SOILS
Soil = mixture of mineral grains,
organic material and pores spaces
filled with variable amounts of air
and water. Soil development = part
of the process of erosion of rock :soil is largely derived from
weathering of rock (the physical
disintegration and/or chemical
decomposition of rock
- usually weaker than rock
- therefore more easily removed by
erosional processes.
Soils are made of four components – mineral matter, organic
matter, air and water. Air and water occupy pore spaces in
the soil. The example above is fairly typical.
Soil Parent Materials. Residual soil = soil developed on underlying bedrock
Transported soil = soil developed on unconsolidated deposits (sediment).
Well-drained
Poorly drained
Controls On Soil Characteristics
The characteristics of soil depend on: parent material; climate;
vegetation; slope, time.
1. Parent material: influences soil composition (e.g. shales produce a lot of
clay; sandstone produces sandy soil) and physical properties of soil e.g.
permeability/drainage (number, size and connectivity of pore spaces); shrinkswell potential (amount of expansive clay); cohesive strength (clay content clayey soils are "sticky" - this aids cohesion).
2. Climate: influences type and rate of weathering, amount of water moving
through and over the soil; type of vegetation.
3. Vegetation: influences organic content of soil, strength of soil (roots
increase cohesion).
4. Slope: Steeper slopes -> accelerated erosion, if rate of erosion > rate of soil
development -> thin or no soil. Soils on steeper slopes also have lower water
contents (lower infiltration) -> less weathering, less vegetation. Soils in lowlying areas have higher water contents, more weathering, thicker soils, more
vegetation.
5. Time: soils become more well-developed over time.
What is meant by a "well-developed" soil? A soil is more than a heap of rock
fragments, organics, air and water thrown together; it has INTERNAL
ORGANIZATION created by soil-forming processes:
This soil is organized into
distinct layers or horizons.
PEDOGENIC (SOIL-FORMING) PROCESSES:
The most important soil-forming processes are vertical movements of soil
materials which result in HORIZONATION or layering of the soil. These
movements are caused by percolating soil water.
1. Organic activity: concentrated at the surface -> HUMUS (decayed organic
material), which mixes in with the mineral soil.
2. Translocation: Downward movement of material due to percolating soil
water
(a) Leaching: removal of material in solution eg. iron and aluminum. Stable
materials eg. silica (quartz) are more resistant to leaching therefore tend to
remain in upper soil.
(b) Lessivation: downward flushing of solid particles - mainly clay - through
pore spaces in soil.
Together, these processes are known as ELUVIATION and result in an upper
eluviated horizon.
Some of the eluviated
material may be deposited
further down in the soil i.e.
accumulation of clay
particles; precipitation of
iron and aluminum oxides these processes are known
as ILLUVIATION and
result in a lower illuviated
horizon.
The effect of the above is
the formation of a soil
profile:
O horizon: accumulation of organic matter at the surface (becomes more
decomposed with depth forming humus (black)).
A horizon: mixture of rock/mineral fragments and organic material.
E horizon: This horizon is subject to LEACHING - material is removed
in solution by acidic soil water and fine particles (clays) are flushed
down through the soil. This results in a porous, low density horizon, rich
in resistant minerals e.g silica (quartz) (Note: the A and E horizons are
sometimes combined into a single A horizon).
B horizon: material removed from the upper soil accumulates here
resulting in a clay-rich, fe and al oxides-rich, dense, orange/yellow
colored B horizon.
C horizon: weathered parent material (regolith).
R horizon: unweathered parent material.
Soils in the DFW Region:
Four major rock outcrops run
through the DFW region:
1. Paluxy sandstone in the
west.
2. A mixture of limestones,
clays (shales) and marls
running through Cooke
County, western Denton
County and western Tarrant
County.
3. Woodbine sandstone
running through central
Denton County and eastern
Tarrant County.
4. Eagle Ford Shale, Austin
Chalk (a type of limestone)
and Taylor Marl running
through Dallas County.
Oak woodlands have developed
on the sandy soils of the Paluxy
and Woodbine sandstones.
Alfisols (from Aluminum Al and
Iron Fe) are found in these
areas. These soils usually have a
thin sandy to loamy surface
layer and a clayey to loamy
subsoil. The thin sandy soils are
readily eroded by surface runoff. Leaching of the surface
layer has produced iron and
aluminum oxides, giving these
soils a "rusty" color. The drier
climate in the west supports
mainly dwarfed post oaks and
short grasses (on the Paluxy
sandstone). Larger trees and a
mixture of short and tall grasses
are supported by the wetter
climate in the east (on the
Woodbine sandstone).
Vegetation and soils in the
Western Cross Timbers.
Mollisols (from the Latin mollis - soft)
have developed on the limestones, clays
and marls lying between the Paluxy and
Woodbine sandstones. These are
calcareous (calcium-carbonate-rich)
clayey soils that support a mixture of tall
and short grasslands, under the fairly
dry conditions. These soils have a dark,
organic-rich surface layer due to large
inputs of organic material from the
dense grass cover.
Vegetation and soils in the
Grand Prairie.
Mainly deep calcareous clayey soils
have developed on the shales,
limestones and marls in the east of
the region. These are Vertisols (from
invert - to turn over), characterized
by expansive clay that swells in the
wet season and shrinks and cracks in
the dry season. These movements
cause vertical mixing of the soil
(literally "turning the soil over").
The slightly wetter conditions in the
east support mainly tall grasslands.
Vegetation and soils in the
Blackland Prairie.
Expansive clay soils (vertisols) cause a lot of damage to
structures in the DFW area – particularly slab foundations of
single family dwellings (houses) and pipe breaks.