Which Layer of Mature Soil Contains Silt and Clay?

Soil Horizons

• Classifying soil according to how its layers develop is a detailed science represented by many types and sub-types. A single type, such as the A horizon, can be broken down into several more categories based on soil production and particle movement within that horizon, as well as from one layer to the next. The basic breakdown of soil horizons starts with the top layer, called the O horizon, and moves through a sequence of B, C and E that feature soil at different stages of its formation. The layers don't always follow a proper sequence as the process of soil formation differs from region to region. If a layer close to the surface contains a higher concentration of organic matter than mineral, it is labeled A. If it exhibits a higher concentration of minerals, such as clay or silt, it is labeled E. The resulting downward movement, or illuviation, of clay particles from the E horizon creates the greatest concentrations of clay particles at the B level located below it.

Characteristics

• All mature soils from the A to the E horizon contain some measure of silt, clay or a combination of the two. Each soil layer exhibits different properties, depending upon the process of vegetative decomposition that occurs in the top two horizons and how this affects the bottom soil layers. Based on their concentrations of decayed organic matter and minerals, horizons A and E are the most mature as they generally contain all soil components in amounts that make successful plant growth possible. In some instances, climate factors and other processes interfere with the downward progression of minerals, resulting in much of the clay content remaining at or near the soil's surface.

Mineral Movement

• As the raw organic matter that forms the O horizon decomposes, it essentially feeds and aerates the layer of soil, or A horizon, immediately below it. Depending upon the topography and the climate in a given area, the E horizon is often one of the uppermost layers, occurring directly beneath the O or A layer. Soil in the A horizon is rich in minerals while the E layer contains high levels of silt left behind as its high clay content diminishes through illuviation.

Considerations

• For gardening purposes, soil is classified according to what it is composed of, a process that involves performing compression tests on the top layer, also called topsoil. This is usually a combination of the four basic soil components -- humus, or decayed plant matter, clay, sand and silt. If a handful of soil sticks together but crumbles easily when released, it has a high sand content but contains enough clay and organic matter to hold it together. If the soil sticks together and doesn't crumble, it has a high clay content but not enough sand to aerate it and make it tillable. Mineral particles range in size from sand, the largest, to silt then clay, with particles that are too small to be seen by the naked eye.