142 



NATURE AND PROPERTIES OF SOILS 



that bring about granulation in a soil, it is difficult to state 

 specifically just why this phenomenon takes place. It has been 

 suggested that much of the granule formation in the soil is 

 due to the contraction of the moisture around the particles 

 when, for any reason, the moisture content is reduced. It 

 is known that the soil particles tend to be drawn together 

 by this reduction in the soil-moisture, due to the pulling power 

 of the thinned films. 



If to this condition is added a material which tends to exert 

 not only a drawing power on loss of moisture, but also a bind- 



Fig. 24. — A well granulated soil and a puddled soil. Organic matter 

 plays an important role in structural condition. 



ing and cementing power when dry, all the essentials for suc- 

 cessful granulation are present. This second force is found 

 in the colloidal material existing in considerable quantities in 

 heavy soils. Such materials have already been shown to deter- 

 mine the cohesion of the soil. The influence of the colloidal 

 material is considered by many authorities as the more im- 

 portant in the structural adjustments of the soil. 



It is evident that if cohesion and plasticity are to function 

 in 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. In 

 a soil subject to such a condition, the cohesive forces being 



