Review of the Aquic Conditions and Hydric Soils Status of the Sharkey and Related Vertisols

Final Report

December 8, 1997

EXECUTIVE SUMMARY

Principle findings are as follows:

1. The Sharkey is a Vertisol and can be classified as a member of the very-fine, smectitic, thermic Chromic Epiaquerts or a similar subgroup. In cases of artificial drainage, the classification should be determined by the soil’s morphology which will reflect conditions prior to drainage.
   
2. Saturation is an idealized concept that is rarely if ever achieved in any natural soil. Vertisols should not be expected to reach a point of saturation. In lieu of saturation, it is proposed that the occurrence of free water be monitored with piezometers, and that after free water has been present for a continuous period of 14 days, the soil at the depth monitored will be assumed to have reached the condition of "hydric-saturation" on the 15th day. Hydric-saturation will be a condition that substitutes for the idealized concept described by the term saturation.
   
3. A solution of a, a’-dipyridyl dye is an appropriate tool to use to identify reduction in soils. However, its use is appropriate only for soils that contain reducible forms of iron. In soils which have no reducible iron then redox potential measurements should be made instead.
   
4. A recommended definition for Artificial drainage is "the use of human effort and devices to remove free water from the soil surface or from the ground, or the use of human effort and devices to prevent surface or ground water from reaching the soil". This definition is intended to include dams and levees as devices that are used to artificially drain soil.
   
5. The phrase "soil formed under conditions of..." does not refer to a set number of years. It means that the soil contains visible features (hydric soil field indicators) that are known to form only under conditions of wetness approaching saturation, or caused by flooding or ponding that lasted long enough such that the soil developed anaerobic and reducing conditions in its upper part. Because artificial drainage removes the free water that is required for aquic conditions, the occurrence of aquic conditions cannot be verified in these soils. The assumption that aquic conditions occurred at some time in the past is shown by the presence of morphological features that formed under such conditions.
   
   Official Series Descriptions of the Sharkey and related Vertisols were examined for the presence of field indicators of hydric soils. Such indicators were identified in 12 (including the Sharkey) out of the 18 series descriptions that were examined. The hydric soils had common field indicators which showed that the soils were anaerobic and reduced in their upper parts at some point in their history.. However, it is not known whether these hydric soils still have wetland hydrology, or whether they are in areas that can be considered jurisdictional wetlands. On-site investigation of the hydrology is needed to make this wetland assessment.
   
6. Definitions of flooding and ponding that are given in the Soil Survey Manual appear to meet the needs for such definition. More field description of the depth, duration, and possibly the source of the water for flooding and ponding is warranted.

INTRODUCTION

Background

The hydric soils status and classification of certain clayey soils, mainly those of the Sharkey series, have recently been challenged. Questions have also arisen about the need for a uniform and mutually acceptable method for monitoring saturation in Vertisols such as those of the Sharkey series. During the week of January 6, 1997, soil and wetland scientists from NRCS and scientists from various land-grant universities and a representative from the U.S. Army Corps of Engineers met in the Mississippi Delta States and participated in a field study of the Sharkey soils to determine if changes in their hydric soils status were warranted. One of the recommendations from this study was that a committee of prominent scientists be assembled to review existing data and clarify the concept of the Sharkey series and agree on a classification.

This committee was appointed on September 4, 1997 by Mr. Horace Smith, Director Soil Survey Division (Sponsor) and Dr. Robert Ahrens, Soil Scientist, National Soil Survey Center (Advisor). The committee will be referred to as the Sharkey Committee in this report. It consisted of the following members:

Committee Members

• Dr. J. L. Richardson, North Dakota State University, Fargo (Co-Chair)
• Dr. M. J. Vepraskas, North Carolina State University, Raleigh (Co-Chair)
• Dr. P. McDaniel, University of Idaho, Moscow
• Dr. W. Hudnall, Louisiana State University, Baton Rouge
• Dr. D. Pettry, Mississippi State University, Mississippi State
• Dr. H. Huddleston, Oregon State University, Corvallis
• Dr. R. Griffin, Prairie View A&M University, Prairie View, TX
• Dr. M. Rutledge, University of Arkansas, Fayetteville
• Dr. L. Wilding, Texas A&M University, College Station
• Dr. P. Veneman, University of Massachusetts, Amherst
• Dr. R. Rabenhorst, University of Maryland, College Park
• Dr. W. Lynn, NRCS, Lincoln, NE
• Mr. C. Fultz, NRCS, Little Rock, AR
• Mr. W. Hurt, NRCS, Gainesville, FL
• Mr. J. Daigle, Alexandria, LA
• Mr. D. Jones, NRCS, Jackson, MS
• Mr. D. Newton, NRCS, Nashville, TN
• Mr. V. Cole, NRCS, Washington, DC
• Mr. M. Whited, NRCS, Lincoln, NE
• Mr. R. Pringle, NRCS, Baton Rouge, LA

Shortly after the committee was assembled the co-chairs agreed to add two more individuals who had extensive experience studying Vertisols. These were: Mr. W. Miller, NRCS, Victoria, Texas and Mr. D. Williams, NRCS-retired, Fort Worth, Texas

Charges

The committee was asked to complete seven charges which were as follows:

1. The concept and classification of the Sharkey series.

2. Agree on a method of monitoring saturation in Vertisols.

3. Agree or disagree that a, a’-dipyridyl is or is not a proper indicator of reduction in soils. If it is not, then a proper indicator for reduced soils must be developed.

4. Agree on a definition of artificial drainage. Does the definition include dams and levees?

5. Examine the series descriptions for Sharkey and related Vertisols and determine the validity of statements such as "...soil formed under conditions of ..." and aquic conditions can be verified "...except in artificially drained soils". How far into the past must we go to satisfy "formed under"?

