SOIL STRUCTURE 173 



scopic water, and dye adsorption as an expression of 

 plasticity become logical on this basis. 



114. The importance of plasticity. — Plasticity assumes 

 considerable importance in a soil when it becomes highly 

 developed, since it promotes ease in puddling. The 

 more plastic a soil is, the more likely it is to become pud- 

 dled by tillage, especially if it has a high moisture con- 

 tent. Thus a clay cannot be plowed wet, since this 

 would allow its particles to be worked into that very intri- 

 cate condition so detrimental to plant growth. A sand, 

 on the contrary, may be stirred even when saturated, 

 and still its structural condition will not be impaired 

 since its plasticity is low or nihil. A very plastic soil 

 is also. likely to become exceedingly hard when dry unless 

 it is well granulated, which shows the great care demanded 

 by soils having high plasticity coefficients. 



The three factors that affect plasticity to the greatest 

 extent are texture, granulation, and moisture. In gen-' 

 eral, the finer the texture of the soil, the higher is the 

 maximum plasticity thereof. The more granular a soil, 

 the lower is the plasticity or the tendency to puddle 

 when plowed. The amount of water is the third vital 

 factor. A soil will exhibit its maximum plasticity at a 

 definite moisture content. This point will lie somewhere 

 between the flowing, or viscous, condition and the point 

 at which a soil refuses to mold, or, in other words, to be- 

 come crumbly. With a soil such as a clay, in which the 

 plasticity is high, plowing should be done when the 

 moisture condition is such that there is no likelihood of 

 puddling, and yet the soil will turn over with a maximum 

 granulating effect. 



115. Cohesion. — Very closely correlated with plas- 

 ticity, but not in exact similarity, is cohesion. By the 



