THE PHYSICAL PROPFR'riHS OF THP SOIL. 



Dr. E\VALT> Wor.T.XY. 



l*AliT I. 



Having already sliowii^ the sigiiificaiice of the physical properties of 

 the soil ill the growth of i)laiits, I shall attempt iu the following- pages 

 to set forth briefly, in the light of investigations made up to the i)resent 

 time, the nature of these properties and the corresponding differences 

 among soils of different kinds nnder varying external conditions. 



I. — THE PRINCIPAL SOIL CONSTITUENTS. 



We may regard quartz, lime, clay, and humus as the principal con- 

 stituents of the soil which intlnence its texture or mechanical character, 

 and hence its behavior toward water, air, and heat, although under 

 certain conditions a similar influence may be exercised by iron com- 

 pounds aud certain salts. In so far as the character of the mass depends 

 on the size of its component j)articles, the coustituents may be divided 

 into sand (fine and coarse), ftner particles which can be easily washed 

 away (silt) and humus. The fact that these substances have very 

 different properties, and occur in widely varying proportions, explains 

 the marked differences that exist iu the physical properties of natural 

 soils, and a knowledge of the principal peculiarities of these individual 

 coustituents, and of their mutual relations, as well as of the proportions 

 iu which they are present in any particular instance, is therefore of 

 primary interest aud importance. 



II. — GENERAL PHYSICAL PROPERTIES OF THE SOIL CONSTITUENTS 

 AND OF THE SOIL AS A WHOLE. 



THE SIZE OF THE SOIL PAKTICI.ES. 



The size of the soil particles varies within wide limits. In plastic 

 clays the size of the grains is between 0.0001 and 0.005 nim.;^ in the 

 coarsest kinds of sand they are as high as 2 mm. in diameter. The 

 extreme variation is therefore between 0.0001 and 2.0 mm. That part of 

 the soil consisting of particles 2 to 5 mm. in diameter is called gravel, 

 5 to 10 mm. coarse gravel, and above 10 mm. stones. The portion made 

 up of particles over 0.23 mm. in diameter is termed the "soil skeleton," 

 that of finer grains the "fine earth." According to the preponderant 

 size of the grains we distinguish between gravelly, sandy, coarse- 

 grained, fine-grained, and dusty soils. 



»E. S. R., 4, pi).528, 627. 



»M. Whitney, U. S. Dept. Agr., Weatliev Uureau Bui. 4. 



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