Hydric
Soils Defined
Hydric
soils are defined as soils formed under conditions of saturation, flooding or
ponding long enough during the growing season to develop anaerobic conditions
within the upper part.
The saturated and anaerobic
conditions do not have to be occurring at the time
of examination. Similarly, soils formed under these conditions that have been
drained are still considered hydric soils.
A soil is defined as saturated when all
pores are filled with water, excluding all air. The saturated soil closest to
the soil surface indicates the level of the water table.
Flooding refers to submergence by moving water, while ponding refers to
submergence by stagnant water.
The
growing season is determined from the dates when soil temperatures at 20 in (50
cm) below the soil surface are warmer than biological zero (40°F, 5°C). When temperatures are
below biological zero, it is assumed that the growth and functions of plants
and microorganisms are negligible.
Anaerobic
conditions exist when oxygen is absent from the soil.
The upper part is not technically defined to
allow some freedom in decision making, but is understood to be within 12 in (30
cm) of the surface for loams and clays and 6 in (15 cm) for sands.
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Landscape Role in Hydric Soils
Positions
in the landscape that have high water tables are more likely to have wet and
potentially hydric soils. The same is true for soils that are prone to flooding
or ponding. The following landscape positions are locations that may contain
hydric soils, based on the timing and duration of saturation and anaerobic soil
conditions.
Depressional
areas
collect and store runoff water from the surrounding landscape after rain
events. Saturation is not sustained for long periods after rain events. Mineral soils are likely to be present and may or may not
be hydric. Vegetation can consist of trees, shrubs and herbs.
Flood
plains
that are seasonally flooded may contain hydric soils. Hydric soils usually form
in the backwater area, where water is retained for
extended periods of time. Soils are typically mineral with trees and shrubs
being the dominant vegetation.
Seeps occur at the
bases of slopes where the groundwater table intersects the soil surface (Figure
1). They are often found where a slope grades into flat land. The high water
table in seeps is sustained by groundwater discharge. There can be mineral or organic soils, and the vegetation in and around the seep
can consist of trees, shrubs and herbs.
Figure 1. Diagram of a seep
showing that saturated soils are likely to occur at the base of the slope where
the water table intersects the land surface (Source: Vepraskas, M.J. NCSU, SSC
470 Wetland Soils Lecture, Unit 11,
Figure 6).
Regional
landscape
differences also influence the location of hydric soils. In the Piedmont region
of North Carolina, soils adjacent to rivers or streams have high water tables,
resulting in poorly drained soils that could potentially be hydric (Figure 2).
The Piedmont soils upslope from rivers or streams are well drained and not
likely to be hydric because of the greater depth to the water table.
In the Coastal Plain region, the water table is near the surface in the
broad flat areas between the rivers or streams (Figure 3). These areas
typically have organic soils and shrubby or herbaceous vegetation. The soils
adjacent to the streams are also saturated and may be hydric. The rivers or
streams act as drainage ditches in the Coastal Plain soils, drawing the water
table down in the soils on the edge of the broad flats. Those soils are well
drained and are not expected to be hydric.
Figure 2.
Landscape of the Piedmont region showing that the water table is
closest to the surface near water bodies, indicating that the poorly
drained soils will be located in those areas (Adapted from: Buol et. al.,
1997 pg 155 Figure 4.7a).
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Figure 3.
Landscape of the Coastal Plain region showing that the water table
is near the surface in the broad interstream divides, where the poorly
drained soils are located (Adapted from: Buol et. al., 1997 pg 155 Figure
4.7b).
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A perched
water table may occur in any landscape that has an underlying soil layer
high in clay content. The clay layer drastically slows the downward movement of
water through the soil, causing the water to pond or perch. This ponding will
saturate the soils above the clay layer and may produce hydric soils.
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Formation of Hydric Soil
Characteristics
Hydric
soils are found in wet areas in the landscape.
Physical processes occur in hydric soils if the four factors of soil
reduction are met. The four factors are
listed below.
Soil must be saturated
by water. This forces out air, and more importantly oxygen, from soil
pores.
Soil must contain active microorganisms, which is dependant on
soil temperature. Most of these small organisms can function as long as the
soil temperature remains above 40°F (5°C). In North
Carolina, soil temperature usually stays above this level year round.
Microorganisms require a food source. Soil microorganisms rely
on dead or decomposed plant or animal material to sustain their respiration.
This is commonly in the form of dead leaves or dead plant roots.
Soil water must not contain oxygen. This occurs when the water is slow moving or stagnant. Slow
moving water provides adequate time for microorganisms to consume oxygen by
respiration.
If
these four factors are met, the soil environment will change from an aerobic,
or oxygen-rich, to an anaerobic, or oxygen-deficient, state. Several transformations take place as a
result, which cause the formation of hydric soil characteristics.
Soil
organic matter accumulates because the microorganisms decompose plant and animal
material slower in anaerobic soils. This decrease in decomposition causes
organic matter to build up at the surface. As a result, anaerobic soils usually
have a dark and almost black surface. Common rates of organic accumulation may
average 10.7 in. every 100 years (5 cm. every 100 years.)
