174^ SOILS: PROPERTIES AND MANAGEMENT 



cohesion of a soil is meant the tendency that its particles 

 exhibit in sticking together and in conserving the mass 

 intact. In general, the greater the plasticity of a soil, 

 the higher is its cohesion, especially when it is dry or 

 only slightly moist. For that reason, cohesion might 

 be made a rough measure of plasticity. Cohesion of a 

 soil occurs under two general conditions, the wet and the 

 dry. When a soil is moist its cohesion is developed by the 

 moisture films and the colloidal materials that may be 

 present. This form of cohesion is often spoken of as 

 tenacity. When a soil is dry its cohesion is developed to 

 some extent by the interlocking of its grains and the dep- 

 osition of cementing salts. The greatest force is devel- 

 oped, however, by the drying and shrinking .of the 

 gelatinous colloidal matter. As a general rule, the greater 

 the amount of colloidal material, the more firmly the soil 

 is bound together when dry, or, in other words, the greater 

 is its cohesion. 



Cohesion is important in tillage operations, in that 

 soils having a high coefficient of cohesion tend to become 

 cloddy when plowed and may thus be rendered poor in 

 physical condition. This may be avoided by timing the 

 operation so that the moisture content is somewhere 

 above the point at which excessive cohesion is exerted. 

 As cohesion is not greatly developed, except in a heavy 

 soil, it is only where fine texture is found that such a 

 danger exists. As already shown, the, danger is a double 

 one, for, since high plasticity and high cohesion go to- 

 gether, a soil plowed too wet may puddle while one 

 plowed too dry may clod. 



116. Methods of determining cohesion. — A number 

 of methods have been devised for determining the co- 

 hesion of clays and other soils. One of the earliest was 



