Soils

A soil is a body of weathered mineral and organic materials that has been altered through time into a series of discrete layers. The movement of water through the materials facilitates the development of layers (horizons) that are distinguished by whether they have lost or accumulated different mineral and organic constituents (Figure 1). A number of strongly interrelated variables contribute to the development of a soil; these are most succinctly summarized by Hans Jenny's clorpt equation, which emphasizes the interaction of the five soil-forming factors of climate, organisms, relief, parent material, and time.

Every soil starts from an accumulation of some type of parent material (fractured bedrock, river alluvium, glacial till, wind-blown loess, plant residue, etc.) that is then altered through time by weathering processes that are highly dependent [Page 2586]on the climate (long-term patterns of precipitation and temperature). The organisms (plants and animals of all sizes, including microbes) involved in soil formation are also closely linked with the climate. The landscape position (relief) affects the potential energy for movement of materials into and out of the original parent material. Additions to the parent material can include mineral particles (e.g., dust) and organic materials from decaying plants and animals. These additions and the original parent material are altered by physical and chemical weathering, and through time, the various weathered products are moved (translocated) throughout the parent material, forming what is referred to as a soil profile.

Figure 1 Soil profile illustrating the typical sequence of horizons. O horizons are dominated by decomposing organic material; A horizons are mineral horizons enriched with organic matter; B horizons are most often illuvial horizons, with the illuviated material largely dependent on the interaction of the soil parent material, climate, and time; C horizons represent relatively unaltered parent material. If present, unaltered parent material would be represented by an R horizon.

Source: F. D. Wilkerson.

Regardless of the dominating soil-forming factor, the presence of water is critical to the formation of a soil profile and its characteristic horizons (layers). A desert soil exhibits very different attributes from a soil developed in a humid environment, but water is a crucial component in both. On the planet Mars, even without what we recognize as plants, the presence of water supports the existence of Martian soils. Water-related soil processes point to its significance in chemical reactions within the soil and its role as a medium for the movement of different materials in solution (leaching) or in suspension. The movement of water is driven by gravity, so the majority of translocation within a soil profile proceeds from the top of the profile downward through time. This movement is facilitated by the presence of soil pores (voids or air spaces), which occur within the profile in a variety of shapes, sizes, and connectivities. As translocations proceed over time, soil components are removed from certain horizons and concentrated in others. Horizons that have lost a certain component are called eluvial horizons (soil components have exited that horizon), while horizons with an accumulation of translocated components are illuvial horizons (soil components have moved into the horizon).

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