164 SOILS: PBOP£JBTIJES AND MANAGEMENT 



is generated ; thus the effects may be marked, although 

 the amount present at any one time is extremely minute. 



109. Colloids and soil properties. — As may naturally 

 be inferred, the influence of the colloidal matter on soil 

 conditions, especially as related to plants, are extremely 

 important. This influence is exerted in two ways. First, 

 on cohesion and plasticity ; and, secondly, on the adsorp- 

 tive power of the soil. Both these qualities must be con- 

 sidered, not only in the physical, but also in the chemical 

 and the biological, study of the soil as a medium for crop 

 production. 



In general it is found that, other conditions being equal, 

 an increase of colloidal matter increases plasticity; in 

 other words, the ease with which a soil may be worked 

 into a puddled condition becomes greater. This is a rather 

 undesirable quality when too pronounced, and in clays 

 in which it is most likely to be developed some means 

 of decreasing the colloidal influence is advisable. This 

 great plasticity is developed because the colloids, espe- 

 cially those of a gelatinous or viscous nature, facilitate 

 the ease with which the particles may move over one an- 

 other and yet cohere sufficiently to prevent disruption 

 of the mass. In general, also, the greater the plasticity 

 of a soil, the greater is the cohesion when dry. In soils, 

 then, in which the colloidal material is very high, clodding 

 may occur if the soil is tilled too dry because of the great 

 tendency of the particles to cohere. This cohesion and 

 plasticity, as factors in soil structure, soil granulation, 

 and tilth, will be discussed in the succeeding chapter. 

 It is sufficient at this point only to observe the relation- 

 ship of colloidal materials to the development of such 

 qualities. 



The second important attribute imparted to soil by 



