18 



BULLETIN 355, U. S. DEPARTMENT OF AGEICULTURE. 



is used for separating the finest sands. To separate the still finer 

 particles constituting silts and the clays it is necessary to shake the 

 remaining portion of the soil thoroughly in water and then at different 

 periods of time to draw off that which remained suspended during 

 the previous period, allowing it to stand in another vessel for a longer 

 time. By using these methods any number of different grades may 

 be established. As a rule, however, but seven grades are separated. 

 These have the following names and diameters expressed in milli- 

 meters and inches. 



Table I. — Grades and size of soil particles. 



Grade of soil. 



Millimeters. 



Inches. 



Grade of soil. 



Millimeters. 



Inches. 





3 tol 

 1 to .5 

 .5 to .25 

 .25 to .1 



0. 12 to 0. 04 

 .04 to .02 

 .02 to .01 



.01 to .004 



Very fine sand 



Silt 



0. 10 to 0. 05 

 .05 to .005 



.005 



0. 004 to 0. 002 



Cnarsft pand . , 



.002 to .0002 



Medium sand 



Clay, all particles less 

 1 than 







.0002andless. 









The measurement of the diameter of these particles is made by 

 means of a microscope. 



Mechanical composition of various soils. — All soils contain some 

 particles of each of the seven gi^ades as previously given, but the pro- 

 portion varies greatly. Heavy clay soils are largely made up of silt 

 and clay particles mth small quantities of the different-sized sands, 

 while sandy soils are made up of relatively large quantities of the 

 various grades of sand and correspondingly smaller quantities of silt 

 and clay. It is therefore deshable to subdivide soils on the basis of 

 the relative proportions of the different-sized grains. Soil investi- 

 gators recognize on this basis coarse sand, sandy loam, fine sandy 

 loam, loam, silt loam, clay loam, and clay. 



These different classes of soils have the average mechanical com- 

 position or texture shown in Table II. 



Table II. — Average texture of important classes of soils. 



Class of soil. 



Coarse sand 



Sandy loam 



Fine sandy loam . 



Loam 



Silt loam 



Clay loam 



Clay soU 



Mechanical analysis giving average percentage of soil separated in 

 each class. 



Fine 

 gravel. 



Coarse 

 sand. 



Medium 

 sand. 



Fine 

 sand. 



Very fine 

 sand. 



Silt. 



Clay. 



Quantity of surface exposed in soils. — The area of the total surface 

 of the particles in a soil of fine texture is much larger than in one of 

 coarse texture. The principle is illustrated by considering the effect 



