|WikiProject Soil||(Rated Start-class, Low-importance)|
Why are hydric soils important?
The environmental conditions that create hydric soils (water remaining at or near the soil surface for extended time periods during the growing season) also favor the formation of many types of wetlands.
Wetlands play important roles in the environment, some of which we have only begun to understand and appreciate. Groundwater discharges (exits) to become surface water through wetlands. During periods of heavy rains or melting snow, flooding can present a real danger to people and property; but because wetlands occupy depressions in the landscape they can trap and thereby detain flood waters, thus reducing downstream damages. Wetlands are often difficult places for humans to physically move around in, so most people avoid them; this is one reason that they provide critical habitat for many rare and endangered species of flora and fauna. Because wetlands often occur in relatively low elevations, they commonly receive polluted waters from man's activities on higher, drier ground; wetlands can effectively filter these waters and retain excess nutrients. Wetlands are also valuable for recreation, including nature appreciation, hunting, fishing, canoeing, etc.
The number and extent of wetlands has been greatly diminished in the United States since the time when the first white settlers arrived. Political debates and new regulations have focused on methods to conserve and rehabilitate wetlands. Because they are formed in association with wetlands, hydric soils can be used to identify the presence and boundaries of wetlands. In fact, hydric soils were defined so that they help identify wetlands. Along with unique vegetation and hydrology, hydric soils are one of the three required indicators for wetland identification. As a result, hydric soils are a very important issue in land management and land planning across the United States due to their role in the identification of wetlands and their function in wetland ecology.
Defining hydric soils
Various state and federal government agencies are involved with wetland protection. The US Department of Agriculture - Natural Resources Conservation Service identifies and protects wetlands that have been used for agriculture. The US Army Corps of Engineers protects wetlands of practically any size. With the help of soil scientists, they have defined hydric soils, which they consider to be those soils which are developed under sufficiently wet conditions to support the growth and regeneration of hydrophytic vegetation:
A hydric soil is a soil that is saturated, flooded, or ponded long enough during the growing season to develop anaerobic conditions in the upper part. This definition can be broken up into three component parts:
- The soil is saturated, flooded or ponded. Saturated conditions are often the result of a high water table. Flooded conditions are produced by overflowing streams, runoff from higher surrounding slopes or from high tides that inundate coastal wetlands. Ponded conditions are produced by higher water inflow than water outflow from a closed depression.
- Wet conditions occur during the growing season. This is the period of time when the soil is above 5oC or approximately 40oF. Above this temperature, biological activity is significant and many plants are able to grow.
- The soil is wet long enough to develop anaerobic conditions in the upper part. The vast majority of soil biological activity occurs at or near the soil surface. When the soil is biologically active, a few weeks of wet conditions is usually adequate to use up available oxygen; however, this can be affected by many factors (e.g. soil and water temperature, the oxygen content of the water, soil organic matter content, soil permeability, etc.). The important thing is that anaerobic conditions result often or long enough to support mostly hydrophytic (water-loving) plants. Further, much of the biological activity in soils is engaged in the decomposition of organic matter either deposited within or on the soil surface. When oxygen is not available to the soil flora and fauna, biological activity is greatly reduced. As a result, organic material builds up in the soil. Additionally as a result of the wet, anaerobic environment the soil takes on a characteristic reducing condition and undergoes chemical reactions that are different than non-hydric soils.
Hydric soil properties and indicators
Hydric soils usually have a water table, or the top of a zone of saturation, within one foot from the soil surface during the growing season. This shallow water table excludes oxygen and so creates a reducing environment, especially in the upper part of the soil profile. As a result, mostly hydrophytic plants proliferate -- such as rushes, cattails, sedges and skunk cabbage.
Most soils, including hydric soils, are dominantly composed of minerals such as quartz, feldspars, clay minerals, etc. However, hydric soils commonly have a build-up of organic matter at the soil surface, for reasons described above, which can make the surface horizon dark colored. If the organic matter content (measured as organic carbon) is greater than 20 to 30% of the soil's weight (depending upon clay content) and this organic-rich layer is over 16 inches thick, then it is considered an organic soil. Most soil organic matter originates as plant tissue, so organic soils are called Histosols (the Greek word for tissue is histose). Many types of organic soils exist, but they can be classified by their thickness and degree of decomposition. Peat, such as common "peat moss", is mostly composed of recognizable plant fragments that are only partly decomposed. Muck contains highly decomposed organic matter and, when drained of excess water and carefully managed, these black and spongy soils comprise some of the more fertile soils in mid-latitudes.
Another property characteristic of hydric soils is their color or color patterns. Besides the dark shading from the presence of organic matter, iron compounds are the most important coloring agents in soils. Hydric soils tend to exhibit gray or blue-gray colors (known as gleying or gleyed colors) especially just beneath the topsoil or surface horizon (see lower portion of photograph). This results from the chemically reduced oxidation state of iron compounds, as opposed to the rusty red (oxidized) and brown colors of drier, non-hydric soils. Where shallow water tables fluctuate, gray, yellow and red colors can also occur as small splotches, threadlike or network patterns, created by accumulations or depletions of iron and manganese (orange colors in photograph). Because they result from processes of reduction and oxidation these color indicators of wetness are collectively termed redoximorphic features.
- I redirected "Reducing environment" to "Redox." --220.127.116.11 (talk) 11:43, 10 November 2008 (UTC)
Vepraskas, M. J. and J. L. Richardson, Editors, 2001, Wetland Soils: Genesis, Hydrology, Landscapes, and Classification