. In adding an electrolyte to completely precipitate a colloid 

 a sufficient amount must be added so that the charge of one exactly 

 neutralizes the charge of the other. 



(g) Adsorption. Adsorption is a surface phenomenon and 

 hence any increase in the total amount of surface area will increase 

 the adsorption. Colloids possess this property to a high degree be- 

 cause of the large total area of the small particles. When a solid 

 is exposed to a gas a certain amount of gas adsorption occurs. When 

 a solid and a liquid come in contact, concentration occurs on the in- 

 terface between the two. This concentration is known as adsorp- 

 tion. All substances are not equally adsorbed by colloids. The 

 same is true of all ions. If potassium chloride is passed through a 

 soil more of the potassium ions will be adsorbed than of the chlorine. 



(h) Shrinkage. The property of shrinkage is very character- 

 istic of cololids (see Fig. 72). 



Colloids in Soils. The -colloids in soils consist of both organic 

 and inorganic or mineral substances. 



(a) Organic Colloids. Some of the various forms of humus 

 constitute the organic colloids and probably form the larger part 

 of colloids in many soils. These are formed as a result of bac- 

 terial action in the process of humification. Part of the organic 

 matter is broken down into such minute particles as to form colloids. 

 The amount is constantly changing in the same soil. Since granu- 

 lation takes place more perfectly in the spring than at any other 

 time of the year, it would seem that there is a greater supply in the 

 soil at this time than at other periods. This may apply to mineral 

 colloids as well. The adsorptive power of these organic colloids for 

 water is of great economic importance in soils. Schlossing states 

 that one per cent of calcic humate (colloidal) has as much cement- 

 ing power as 11 per cent of plastic clay. 



(b) Mineral Colloids. Mineral colloids are found most abun- 

 dantly in fine-grained soils such as clays and clay loams. The col- 



