SOIL STRUCTURE 181 



the binding power of the colloidal material is greatly 

 augmented by desiccation. In a coarse soil such as 

 quartz, in which there is very little colloidal matter, the 

 cohesion is developed principally by the water film. As 

 this thins, its pulling power increases £nd the curve 

 ascends; but when the soil dries this film disrupts and 

 the curve drops again, having no colloidal binding ma- 

 terial. The same relationships are shown by curves (see 

 Fig. 27) adopted from recent determinations by Atter- 

 berg. 1 



118. Moisture limits for successful tillage. — In heavy 

 soils, in which the colloidal content is usually high, plas- 

 ticity and cohesion also are high. This means that the 

 soil when too moist will be puddled by tillage implements, 

 especially such as the plow, and when too dry clodding 

 will occur because of very high cohesion. A moisture 

 limit must therefore exist on a heavy soil, within which 

 successful plowing may be done and maximum granu- 

 lation results may be secured. That this moisture limit 

 is narrow is obvious, since high cohesion and high plas- 

 ticity bound it so closely on either hand. Such a rela- 

 tionship must be kept in mind not only by the farmer 

 but by the technical man as well, since so much depends 

 in any work upon good soil tilth. The relationship is 

 clearly shown by the following curves (Fig. 28) partially 

 adopted from Atterberg. 1 The cohesion and plasticity 

 curves are seen to cross near the center of the diagram 

 and indicate the existence of a zone where neither is 

 exceedingly high or low. 



In a clay soil a study of the optimum moisture condi- 



1 Atterberg, A. Die Konsistenz Kurven der Mineralboden. 

 Internat. Mitt. f. Bodenkunde, Band IV, Heft 4-5, Seite 418- 

 431. 1914. 



