TEXTURE AND STRUCTURE OF SOILS 



31 



If all the particles were spheres, it is estimated that each 

 cubic foot of soil would have a surface area on its particles 

 amounting to from two to three and one-half acres. 



31. Mechanical analysis of soils. — A separation of the 

 particles of a soil into groups, each of which comprises 

 particles whose sizes fall within certain definite limits, is 



-Firm gravel 



•COARSE 5AHD 

 -MEDIUM - 

 -FINE 

 VERTHME;'' 



SILT 



CL/T/ 



Fig. 2. — Relative sizes of soil particles in the various grades into which 

 a mechanical analysis separates a. soil. All are enlarged many times. Par- 

 ticles of fine gravel may vary in size from the largest circle to the next largest ; 

 coarse sand from the second to the third ; medium sand from the third 

 to the fourth, and so on. The dot in the center represents the largest clay 

 particles ; the smallest cannot be shown in a figure of this magnification. 



called a mechanical analysis of the soil. The size limit 

 of these groups is a purely arbitrary matter, consequently 

 it is desirable that a universal system shall be adopted. 

 The classification in general use in this country is one pro- 

 posed by members of the Bureau of Soils of the United States 

 Department of Agriculture. It provides for groups of the 

 following sizes : 



