186 SOILS: PROPERTIES AND MANAGEMENT 



the essentials for successful granulation are present. This 

 second force is found in the colloidal material present in 

 considerable quantities iTi heavy soils. These are the same 

 forces that have already been shown to determine the 

 cohesion and plasticity of the soil, except that in granu- 

 lating operations they are localized at numberless foci, 

 and clodding or puddling is thereby prevented. It is 

 evident that if cohesion and plasticity forces are to func- 

 tion for granulation or, in other words, locally in the 

 soil instead of generally and uniformly as when clodding 

 or puddling occurs a certain moisture content must 

 be maintained. From what has already been shown, 

 it is hardly necessary to restate that this moisture condi- 

 tion is near the optimum moisture content for plant 

 growth. 



Warington * attributes granulation to unequal expan- 

 sion and contraction of the soil mass, due to the imbibi- 

 tion and loss of water. In a soil subject to such a condi- 

 tion, the cohesive forces being localized, the internal 

 strains and pressures are unequal and a tendency arises 

 for the mass to divide along lines of weakness into groups 

 of particles. The binding capacity of colloidal material, 

 as well as of salts deposited from the soil solution, tends 

 to make such a crumb structure more or less permanent. 

 Tillage operations, development of roots, burrowing 

 of animals and insects, the presence of humus, and the 

 formation of frost crystals, may assist in further develop- 

 ing these lines of weakness in the soil mass, on which the 

 tension of the moisture films around the soil particles is 

 brought to bear. The flocculation of soil particles may 

 also develop lines of cleavage by their aggregation around 



1 Warington, R. Physical Properties of Soils, pp. 36-41. 

 Oxford. 1900. 



