What are Hydric Soils?

Silty Loam Hydric Soil

Hydric soil conditions generally develop in areas that have continuous or occasional saturation and reduction in the mineral layers.  

Direct evidence of these conditions is often seen as redoximorphic features, except in histosols, and can also be verified by measuring or observing saturation and reduction of iron (evidenced by the coloration of the soil, or chemical tests). There are some great books on the topic if you’re the reading type.  

Saturation in a soil pit is the easiest way to determine if the hydrology is currently there, however water present does not necessarily mean that the soil is hydric. The converse is also true, lack of hydrology does not indicate a non-hydric soil.  

Saturation is generally determined to be present if a small pedon (ped, or piece) of soil is examined to have “free water” available. Simply being damp is not wet enough to be called saturated.

Soils are considered hydric if they have a pressure head greater than -1 kilopascal (kPa). Tensiometers are not used very often to measure the exact saturation level of soils, generally field observations are adequate when  delineating wetlands.

The length of time required to develop hydric soil conditions varies depending on the environment and is not specifically stated for any one place. This article should help you answer the question of What are Hydric Soils? 


There are three types of saturation defined by the USDA

  • Endosaturation
  • Episaturation
  • Anthric saturation

Endosaturation is a soil saturated with water in all layers from the upper Horizon to a depth of approximately 75 inches deep.

Episaturation is when the soil is saturated with water in potentially only one or a couple layers within that upper 75 inches of the soil surface but it also has one or more unsaturated layers.  the zone of saturation (or what is often called the water table) is perched on top of a relatively impermeable layer.

Anthric saturation is a special kind of hydric soil condition that occurs in areas that are cultivated and irrigated, such as with flood irrigation. There is a list of other conditions that the soils must meet in order to be anthrac saturation, but generally if it looks hydric and has been irrigated long enough to develop indicators, it should be considered hydric.

What are Hydric Soils?

Reduction

Visual Evidence in the form of redoximorphic features is often  present in most hydric soils. These redox features can look like small orange worms working their way through the poor Linings of Relic route channels, or they may be borderless blotches spread throughout the face of the soil. Or they could be very dark nodules or concretions of Manganese that has gone into solution and then come out of solution quickly in one spot as the water table evaporated or fell.

Reduction occurs when soil is saturated with water, and oxidation occurs when the soil is aerated and not not saturated.  the reduced iron and manganese II ions are mobile and may be transported by water when it moves through the soil. Certain redox patterns occur as a function of the patterns in which the ion carrying water moves through the soil such as for lining or fissures in the soil. patterns are also affected by the fact that manganese is reduced more rapidly than iron,  however iron oxidizes more rapidly upon aeration.

When both iron and or manganese are oxidized and precipitated, they will form either soft masses or hard concretions, or nodules. movement of these two minerals as a result of the redox process and its oil May create redoximorphic features defined below:

Redox concentrations

  1. Nodules and concretions are cemented small bodies of mineral that can be  separated from the soil on their own, like a small pebble. However they usually can be crushed with medium pressure using your fingers, unlike a small stone.
  2. Masses, which are non cemented or soft boundary concentrations in the soil Matrix.
  3. Pore Linings, the areas along old root channels often (or cracks in the soil).

Redox depletions

These are parts of the soil that Show Low chroma or either iron or manganese oxides have been stripped out of the soil.

Why do we care about Hydric Soils?

These kinds of soil usually represent wetlands or areas that recharge rivers with water. The soils themselves may or may not be the most important part, however the hydric soils indicate an area where water congregates and influences a system.  

Often new wetland delineators are most intimidated by the soils aspect. People feel uncomfortable making a judgement call on a wetland boundary, as required, using indicators such as color and chroma from the Munsell Soil Color Chart or the Globe soil color chart.  Also knowing the broader choices, and reasons a certain landform or landscape may have certain soils requires a different perspective.  Knowing why a soil might show the characteristics that it does can help you determine IF the characteristics you’re seeing are a result of saturation, or possibly something else (such as parent material, or a burn).  

However just as botanists have an easier time with the wetland vegetation, geologists and geographers who have learned their soils and landforms may have just as easy a time with the soils. I actually find that if the vegetation is grazed or there are some species I don’t know than the soils are the next best indicator that I turn to to help me make my decision in a wetland delineation.

It’s important to have the correct tools and equipment with you to properly evaluate a soil. A good sharp shovel with a blade at least 6 or 8 inches deep, with a strong handle so you can really put some leverage onto it, is a must. In some soils of soil probe will do the trick, but in rocky soil or hard packed clay or even soils that are to sandy a probe won’t work. Also a ruler or some measurements on your shovel marked out is important to get your actual depths. Often a hydric soil indicator falls within a half inch or an inch of given threshold from the criteria. Honest measurements are important. Thirdly you need a Munsell or Globe soil color chart book, as simply guessing what the soil colors are is nowhere near defensible legally or repeatable scientifically.

Some examples of both Hydric soils and non-hydric (upland) soils:

Hydric soil examples from wetland areas

Sandy loam hydric soil:

What are Hydric Soils
Sandy clay loam hydric soil
Sandy Clay Loam Hydric Soil
What are Hydric Soils

Silty loam hydric soil:

Silty Loam Hydric Soil
What are Hydric Soils

Clay loam – This soil has redox concentration visible as red areas in the center.  This come from a wetland with palustrine emergent (PEM) wetland vegetation. Which simply means dominated by grasses and rushes, sedges (no shrubs or trees):    

Clay Loam Hydric Soil
What are Hydric Soils

Upland soils

This soil was found in an active river floodplain, it is so Sandy as to be well drained and well aerated. This is a non hydric soil, and prove to be an area of Upland in this case.

Sandy Upland Soil

This soil came from a large agraded area in a sandy creek system:

Sandy Upland Area

Loam:

Loam Upland

Even though that soil above came from a drainage swale feature, it was well drained, and well aerated (oxidized):Upland Swale

 

I hope that helped clear up what Hydric Soils are for you!  Please let me know if you have any questions in the comments section below.  Like and share if I’ve helped you learn something.  Got a cool photo of a hydric soil that you’ve seen?  Paste a link, we want to see it!

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