1. 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 (919) $5 / copy.
Presentation developed by:
Michael Whited
NRCS - Wetland Science Inst.
August, 2000
This is a complete presentation on redox. features.
It would be beneficial to the viewer to have a copy of “Redoximorphic Features for Identifying Aquic Conditions”, Technical Bulletin 301, North Carolina State University, Raleigh, North Carolina.
Order from: Dept. of Ag. Comm., Box 7603, NCSU, Raleigh, NC
(919) $5 / copy.

2. Objectives Explain how redoximorphic features form
Upon completion of this section, participants will be able to:
Explain how redoximorphic features form
Identify and describe redoximorphic features
Understand the use of a,a’-dipyridyl to confirm soil reduction

3. References
Vepraskas, M.J. 1995(revised). Redoximorphic Features for Identifying Aquic Conditions. Tech. Bull. 301, North Carolina State University, Raleigh, NC.
Mitsch, W.J. and J.G. Gosselink Wetlands. Van Nostrand Reinhold, New York, NY. pp
Soil Survey Staff Soil Survey Manual. USDA Handbook No. 18. pp
Mostly this is a shortened version of Mike Vepraskas’s Redoximorphic Features slide set.

4. 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 characteristics
most are readily observable changes in soil color
In order for a soil to develop anaerobic conditions it must be saturated so that (most) all pores are filled with water. When this occurs, any dissolved O2 that may be present in the soil water is rapidly removed (couple of days, or so) by respiration of micro-organisms, roots, and soil fauna. An anaerobic condition is a condition in which oxygen is virtually lacking from the soil.
Prolonged anaerobic conditions changes biogeochemical processes in the soil. This change in chemical processes results in distinctive soil morphological characteristics being present in most hydric soils. The most important of these are the results of:
reduction, movement (translocation) and subsequent oxidation of iron and manganese, and
accumulation of organic matter under anaerobic conditions. In most aerobic environments OM is rapidly oxidized and does not accumulate as readily as in wet soils.
The first process results in the formation of redoximorphic features.

5. Reduction Sequence
Review of Bill Patrick’s (LSU) research.

6. Soil Color and Oxidation / Reduction
In subsoil horizons, Fe and Mn oxides give soils their characteristic brown, red, yellow colors
When reduced, Fe and Mn are mobile and can be stripped from the soil particles
Leaving the characteristic mineral grain color
usually a “grayish” color
Most mineral soil grains are dominantly silica, so they are generally “colorless”, i.e. gray or whitish, unless they are coated by OM or Fe.

7. Redox Concentrations A review of formation
Review concentration formation, note how similar appearing features can form in different ways.
See pages in Vepraskas (1995).
Fig. 9 from Vepraskas 1995

8. Types of Redoximorphic Features
Redox Concentrations
Masses
Pore Linings
Nodules and Concretions
Redox Depletions
Depleted Matrix
Reduced Matrix
Types of redox features.
I have added Depleted Matrix as a “subset” of redox depletions, after all it is a big depletion.

9. Redox Concentrations Bodies of apparent accumulation of Fe-Mn oxides
Masses
Pore Linings
ped faces
root channels
Nodules and Concretions
Redox concentrations, most often (in upper horizons) they occur as pore linings in root channels and pore linings on ped faces.
Note the difference in soil terminology vs. hydrology indicators. Soil scientists say redox concentrations as pore linings in root channels, NOT the same as oxidized rhizospheres which require living roots. In fact, the rhizosphere is on / immediately adjacent to root and the roots should have an iron “plaque” on them in order to be used as a hydrology indicator.
Picture is redox concentrations occurring as pore linings on ped faces and in root channels.

10. Soft Masses Soft bodies frequently in the soil matrix variable shape
can usually be removed from the soil “intact”
Darker colored masses are dominated by Mn.

11. Soft Masses in Sand
The masses have diffuse reddish boundaries

12. Pore Linings Zones of accumulation coatings on a pore surface
impregnation's of the matrix adjacent to the pore
Picture is of pore linings along root channels. This is an obvious case of “oxidized rhizospheres.”

13. More pore linings in a dark A horizon.

14. Nodules and Concretions
Firm to extremely firm bodies
often relict
should be irregular shape
diffuse boundary
“halo” or “corona”
Screened Fe nodules from a soil in Texas. In Texas these features are sometimes used on “gravel” roads. These are most likely relict.
Contemporary concretions / nodules should have “halos”, diffuse boundaries, or be irregular in shape.
Concretions have concentric rings, nodules do not.
Photo from M. Vepraskas.

15. Relict Fe/Mn concretion from Chesterfield soil, near San Diego, CA.

16. Fe / Mn concretion with reddish Fe “halo”, a contemporary feature from a depressional wetland in south-central Nebraska (known locally as rainwater basin wetlands).

17. Representation of relict concretion. No “halo.”

18. Representation of contemporary concretion, with iron halo, in this case the yellow could be representative of the Fe oxide goethite.

19. Redox Depletions
Bodies of low chroma where Fe-Mn oxides have been stripped out
generally value  4
chroma  2
formerly called
“gray mottles”
Picture shows redox depletions along root channels. Redox depletions are most common along root channels because this is where the OM is available to energize the microbial reduction process.
Redox depletions are usually not used to identify hydric soils unless the depletions dominate and therefore become the matrix color which is defined by the term “depleted matrix.”
Note: Although not defined in any of the present soil survey guidance it is possible to have redox depletions that are not so gray. For example a 4 chroma (possibly red parent material) soil with 3 chroma “mottles.” If one was sure these features formed as a result of saturation and reduction they should be considered redox depletions. (Mike Vepraskas, pers. commun.).

