Originally, this rock lay exposed at the surface where it weathered rapidly into soil. The R layer (D horizon) is the bedrock, or sometimes, the sediment from which the other horizons develop. Some of the original rock is intact, but other parts have been chemically changed into new minerals. Variously sized chunks of the rock below are surrounded by smaller bits of rock and clay weathered from those chunks. Partially weathered bedrock composes the C horizon. The A and B horizons together make up the solum, or true soil. If composed of iron oxides, it is called an "ironpan." Fragipans are extremely difficult for crop roots and water to penetrate. 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 claypan (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 which 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 clay-like. 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 like balls in a Japanese pachinko game. 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 therefore, the A is known as the Zone of Leaching. Frolicking earthworms, 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 (minus the cow) compost or humus. The O horizon (sometimes known as the A 0) consists of freshly dead and decaying organic matter-mostly plants but also small (especially microscopic) animals or the occasional rigid cow. It takes many thousand to a 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 rust-like iron oxides, or soil particles that differ from adjacent layers. A soil horizon is a horizontal layer of soil with physical or chemical characteristics that separate it from layers above and below.
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