6. Develop a complete definition for flooding.

7. Develop a complete definition for ponding

Mode of Operation

The co-chairs decided to combine charges 6 and 7 into one charge, and then divided the six charges equally. Each co-chair then selected members of the committee to address a given charge. The co-chairs formulated questions that needed to be answered to address a particular charge, and then contacted members by email or fax to address the questions posed. Answers to questions were returned to the co-chairs in a similar manner. Informal face-to-face meetings were held at the Soil Science Society of America’s Annual Meeting in Anaheim, CA in November, 1997.

When the committee addressed the charges to the maximum extent possible, all responses were forwarded to M. Vepraskas who reviewed them and summarized the opinions of the majority in this final report. It must be remembered that individual committee members may not agree with all recommendations made here.

RESPONSE TO CHARGES

CHARGE 1: Agree on the concept and classification of the Sharkey series.

I. Background

A. History

1. Background information is summarized from that presented by Pettry and Switzer (1996).

1. The Sharkey series was established in Yazoo County, MS in 1901 and it is the dominant soil in the MS Delta region.
2. Sharkey soils occur on nearly level topography on lower parts of natural levees, terraces, and flood plains of the Mississippi River and its tributaries.
3. They have been mapped on slopes ranging from 0 to ½%, to 2 to 5%, which included slopes up to 8%.
4. Sharkey soils were classified as Grumusols prior to Soil Taxonomy, based on shrinking, swelling, and cracking properties.
5. Sharkey soils were reclassified without field studies as very-fine, montmorillonitic, non-acid, thermic Vertic Haplaquepts despite numerous data indicating a Vertisol classification was appropriate.

B. Classification of the Soil Order

1. Detailed field and laboratory studies of Sharkey soils in the MS Delta over the past 20 years clearly indicate the soils are Vertisols based on the following:

1. Extensive cracking occurred when evaporation exceeded precipitation each year with cracks typically appearing in May and periodically closed and opened at the surface after precipitation events until November. Cracks ranged to 8-cm in with at the surface and extended to depths of 1-m and greater.
2. Soils contained clay contents that were more than 30% between the surface and the 18 cm depth, and more than 30% weighted average clay between depths of 18 and 50 cm. Clay contents exceeded 60% in the sola and ranged to 86%.
3. Soils contained well-developed compound structure with prismatic parting to angular and/or subangular blocky structure.
4. Soils contained prominent intersecting slickensides within 100 cm of the soil surface which extended to depths of 3 m and greater. Tongue and groove slickensides increased with depth.
5. Soils possessed coefficients of linear extensibility (COLE) greater than 0.09 in the sola.
6. Clay fractions were dominated by montmorillonite with fine clay (<0.02 µm) exceeding coarse clay (2-0.02 µm).

2. These properties alone are sufficient to justify classifying the Sharkey series in the Vertisol order.

II. Suborder and Lower Classifications

A. Overview

1. The committee feels that no one taxonomic classification will capture all of the Sharkey series as it is presently mapped. Major reasons for this include:

1. Micro-low and micro-high topography
2. Effects of artificial drainage on the classification

2. Micro-low and micro-high topography

1. Many Vertisols, but not all, have a characteristic surface topography that consists of bowl-like depressions separated by ridges (Fig. 1).
2. As shown in Fig. 1, this pattern is caused by the expansion and contraction of the soil on repeated wetting and drying.
3. The ‘bowls’ or micro-lows may be 2 to 5 m across.
4. The micro-highs, or ridges, may be 30 to 60 cm above the lowest point of the bowls.

3. Hydrology

1. Hydrology differs between the micro-highs and micro-lows, in part because the highs shed water and the lows trap it.
2. Soil morphology and the soil classification may also vary between micro-highs and micro-lows (Fig. 1F).

B. Recommendations

1. In cases where soil classification differs between micro-high and micro-low, two classifications must be given for the series.
2. If one classification must be used to represent the series, then it should be taken from the micro-low because the hydrologic conditions of the micro-low will most likely limit how the soils are used.

Figure 1. Illustration of the formation micro-low and micro-high (or gilgai) topography in Vertisols as a result of shrinking and swelling on wetting and drying (A to E). In this diagram the micro-lows form around the large vertical desiccation cracks. Horizon development can differ between the micro-lows and micro-highs (F). Diagram obtained from Wilding and Tessier (1988).

III. Artificial Drainage

A. Present Condition

1. The Sharkey series, in many instances, has the morphology of soils that experience aquic conditions, and based on morphology alone it is reasonable to conclude these soils currently experience periods of seasonal saturation and reduction.
2. However, the Lower Mississippi Valley has undergone extensive hydrologic modification such that it is believed that at least some members of the Sharkey series no longer become seasonally saturated or reduced due to the construction of levees, dikes, or other structures.
3. Some members of the Sharkey committee believe strongly that the soil classification selected should reflect the current or modified hydrology, but this position cannot be endorsed.
4. Soil Taxonomy clearly requires that soils with artificial drainage be included with soils that are currently saturated and reduced or in other words have aquic conditions.

1. It is recommended that the Sharkey soils be classified on the basis of their current morphology.
2. Effects of artificial drainage, if known, can be included in the series description but should not be used to classify the soil.

B. Original Hydrology

1. It is assumed by many members of the committee, but not all, that the Sharkey series was at one time subject to surface inundation of water.
2. Dikes and levees were constructed to prevent this process. The classification of the soil must reflect this process.
3. Therefore, if the soil morphology indicates that the soil had aquic conditions prior to artificial drainage, it can be assumed that the type of saturation was episaturation.

IV. Recommended Classification

A. Present Status

1. The Official Series Description for the Sharkey describes the Sharkey as an Inceptisol.
2. There is no apparent justification for this classification

B. Recommended Classification

1. The soil can be classified as a Vertisol as follows:  Very-fine, smectitic, thermic Chromic Epiaquert.
2. It is not clear whether this classification pertains to the micro-low only, or both the micro-low and micro-high.

