PHYSICAL PROPERTIES OF SOILS 117 



Shrinking on Drying. Some soils increase in bulk when they 

 become wet, and shrink on drying. The shrinkage is very per- 

 ceptible in some clay soils. They become full of cracks and rifts 

 on drying, and, since they harden about the rootlets imbedded in 

 them, the roots may become ruptured during dry weather. Heavy 

 clays may thus lose one-tenth or more of their volume. 



Sand does not change in bulk on wetting or drying, and, when 

 present to a considerable extent in the soil, its particles prevent 

 the adhesion of the clay particles. Although a loam shrinks on 

 drying, the lines of separation are more numerous and less wide 

 than in a clay. 



Some soils crack into comparatively small masses on drying. 

 These are often termed buckshot soils. Others crack into larger 

 masses, several feet in size. In others irregular cracks are found, 

 sometimes an inch in width. Schubler prepared cubes of various 

 soil constituents and determined the contraction in volume on 

 drying. Pure clay contracted 18.3 per cent, of its volume; humus 

 from the center of a decayed tree 20 per cent. ; a sandy clay 6 per 

 cent. ; an arable soil 12 per cent. ; a garden soil 14.9 per cent. The 

 rifts allow an easier drying of the subsoil. The results of drying 

 are afterwards favorable in a clay soil, the fissues affording 

 drainage lines. The texture is improved, drying and moistening 

 being favorable to formation of compound particles. Air is also 

 admitted to the subsoil, and oxidation in the subsoil is promoted. 



Number of Particles in Different Types of Soil. These have 

 been calculated by Whitney 1 as follows : 



Early truck 1,955,000,000 



Truck and small fruit 3,955,000,000 



Tobacco 6, 786,000,000 



Wheat 10, 228,000,000 



Grass and wheat 14,735,000,000 



Limestone 19,638,000,000 



The basis of this calculation is the assumption of a certain 



average size for each grade of soil particle. Knowing the size 



and the specific gravity of the soil, the weight of each grade of 



particle can easily be calculated. Then the numbers of particles 



1 Bulletin 21, Maryland Station. 



