Soil horizon
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A soil horizon is a specific layer in the soil.
- Soil is an intricate substance, and contains six primary components:
- 1. Rocks and rock particles, constituting the greatest portion of the soil. Mineral nutrients essential for plant growth are freed as these weather.
- 2. Humus — dead and decaying plant and animal matter that retains water, sustains soil organisms, and provides nutrients.
- 3. Dissolved substances, including phosphorus, potassium, calcium, and other nutrients needed for plant growth.
- 4. Organisms, including animals such as insects and worms, as well as many micro-organisms such as bacteria and fungi.
- 5. Water from precipitation, needed for plant growth and helping to circulate other materials through the soil.
- 6. Air, sharing the porous spaces in the soil with water, necessary for respiration by the roots of plants and organisms living in the soil.
These materials are not evenly distributed in soils, but are found in layers, called soil horizons. Soil horizons are made by the movement of water, minerals, and organic matter vertically through the soil, and by changes in biological and chemical processes at different depths in the soil. In this way they are dissimilar to layers in sediments, which are deposited in sequence from the bottom up.
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[edit] Master Horizons
The master horizons are layers in the soil used to differentiate among different soils. While they usually occur in the order presented from top to bottom, in any given area a horizon may be out of order or not present.
[edit] Solum
The solum is the part of the soil in the process of being formed. The O, A, B or E horizons are included in the solum.
[edit] O
The O horizon, is comprised of accrued litter — leaves, twigs, peat soil and organic materials. This is called the O (for organic) horizon. The O horizon has no mineral materials. The deepest part of the O horizon may have organic soils, but they are mostly composed of forest litter and muck. A layman may not consider most O horizons to be a part of the solid ground.
[edit] A
The A horizon is a surface horizon composed of minerals and organic matter. The organic matter is accumulated from growing plants and decomposed organic matter by organisms. Plants add organic matter to the A horizon as their roots grow. Organisms add organic matter to the A horizon as they decompose organic matter from the O horizon. For instance, earthworms decompose matter from O horizons, adding to A horizons, often drastically changing the soil profile. The A horizon is the layer of soil at the mineral soil surface, and is roughly equivalent to topsoil. Plant roots and seeds grow in this layer, which is primarily composed of humus and minerals. The A horizon is usually below the O horizon and above the B horizon.
[edit] E
The E horizon is named for being the zone of eluviation, or leaching out. Minerals are translocated, or eluviated out of this zone, leaving mostly silica. Because the remaining silica is white, the E horizon may appear lighter in colour than its surrounding horizons. The E horizon is usually lighter in colour than the A or B horizon due to leaching that takes place in the soil. Because a B horizon is the area that receives all the leached materials from the E horizon, the E and B horizon must coexist with each other. Younger soils that have no B horizon can have no E horizon.
[edit] B
The B horizon is the zone of illuviation, or translocation in. It is usually located below an A, E, or O horizon. Clay, iron, humus or carbonates accumulate in B horizons, giving them more color than surrounding horizons. This leaching in alters the character of the parent material. In ABC soil, the second or subsoil zone of soil made of clay and oxidized materials and organic matter obtained from the A horizon by leaching; subsoil. Also called zone of accumulation, zone of illuviation.
[edit] C
The C horizon is little affected by soil forming proceses. It is characterized by lacking properties of the A, O, B or E horizons and of the parent material.
[edit] R
The R horizon is the rock horizon, composed of hard bedrock.
[edit] D
Some soil scientists use the term "D horizon" for the consolidated parent rock (bedrock).
[edit] Other Layers
In addition to these soil horizons, other layers are distinguished. Thus, the layer of plant material on the soil surface is classified as: the L horizon (fresh litter); the F horizon (decomposing litter); the H horizon (well decomposed litter)
[edit] Transition horizons
If a horizon has properties of one master horizon for the most part but also properties of another are called transition horizons. They are notated by the dominant horizon property, followed by the second. A horizon most like an A but somewhat like a B would be called an AB horizon, while a horizon that was mostly like a B, but somewhat like an A would be called a BA horizon.
Sometimes the boundary between horizon bodies is complex. A horizon with sufficient tonguing and fingering of the A or the E material into the B master horizon is designates an A/B or an E/B horizon.
[edit] Horizon Subscripts
The A horizons:
- Ah horizons are found under uncultivated land,
- Ahp horizons are under cultivated land, and
- Apg horizons are on gleyed land. P stands for plowed
- Ab horizons are buried
The O horizons
- Oa sapric horizons are well decomposed
- Oe hermic horizons are moderately decomposed
- Oi fibric horizons are not decomposed
The B horizons:
- Bk horizons have an accumulation of calcium carbonate
- Bm horizons are hard from cementation
- Bq horizons are largely silica, being old and weathered
- Bf horizons are permafrost, frozen all year round
- Bg horizons are gleyed, with low O2 content and gray color
- Bh horizons have accumulations of humus,
- Box horizons have a residual accumulation of sesquioxides,
- Bs horizons are areas of sesquioxide accumulation, and are red
- Bt horizons contain clay minerals,
- Bw horizons have weak color or structure development
- Bx horizons, or fragipans contain a dense but brittle layer caused by compaction.
The C horizons are also subdivided:
- Cu horizons show little evidence of gleying, salt accumulation, or fragipan;
- Cr horizons are too dense for root penetration; and
- Cg horizons are gleyed.
[edit] Processes
Water can erode substances from the surface of the soil, or carry some down from the A horizon the B horizon. Chemical-weathering-formed clay minerals often aggregate in the B horizon. In some environments organisms help to transport materials between the A and B horizons. In arid climates, soluble substances like calcium accumulate in the B horizon. The C horizon, beneath the B horizon, contains weathered parent materials which are less altered by soil-forming processes than the upper horizons are.