C. Additional Recommendations

1. Two classifications should be given if micro-low and micro-high topography is evident from surface examination, or can be seen in the examination of soil profiles.
2. The potential effects of artificial drainage on current hydrology should be discussed in the official series description but not used to classify the soil.

CHARGE 2: Agree on a method to measure saturation in Vertisols.

  I. Concept and Definition of Saturation and Free Water

A. Saturation Definitions

1. The Glossary of Soil Science Terms (SSSA, 1996) defines the term saturate as "to fill all voids between soil particles with a liquid".
2. Freeze and Cherry (1979) define a "saturated porous medium" as one where all voids are filled with water.
3. The Soil Survey Manual (Soil Survey Division Staff, 1993) also defines saturation as "zero air-filled porosity, and a similar concept for saturation can be found in Hillel (1980, p. 13).
4. These definitions mean that the term saturated refers to a specific water content in the soil.

1. A volume of soil is "saturated" with water when its water content (on a volumetric basis) equals its porosity.
2. In other words when the volume of water in the soils exactly equals the volume of pores the soil has.

B. Application of Definition to Natural Soils

1. While this definition for saturation is simple and easy to visualize, it is actually difficult to achieve this moisture state in the field or even in the laboratory.
2. Hillel (1980) believes that complete saturation is seldom achieved in soils because "some air is nearly always present and may become trapped in a very wet soil."
3. Klute (1986) point out that it is difficult to saturate soil samples even in the laboratory.

1. To achieve saturation he recommends that samples be wetted under vacuum, or that samples be flushed with carbon dioxide gas because this gas dissolves in water.
2. Under field conditions, Klute (1986) estimated that soils rarely, if ever, achieve saturation, and at best the maximum water content the soil achieves is between 80 to 90% of saturation.
3. For most laboratory measurements a sample of soil is "saturated" by placing it in a tub of water that surrounds the sample for 12 hrs. The water content achieved is termed "natural saturation", because it is assumed that some air is entrapped in the sample..
4. While it is expected that complete saturation has not been reached by this procedure the water content of this "naturally saturated" soil is as close to saturation as a soil would likely get under field conditions.

C. Need For Saturation

1. When a soil is saturated, or nearly so, the movement of atmospheric oxygen into the soil is prevented.
2. This allows biological activity in the soil to deplete the soil of entrapped oxygen, and the soil then becomes anaerobic and chemically reduced (Meek et al., 1968).
3. The major reason hydric soils need to be "saturated" is simply for their water content to be high enough such that atmospheric oxygen cannot enter the soil.

1. The oxygen generally enters along large voids such as root channels or cracks that extend from the surface into the soil.
2. For this report, these large voids will be called macropores.

D. Free Water

1. Free water is that which is not held under a tension or suction.
2. It flows under the influence of gravity because it has a pressure that it equal to or greater than atmospheric pressure.
3. As a result of its pressure, free water flows from the soil and fills auger holes, piezometers, and wells.
4. The top of the level of free water is the water table.
5. When free water is present in soils for a period longer than several days, the soil water content is probably near the point of "natural saturation".

1. It is probably not saturated because air could be in at least some pores.
2. On the other hand, when free water fills root channels and large cracks (macropores) the entry of atmospheric oxygen is stopped and the soils can become anaerobic or reduced if biological activity is occurring.

E. Saturation for Soil Taxonomy

1. Soil Taxonomy requires that saturation occur in soils for them to have aquic conditions.
2. Soils are considered to be saturated if they contain free water as defined above.
3. It is not required that complete saturation of the soil occur.

F. Piezometers

1. The occurrence of free water in soils is relatively easy to detect , even clayey soils such as Vertisols, using piezometers.
2. Piezometers are open-ended pipes that are installed (vertically) in the soil to a depth at which free water detection is needed.
3. When water is found in the pipe or piezometer, it indicates that the soil at the bottom of the pipe contains free water.
4. The soil’s water content is probably close to saturation, but how close is not know because there is no way to detect the amount of entrapped air.
5. The wetness state of the soil should be described as ‘free water is present’ rather than ‘saturated’.

II. Saturation in Clayey Soils

A. Background

1. Free water in clayey soils such as Vertisols that wet up by rainfall or inundation of the surface enters the soil at the surface and runs down cracks or large root channels (Fig. 2).
2. The cracks or channels fill with free water, but because the clayey matrix will slowly absorb the water, the matrix will not contain free water until the free water has had ample time to flow into the matrix (W in Fig. 3).
3. This process of water movement is called by-pass flow, and causes free water to occur in the macropores for much longer periods than it occurs in the matrix.
4. The time period required for most of the pores in the soil matrix to fill with free water is not known but may require several months in some cases.
5. Experiments from fist-sized samples of Vertisol material that are placed in contact with free water suggest it can take up to 4 months before the matrix will contain free water (R. Griffin, Prairie View A&M University, personal communication).
6. This wetting scenario is assumed to apply to virtually all clayey soils such as Vertisols because the soil matrix conducts water very slowly.
7. Saturated hydraulic conductivities of the soil matrix of Vertisols can be less than 1 mm/day (Ritchie et al., 1972).
8. The saturated hydraulic conductivity of the soil matrix in Vertisols is probably among the lowest for soil materials except for cemented layers.

Figure 2. Diagram showing the network of cracks in a Vertisol that are formed by desiccation (shrinkage cracks) and wetting (slickensides). Dashed lines represent minor slickensides. Note that even though the surface is level, the pattern of slickensides still defines the micro-lows and micro-highs. Centers of adjacent micro-lows were separated by distances of 3 to 10 m. Thickness of A and AC horizons ranged from 125 to over 250 cm. Diagram from Ritchie et al. (1972).

