Soil is an intricate substance, and contains six primary components:
1. Rocks and rock particles, constituting the greatest portion of the soil, and may become weathered, manumitting nutrients essential for plant growth.
2. Humus, dead and decaying plant and animal matter that retains water, sustains soil oranisms, and providing nutrients.
3. Dissolved Substances, including phosphorous, potassium, calcium, and other nutrients needed for plant growth.
4. Organisms, including animals, like insects and worms, and many microorganisms, including 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 with the movement of water, minerals, and organic matter vertically through the soil, and by changes inn biological and chemical processes at different depths in the soil. In this way they are disimilar to layers in sediments, which are deposited in sequence from the bottom up.
The first horizon, located at the top, is comprised of accrued litter--leaves, twigs, dead bugs and such organic matter, and is called the O (for organic)horizon. As the litter decays, insects, worms, bacteria, and other oranganisms, consume and carry it underground, where it contibutes to the A horizon. The A horizon contains many of plant-supporting nutrients in many soils.
Water can erode substances from the surface of the soil, or carry some down from the A horizon the the B horizon. Chemical-weathering-formed clay minerals often aggregate in the B horizon. In some environments organisms help to trnsport 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 that more remain unaltered by soil-forming processes than the upper horizons.
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