PHYSICAL INTER-RELATIONS IN SOILS 55 



clays, after their pore space has been reduced far below 

 that desirable or necessary to permit the support of crops, 

 shrink when their 

 water content is 

 reduced. The cleav- 

 ages extend in differ- 

 ent directions but at 

 first chiefly verti- 

 cally. The chief re- 

 sult is that the mass 

 of drying soil is thus 

 reduced to blocks, 

 app r oxim ately 

 cubes, in form. 



With the next 

 thorough wetting of 

 the soil that occurs 

 because of a heavy 

 rain or melting snow 

 or flooding (for ir- 

 rigation purposes), 

 these cubes of soil 

 will swell until most 

 of the cracks are 

 closed or nearly so. 



FIG. 21. A column of heavy clay, showing 

 the cleavages which take place upon air 

 drying. It illustrates the way in which 

 saturated clays check upon the removal of 

 the excess of moisture. It also indicates 

 that this soil would respond readily to 

 tile drainage. With successive wettings 

 and dryings the checking would multiply. 



They will not be 



closed so tightly, 



however, but that, 



as the excess of 



water causing the 



wetting passes downward, these cracks will open again. 



With this second shrinking, new cleavages occur so that, 



when this shrinking ceases, the blocks of soil will be 