Figure 3. Diagram showing the effects of cracks or macropores on the movement of free water in a clayey soil. Ped interiors are unsaturated or do not contain free water. Free water flowing from the surface fills cracks and surrounds the slowly permeable, unsaturated peds. "A" shows the effect of water flowing down an open borehole. If free water remains in the cracks for 14 continuous days, the soil is assumed to be at the state of hydric-saturation from the point shown by the "W" downward. Hydric-saturation allows for the peds to remain unsaturated as shown. Diagram form Bouma and Loveday (1988).

B. Use of Piezometers in Clayey Soils

1. When an initially dry clayey soil is wetted by rainfall or surface inundation, free water that is detected in piezometers during or within a day of the rainfall must be assumed to have flown into the piezometer along cracks or large root channels.
2. The soil matrix will probably not contain free water until it has been absorbed over time from the free water contained in the cracks and channels.
3. The wetting period for the Vertisol matrix, where the matrix begins to absorb free water, starts when free water first appears in a piezometer that intersects a crack or macropore.
4. The matrix may be able to absorb the water from the channels and drain the water out of the piezometer in a matter of days to weeks.
5. Thus, the first occurrence of free water in piezometers placed in Vertisols must by interpreted differently than in more permeable soils.

1. Free water in piezometers placed in soils more permeable than Vertisols indicates the soil is at or near natural saturation.
2. In Vertisols the first occurrence of free water marks the start of a "wetting period" where the matrix is unsaturated but absorbing free water.

C. Duration of Wetting Period

1. The amount of time required for free water to enter and fill the soil matrix in clayey soils is not known.
2. Calculations based on equations governing water movement provide little guidance because input parameters for hydraulic conductivity, hydraulic head gradient, and distance of wetting, all of which change over time, are not known and widely different results can be obtained when even reasonable assumptions are made.
3. The scientists who contributed to this report suggest that a wetting period of 14 days be adopted for the time that free water must be present in a Vertisol before it can be concluded that the soil, at the depth at which the free water occurs, has reached a wetness state that substitutes for saturation.
4. This 14 day period is justified because if free water remains in the macropores for this time, the soil water could become anaerobic and other reducing chemical reactions could occur if biological activity is occurring in the soil.
5. The water content of the soil after the 14 day wetting period will not be known, but it should be sufficient for the soil to become anaerobic in the zone where most roots grow which is along the cracks and channels.
6. The 14-day wetting period is a compromise.

1. Researchers in Texas suggested a 21 day wetting period.
2. Researchers in Louisiana felt a 7-day wetting period was sufficient.
3. It is possible that further research will show that the wetting period duration will have to be adjusted for different regions. This adjustment is beyond the scope of this committee.

D. Concept of Hydric-saturation

1. The moisture content of the portion of the Vertisol that contains free water for 14 continuous days will probably not reach true saturation.
2. However, the 14 day wetting period has brought the soil to the point that reducing chemical reactions could occur in the soil because atmospheric oxygen is prevented from penetrating the soil along macropores. In other words, chemical reactions could be occurring in the Vertisol that commonly occur in wetland soils (Meek et al., 1968; National Research Council, 1995).
3. It is recommended that the concept of hydric-saturation substitute for "saturation" in the definition of hydric soils.
4. Hydric-saturation is reached after free water has occurred in the Vertisol for a period of 14 continuous days.
5. When saturation, as originally defined, occurs is not known.

E. Water Table Fluctuations

1. The free water surface is normally called the water table, and it marks the point where the soil water pressure is equals atmospheric pressure.
2. This concept is simple and useful, but it is difficult to determine the exact depth to the water table from piezometer measurements (see Fig. 3).
3. The surface of water in a piezometer represents the water pressure at the base of the piezometer, and this is usually greater than atmospheric pressure.
4. The depth to the point where water pressure is zero can be estimated, but the exact depth is best determined with a well.
5. Wells are not recommended for use in Vertisols because of the chance of by-pass flow.
6. Depth to the water table should not be estimated from piezometer data where exact depths are required.

III. Procedure for Monitoring Free Water and Hydric-Saturation

A. Site Selection

1. Micro-highs and Micro-lows

1. Vertisols, and similar clayey soils, that have not been modified for agriculture may have the topography described as "micro-high and micro-low" described earlier.
2. A given area will generally have equal areas of micro-lows and micro-highs. Plowing and land planing can nearly equalize the topographic differences between micro-highs and micro-lows, but examination of the soil profiles will generally show the location of micro-lows and micro-highs.
3. The occurrence of free water and hydric-saturation is known to differ between micro-highs and micro-lows, and therefore water measurements must be made in each position at a site.

2. Distances from Drainage Structures and Field Edges

1. The hydrologic conditions that occur in the majority of the field should be monitored.
2. Therefore, all measurements of hydrology should be made no closer than 40 m to ditches, dikes or levees, or other structures designed to alter soil hydrology.
3. Whenever possible, the hydrology should not be measured at the edges of fields.

B. Equipment

1. Piezometers

1. Piezometers should be constructed from rigid pipe approximately 1.5 in. ID. Actual diameters will vary, but large diameters will likely intersect cracks which is preferable.
2. Pipe length should allow approximately 60 cm (24 in.) to extend above ground. Greater heights above ground are warranted if depth of flood water is higher.
3. The bottom 15 cm (6 in.) of pipe will be perforated to allow water entry.
4. The perforations will be covered by a permeable fabric that keeps soil from filling the piezometer. The fabric will be held in place with waterproof tape or other suitable material.
5. The open top of the piezometer needs to be covered by a cap to prevent water entry. If the cap fits tightly, an air-hole must be drilled in the side of the pipe.

