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SOILS
Soils are an important
element of a watershed. They help establish vegetation as well as
create a filter for both surface and ground water run off.
A “soil” is defined by
the Soil Science Society of America as:
(i) The
unconsolidated mineral or organic material on the immediate surface of
the earth that serves as a natural medium for the growth of land plants.
(ii) The
unconsolidated mineral or organic matter on the surface of the earth
that has been subjected to and shows effects of genetic and
environmental factors of: climate (including water and temperature
effects), and macro- and microorganisms, conditioned by relief, acting
on parent material over a period of time. A product-soil differs from
the material from which it is derived in many physical, chemical,
biological, and morphological properties and characteristics.
Soils are classified
by the composition of their particle size or soil separate. The chart
below provides the USDA classification scheme.
|
Name of soil separate |
USDA Classification
Diameter limits (mm) |
|
Clay |
less than 0.002 |
|
Silt |
0.002 - 0.05 |
|
Very fine sand |
0.05 - 0.10 |
|
Fine sand |
0.10 - 0.25 |
|
Medium sand |
0.25 - 0.50 |
|
Coarse sand |
0.50 - 1.00 |
|
Very coarse sand |
1.00 - 2.00 |
The United States Department of Agriculture
Sand particles
hold little water but allow good aeration.
Silt particles
are medium sized and have properties in between those of sand and clay.
Clay particles
are very small in size and tend to pack down so that water does not
drain well and little or no air can penetrate.
Loamy soils,
combine all three of these types of particles in relatively equal
amounts. Loamy soil holds plenty of moisture but also drains well so
that sufficient air can reach the roots.
Loam is a soil
composed of a relatively even mixture of the three particle size groups:
sand silt and clay. Loams are generally gritty, plastic when moist, and
retain water easily. Yet they drain well where the topography allows.
They generally contain more nutrients than sandy soils.
This
classification is further defined by textural classes:
Soil
Textures
The
relative proportions of the various soil separates in a soil. The
textural classes may be modified by the addition of suitable adjectives
when rock fragments are present in substantial amounts; for example,
"stony silt loam." (For other modifiers see also rock fragments.) The
sand, loamy sand, and sandy loam are further subdivided on the basis of
the proportions of the various sand separates present. The limits of the
various classes and subclasses are as follows:
Clay
- clay - Soil material that
contains 40% or more clay, <45% sand, and <40% silt.
- clay loam - Soil material that
contains 27 to 40% clay and 20 to 45% sand.
Silt
- silt - Soil material that
contains 80% or more silt and <12% clay.
-
- silty clay - Soil material that
contains 40% or more clay and 40% or more silt.
- silty clay loam - Soil material
that contains 27 to 40% clay and <20% sand.
- silt loam - Soil material that
contains 50% or more silt and 12 to 27% clay (or) 50 to 80% silt and
<12% clay.
Sand
- sand - Soil material that
contains 85% or more of sand; percentage of silt, plus 1.5 times the
percentage of clay, shall not exceed 15.
- coarse sand - Soil material
that contains 25% or more very coarse and coarse sand, and <50%
any other one grade of sand.
- sand - Soil material that
contains 25% or more very coarse, coarse, and medium sand, <25%
very coarse and coarse sand, and <50% fine or very fine sand.
- fine sand - Soil material
that contains 50% or more fine sand (or) <25% very coarse,
coarse, and medium sand and <50% very fine sand.
- very fine sand - Soil
material that contains 50% or more very fine sand.
- sandy clay - Soil material that
contains 35% or more clay and 45% or more sand.
- sandy clay loam - Soil material
that contains 20 to 35% clay, <28% silt, and >45% sand.
- sandy loam - Soil material that
contains 7 to 20% clay, more than 52% sand, and the percentage of
silt plus twice the percentage of clay is 30 or more; or less than
7% clay, less than 50% silt, and more than 43% sand.
- coarse sandy 1oam - Soil
material that contains 25% or more very coarse and coarse sand
and <50% any other one grade of sand.
- sandy loam - Soil material
that contains 30% or more very coarse, coarse, and medium sand,
but <25% very coarse and coarse sand, and <30% very fine or fine
sand, or <15% very coarse, coarse, and medium sand and <30%
either fine sand or very fine sand and 40% or less fine plus
very fine sand.
- fine sandy loam - Soil
material that contains 30% or more fine sand and <30% very fine
sand (or) between 15 and 30% very coarse, coarse, and medium
sand, or >40% fine and very fine sand, at least half of which is
fine sand, and <15% very coarse, coarse, and medium sand.
- very fine sandy loam - Soil
material that contains 30% or more very fine sand and <15% very
coarse, coarse, and medium sand (or) >40% fine and very fine
sand, more than half of which is very fine sand and <15% very
coarse, coarse, and medium sand.
Loam
- loam - Soil material that
contains 7 to 27% clay, 28 to 50% silt, and <52% sand.
- loamy sand - Soil material that
contains between 70 and 91% sand and the percentage of silt plus 1.5
times the percentage of clay is 15 or more; and the percentage of
silt plus twice the percentage of clay is less than 30.
- loamy coarse sand - Soil
material that contains 25% or more very coarse and coarse sand,
and <50% any other one grade of sand.
- loamy sand - Soil material
that contains 25% or more very coarse, coarse, and medium sand,
<25% very coarse and coarse sand, and <50% fine or very fine
sand.
- loamy fine sand - Soil
material that contains 50% or more fine sand (or) <25% very
coarse, coarse, and medium sand and <50% very fine sand.
- loamy very fine sand - Soil
material that contains 50% or more very fine sand.
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