How Soil Forms
Soil is a combination of mineral and organic matter, water and air. It is created by external processes that break rock into fine particles. Without these processes, the earth would not be able to support plant life, or us.
Over time rock breaks up and these smaller particles mix with decaying animals and plants to create soils.
About ½ of the total volume of good soil is a mixture of disintegrated and decomposed rock (mineral matter) and humus, the decomposed remains of animals and plants (organic matter). The remaining half is space where air and water circulate. The nature of soil in any given environment depends upon its parent material, time, climate, plants and animals, and slope.
The parent material is the rock material that has formed the soil due to weathering, erosion etc. Soils may form on bedrock (that is, from the underlying rock) or may be transported to their location by erosion and other processes. The chemical composition of the parent material will determine the rate of weathering, and therefore, of soil formation, and also its chemical content.
As time passes, the soil will be changed to be less similar to its parent material less. In general, the longer a soil has been forming, the thicker it becomes and the less it resembles its parent material.
Chimate is considered the most factor affecting formation of soils, since it determines how most of the rock will be weathered (chemically or mechanically) and at what rate. A hot, wet climate can produce a thick layer of soil due to chemical weathering, whereas a cold dry climate might produce a think layer of mechanically weathered soil. Rainfall will determine whether minerals are frequently leached (washed) from the soil. Climate will also determine what kinds of animals and plants are present.
Life forms are major contributors to soil, providing the organic matter that is needed to sustain further life. While micro-organisms and animals are important, the main source of organic matter is plants. Soil enriched with decayed life forms is called humus. In addition to being an important source of recycled plant nutrients, humus improves the ability of soil to retain water, and provides tiny spaces for the air that gives oxygen and carbon dioxide to the many life forms in soil.
Earthworms and other burrowing animals mix the mineral and organic parts of the soil, and aid the circulation of water and air through the soil. Earthworms can enrich many tons of soil per acre in a year.
Slope will affect the amount of erosion and water content in the soil. Steep slopes often have poor soils, which may be thin or quickly washed away. On the other hand, poorly drained soils found in some low regions may be thick and very dark, and too dense to support much plant life. The ideal slope of soil development is flat-to-undulating surface above sea level, with good drainage, minimum erosion and good water retention. The direction of slope is also important. Many mountain ranges have completely different soil cover on their alternate sides because they receive different degrees of sunlight and/or winds.
This is the vertical arrangement of the different layers (horizons) of mature soils. The four basic horizons are the O, A, B, and C horizons. The most important of these for life is the O horizon, which is composed of organic matter and humus. Rain, irrigation and other factors can leach (carry) nutrients from this layer into the other layers, thereby enriching them.