2. Piezometer Installation
   
   1. A 8 cm (3 in.) diameter hole will be bored to a depth that is 18 cm (7 in.) above the depth the piezometer base is to be placed.
   2. A smaller diameter hole should be bored at the bottom of the 8 cm (3 in.) diameter hole, to seat the piezometer at the desired depth. The diameter of the smaller hole should be selected to provide a tight fit for the piezometer.
   3. If a tight fit between the perforated portion of the piezometer with surrounding soil cannot be achieved, the space separating the perforated portion of the pipe should be filled with sand to cover the perforations.
   4. The hole will be backfilled with bentonite, purchased commercially, to ensure that water cannot flow down the area excavated for the piezometer.
   5. A layer of bentonite should be at least 2.5 cm (1 in.) thick, and the remainder of the hole filled with soil.
   6. Alternatively, bentonite may be used to fill the hole if the excavated soil is too dry and difficult to work to seal the hole.
   7. Above the surface the soil (or bentonite) should be mounded to slope away from the piezometer.

C. Depths of Examination

1. To determine whether aquic conditions occur for soil classification, piezometers should be installed at depths of 30, 50, and 100 cm (approximately 12, 20, and 40 in.).
2. To determine whether a soil is a hydric soil, piezometers should be installed at 30 and 50 cm.

D. Piezometer Locations

1. Piezometers should be installed to monitor representative soil conditions. they must be placed no closer than 36 m (120 ft.) from ditches, dikes, or other structures designed to alter the soil hydrology.
2. Piezometers should be installed in both a micro-low and micro-high, at the depths noted above, for each site.

E. Replication

1. One replicate of instruments will consist of piezometers installed in the center of the one micro-low and at the top of an adjacent micro-high at the depths noted above.
2. If the hydrology data are used to settle controversial issues then up to three replicates of piezometers are recommended for each micro-low and micro-high.
3. It is recommended that, whenever possible and especially in controversial cases, three such plots be instrumented in a single field.

F. Measurements

1. Measurements of water in each piezometer should be made weekly whenever possible. In controversial cases, daily measurements are recommended using pressure transducers and data-storage devices.
2. The depth to free water from the soil surface in each piezometer will be determined.
3. With one exception, piezometers should not be pumped dry either before or after making measurements, because it is not known how long a period is required for refilling.
4. When flood water has entered a piezometer from the surface, then the piezometer must be pumped dry and allowed to refill before measurements are made.

G. Data Interpretation

1. Wetting Phase

1. Soil matrix has been unsaturated (free water not present) for several months and now begins to wet up as rainfall becomes frequent
2. Piezometers containing water for continuous periods of 14 days or less will be considered to be in unsaturated soil.
3. It will be assumed that any free water in piezometers represents free water in cracks that is seeping into the matrix during this period.
4. Piezometers containing free water for continuous periods exceeding 14 days will be considered to be in soil that meets criteria for "hydric-saturation".
5. The duration of hydric-saturation will be determined after the 14 day wetting period has been met.
6. For example, if a piezometer contained free water for a period of 20 days, then hydric-saturation would occur for a period of 6 days, while the soil would be considered unsaturated during the first 14 day period.
7. The depths in the soil where hydric-saturation occurs will be determined from the piezometers.
8. The depth to the water table cannot be determined easily with piezometers, and in controversial cases it is recommended that data be presented as the duration that hydric-saturation occurs at the depths of the perforated portions of the piezometers.

2. Other conditions

1. If a Vertisol is at hydric-saturation but drains to allow oxygen to enter the soil, the wetting period will probably not have to recur for the soil to reach hydric-saturation because the matrix will not have lost much water.
2. A 14-day wetting period is required in initially dry soil that becomes inundated by flooding or ponding.

H. Other Data

1. Rainfall must be measured at the site at least weekly.

1. Monthly rainfall data should be compared to monthly data for the long-term 30th and 70th percentiles that can be found for the nearest available weather station.
2. This will assess whether hydric-saturation is being determined for periods of normal, drier than normal, or wetter than normal rainfall amounts.

2. Crack width and frequency (number of cracks per meter) should be estimated.
3. Inundation depth must be measured weekly for both micro-lows and micro-highs. This can be done with a tape measure or by placing depths on the piezometer shafts that extend above ground.

IV. Summary and Principle Recommendations

1. The concept of saturation, which refers to a specific soil water content, is idealized and is not expected to be reached in soils under natural conditions. Definitions requiring that saturation be attained should be revised to indicate that this idealized moisture state is not a requirement.
2. The occurrence of free water in Vertisols should be monitored in place of the water content.
3. When free water is first detected in a Vertisol after an extended dry period, a 14 day wetting period is required before the soil can be considered wet enough for the soil to become anaerobic.
4. After the 14 day wetting period has been completed, the Vertisol will be considered to be at "hydric-saturation". This concept will substitute for the saturation requirement of hydric soils and aquic conditions.
5. In cases which require specific durations for saturation, flooding, or ponding, it is recommended that the durations will begin to be measured in Vertisols after the 14-day wetting period has been completed. For example, if a soil needs to be saturated continuously for a period of 13 days to meet the requirement for wetland hydrology, then this requirement will be met in a Vertisol after free water has been detected in a piezometer for at least 27 days at the appropriate depth.
6. Piezometers can be used to accurately detect the occurrence of free water. Depth to the water table can be approximated but the exact depth cannot be determined with piezometers alone.
7. All researchers who contributed to this study used piezometers to assess free water occurrence in Vertisols. Most researchers felt piezometers were not the ideal tool to use, however no other tools were recommended. No researcher claimed his data were invalid because piezometers were used.
8. The data collected with piezometers can be made more useful and reliable when measurements are made weekly, and preferably daily. Technology is now available to allow piezometers to collect and store daily data. Such technology needs to be employed to collect daily data in controversial cases.

