|One of the identifying characteristics of
wetlands, from both ecological and statutory points of view, is the presence of
hydric, or wet, soils. Hydric soils are defined by the U.S.D.A. Natural
Resources Conservation Service (NRCS) as "soils that formed under conditions of
saturation, flooding or ponding long enough during the growing season to
develop anaerobic conditions in the upper part".
The three critical factors that must exist for the soil to be classified as hydric soil are saturation, reduction and redoximorphic features. When a dominant portion of the soil exhibits these three elements the soil is classified a hydric soil.
Saturation, the first factor, occurs when enough water is present to limit the diffusion of air into the soil. When the soil is saturated for extended periods of time a layer of decomposing organic matter accumulates at the soil surface.
Reduction, the second factor, occurs when the soil is virtually free of elemental oxygen. Under these conditions soil microbes must substitute oxygen?containing iron compounds in their respiratory process or cease their decomposition of organic matter.
Redoximorphic features, the third factor, include gray layers and gray mottles both of which occur when iron compounds are reduced by soil microbes in anaerobic soils. Iron, in its reduced form, is mobile and can be carried in the groundwater solution. When the iron and its brown color are thus removed, the soils show the gray color of their sand particles. The anaerobic, reduced zones can be recognized by their gray, blue, or blue?gray color. The mobilized iron tends to collect in aerobic zones within the soil where it oxidizes, or combines with additional oxygen, to form splotches of bright red?orange color called mottles. The mottles are most prevalent in the zones of fluctuating water and thus help mark the seasonal high water table.
The blue-gray layer with mottling generally describes wetland mineral soils. However, where saturation is prolonged, the slowed decomposition rate results in the formation of a dark organic layer over the top of the blue?gray mineral layer. Although classification criteria are somewhat complex, soils with less than 20 percent organic matter are generally classified as mineral soils and soils with more than 20 percent organic matter are classified as organic soils. For the purposes of this document, the organic layer becomes important when it reaches a thickness of approximately 16 inches. Under the right conditions, the layer can grow to many feet in thickness.
The organic soils are separated in the soil survey into Fibrists, Saprists, and Hemists. The Fibrists, or peat soils, consist of soils in which the layer is brown to black color with most of the decomposing plant material still recognizable. In Saprists, or muck soils, the layer is black colored and the plant materials are decomposed beyond recognition. The mucks are black and greasy when moist and almost liquid when wet. Mucks have few discernible fibers when rubbed between the fingers and will stain the hands. The Hemists, mucky peats, are in between in both color and degree of decomposition.
Identification of larger areas of hydric soil has been simplified. They are identified on maps available at U.S.D.A. Natural Resources Conservation Service (NRCS) county offices.
The amount and decomposition of the organic component determines several important differences between mineral and organic wetland soils.
|Profile #1 is a hydric soil and the wettest of the soils in this photo series. It shows two hydric soil characteristics, a thickened organic layer, explained below, and a gray matrix explained under the second profile. Profile #1 occurs at the lowest point in the wetland and is inundated for extended periods of time. The ponded water fills the soil pores preventing air from entering the soil. A few days of soil saturation is usually sufficient for soil microbes to exhaust the supply of dissolved oxygen in the soil water. The lack of oxygen slaws the process of microbial decomposition causing partially decomposed organic matter to accumulate above the mineral layers of soil creating the thickened 0 and A layers apparent in this soil profile. The thick organic layer at the soil surface indicates a hydric soil.||Profile #2, also a hydric soil, is somewhat higher in the
landscape and although still subject to fluctuating water rabies and lengthy
periods of saturation, is not inundated for long periods. Therefore, the soil
shows the gray matrix color in the Band C layers, but not the thickened organic
surface. Without oxygen, microbes must utilize iron compounds to obtain energy
from organic matter. In the process, iron compounds are converted from
insoluble to soluble and flushed out leaving the gray background color. As iron
precipitates in the aerated zones, additional soluble iron migrates from
anaerobic sites unitl they become so depleted of iron that the gray color
predominates. The term matrix is used to describe conditions in the dominant
volume of soil within a layer. A soil layer is considered to be anaerobic when
the sail matrix is dominated by gray depletion color. Layers B and C show the
gray matrix indicating that these layers are saturated for long periods. The
dull gray matrix extending to the soil surface indicates a hydric
