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HYDRIC SOIL CRITERIA

and

Basic Field Indicators

Definition of a Hydric Soil

A hydric soil is a soil that formed underconditions of saturation, flooding, or pondinglong enough during the growing season todevelop anaerobic conditions in the upper part.

AnaerobicWaterlogging process

- Microbes use up O2- H2O retards oxygen diffusion to soil

Technical Definition- Free of all O2- Depleted enough to stress plants

Measurement- O2 meters, Pt electrodes, or a, a’ dipyridyl

Problems- Difficult to measure O2, climate patterns,

Reduction is “lower” than anaerobiosis

“long enough” “the upper part”

Don’t know exactly how long to develop anaerobic conditions- as little as 2 days in a lab- possibly in as little as 1 week in the field

The upper part is- the major portion of the rooting zone- usually 6 (sandy soils) to 12 inches (loamy

soils)

“formed under conditions of”

Artificial drainage does not alter hydric soil status - however, it can alter soil properties

less OMcompactionshift microbial populationschange pH and salinity

One Corps Serving the Armed Forces and the NationFebruary 2009

Indicators of Start of the Growing Season

Indicators of Start of the Growing Season

1. “Green-Up” indicator (new)

2. Soil temperature at 12 inches

is 41 degrees F. or higher

(modified)

3. In the absence of site-specific information (e.g., no site visit), use the 28-degree F., 5 years in 10, per WETS tables (default) © Steve D. Eggers

Box Elder Leaf Bud

Burst

Box Elder Leaf Bud

Burst

© Steve D. Eggers

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Criteria for Hydric Soils

Designed to generate a list of hydric soils fromthe national database of soils interpretationrecords. Not intended for field identification ofhydric soils (except criteria 3 and 4).

The four broad criteria are:1. Organic soils2. Mineral soils with high water tables3. Ponded soils4. Flooded Soils

Criteria, cont.

Criteria are not intended for on-site application

Criterion 1 is an indicator

Data that proves criteria 3 or 4 exists can be used to document the presence of a hydric soil

Criteria for Hydric Soils

3. Soils that are frequently ponded for longduration or very long duration during thegrowing season, or

4. Soils that are frequently flooded for longduration or very long duration during thegrowing season

Ponded or Flooded Hydric Soils

FREQUENTLY = 5 years in 10 or a 50% probability in any one year.

LONG DURATION = a single event that lasts from 7-30 days.

Local Lists of Hydric Soils

• These are lists of map units that are namedfor soil series on the national list, for wetmiscellaneous areas, or for wet soilsclassified at levels higher than the series.

• Also listed are map units that potentiallycontain hydric soil inclusions.

• These are the only lists that should be usedfor wetland delineation purposes, aspreparation before the field visit.

Summary

All hydric soils must meet the definition- Criteria are used to generate lists

Lists are used off-site to determine if an area is likely or not to contain hydric soils

- Indicators are used to identify hydric soils on site

- Technical standard is used with dataTo prove a hydric soil exists in the absence of an

indicator, and to develop new indicators

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Redoximorphic Features

“Redoximorphic Features for Identifying Aquic Conditions”,

Technical Bulletin 301 NCSU, Raleigh, North Carolina.

Order from: Dept. of Ag. Comm., Box 7603, NCSU, Raleigh, NC 27695-7603

(919)-513-3173 $5 / copy.

What we are looking for in the field! Formation of Redoximorphic Features

Anaerobic conditions-soil is saturated so most all pores are filled with

water, absence of oxygen

Prolonged anaerobiosis-changes the chemical processes in the soil

Reduction of Fe and Mn oxides-results in distinct soil morphological

characteristicsmost are readily observable changes in soil color

Oxidation - Reduction

• Electrons (e ) are taken out of one substance and given to another.

• Most electrons come from organic matter as it rots (decomposes).

• If organic matter is not present, oxidation-reduction reactions don’t occur.

Oxidation (Producing e )

Organic Matter

e

H+

Bacteria

Kinds of O.M. for Reduction

- Dead roots- Plant debris- Dissolved organic matter

When air is present in soil, all eproduced by O.M. decompositionare grabbed by O2 to make water

Air present - e taken by 02 to make water

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Reduction (Consuming e ) air in pore, bacteria uses O

4e + O2 + 4H 2H2O

O2 can’t enter waterlogged soils

Reduction in SoilsAerated conditions:

O2 + 4 e¯ + 4 H+ 2 H2O

Anaerobic conditions (for example):

Denitrification

2 NO3- + 10 e¯ + 12 H+ N2 + 6 H2O

Iron Reduction

Fe2O3 + 4 e¯ + 6 H+ Fe+2 + 3 H2O

(Ferric) (Ferrous = colorless)

Soil Color and Oxidation / Reduction

-In subsoil horizons, Fe and Mn oxides give soils their brown, red, yellow colors

-When reduced, Fe and Mn are mobile and can be stripped from the soil particles

-Leaving the mineral grain color (usually “grayish”)

Sulfate Reduction

8e¯ + 10H + SO4

H2S + 4H2O

+ =

Gas (rotten egg)

Oxidation/Reduction Sequence

Redox Potential

Element

Reaction

+350 mV Oxygen O2 H2O

+220 mV Nitrogen NO3¯ NO2

¯, N2, NH4+

+200 mV Manganese Mn+4 Mn+2

+120 mV Iron Fe+3 Fe+2

-150 mV Sulfur SO4-2 H2S

-250 mV Carbon CO2 CH4

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Laboratory Studies

O2

NO3¯

Mn+2

Fe+2

Conditions Needed to Reduce Fe2O3

1. Organic matter must be present (source of e )

2. Air must not enter soil (soil-waterlogged).

Cont.

