Soil Forming Processes and Factors

Soil forming processes include additions to soil bodies, such as sand blown up onto a sand dune; losses from soil bodies, such as soil erosion; translocations within a soil body, such as downward movement and accumulation of clay-sized particles in the subsoil; and transformations of material within a soil body, such as weathering of sand-sized mica particles into clay-sized kaolinite minerals.

The five soil-forming factors are parent material, relief, organisms, climate, and time.

  1. Parent material is the rock or other matter which degrades into soil. Soils are very reflective of their parent material. For example, a soil developed from granite rock will always have a coarse texture and a relatively low pH.

  2. Relief refers to both the slope of land and the aspect (the direction in relation to the sun) of the surface. The most obvious influence of relief is through slope. Slope affects losses and additions and thus causes changes in soil depth.

  3. Organisms refer to the biological agents such as plants, fungi, and microor­ganisms that break down parent material into soil particles and also contrib­ute organic matter to the soil. For example, the distribution, quantity, and type of organic matter in a soil developed under prairie vegetation is very different from a soil developed under forest vegetation.

  4. Climate encompasses rainfall and snowfall, evaporation, and temperature. Climate controls some chemical and physical reactions and it can also affect the type of organisms in and on a given soil. Weathering of a soil is either hastened by a hot, moist climate, or retarded by a cold, dry climate.

  5. Time is an important soil-forming factor because it modulates the other four factors. For example, a younger soil has had less time for its parent material to be changed, and for climate, relief, and organisms to affect the soil forming processes.

From the North Carolina Onsite Guidance Manual