Partially weathered bedrock composes the C horizon. Fragipans are extremely difficult for crop roots and water to penetrate. If composed of iron oxides, it is called an ironpan. If the hardpan is composed of the calcium-rich mineral calcite, it is called caliche. This accumulation creates a hardpan impenetrable to any rain percolating (sinking) downward, resulting in easily evaporated pools or rapid runoff. In arid climates, intense evaporation sucks water and its dissolved minerals upward. Sometimes the top of the B horizon develops a dense layer called a fragipan (a dense but not cemented layer inhibiting penetration of roots), a claypan (often compacted by vehicles), or a hardpan (cemented by minerals). If the upper horizons erode, plant roots have a tough time penetrating this clay and rain that falls on the exposed clay can pool on the surface and possibly drown plants or flood basements. The B horizon, stained red by iron oxides, tends to be quite claylike. The leached material ends up in the B horizon, the zone of accumulation. Sometimes the lower half of the A horizon is called the E (Eluvial) horizon meaning it is depleted of clay and dissolved minerals, leaving coarser grains.
These tiny clay particles zigzag downward through the spaces (pores) between larger particles. Therefore, the A is known as the zone of leaching. Water forced down through the A by gravity carries clay particles and dissolved minerals (such as iron oxides) into the B horizon in a process called leaching. Earthworms, other small animals, and water mix the soil in the A horizon. Below the O lies the A horizon, or topsoil, composed of organic material mixed with soil particles of sand, silt, and clay. A gardener would call this organic matter compost or humus. The O horizon (sometimes known as the A0) consists of freshly dead and decaying organic matter -mostly plants but also small (especially microscopic) animals. It takes many thousand to one million years to achieve a mature soil with fully developed horizons. Horizons usually form in residual soils: soils not transported to their present location by water, wind, or glaciers but formed in place by the weathering of the bedrock beneath them. Considered together, these horizons constitute a soil profile. Soil scientists generally name these horizons (from top to bottom) O, A, B, C, and R, and often subdivide them to reflect more specific characteristics within each layer. More simply, each horizon contains chemicals, such as rustlike iron oxides, or soil particles that differ from adjacent layers. Horizon is often used in geology, for example, as a term that means a separation between two layers of soil.Ī soil horizon is a horizontal layer of soil with physical or chemical characteristics that separate it from layers above and below. However, more distinct differences can occur during scientific applications or in such endeavors as aviation. The difference in the two is very small when viewed casually. Secondly, it is defined as a (actual) spherical surface, what is called a true horizon.
First, it is defined as a (hypothetical) flat, infinite plane what is called a geometrical horizon. Horizon can be defined in two ways when used to separate soil and sky. That is, it is not blocked out (or obscured) by natural or artificially made objects such as trees, mountains, skyscrapers, and trucks. When used in this sense, the visible horizon is one that is visible to see. It is oftentimes used to mean the line that separates Earth (its soil) from the sky (its atmosphere). It originated from the Greek word orizein, which means to limit. The word horizon generally is defined as the line that separates two layers or divisions.
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