||Profile #3 is also a hydrie soil and although saturated far shorter periods of time, still shows the strong gray matrix color all the way to the soil surface. Fluctuating water levels in the soil allow air to fill the larger pores as the water level lowers and then trap the air as the water level rises again. When the iron dissolved in the water encounters a zone of trapped air, it farms a strong, red-brown colored precipitate. The precipitated colors are known as concentrations or mottles and are evident here in the AB and B layers. The gray matrix with bright mottles extending to very near the soil surface indicates a hydric soil.|
|Profile #4 is significantly higher in the landscape and though it weakly displays wetness characteristics it is not a hydric soil. In both the A and B layers the matrix has medium colors associated with the original evenly distributed oxidized iron coatings on sand grains. Small areas of iron depletion and concentration exist, however the segregation process and attendant gray color is not dominant in any layer. This soil is probably saturated to the surface for only very short periods of time and is, therefore, not a hydric soil.||Profile #5 is not a hydric soil and shows evidence of saturation only in the deeper layers of the soil. As stated previously, iron depletions and concentrations are evidence of iron segregation due to saturation. The AB layer is saturated and anaerobic so infrequently that gray depletions are almost nonexistent. The deeper B layer shows faint depletions indicating that the soil is saturated only at depth and only for relatively short periods. Since the sail is seldom saturated to the surface and only saturated at depth for short periods of time, it is not a hydric soil.||Profile #6 occupying the highest landscape position in this
series, shows essentially no evidence of saturation and is not a hydrie soil.
In this soil.the vertical redistribution of iron into horizons is associated
with water percolating down through the soil profile. As a result, iron is
evenly distributed within each of the soil layers. Saturation is either absent
or restricted to such brief periods that the soil dues not develop anaerobic
conditions, thus preventing iron segregation and development of the gray matrix
within the soil horizons. The absence of inundation also prevents the
development of organic accumulations on the surface.
|Organic soils have lower bulk densities, that is lower weight per
unit of volume, than mineral soils. Consequently, organic soils have more pore
space and greater water holding capacity and while flooded can be more than 80
percent water by volume. By contrast, minerals soils are usually less than 55
percent water. However, water holding capacity has little effect on flood
storage because the pores are usually filled and do not readily release
moisture from the less porous lower layers.
The hydraulic conductivity, a measure of the speed at which water can move through the soil, varies considerably both within and between organic and mineral soils. While organic soils may have a larger water storage capacity, water movement may be considerably slower than in mineral soils. Much depends on the degree of decomposition of organic matter. However, the effect tends to extend the response time or period of time between the onset of a storm event and the resulting peak streamflow as discussed in the hydrology section.
Decomposition is also important in determining the location of the levels of greatest flow with respect to the surface in organic soils. The chart by Verry shows that more than 90 percent of the horizontal water flow in organic soil wetlands occurs at a depth of less than twelve inches below the surface. Relatively undecomposed organic matter near the surface creates larger pore spaces permitting greater Boelter and Verry 1977
flows. As depth increases and organic matter is more completely decomposed, pore spaces are blocked by ever finer particles of organic matter and flow is reduced.
Organic soils tend to be richer than mineral soils in the nutrients important to plant productivity. But organic soils often have very low productivity because the nutrients present are bound in organic compounds and thus unavailable for plant growth. Therefore, unless the wetland receives an inflow of nutrients from other sources, the plant forms present are apt to be those with low nutrient requirements or special adaptations such as carnivorous plants.
When not flooded, organic soils generally have more hydrogen cations available, tending to make them more acid than mineral soils. Hence, acid loving plants are associated with organic soils. An example is the sphagnum mat or ring that forms around a bog lake. Exceptions are those fens which are influenced by limestone geology and thus receive calcium bicarbonate in groundwater. The bicarbonate easily removes the free hydrogen cations by forming water and carbon dioxide and results in fens that are neutral to basic.
Organic soils have a greater potential for removal of excess nutrients and other pollutants. Small soil particles with large surface to volume ratios have the ability to attract and hold positively charged ions, known as cations such as ammonium (NH4++) and calcium (CA++). The cations are adsorbed or loosely held by electrical attraction. Cations held in this way may be stored for extended periods in sediments or removed and incorporated into other natural compounds by chemical or microbial activity. When the adsorbed cations are incorporated into other compounds the soil particles become available to adsorb additional cations. In this way, wetland soils maintain their ability to remove and recycle excess nutrients and other pollutants. Cation exchange capacity is one measure of the potential for wetland soils to alter the chemistry of the waters moving through them and to transform nutrients into other forms.
Go To Next Chapter
Back to Table of Contents