CHARGE 3:    Agree or disagree that a, a’-dipyridyl is or is not a proper indicator of reduction in soils. If it is not a proper indicator for reduced soils we must develop one.

I. Background

A. Definition of Reduction

1. Reduction is defined as the gain of one or more electrons by an ion or molecule (SSSA, 1996).
2. For soil classification purposes a reduced soil is one which has reduced iron (Fe(II)) in solution.
3. Soils that do not contain any Fe minerals are considered to be reduced when chemical reactions are occurring that would reduce Fe oxides if the oxides were introduced into the soil.

B. Use of Fe to Define Reduction

1. The committee agreed unanimously that Fe(II) should be the basis by which we identify a reduced soil.
2. When Fe is reduced the soil is anaerobic, and redoximorphic features should be forming (Vepraskas, 1992).
3. At least two different field tests are available to determine if Fe(II) is present in the soil solution:

1. a, a’-dipyridyl dye
2. redox potential.

C. a, a’-dipyridyl Dye

1. A fresh solution of a, a’-dipyridyl dye is colorless.
2. When applied to a field moist sample of soil that contains Fe(II) the solution will turn red or pink within one minute of application.
3. If no color change is detected, then no Fe(II) is present. This means that the soil is either not reduced, or that no Fe is present in the soil, and hence reduction cannot be detected using the dye.

D. Redox potential

1. This is an electrical measurement where a voltage is measured between a platinum wire buried in the soil and a reference or standard electrode which is also in contact with the soil solution.
2. The magnitude of the voltage can be related to the chemistry of the soil solution, and in particular can be used to estimate whether Fe(II) is likely to be present.
3. Redox potential measurements are a more difficult technique to use, but it can be used on any soil whether Fe is present or not.

II. Evaluation of a, a’-Dipyridyl Dye

A. Reliability

1. The committee agreed that the dye is a reliable indicator of reduction for soils that contain free Fe oxides that can be reduced.
2. The dye only reacts with Fe(II). If the soil contains no reducible forms of Fe, then no reaction to the dye will ever be found.

B. Solution Formula

1. The dye mixture used throughout the U.S. should be prepared using the same recipe.
2. The National Soil Survey Laboratory prepares a 0.2% solution of a, a’-dye as follows:

Dissolve 77 g of ammonium acetate in 1 liter of distilled water, and add 2 g of a, a’-dipyridyl powder and stir until dissolved. Store the solution in the dark until used.

C. Shelf Life

1. The dye solution has a useful lifespan that is probably less than one year.
2. Before it is used in the field the solution should be checked for potency by spraying it onto the worn edge of a ‘sharpshooter’ (shovel).
3. Potent dye solution will turn red within approximately 30 seconds of application.

III. Redox Potential

A. Justification for Use

1. Redox potential measurements must be used to assess reduction in soils that contain little or no reducible Fe oxides.
2. Such soils are sands that have a gray color, or other soils forming in gray colored parent materials.

B. Equipment and Procedures

1. Redox is measured in the field using platinum microelectrodes, a volt meter, and a standard reference electrode.
2. Platinum electrodes should be installed in the soil at the depth of interest, and left in place.
3. At least five electrodes should be installed at one location at a given depth. Electrodes should be placed approximately 30 cm apart at the same depth and elevation.
4. Depth of installation will be dictated by the purpose of the study.

1. Soil Taxonomy should be consulted for the critical depths needed for soil classification.
2. For hydric soil assessment, electrodes should be installed a depth of approximately 15 cm.

5. Measurements of redox potential are made by measuring the voltage between the reference electrode and each platinum electrode, and then computing a mean potential for the electrodes at a given depth and time of measurement.
6. The mean voltage determined in the field is converted to Eh or redox potential by adding a correction factor (approximately 200 mv) that is determined by the type of reference electrode used.
7. The pH of the soil must also be determined at the time the voltages are measured.
8. To determine whether a soil is reduced using redox potential measurements, the average redox potential must be below the critical point at which Fe(II) appears in solution.

1. This critical point varies with soil solution pH. For a pH of 7, the committee feels a value of 150 mv should be used. Thus, when the mean redox potential is at or below 150 mv, and the soil solution pH is 7, then the soil should be considered reduced.
2. Agreement was not reached on the redox potential at which Fe(II) appears for lower pH’s, but it appears that it will be in the range of 320 to 350 mv for solution pH’s between 5 and 5.5. A value of 320 mv is suggested for use for soil solution pH’s of 5.5
3. Critical Eh values that can be used to assess reduction can be estimated using the following equation for solution pH’s less than 8:  Eh = 943 - 113(pH)
4. Reports from field experiments conducted in NC, IL, and TX indicate that when redox potentials are at or below the values computed with the above equation a positive reaction to a, a’-dipyridyl dye has been observed.
5. There is not universal agreement yet on which exact Eh values should be used to identify reduction in soils, however the above values are close to those preferred by most researchers surveyed.
6. The committee did agree that the critical Eh values selected to determine soil reduction should correspond to where positive reactions to a, a’-dipyridyl dye have been found.

IV. Duration and Minimum Volume of Reduction

A. Duration of Reduction

1. The committee recommends that a minimum time period should be specified for how long reduction must occur.
2. The National Technical Committee for Hydric Soils has tentatively proposed a period of 14 days be required for a soil to be considered hydric.

1. This time period seems adequate for soil classification as well.
2. It is known that redoximorphic features can begin to form in soils that are reduced for 14 days or longer.

