A soil horizon is a horizontal layer of soil with physical or chemical characteristics that separate it from layers above and below. More simply, each horizon contains chemicals, such as rust-like iron oxides, or soil particles that differ from adjacent layers. 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. Considered together, these horizons constitute a soil profile.
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. It takes many thousand to a million years to achieve a mature soil with fully developed horizons.
The O horizon (sometimes known as the A0) consists of freshly dead and decaying organic matter—mostly plants but also small (especially microscopic) animals or the occasional rigid cow. A gardener would call this organic matter (minus the cow) "compost" or "humus." Below the O lies the A horizon, or topsoil, composed of organic material mixed with soil particles of sand, silt, and clay. Frolicking earthworms, other small animals, and water mix the soil in the A horizon. 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. These tiny clay particles zigzag downward through the spaces (pores) between larger particles like balls in a Japanese pachinko game. 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.
The leached material ends up in the B horizon, the Zone of Accumulation. The B horizon, stained red by iron oxides, tends to be quite clayey. 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.
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). In arid climates, intense evaporation sucks water and its dissolved minerals upward. This accumulation creates a hardpan impenetrable to any rain percolating (sinking) downward, resulting in easily evaporated pools or rapid runoff. If the hardpan is composed of the calcium-rich mineral calcite, it is called "caliche." If composed of iron oxides, it is called an "ironpan." Fragipans are extremely difficult for crop roots and water to penetrate.
Partially weathered bedrock composes the C horizon. Variously sized chunks of the rock below are surrounded by smaller bits of rock and clay weathered from those chunks. Some of the original rock is intact, but other parts have been chemically changed into new minerals.
The R layer (D horizon) is the bedrock or, sometimes, the sediment from which the other horizons develop. Originally, this rock lay exposed at the surface where it weathered rapidly into soil. The depth from the surface to the R layer depends on the interrelationships between the climate, the age of the soil, the slope, and the number of organisms. Most people do not consider the R layer as soil, but include it in the profile anyway, since the weathering of this bedrock usually produces the soil above it.
In a perfect world, all soils demonstrate these horizons, making the lives of soil scientists and soil students blissful. In reality, however, some soils, like transported soils (moved to their present locations by water, wind, or glaciers), lack horizons because of mixing while moving or because of youth. In other soils, the A and B rest on bedrock, or erosion strips an A, or other complicated variations. Around the world, scientists classify soils by these horizonal variations.