5 factors influence soil development
THE FIVE SOIL FORMING FACTORS
Soil formation and the properties of the soil are the result of five key factors:
- parent material - this could be rock, organic matter, or an older soil. The composition and structural state (fragmented or not) affect the result and the rate of soil formation. For example, the smaller the grain size the faster the weathering.
- topography - steep slopes are subject to more erosion and thus soil removal. Also the location of a soil in a landscape can affect how climate impacts it. Soils at the bottom of a hill will get more water than soils on the slopes, and soils on the slopes that directly face the sun will be drier than soils on slopes that do not.
- climate - moisture and warmth promote chemical weathering. Where there is no rain, in deserts, there is often no soil. In the tropics where it is warm and there is much rain in tropics there is much leaching so chemical change is significant. Also chemical processes happen much more rapidly where it is warm than where it is cold.
- life forms All plants and animals living in or on the soil (including micro-organisms and humans!). - The dead remains of plants and animals become organic compounds which enrich the soil. Also animals living in the soil affect thedecomposition of waste materials and how soil materials move around in the soil profile.
- time All of the above factors act at Earth's surface over time, often hundreds or thousands of years.
Many of these factors are closely associated - for example much moisture generally is accompanied by abundant plant life.
Soils - consist of several layers
A, B, C horizons - these three major layers or horizons: top soil, subsoil and parent material are almost universally present
A- is the zone where soluble material is removed, organic material accumulates. Surface-dark gray colored-high organic matter, high biotic activity, abundant roots, commonly leached. Subsurface-moderately dark-many roots, moderate organic matter, commonly leached.
C- Parent material-unconsolidated-slightly weathered rocky mass from which soil develops. No biotic activity, few roots. This horizon grades downward to unweathered rock
B- is intermediate - here the composition is determined by the balance between the intensity of chemical weathering in C and intensity and volume of leaching in A. B is also the layer in which some of material dissolved out of A is precipitated. It has fewer roots and less aeration than A.
Important properties of soils:
Porosity - amount of open space in material expressed in % as the ratio of pore volume to total volume
Permeability - degree of ease with which water moves through the available open spaces. This does not always correlate directly with porosity.
Both porosity and permeability tend to decrease with depth. The pressure of increasing weight of rock load tends to close up pore spaces.
rocks where both porosity and permeability are high are good fluid reservoirs -
- for natural fluids which can be extracted or
- as places to store fluids such as wastes or oil reserves