18 
BULLETIN 355, U. S. DEPARTMENT OF AGRICULTURE. 
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 
. 002 to . 0002 
Medium sand 
Clay, all particles less 
.0002 and less. 
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 grades as previously given, but the pro- 
portion varies greatly. Heavy clay soils are largely made up of silt 
and clay particles with 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 desirable 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 soil 
Mechanical analysis giving average percentage of soil separated in 
each class. 
Fine 
gravel. 
Coarse 
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 