3. Bacteria must decompose organic matter (warm temp., pH between 5 & 8).

4. Dissolved O2 in water must be removed.

Types of Redoximorphic Features

1. Redox Concentrations- Masses- Pore Linings- Nodules and Concretions

2. Redox Depletions- Depleted Matrix

3. Reduced Matrix

Redox Concentrations

Bodies of apparent accumulation of Fe-Mn oxides- Masses- Pore Linings

ped facesroot channels

- Nodules and Concretions

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Pore Linings

Coatings on a pore surface

Impregnations of the matrix adjacent to the pore

Pore Linings

Ped Faces Redox Depletions

- generally value 4- chroma 2- formerly called

“gray mottles”

Bodies of low chroma where Fe/Mn oxides have been stripped out

Depleted Matrix

Dominant color of the soil is “gray”

Commonly used to identify hydric soilsDiscussed more in hydric

indicators section

Reduced Matrix

Soils have high value, low chroma in situ but color changes when exposed to air• reduced Fe is present• Fe+2 (ferrous iron) is

oxidized to Fe+3 (ferric iron) upon exposure to oxygen

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a, a’ - Dipyridyl

A dye used to test for the presence of reduced Fe-pink reaction to Fe+2

-dye sensitive to light and heat

-apply to freshly broken open soil ped

Describing Redoximorphic Features

• Concentrations and Depletions– Describe type, color, abundance and location

(i.e., along macropores or within matrix)

• contrast can be obtained from color charts

• Reduced Matrix– Describe reduced matrix color, oxidized color,

and time for color change to occur

• alfa, alfa-Dipyridyl– Describe % of soil that reacts and location

Interpretation Problems

Redoximorphic features do not occur in all soils

• Low amounts of soluble Organic Carbon

• High pH• Cold temperatures• Low amounts of Fe• Aerated groundwater

Age of FeaturesRedox features do not always indicate current hydrologic condition- commonly found in drained (historic) wetlands- can be relict of past climates

terraces & benches along riversrelict features may have sharp edges and abrupt

boundaries with the soilrelict nodules and concretions are often rounded

- contemporary features should have diffuse boundaries and / or be associated with ped faces or root channels

Relict vs Contemporary

Relict Contemporary

Relict features are often firm to extremely firm and have abrupt boundaries with the soil matrix

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Introduction

• Field indicators are soil morphological features used to identify hydric soils

• The features result from soil genesis in the presence of “anaerobic conditions”

• They are used for on-site verification of hydric soils and are “test positive”

• The list of indicators is dynamic and are subject to revision with new research and field testing

Development of the Indicators

Continuous process– ongoing since the mid-1980’s

Interagency– including universities, private sector,

federal, state, and local agencies

Multi-disciplinary– soil scientists, hydrologists, botanists

Recent development– COE implementation

Field Indicators of Hydric Soils in the U. S.

- Available on-line at www.soils.usda.gov

Click on Hydric Soils (left column, top) Scroll down to “Links” Click on Field Indicators of Hydric Soils v

7.0Also check “Errata” for changes & updates

New Concepts and Terminology

• Regionalized

• Control Sections

• Depleted Matrix

• Gleyed Matrix

• Test Indicators

Regional

Lists by Land Resource RegionAddresses “Problem” Soils

- Spodisols- Mollisols - Sandy Soils- Flooded and Ponded Soils- Red Parent Materials

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Control Sections or Zones

1) Layers with:-high value, low chroma or;-redoximorphic features or;-organic matter accumulations

2) at a depth3) of certain thickness

Redox Morphology

Depleted Matrix-Value 4, Chroma 1 or 2 with redox “mottles”-Value 5, Chroma 2 with redox “mottles”-Value 5 or more, Chroma <1 w/ or w/o redox-Value 6 or more, Chroma <2 w/ or w/o redox

Gleyed Matrix-All Gleyed Pages Value 4 or more

Depleted / Gleyed Matrix

4/1, 4/2, 5/2, 5/1 or 6/2 Value >=4with 2% redox with or without Gley pagesconcentrations redox features

Depleted Matrix

Value >5 chroma 1,

Value >6, chroma 2

with or without redox concentrations

4/1, 4/2, 5/2 with >2%

redox concentrations

Gleyed Matrix

Gley Pages

Value 4 or more

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Gleyed Matrix

Three Major Divisions

All SoilsUse regardless of soil textureMostly surface layers of organic material

Sandy SoilsLoamy Soils

Use sandy indicators in sandy layers, loamy indicators in loamy layers

Indicator Format

1) Alpha-Numeric Designation2) Short Name3) Applicable Land Resource Regions (LRR)4) Description of the Indicator5) User Notes

Example Indicator Format

A1 – first indicator for All SoilsHistosol – short nameFor use in all LRRs – applicable

regionsClassifies as a Histosol, except

Folists – indicator description“A Histosol has 16 inches …” –

user notes

Houghton MuckA1, Histosol

“All Soils”

A2, Histic Epipedon

A layer of organic soil material 8 inches or more thick, underlain by mineral soil with a chroma of 2 or less. Aquic conditions or artificial drainage are required.

