Alabama Landscapes
Soils
If you ask most people what characterizes soils in Alabama, they would say two things: a red color and clay-rich. This is a good generalization, but there are differences among soils in Alabama.
Soil Components
Soils are exceedingly complex materials, formed by the interaction of rocks, climate and microorganisms. They typically contain 45% minerals (released or formed during weathering), 50% air and water, and 5% organic matter. The organic matter includes roots, decaying plant material (called humus), and a variety of microscopic animals that live in the soil.
The mineral material in soil varies considerably in size:
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sand 2 and 1/16mm
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silt: 1/16 and 1/256mm
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clay < 1/256mm
A soil’s texture is described by the percentage of sand to silt to clay. Soils with approximately equal amount of sand, silt and clay are called loams.
Most soils have obvious layers or horizons. The lowest horizon (the C horizon) consists of decaying rock material. Immediately above that is a layer composed mainly of minerals, mostly clay (the B horizon). This layer is often called the subsoil. The uppermost horizon (the A horizon) is composed of minerals and humus and is the site of most root growth. Also called the topsoil, this layer contains most of the nutrients used by plants and most of the water stored in the soil that roots use during dry times.
The ability of soil to retain water for
later use is largely determined by the amounts of clay, silt and sand in
the soil. The ideal soil, from the point of view of water retention, is
called a loam, which consists of about equal amount of all three. Too
much sand and the water will drain quickly out of the soil; too much
clay and the soil will become waterlogged. The surface horizon consists
of plants, grasses and humus.
The acidity or pH of a soil is an important characteristic. Soils with
high hydrogen ion concentrations or pH < 7 are considered acid, and
those with low hydrogen ion concentrations or pH > 7 are considered
alkaline. Soils with pH < 6 require lime to raise their pH and make them
suitable for crops.
Soil Formation
The three main variables that determine soil type are the nature of the
parent material, time, and climate.
Soils that are young--immature if you like--closely resemble the rock or
material on which they form. They usually have only one or very weakly
developed horizons. Immature soils evolve as a result of intense
weathering and the activity of microorganisms. Climate now begins to
influence soil characteristics: temperature controls the rate of
weathering and decomposition and the amount of rainfall determines how
much of the nutrient are retained in the soil or flushed away to lower
levels. Mature soils, ones with horizons, have reached equilibrium with
the climate and the parent material.
Soil Classification
Soils are classified on the nature of their horizons or lack of horizons.