B. Minimum Soil Volume

1. A minimum volume of soil in which reduction must occur should be specified.
2. Soil Taxonomy specifies that redox depletions must occupy 50% of the horizon volume or, as in the case of Vertisols which have most roots on ped surfaces, 50% of the ped surfaces must contain redox depletions.
3. The committee recommends that the 50% requirement for redox depletions be adopted as the minimum volume required for reduction in soils before the soil can be considered reduced.
4. The 50% requirement should be applied to either the soil matrix or ped surfaces depending upon where most roots occur in the soil of interest.

CHARGE 4: Agree on a definition of artificial drainage. Does the definition include dams and levees?

I. Concept

A. Definitions

1. The committee agrees that the term ‘artificial’ refers to something brought into being not by nature but by human art or effort.
2. Drainage also refers to the removal of free water from the soil. This can be ground water or surface water flowing over the soil that originated from rainfall or flood water.

B. Inclusion of Surface Free Water Impacts

1. The concept of artificial drainage needs to be broadened to include construction of devices that prevent free water from reaching the soil.
2. Thus, a soil on a flood plain can be protected from flooding by construction of dams, dikes, or levees. This is another form of artificial drainage because it prevents free water from entering the soil.

C. Principle Activities Included

1. The committee agrees that Artificial Drainage occurs when objects or activities such as the following are constructed to either remove surface water from the soil or to prevent surface water from reaching the soil reaching the soil:

1. Ditches, canals, subsurface tile drains, surface crowning or land leveling all of which remove free water from the land.
2. Dams, levees, dikes, or similar obstructions which prevent flood water from inundating a soil.
3. Subsurface pumping of ground water which can prevent ground water from entering the soil or speed its removal from the soil.

2. This list is not intended to identify the only features that constitute artificial drainage, but simply to give examples of activities that achieve artificial drainage.

 II. Proposed Definition

Artificial Drainage can be defined as follows:

The use of human effort and devices to remove free water from the soil surface or from the ground, or the use of human effort and devices to prevent surface or ground water from reaching the soil.

CHARGE 5:   Examine the series descriptions for Sharkey and related Vertisols and determine the validity of statements such as "...soil formed under conditions of..." and aquic conditions can be verified "...except in artificially drained soils". How far into the past must we go to satisfy "formed under"?

I. Background

A. Meaning of "...soil formed under conditions of..."

1. This phrase comes from the definition of hydric soil. The complete definition is: A hydric soil is one that formed under conditions of saturation, flooding, or ponding long enough during the growing season to develop anaerobic conditions in the upper part"
2. To determine whether a soil formed under these conditions, the National Technical Committee for Hydric Soils has approved a list of field indicators that are known only to form in soils that have been "...anaerobic in the upper part" (NRCS, 1996).

1. If a soil has these field indicators present, then it is safe to assume that the soil formed under the conditions specified in the definition of hydric soils.
2. The definition of hydric soils also states, that the soil need not be saturated, flooded, or ponded today to qualify as a hydric soil. As long as at least one hydric soil field indicator is present the soil is considered to be a hydric soil.

3. The phrase "formed under" does not imply a set number of years. It means that the soil contains visible features (hydric soil field indicators) that are known to form only under conditions of saturation, flooding, or ponding that lasted long enough.... to develop anaerobic conditions in the upper part.

B. Verifying Aquic Conditions

1. Aquic conditions require that the soil contain free water within 50 cm of the surface and be reduced within the same depth.
2. These conditions can be verified using the methods described for charges 2 and 3, as long as the soil has not been artificially drained. As noted earlier, artificial drainage is designed to remove free water from the soil, or as in the case of flooding, to prevent it from reaching the soil.

It is assumed that artificial drainage will effectively lessen or prevent the occurrence aquic conditions.

3. It has been a long-standing policy of Soil Taxonomy that a soil’s moisture regime cannot be changed by artificial drainage for classification purposes even though the soil may no longer be saturated or inundated as a result of the drainage practices.

1. The philosophy behind this policy has been that should the artificial drainage system fail, then the soil’s can become inundated or saturated again by virtue of its topographic position and climate.
2. Justification for this policy can be found by studying areas along the Mississippi River that flooded in 1993 in Monroe County, IL. The soils on this portion of the flood plain were protected by a large levee that was assumed to protect the area from flooding. The levee broke in 1993 due to high water in the Mississippi River. The town of Valmeyer was completely inundated. Some soils in the area are classified as Fluvaquentic Hapludolls which indicates they are subject to flooding.  Thus classification is justified and advisable.

4. Aquic conditions are assumed to have been present in soils that are artificially drained if the soils retain the morphological features that formed under conditions of saturation and reduction. These are usually redoximorphic features (Vepraskas, 1992) which have been extensively studied and associated with conditions of saturation and reduction.

II. Examination of Series Descriptions

A. Series Examined

1. Official series descriptions of the Sharkey and related soils were examined to determine whether they contained hydric soil field indicators and thereby met the definition of hydric soil in that they "formed under conditions of saturation, flooding, or ponding long enough....to develop anaerobic conditions in the upper part".
2. The series selected included the Sharkey, its competing series, and also geographically associated soils.
3. Hydric soil identification procedure:

1. A series in this set was considered a "Hydric Soil" if it contained one or more indicators found in USDA-NRCS (1996).
2. A series was considered "Not Hydric Soil" if it did not meet an indicator.

B. Results of Series Examination

Hydric Soils

• Sharkey ( Indicator F3 present)
• Perry (F3)
• Alligator (F3)
• Harahan
• Jackport
• Tunica
• Beumont (F3)
• Fausse (F3, A9 or A10)
• Garner (borderline F3)
• Iberia (F13)
• Kobel (F3)
• Tuscumbia (F3, TF4)
• Una (F3, TF4)
• Barbary (A9 or A10, F2, TF4)
• Mhoon (F3)

Not Hydric Soils (no indicator met)

• Bowdre
• Commerce
• Newellton

Comments:

1. Symbols in parentheses show the specific indicator found in USDA-NRCS (1996).
2. All assessments were made assuming the series could be found in Land Resource Regions T, P, O, and N.

