Swamp Stomp
Volume 18, Issue 43
Hydric Soils Primer
By Marc Seelinger
I thought we would revisit some of the more fun aspects of wetland science. This week we are going to talk about soils.
One of the most fundamental and often confusing topics concerning soils are those darn hydric soil indicators. There are just so many of them. Each regional supplement can have different ones and sometimes there are tweaks that are region or sub-region specific.
The most basic concept surrounding hydric soil indicators is that they only apply to hydric soils. Now, this may seem a bit obvious but it is critical to the understanding of how the indicators work. Non-hydric soils do not exhibit any of the listed indicators. However, if an indicator is present, it is a positive test for hydric soils. Once that happens it is not usual to find multiple indicators in the same soil profile. If there are no indicators, the soil is not hydric, and no indicators should have been found. This becomes a bit tricky when dealing with remnant hydric soils. Shadows of indicators might be present. However, the soil is not actively hydric. The lack of hydrology indicators may help to confirm this.
The next topic is, “what is it we are looking for?” The hydric soil indicators are based on how three groups of elements respond to the presence of water. It is not just the presence of water, but the anaerobic environment the water creates. These element groups are:
Carbon
Iron and Manganese
Sulfur
The easiest one to spot is sulfur. The soil stinks like rotten eggs. If you have stinky soil you meet one of the hydric soil criteria. Be careful to not misdiagnose the smell. There are lots of stinky things out there. Make sure what you are smelling is hydrogen sulfide.
Iron and manganese are also fairly easy to spot. There is a distinct color change from orange-red to grey in the case of reduced iron. The anaerobic environment chemically changes the color of the soil. Manganese tends to turn black in this wet environment. However, the problem with these is that the color change back to the brighter colors in an aerobic environment may not happen quickly or at all in some cases. Consequently, you need to make sure that you have an active reducing environment by cross-checking your hydrology indicators.
Carbon is perhaps the trickiest. A simple explanation is that a significant amount of organic material (a.k.a. carbon) is present due to the lack of oxygen in the environment. The soil microbes are not able to break the organic material down because they need oxygen to do this. The more the soil is subjected to anaerobic conditions the thicker the layer of undigested carbon becomes. The more organic matter, the more likely the soil will be hydric. It probably stinks too.
To help organize all of the indicators the Corps uses USDA texture classes. Each indicator is grouped based upon its dominant texture. These include sand, loam, and no specific texture.
Sand is the easiest. The texture is sandy like beach sand. All of the indicators have this in common. The funny thing about this one is that the presence of organic matter is a big part of the “S” indicators.
Loam is denoted by the letter “F.” It stands for fine sand or finer. This includes silts and clays. Most of the indicators in the F category are related to iron and manganese color changes.
“All soils” is the last category and is listed as not specific to any one texture type. Many of the poorly-drained organic soil types fall into this category. However, stinky soil also is an “A” indicator. These “all soils” indicators all sort of fall into the category of “other” but with a strong emphasis on organic soils.
One last thought on this soil overview. The thickness of the feature is a new concept. Many of the indicators have thickness requirements. A given soil feature must be a specified thickness in order to count. It may also have to occur at a specified depth, otherwise, the feature does not count. Oh, and by the way, you sometimes can combine features if present, to meet these thickness thresholds.
Have a great week!
– Marc