SOIL STRUCTURE 187 



certain centers. This movement of the soil particles is 

 in every case facilitated by the presence of a moderate 

 amount of moisture. 



122. Forces facilitating granulation. — Granulation is 

 nothing more or less than a condition brought about by 

 the force exerted by a variable water film and the pull- 

 ing and binding capacities of colloidal material, operating 

 at numberless localized foci. It is evident that any influ- 

 ence or change in the soil which will cause a greater locali- 

 zation of these operative forces will promote increased 

 granulation. The addition of materials from extraneous 

 sources is also a practice that may tend to develop lines of 

 weakness and thus cause a more intense localization of 

 the forces at work. 



The conditions, additions, and practices tending to 

 develop or facilitate a granular structure in soils may be 

 listed under six heads : (1) wetting and drying of the soil, 

 (2) freezing and thawing, (3) addition of organic matter, 

 (4) action of plant roots and animals, (5) addition of 

 lime, and (6) tillage. 



123. Wetting and drying. — The drying of a soil has 

 been shown to result in a drawing together of the particles 

 into aggregates. When this process is repeated again 

 and again by alternate wetting and drying, the influence 

 on granulation becomes marked. 



In drying, the small particles are moved into the spaces 

 between the larger ones, thereby reducing the volume 

 as is shown by the checks produced. These checks that 

 result from shrinkage are due to the unequal contraction. 

 There comes a time when the general film around the 

 whole mass must rupture, and it breaks along the lines 

 of least resistance. If the soil mass is very uniform, there 

 will be few breaks and the shrinkage will be mainly around 



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