Soil Formation (Open Notes)
Soil: What is it?
- Soil. Earth. Dirt. No matter what we call it, it’s the material that constitutes the outermost solid layer of the planet. We build on it. We raise food in it. We mine mineral resources from beneath it.
- Apparently unchanging and lifeless, soils are dynamic mixtures, teeming with life. One teaspoon of soil in the temperate regions can contain billions of organisms, ranging from simple bacteria and fungi to more advanced forms. Earthworms, insects, and spiders are examples. Bedrock is continually fractured, dissolved, and changed into soil, but the process occurs slowly so we usually never notice.
- Soil is a naturally occurring mixture of mineral and organic ingredients with a definite form, structure, and composition. The exact composition of soil changes from one location to another. The following is the average composition by volume of the major soil ingredients:
- 45% mineral matter (clay, silt, sand, gravel, stones)
- 25% water
- 25% air
- 5% organic matter or humus (both living and dead organisms)
Formation of Soil:
- There are thousands of different soils throughout the world. Soils are formed through a combination of five important factors.
- Parent Material - Earthy materials, both mineral and organic. Parent material can be a volcanic deposit such as ash; it can be a sediment that has been transported and deposited by wind or water; or it can be a deposit left by glaciers.
- Climate - Parent material is broken down into finer particles by a process called weathering, which is controlled by the climate of a given location. Temperature and water are major climatic forces that influence weathering.
- Living Organisms - Both plants and animals help to create soils. As they die, plants and animals add organic matter to weathered parent material to help form subsoil and topsoil. As animals dig through the soil, they break it up, permitting more air and water to enter.
- Topography - Topography is the hilliness, flatness, or amount of slope of the land.
- Time - The age of a soil must be considered in thousands and even millions of years since it may take hundreds of years for these factors to form one inch of soil from parent material.
- Soils develop into layers. These layers, called horizons, are usually seen along road cuts and other areas where the soil is exposed. In the hypothetical situation, there are four horizons in a soil profile. The thickness of each varies with location, and under disturbed conditions -- heavy agriculture, building sites, or severe erosion, for example -- not all horizons will be present.
- O = Organic. It consists of fragments of leaf litter, twigs, roots, and other organic material lying on the surface of the soil. This layer is not present in cultivated fields.
- A = Topsoil. It is usually darker than lower layers, loose, and crumbly with varying amounts of organic matter. This is generally the most productive layer of soil.
- B = Subsoil. It is usually light colored, dense, and low in organic matter.
- C = Parent Material. The unconsolidated organic and mineral material in which soil forms..
- R = Bedrock. The solid rock that underlies the soil and other unconsolidated material