20. Redox depletions along root channels in an episaturated (perched water table) soil. The upper part of the profile is depleted. Water perches on the clay layer and as it trickles down through root channels into subsoil reduction occurs along root channels.
Photo by Warren Lynn - NRCS.

21. Redox depletion along ped face in subsoil
Redox depletion along ped face in subsoil. This is common morphology in subsoils with fragipans.
Picture is looking down at a horizontal cross-section.

22. Depleted Matrix Dominant color of the soil is “gray”
Commonly used to identify hydric soils
Discussed more in hydric indicators section
Depleted matrix in a Fragiaquept. This profile may be close, but (I believe) the dominant color is “gray.” Might be used as an opportunity to ask the class what they perceive as the dominant color.

23. Reduced Matrix
Soils have high value, low chroma in situ but color changes when exposed to air
reduced Fe is present
Fe+2 is oxidized to Fe+3 upon exposure
Reduced matrix in Missouri River flood sediments. Large polygons were cracked so that outside surface had been exposed to air (brown color), when broken open the interior color was gleyed.

24. Redox Depletions A review of formation
Old Root Channel
Depletion Zone Fe++
Concentration Zone Fe+++
Soil Matrix
Formation of redox depletions and concentrations along root channels
Explain based on “Redoximorphic Features...” booklet.
See pages in Vepraskas (1995).

25. 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
Important to note that dye will react to light so the Rx needs to take place almost instantaneously when dropped on soil.
Dye needs to be carried in a dark colored bottle, do not leave in heat and sun (i.e. never leave on the dashboard of vehicle). Dye should be replaced about once per year. Also, dye will give a false positive reading on soils that have been in contact with steel (i.e. scraped by knives, shovels or augers) - a freshly broken piece of soil for testing is imperative.
Never use dye on any soil that has been treated with HCl. Hydrochloric acid (10% solution) is used by soil scientists (generally west of the Ohio Valley) to test for the presence of carbonates.
Reference tech. notes on a,a’ - Dipyridyl included in student materials.

26. 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
a a’ - Dipyridyl
Describe % of soil that reacts and location

27. Redox Concentrations
Hard Fe/Mn nodule in matrix (likely relict)
Pore linings on root channel
Pore linings on ped surface
Review this adaptation of Fig. 1, pg. 7 in Vepraskas (1995)
Soft Fe mass in matrix
Hard Fe/Mn concretion in matrix
Hard Fe/Mn nodule in matrix (likely contemporary)
Adapted from Fig 1, Vepraskas 1995
Schematic illustration showing different kinds of redox concentrations and their relationship to soil macropores and matrices

28. 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
Low amounts of OC, generally , < 2% OM (e.g. sandy soils)
High pH, > 8 to 8.5, redox status is pH dependent, explain chart.
Cold temperatures, below “biological 0” (<50C). Research has shown that the concept of biological zero may not be valid as reduction occurs in “frozen” soils in Alaska; however, the rate of reduction is very slow.
If no Fe in the soil system can’t form redox features (e.g E horizons of Spodosols, some parent materials such as sand).
Aerated groundwater - soil may be wet (even saturated) but O2 in water prevents reduction (fast moving floodwaters, slope wetlands).
(more of this in “Problem Soils” module).

29. Rate of Feature Formation
A 2 mm thick Fe depletion around a root channel ranged from less than 1 to greater than 100 years depending upon how long reducing conditions occurred and how much Fe was in solution each day
Recently constructed wetlands should have redox depletions evident within a couple of years if wetland hydrology is present during the “growing season”
Everyone asks this question, the basic answer is “it depends”.
A soil can become anaerobic under the right conditions in as little as 1 day (warm temperatures, stagnant water, fresh OM, etc.) and reduced in less than a week.
Note: When soil scientists say “reduced”, they are talking about the level of reduction necessary to change ferric Fe to ferrous Fe.

30. Age of Features
Redox features do not always indicate current hydrologic condition
commonly found in drained (historic) wetlands
can be relict of past climates
terraces in LMV, Texas Coastal Prairie
relict features may have sharp edges and abrupt boundaries with the soil
relict nodules and concretions are often rounded
contemporary features should have diffuse boundaries and / or be associated with ped faces or root channels
Redox features can persist for 1000’s of years.

31. Relict vs Contemporary
Relict Contemporary
Relict features are often firm to extremely firm and have abrupt boundaries with the soil matrix
On left is a relict pore lining in deoxidized loess from eastern Nebraska. Commonly called pipe stems, these features formed at least 10,000 years ago (Pleistocene). On right contemporary depletions along root channels and a contemporary Fe/Mn soft mass in lower left.

32. Quiz time Clockwise from upper left:
Redox depletions and Mn/Fe concentrations in a reddish brown soil matrix.
Classic depletion in root channel with Fe concentration impregnating the soil matrix.
Redox concentrations on ped faces in the E horizon of a hydric Argialboll.
Relict features in bedrock, Torrey Pines State Park, San Diego, CA. Great slide for showing depletions in root channel, adjacent concentrations and tan colored “soil” matrix.

33. Don’t you just love your job?
Photo courtesy of Warren Lynn, NRCS-NSSC.