HYDRIC SOILS. By Neal Svendsen Resource Soil Scientist Natural Resources Conservation Service Sept 2006

Transcription

1 HYDRIC SOILS By Neal Svendsen Resource Soil Scientist Natural Resources Conservation Service Sept 2006

2 Wetlands and Hydric Soils The term hydric soils proposed by Cowardin in 1979 Classification of Wetlands and Deep Water Habitats. Hydric soils, wetland hydrology, and hydrophytic vegetation, were the three characteristics all wetlands were thought to have. Hydric soil criteria developed by SCS (NRCS) to identify hydric soil series from database information. Criteria defined only for searching soil database, not for routine field use. Published lists of Hydric Soils of the United States in the Federal Register updated annually.

3 Hydric Soil Definition Soils that formed under conditions of saturation, flooding, or ponding, long enough during the growing season to develop anaerobic conditions in the upper part of the soil. Hydric soils (usually) exhibit characteristics that result from repeated periods of saturation and/or inundation for more than a few days.

4 Hydric Soil Criteria 1. All Histels except Folistels and Histosols except Folists, or 2. Soils in Aquic suborders, great groups, or subgroups, Albolls suborder, Historthels great group, Histoturbels great group, Pachic subgroups, or Cumulic subgroups that are: a. Somewhat poorly drained with a water table equal to 0.0 foot from the surface during the growing season, or b. Poorly drained or very poorly drained and have either: i. water table equal to 0.0 ft during the growing season if textures are coarse sand, sand, or fine sand in all layers with 20 inches, or for other soils: ii. water table at less than or equal to 0.5 ft from the surface during the growing season if permeability is equal to or greater than 6.0 in/hour in all layers within 20 inches, or

5 Hydric Soil Criteria (cont.) iii. water table at less than or equal to 1.0 ft from the surface during the growing season if permeability is less than 6.0 in/hour in any layer within 20 inches, or 3. Soils that are frequently ponded for long or very long duration during the growing season, or 4. Soils that are frequently flooded for long or very long duration during the growing season. Remember, this criteria is for database retrieval. Not for field identification.

6 How Do Hydric Soils Form? Two main things have to happen: 1) Soil must be saturated near the surface. (typically within 6 to 12 inches) 2) Oxidation-Reduction (redox) reactions must occur in the waterlogged soil.

7 Upper Part of Soil May be Saturated by: High Water Table Flooding or ponding Restrictive layers Stratified layers

8 High Water Table Capillary Fringe Water Table A soil is considered saturated when the soil water pressure is zero or positive. Water will flow from the soil matrix into unlined auger holes. The most typical situation in western Montana.

9 Flooded or Ponded Soils Ponding in potholes and undrained basins, and inundation from flooding commonly form hydric soils in Montana. Flood irrigation can also form hydric soil characteristics.

10 Restrictive Layers Sandy Loam Clay Impermeable subsoil will also perch water. This can form hydric soils in Montana but is more common in higher rainfall areas.

11 Stratified Layers Clay Sand Water will perch above the clay-sand boundary. This does not usually form hydric soils in Montana but can in areas with higher rainfall.

12 Oxidation-Reduction Reactions (Redox) Redox reactions transfer electrons from one element to another. Oxidation: electrons removed, valence state increases (Fe +2 to Fe +3 ) Reduction: electrons added, valence state decreases (Fe +3 to Fe +2 ) Organic matter is the source of most electrons used in soil redox reactions. Organic matter decomposition is a type of oxidation that provides electrons by bacterial respiration. Redox reactions in waterlogged soils are biochemical reactions.

13 The Biochemical Reaction Electrons are transferred along with a proton in the form of H + ion. Most common reduction reaction in soils is molecular O 2 to water (O 2 + 4e - + 4H + 2H 2 O) When all the O 2 in the soil and soil water has been removed the soil is anaerobic. Despite the waterlogging, bacterial respiration still occurs and organic matter decomposes.

14 The Biochemical Reaction (cont.) Electrons produced are now transferred to NO 3, Mn and Fe oxides, SO 4, and CO 2. The reducing reactions now produce N 2, Mn +2, Fe +2, H 2 S, and CH 4. Nitrogen gas Soluble Mn and Fe Hydrogen sulfide gas (rotten eggs) Methane (swamp gas)

15 Soil Organic Matter Effects Plant litter and dissolved organic compounds provide the energy source. Leaves, pieces of live and dead roots, root secretions, compounds leached from leaves Reduction can occur within days of saturation. Humus, humic & fulvic acids, and compounds containing large amounts of lignin require several months or years to decompose, and do not contribute much to the process.