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NRCS/WLI/2000 M. Whited

A2 - Histic

Epipedon

All - Depleted below Dark Surface

• A layer at least 6” thick with a depleted matrix that has 60% or more chroma 2 or less starting within 12” of the surface. The layer(s) above the depleted matrix have value 3 or less and chroma 2 or less. Or if the overlying layers are sandy they are 3/1 or darker.

• Occurs in Mollisols

NRCS/WLI/2000 M. Whited

A12, Thick Dark Surface.A layer at least 6” thick with a depleted or gleyed matrix that has 60% or more chroma <2 starting below 12” of the surface. The layer(s) above the depleted / gleyed matrix have value <3 and chroma <1 in upper 12” and value <3 and chroma <1 in the remainder of the epipedon (think Mollisols!)

Sandy Soils

Have a USDA texture of loamy fine sand or coarser

“Control Section” <6 inches in depthIndicators include:

organic surface layers differential translocationstreaking of OMFe stripped matrix

Sandy Soilswith High OM surface layers

S1, Sandy Mucky Mineral

Lab testing or professional soil scientist recommended

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Key for determining Contrast

Loamy and Clayey Soils

• Texture is loamy very fine sand and finer

• Control section usually starts within 10-12 inches

• Most indicators are based upon the reduction/oxidation of Fe

•A layer of mucky modified loamy or clayey soil material 4” or more thick starting within 6” of the surface.

Loamy Soils

F1, Loamy Mucky mineral

Even expert soil scientists can not consistently apply this indicator without lab data

“Test” Indicators

• Indicators needing further study

• Designated for use for problematic hydric soils and require additional documentation

• TF12

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A16, Coast Prairie Redox (old TS5 Chroma 3 Sandy Redox)

use with problem soils

A layer starting within 6” of the soil surface at least 4” thick with a matrix chroma <3 with >2% redox concentrations.

No texture requirements

F21, Red Parent Material(Old TF2)

A layer derived from red parent material at least 4” thick starting within 10” of the soil surface with a hue of 7.5YR or

redder. Matrix value and chroma > 2 and <4 with >10% redox depletions and/or concentrations.

Hydric Soil Indicators for LRR K & M

• Histosol A1

• Histic Epipedon A2

• Black Histic A3

• Hydrogen Sulfide A4

• Stratified Layers A5

• Depleted Below Dark Surface A11

• Thick Dark Surface A12

• Sandy Mucky Mineral S1

• Sandy Gleyed Matrix S4

• Sandy Redox S5

Hydric Soil Indicators for LRR K & M

• Stripped Matrix S6• Loamy Mucky

Mineral F1• Loamy Gleyed Matrix

F2• Depleted Matrix F3• Redox Dark Surface

F6

• Depleted Dark Surface F7

• Redox Depressions F8

• Coast Prairie Redox A16

• Red Parent Material F21

Universal Indicators

• A11 Depleted Below Dark Surface

• A12 Thick Dark Surface

• F3 Depleted Matrix

• F6 Redox Dark Surface

• F21 Red Parent Material

GET TO KNOW THESE!

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Hydric Soils Tech Note 4NC Supplement permits combining

Indicators if all requirements are met except thickness.

• S5 – Sandy Redox & S7 – Dark Surface• F1 – Loamy Mucky Mineral• F3 - Depleted Matrix• F6 – Redox Dark Surface• F7 – Depleted Dark Surface

Hydric Soils Tech Note 4

0-3, 10YR 2/1, Loam3-6, 10YR 3/1, cmp 7.5YR redox, Loam6-10, 10YR 5/2, cmp 7.5YR redox, Loam10-20, 2.5Y 4/2, Loam

• F6 needs 4 inches w/in upper 12 inches• F3 needs 6 inches starting w/in 10 inches

• Combine layer 2 & 3 to meet the more restrictive 6 inch requirement of F3

The ‘Professional Judgment’ Clause

• The indicators are used to help identify the hydric soil component of wetlands; however, some hydric soils do not have any of the currently listed indicators. The absence of any listed indicator does not preclude the soil from being hydric. – Some “wet” sites will not meet an indicator. What do we

do if we believe a soil is hydric?

• Guidance for identifying hydric soils that lack indicators can be found in Chapter 3 (see the sections on documenting the site and its soils) and in Chapter 5 (Difficult Wetland Situations in the Region).

SUMMARY“Field Indicators of the U.S.”

• are based upon soil genetic processes

• use hydromorphic features

• are “test positive”

• represent “state of the science”

• are regionalized

• will require further development, testing, & validation

• are recognized in the Regional Supplements