In common usage, the term soil is sometimes restricted to only the dark topsoil in which we plant our seeds or vegetables. In a more broad definition, civil engineers use the term soil for any unconsolidated soft when wet material that is not considered bedrock.
Under this definition, soil can be as much as several hundred feet thick! Ancient soils, sometimes buried and preserved in the subsurface, are referred to as paleosols Figure 2 and reflect past climatic and environmental conditions. Soil is important to our society primarily because it provides the foundation of agriculture and forestry.
Of course, soil is also a critical component for terrestrial ecosystems, and thus important to animals, plants, fungi, and microorganisms. Global cycling of key elements such as carbon C , nitrogen N , sulfur S , and phosphorous P all pass through soil.
In the hydrologic cycle, soil helps to mediate the flow of precipitation from the surface into the groundwater. Microorganisms living in soil can also be important components of biogeochemical cycles through the action of decomposition and other processes such as nitrogen fixation. The fundamental factors that affect soil genesis can be categorized into five elements: climate, organisms, relief, parent material, and time.
One could say that the relief, climate, and organisms dictate the local soil environment and act together to cause weathering and mixing of the soil parent material over time. The role of climate in soil development includes aspects of temperature and precipitation. Soils in very cold areas with permafrost conditions tend to be shallow and weakly developed due to the short growing season.
Organic rich surface horizons are common in low-lying areas due to limited decomposition. In warm, tropical soils, soils tend to be thicker, with extensive leaching and mineral alteration. In such climates, organic matter decomposition and chemical weathering occur at an accelerated rate. Worms, nematodes, termites, ants, gophers, moles, etc. Plant life provides organic matter to soil and helps to recycle nutrients with uptake by roots in the subsurface.
The type of plant life that occurs in a given area, such as types of trees or grasses, depends on the climate, along with parent material and soil type. A soil profile may have soil horizons that are easy or difficult to distinguish.
Some soils also have an O horizon mainly consisting of plant litter which has accumulated on the soil surface. The properties of horizons are used to distinguish between soils and determine land-use potential. Soil forms continuously, but slowly, from the gradual breakdown of rocks through weathering.
Weathering can be a physical, chemical or biological process:. The accumulation of material through the action of water, wind and gravity also contributes to soil formation.
These processes can be very slow, taking many tens of thousands of years. Five main interacting factors affect the formation of soil:. Soil minerals form the basis of soil. They are produced from rocks parent material through the processes of weathering and natural erosion.
Water, wind, temperature change, gravity, chemical interaction, living organisms and pressure differences all help break down parent material. The types of parent materials and the conditions under which they break down will influence the properties of the soil formed. For example, soils formed from granite are often sandy and infertile whereas basalt under moist conditions breaks down to form fertile, clay soils.
Soil formation is influenced by organisms such as plants , micro-organisms such as bacteria or fungi , burrowing insects, animals and humans. As soil forms, plants begin to grow in it. The plants mature, die and new ones take their place. Their leaves and roots are added to the soil. Animals eat plants and their wastes and eventually their bodies are added to the soil. This begins to change the soil. Bacteria, fungi, worms and other burrowers break down plant litter and animal wastes and remains, to eventually become organic matter.
This may take the form of peat, humus or charcoal. Temperature affects the rate of weathering and organic decomposition. Though the soil profiles in Figure 6 belong to two very different soils, both contain distinct surface and subsurface soil layers. Scientists have developed methods to describe the various components and characteristics of the soil profile.
Technical descriptions of the soil are not only useful for farmers, but for scientists, ecologists, soil engineers, hydrologists and land use planners. Figure 5. View of a road cut in Maui. Road cuts are excellent ways to observe the layers, or horizons, within a soil profile. This particular soil profile is well developed and consists of many layers. A soil horizon makes up a distinct layer of soil. The horizon runs roughly parallel to the soil surface and has different properties and characteristics than the adjacent layers above and below.
The soil profile is a vertical section of the soil that depicts all of its horizons. The soil profile extends from the soil surface to the parent rock material. The regolith includes all of the weathered material within the profile. The regolith has two components: the solum and the saprolite.
The solum includes the upper horizons with the most weathered portion of the profile. The saprolite is the least weathered portion that lies directly above the solid, consolidated bedrock but beneath the regolith.
There are 5 master horizons in the soil profile. Not all soil profiles contain all 5 horizons; and so, soil profiles differ from one location to another. O : The O horizon is a surface horizon that is comprised of organic material at various stages of decomposition. It is most prominent in forested areas where there is the accumulation of debris fallen from trees.
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