16 Reduced Soils Soils are reduced when they are anaerobic and contain one or more of the chemically reduced forms of N, Mn, Fe, or S. Three things must occur for a soil to become reduced: Atmospheric O 2 must be prevented from entering the soil (soil is saturated). Biological activity must occur to produce electrons. Soil must remain saturated long enough to deplete it of oxygen.

17 Summary of What Makes a Hydric Soil Saturated (and reduced) soil Organic matter (energy source) Bacteria (to decompose the organic matter) Anaerobic Conditions (to allow reduction) Soil temperatures above 5 o C

18 What s a Hydric Soil Look Like? Redox reactions produce visible changes that can be seen in the field. The morphological indicators (what you can see) are produced by reactions involving four elements: O, Mn, Fe, and S. O 2 reduction doesn t produce a hydric soil indicator directly but results in buildup of C in the form of organic soil layers. Mn and Fe frequently occur together and provide the most visible indicators. Rotten egg smell of H 2 S is one of the most reliable indicators.

19 Soil Color and Redox Reactions In subsoil horizons, Fe and Mn oxides give soils their characteristic brown, red, yellow colors. Soil Color When reduced, Fe and Mn are mobile and can be stripped from the soil particles, leaving the characteristic mineral grain color. Red Soil Coating of Fe2O3 Mineral grain (gray) Remove Fe Gray Soil usually a grayish color

20 Upland Soil Not Hydric Note brown colors Fe oxides evenly coating soil particles

21 Wetland Soil Hydric Soil Note gray matrix where Fe has been removed And rust spots where Fe oxides have accumulated Both are Redoximorphic Features

22 Redoximorphic Features Formed by the reduction, movement, and oxidation of Fe and Mn oxides. Large enough to be seen by the naked eye. 3 basic kinds: 1) Redox concentrations (nodules, masses, pore linings). The reddish brown rust spots (mottles). 2) Redox depletions (zones of gray colors where Fe and Mn oxides have been removed). 3) Reduced matrix (has a gray color in situ because of presence of reduced Fe but changes color when exposed to air, oxidizing the Fe. Usually the wettest soils.)

23 TYPES OF REDOXIMORPHIC Redox Depletions (gray areas Fe removed) FEATURES Redox Concentrations (reddish brown areas Fe oxidized)

24 Field Indicators of Hydric Soils A guide to help identify and delineate hydric soils in the field. Do not replace or modify the requirements in the definition of a hydric soil. List of indicators is dynamic, changes and additions are made. Generally developed for delineation purposes. Best used along the wetland edge.

25 Field Indicators of Hydric Soils Some hydric soils lack any of the currently listed indicators. The lack of any indicator does not prevent classification of the soil as hydric. Some wetland soils may not show distinct hydric soil indicators. Such as soils: formed in grayish or reddish parent materials. with high ph or low content of organic matter. with high concentrations of calcium carbonate. with very dark colored, thick surfaces (Mollisols). with high component of volcanic ash. Relict hydric soil features formed under previously wetter conditions.

26 Organic soil (muck, mucky peat, or peat, more than 16 inches thick). Indicator A1 Histosol

27 Indicator A2 Histic Epipedon Organic soil material 8 to 16 inches thick Histosols and Histic Epipedons are no-brainers for hydric soils

28 A12 Thick Dark Surface High organic matter content creates the thick, dark surface (Mollisols) Have to dig or auger below the dark surface to check for redoximorphic features in the deeper subsoil.

29 Sandy soils require redoximorphic features closer to the surface than loamy or clayey soils. S5 Sandy Redox

30 F3 Depleted Matrix

31 F2 Loamy Gleyed Matrix F3 Depleted Matrix is between the gleyed matrix and the surface layer. Gleyed Matrix

32 F6 Redox Dark Surface Redox concentrations in mineral soils high in organic matter are difficult to see. The organic matter masks most of the concentrations. Careful examination is required and drying of the sample may be necessary.

33 Hydric Soils Hydric soils are just one part of wetland identification. Cannot always verify wetland hydrology due to time of year. Wetland vegetation may have been removed. Hydric soils are sometimes the only obvious wetland indicator. Hyric Soils are

34 For more information Google Hydric Soils Or contact: Neal Svendsen USDA-NRCS 3550 Mullan Road, Suite 106 Missoula, MT (406) , ext 115