Nitrate may be reduced to nitrogen gas. Although this
process is not easily detected without thorough investigation, it is worthy of
mention. Nitrate is a common water contaminant that can lead to water quality
problems such as algal blooms and associated fish kills. When the four factors
for soil reduction are met, nitrate may be reduced to nitrogen gas, which is
the predominant atmospheric gas. This helps prevent nitrate from entering
surface waters.
Soil iron may be reduced.
When iron is reduced, it is transformed from its immobile form to a more mobile
form. Immobile iron coats soil particles, which causes
soils to look red or orange. After reduction, the iron that previously coated
the soil particles is removed, leaving behind gray soil colors. Figure
4 shows a cross-sectional view of a subsoil showing a region where the iron has
been removed (iron depletions) and a region where iron has accumulated (iron
concentrations).
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Figure 4.
Cross section of a soil profile showing iron concentrations and
depletions (Source:
http://www.anr.state.vt.us/dec/waterq/wetdelin.htm).
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A schematic diagram illustrating the formation of
redox depletions and redox concentrations is shown in Figure 5 (Vepraskas,
1999). This process occurs as old root channels become saturated by water,
where iron becomes reduced to its mobile form. The iron can then move laterally
away from the area of saturation (forming iron depletions), where it may
convert back to the immobile, reddish form of iron (forming iron
concentrations).
Sulfate
may be reduced to hydrogen sulfide,
which has a characteristic rotten egg smell. This serves as an easy-to-detect
hydric soil indicator. If the rotten egg smell is present, the chances of the
soil being hydric are high.
All of the processes
described, with the exception of nitrate reduction, can be detected by digging
a hole in the soil and examining the conditions present. The methods for
determining the hydric status of soils are explained in the following section.
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Identification of Hydric Soils
There
are two ways to identify if hydric soils are likely to be found on a piece of
property: searching databases or by examining the soils.
Database Search
The United States Department of
Agriculture – Natural Resources Conservation Service (USDA – NRCS) keeps
records of all soils series mapped in the United
States. These records also contain relevant physical and chemical data about
the series, including the depth to the high water table and flooding frequency
and duration. Hydric soils lists are determined from this database utilizing a
specific set of criterion to search the soil series physical and chemical data.
National, state and county hydric soil lists can be found using the following
web site: http://soils.usda.gov/soil_use/hydric/main.htm.
Field Identification of Hydric Soils
Prior to field identification, it is
helpful to determine the likelihood of encountering hydric soils at a given
property. This can be determined by comparing the soil series at the site with
the national, state or local hydric soil lists. Additionally, the drainage class of the
soils at the site could be determined with a county soil survey. The soil
series description in the survey has information about the soil drainage class.
Soils that are poorly drained or very poorly drained have water tables at or
within 10 inches of the soil surface. Those soils are most often hydric.
Soils are examined by taking a soil
sample 12 in (30 cm) deep, either by augering a hole
or by using a shovel to dig out an intact section of soil. The soil sample is
then examined for hydric soil indicators. Hydric soil indicators are diagnostic
horizons or other unique characteristics that are formed as a result of the
hydric soil forming processes.
A dark surface horizon, underlain by a
gray horizon is one common indicator. Another indicator of a hydric soil is a
horizon that is predominantly gray with accumulations of red material (iron)
along root channels or in masses. In the horizons with accumulated iron, there
are also areas that are depleted, making them lighter than the main horizon
color (NRCS, 1998). Examples of hydric soils are shown in Figure 6.
a) b) c)
Figure 6. Examples of hydric soil indicators: a) organic material
accumulation; b) hydric mineral soil with red iron accumulations and grey redox
depletions; and c) hydric soil with a gray soil matrix.
The indicators that are used vary based
on the type of soil (Table 1). There are indicators that apply to all soil
types (organic, sandy and finer soils). Some of those indicators are based on
the presence of organic matter (like peat) or smelling hydrogen sulfide odor
(rotten egg smell). Other indicators, like those previously mentioned and those
in Table 1, are more specific to sandy soils or finer particle soils (clayey or
silty soils).
Table 1.
Generalization of hydric soil indicators that are applied to the three soil
categories: all soils, sandy soils and fine textured soils.
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All Soils
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Sandy Soils
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Fine (Silty,
Clayey) Soils
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Examples of
Indicators
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Accumulated
organic matter
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Gray
layer with red accumulations (gray layer); dark surface underlain by gray
horizon (dark surface)
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Gray
layer with or without red accumulations (gray layer); dark surface underlain
by gray horizon (dark surface)
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Maximum
Depth to Indicator (in.)
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Starting
at surface
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6
in. (gray layer)
4
in. (dark surface)
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10
in. (gray layer)
6
in. (dark surface)
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Typical
Thickness of Indicator (in.)
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8
– 16 in. OR
greater
than 16 in.
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4
in. (gray layer)
4
in. (dark surface)
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6
in. (gray layer)
6
in. (dark surface)
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Hydric Soil Identification for
Landowners
The following steps are intended to
give landowners an idea of whether or not hydric soils are present on their
property. It does not replace hydric soil identification by professional soil
scientists.
