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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); shrink-swell 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 low-lying 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 run-off. 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).
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.
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.
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.