Though they appear to be static and inert, soils are dynamic; the result of five major factors:
It is important to recognize that these factors do not operate independently, but rather work synergistically. Climate, which directly forms soil, also influences the plant and animal life of a region, which also affect soil formation. Similarly, the effects of climate on parent material will be mediated by topography.
Soil is formed and moved through the ecosystem in a continuous process called pedogenesis. Through freezing and thawing (climate), bedrock (parent material) is broken down, fractured, dissolved, and changed into soil. Plant roots loosen and break up parent material and also stabilize accumulating soil particles. Plants and animals cycle nutrients and move them throughout the ecosystem. Topography influences the direction and rate of runoff, affecting vegetation. Rapidly moving water can remove newly formed soils and also impede soil development. All of these processes occur slowly, and soil develops over time.
Parent material is the organic and mineral material from which soil develops, such as bedrock. The nature of the parent material greatly influences the characteristics of the resulting soil. For example, quartz-rich, coarse-grained rock yields soils with a sandy texture.
Climate plays a key role in degrading parent material into soil through physical weathering. Precipitation, temperature, wind, and sunlight are the major climatic factors affecting soil formation. Water seeping into rock cracks freezes, expands, and fractures rock. The repeated cycle of freezing and thawing breaks rocks down into progressively smaller fragments. The wetting and drying cycle following precipitation events also breaks parent material down as mineral constituents expand and contract at different rates.
Temperature also plays a role in soil pedogenesis. The raw organic material from which some soil is formed is mainly plant material – leaves, bark, roots, and stems of dead plants. This material is broken down mainly by microbes and occurs much more rapidly in warm climates than under cold temperatures.
Living organisms play important roles in soil formation, including initiating the process on bare rock. Lichens, an assemblage of both fungi and algae, grow directly on rock surfaces, beginning the breakdown of parent material through chemical weathering processes. As these plants undergo photosynthesis and respiration, they release acids which break down the rock into smaller fragments, eventually encouraging mosses and grasses to take hold.
As soil develops and larger plants such as grasses and trees take hold, these species promote further soil development by yielding organic matter inputs. The rates of nutrient accumulation and cycling vary by ecosystem type. For example, the deep, extensive roots of Midwestern tallgrass prairie plants contribute rich amounts of organic matter to the upper layers of soil; in contrast, trees in dense forests contribute needles and leaves which result in a thin layer of organic matter. The organic acids from plant decomposition accumulate in deeper soil layers, promoting the development of intermediate soil horizons. Plant cover also reduces erosion and loss of soil from the system.
Worms, termites, insects, and mammals all play a role in soil development by promoting the movement of water and air into soils. These organisms also digest plant matter and make nutrients more available to other organisms in the system. Bacteria, tiny worms in the soils called nematodes, and single-celled organisms also cycle nutrients on a much smaller scale.
Ants cycle soils, bringing deep materials to the ground surface.
Topography refers to the elevation, slope, and orientation of land features. The influence other soil-developing factors such as plant life and climate are mediated by gradient. For example, rainfall mainly runs off a steep slope without infiltrating the soil, but will pool in low-lying areas. Larger plants occupy areas of shallower slope; only hardy plants can grow on steep slopes. As a result, soils on slopes tend to have shallower and less well-developed soils than flat portions of the same landscape.
All of the processes playing a role in soil development take time. Over time, a soil will develop from parent material as a result of physical and chemical weathering as described above. The amount of time required for soils to develop is a function of how rapidly the processes proceed.