Is a county soil survey available to
determine what soils are on the property? A comparison the soil series on the
property to the soils on the national, state or county hydric soils lists will
indicate whether or not hydric soils are present.
Go out and look at the landscape of the
property. Are there low lying areas that seem to be frequently wet? Is a lot of
the property along a water body prone to flooding or saturation during wet
periods? This can indicate where hydric soils may exist.
Examine the soils to see if hydric soil
indicators are present. Take a shovel, dig a hole 12 inches (30 cm) deep and
look at the soil. If indicators like those explained above are found, the soils
might hydric. If there is a rotten egg smell, the soils are probably hydric. If
water fills in the hole that was dug, the soils are likely to be hydric.
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Practical Issues of Hydric Soils
Landowners
should be aware of several issues that are influenced by the presence of hydric
soils on their property. Hydric soils may or may not pose a problem.
Wetlands
According to the U.S. Corps of
Engineers (Environmental Laboratory, 1987) and the NRCS (NRCS,1984), the
presence of hydric soils is one
third of the requirements needed to meet a jurisdictional wetland. The two other
requirements include wetland hydrology
and hydrophytic vegetation. Hydrology refers to the movement of water in
the environment. However, wetland hydrology specifically implies the soil is
saturated to the surface for approximately 5% of the growing season, or is
frequently flooded or ponded. Hydrophytic vegetation is adapted to survive in
saturated and anaerobic soils. If the requirements are met for a jurisdictional
wetland then the federal government protects the area.
If the lands are used for agriculture
the NRCS is responsible for regulation. Any land that has been used for
agriculture prior to December 23, 1985 is exempt from regulation, otherwise
wetlands cannot be drained and used for agriculture purposes. The conversion of
a wetland to agriculture would exclude the landowner from any USDA farm
subsidies.
For nonagricultural land, regulations
are handled by the Army Corps of Engineers, and permits must be obtained before
any alteration may be done to the wetland.
For more specific information on
wetlands and pertinent regulations visit:
http://www.bae.ncsu.edu/bae/programs/extension/publicat/arep/wetlands.html
http://h2o.enr.state.nc.us/ncwetlands/
http://wetlands.fws.gov/
http://www.epa.gov/owow/wetlands/
Agriculture
Lands that contain hydric soils (but do
not meet the requirements for a wetland) may be used in agriculture
production. The level and duration of
the water table may produce soils that are too wet for crop production but may
be altered through drainage ditches or tiles.
On-Site
Waste Disposal
The presence of hydric soils will limit
a landowner’s ability to install an on-site waste disposal
system. On-site waste disposal systems cannot be installed in hydric soils,
areas prone to flooding and areas prone to ponding (ie. depressions). To
receive a permit for on-site systems, gray colors cannot be present in the soil
above a depth of 30 inches.
Before developing a piece of property
or dividing it into building lots, a landowner should take into consideration
the presence of hydric soils on the property. If it seems that there may be a
lot of hydric soils, it is recommended to have a soil scientist identify where
soils are suitable for development.
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Glossary
Augering:
use of a hand held tool that is used to take soil samples
Anaerobic
conditions: conditions when the soils are saturated and
depleted of oxygen
Backwater
Area: low-lying area behind a natural levee in a floodplain where the
flow of water is
slowed
Drainage
Class: degree of natural drainage in the soil determined by the depth to
the water table surface; depth to water
table increases with
increased drainage
Horizons:
layer of soil approximately parallel with the soil surface that has
characteristics that differ from the adjacent horizon above and below
Mineral
Soil: soil predominantly comprised of sand, silt and/or clay
Organic
Soil: soil with greater than 12 to 18% organic carbon when there is 0 to
60% clay if the soils are saturated; soils with more
than 20%
organic carbon if the soils
are not saturated
On-Site
Waste Disposal System: septic system
Reduced
Soils: soils that undergo chemical transformations as a result of
saturation, oxygen
depletion and microbial activity.
Soil
Particle: individual pieces of sand, silt or clay
Soil
Series: classification of soils based on similar characteristics between soils;
each soil series has a unique name and unique properties associated
with it
Water
Table: upper surface of groundwater
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References
Buol,
S.W., F.D. Hole, R.J. McCracken and R.J. Southard. 1997. Soil Genesis and
Classification, 4th Edition. p 155.
Environmental
Laboratory. 1987. Corps of Engineers Wetlands Delineation Manual, Technical
Report Y-87-1, U.S. Army Engineer Waterways Experiment Station, Vicksburg, MS.
NRCS.
1994. National Food Security Act Manual, 3rd ed. US Department of
Agriculture, Natural Resources Conservation Service, Washington, D.C.
NRCS.
1998. Field Indicators of Hydric Soils in the United States: a Guide for
Identifying and Delineating Hydric Soils, Version 4.0. US Department of
Agriculture, Natural Resources Conservation Service, Washington, D.C.
Vepraskas,
M.J. 1999. Redoximorphic Features for Identifying Aquic Conditions. Technical
Bulletin 301. North Carolina Agricultural Research Service, North Carolina
State University.
