Environmental Engineering Reference
In-Depth Information
100
90
80
70
Clay
60
50
Silty
clay
Silty
clay loam
Sandy
clay
40
FIGURE 7.112
Soil texture classification of the USDA.
Chart shows the percentages of clay (below
0.002 mm), silt (0.002-0.005 mm), and sand
(0.05-2.0 mm) in the basic soil texture
classes. (From SCS,
Soil Classification: A
Comprehensive System (7th Approximation)
,
Soil Conservation Service, USDA, U.S.
Govt. Printing Office, Washington, DC,
1960.)
Clay loam
30
Sandy
clay loam
20
Loam
Silt loam
Sandy loam
10
Slit
Sand
Percent sand
Consistence
is caused by adhesion or cohesion and provides the soil with strength.
Descriptive terminology is given for three conditions of soil moisture: dry, moist, and wet:
•
Wet soil is described in terms of stickiness (adhesion to other objects), classed as
0 (none) to 3 (very sticky), and plasticity (the ability to form a thread upon rolling
with the finger), classed from 0 (nonplastic) to 3 (very plastic).
•
Moist soil is graded from 0 (loose or noncoherent) to 2 (friable) to 5 (very firm,
can be crushed only under strong pressure).
•
Dry soil is graded from 0 (loose or noncoherent) to 4 (very hard).
Cemented soils
are described as:
•
Weakly cemented — brittle and hard but can be broken in the hands
•
Strongly cemented — can be broken by hammer, but not by hands
•
Indurated — does not soften upon wetting, extremely hard, hammer rings with
blow
Soil reaction
refers to acidity or alkalinity, given in ten grades in terms of pH:
•
Extremely acid — pH below 4.5
•
Neutral — pH from 6.6 to 7.3
•
Very strongly alkaline — pH 9.1 and above
Symbols used on pedological maps and elsewhere to designate soil horizons and spe-
cial properties are given in
Table 7.14.
General Group Classifications
Soil Profile Development
As described in
Section 6.7.2,
soil profile development depends on five factors:
parent material, climate, topography, organisms, and time. Holding four variables