C. Evaluation

1. The above results show that the Sharkey and many of its related series contain hydric soil field indicators, and should be considered hydric soils.
2. The indicators were in most cases easily identified, and there is no evidence of unusual color patterns or characteristics related to parent material.

D. Effects of Artificial Drainage on Wetland Evaluations

1. To qualify as a jurisdictional wetland, an area must contain hydric soils, wetland hydrology, and hydrophytic vegetation (Environmental Laboratory, 1987). If vegetation is absent, a wetland must have hydric soil and wetland hydrology.
2. If an area has been artificially drained, it likely will not meet the requirements for wetland hydrology.
3. While the above evaluation shows that most of the soils are hydric soils, we do not know whether they have wetland hydrology or not. It is possible that many of these series are in areas that have been artificially drained, and therefore do not meet the requirements for jurisdictional wetlands.

CHARGES 6 AND 7: Develop complete definitions for flooding and ponding.

  I. Existing Definitions

A. Definitions from the Soil Survey Manual, USDA Handbook 18 (Soil Survey Division Staff, 1993):

1. Inundation-the condition that the soil area is covered by liquid free water.
2. Flooding--the temporary inundation by flowing water.
3. Ponding--inundation by standing [stationary] water as in a closed depression.

B. Assessment of Existing Definitions

1. A majority of the committee feel that the above definitions are adequate.
2. Because they have been defined in the Soil Survey Manual, the definitions probably should not be changed.
3. If a modification is needed, then a new term should be coined to represent the new definition.

II. Description of Flooding and Ponding

A. Frequency and Duration

1. Table 3-4 in the Soil Survey Manual (shown below) defines classes for frequency and duration.
2. A majority of the committee feel they are adequate.

B. Depth of inundation should be described when possible.

C. Area of coverage probably should also be described where possible.

Table 3-4: Frequency and Duration of Inundation Classes

Frequency

• None (N) = No reasonable possibility
• Rare (R) = 1-5 times in 100 years
• Occasional (O) = 5-50 times in 100 years
• Frequent (F) = 50 times in 100 years
• Common (C) = Occasional and frequent can be grouped for certain purposes and called common.

Duration

• Extremely Brief (BE) = < 4 hours (flooding only)
• Very Brief (BV) = 4-48 hours
• Brief (B) = 2-7 days
• Long (L) = 7 days-1 month
• Very Long (VL) = 1 month

D. Hydrologic factors:

1. Occurrence in relation to precipitation events: the size of rainfall event needed to initiate flooding or ponding.
2. Occurrence in relation to water additions from surface overland flow. The source of the incoming water should be identified.

III. Summary

A. The definitions of inundation, flooding, and ponding appear to be adequate, although a more formal definition of ponding would be advisable.

B. The major problem expressed with the concept of flooding and ponding appeared to be related to their description rather than their definition.

REFERENCES

• Bouma, J. and J. Loveday. 1988. Characterizing soil water regimes in swelling clay soils. Chapter 5.L. In L. P. Wilding and R. Puentes (eds.) Vertisols: their distribution, properties, classification and management. Technical Monograph No. 18, Texas A&M Printing Center, College Station, TX.
• Environmental Laboratory. 1987. U. S. Army Corps of Engineers wetlands delineation manual. U.S. Army Engineer Waterways Experiment Station Tech. Report Y-87-1, Vicksburg, MS.
• Freeze, R. A. and J. A. Cherry. 1979. Groundwater. Prentice Hall, Englewood Cliffs, NJ.
• Klute, A. 1986. Water retention: Laboratory methods. p. 635-662. In A. Klute (ed.) Methods of Soil Analysis. Part 1. 2nd ed. Agronomy Monograph No. 9, ASA and SSSA, Madison, WI.
• Hillel, D. 1980. Fundamentals of soil physics. Academic Press, New York, NY.
• Meek, B.D., A.J. MacKenzie, and L.B. Grass. 1968. Effects of organic matter, flooding time, and temperature on the dissolution of iron and manganese from soil in situ. Soil Sci. Soc. Am. Proc. 32:634-638.
• National Research Council. 1995. Wetlands, characteristics and boundaries. National Academy Press. Washington, DC.
• Pettry, D. E. and R. E. Switzer. 1996. Sharkey soils in Mississippi. Miss. Agricultural and Forestry Experiment Station Bulletin 1057, Mississippi State.
• Ritchie, J. T., D. E. Kissel, and E. Burnett. 1972. Water movement in undisturbed swelling clay soil. Soil Sci. Soc. Am. Proc. 36:874-879.
• Soil Science Society of America. 1996. Glossary of soil science terms. Soil Science Society of America, Madison, WI.
• Soil Survey Division Staff. 1993. Soil survey manual. U.S. Dep. of Agriculture Handbook No. 18. U.S. Government Printing Office, Washington, DC.
• Soil Survey Staff. 1996. Keys to Soil Taxonomy, Seventh edition. USDA-NRCS, Washington, DC.
• U.S. Department of Agriculture, Natural Resources Conservation Service, 1996. Field indicators of hydric soils in the United States. G. W. Hurt, P. M. Whited, and R. F. Pringle (eds.) USDA, NRCS, Fort Worth, TX.
• Vepraskas, M. J. 1992. Redoximorphic features for identifying aquic conditions. NC Agric. Research Service, Technical Bulletin 301, Raleigh.
• Wilding, L.P. and D. Tessier. 1988. Genesis of Vertisols: shrink-swell phenomena. Chapter 4. In L. P. Wilding and R. Puentes (eds.) Vertisols: their distribution, properties, classification and management. Technical Monograph No. 18, Texas A&M Printing Center, College Station, TX.

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