What are hydric soils?
There are two basic categories or “buckets” of soils in our Smart Wetlands world: hydric or upland. A hydric soil bucket holds soils that are saturated, flooded, or ponded long enough during the growing season to develop anaerobic (or low to no oxygen) conditions. All other soils go in the upland soils bucket.
Hydric soils are created when upland soils become saturated under high water table conditions. The still or slowly moving water combined with the microbe communities that decompose soil organic matter create low-oxygen conditions.
The conditions that create hydric soils also create wetlands—so hydric soils are wetland soils. Hydric soils are one of the three key defining characteristics of wetlands along with hydrology and water-loving (hydrophytic) plants. Those depression areas in your farm field that are wet most springs likely have hydric soils, whereas the rest of the farm that is dry all year has upland soils.
The interesting thing about hydric soils is that they develop specific characteristics over time when they have been continuously waterlogged for short periods (a couple of weeks) during the growing season. These characteristics remain even after the soils dry.
If you look for the signs, it is easy to identify former wetland locations on cropland, even the areas that have been tile-drained. The indicators are specific soil colors (gray or bluish to greenish-gray), muck or soils with highly decomposed organic matter, sulfur or rotten eggs odor, and iron reduction (rust) deposits. Hydric soil indicators are key to your farm’s wetland determination of prior converted or farmed wetlands.
A close up of hydric soil
The three views of this soil provide some clues that this is hydric soil. The first is the soil color – it is bluish-gray. Another clue is that the water table is high (within 2 feet of the surface). The final clue is the small piece of clay tile shown in the second photo. That tile means it was likely saturated soil and in order to farm it, the landowner had to install tile drainage.
As an engineered ecosystem, constructed or tile-treatment wetlands can be built on upland or hydric soils. Since the upland soils will be underwater or at least saturated throughout the year, they will develop characteristics typical of hydric soils over time. But having hydric soil as the base for a constructed wetland definitely has its advantages.
The hydric soil will have the seed bank of the wetland plants that once grew there, even after being farmed, and may still have the microbial communities and soil chemistry needed for certain wetland processes. Hydric soils can have higher soil carbon than non-hydric soils. The presence of soil carbon is needed for several wetland processes such as denitrification, which we rely on to convert the nitrate in the tile water to harmless nitrogen gas in our Smart Wetlands. So, when a tile-treatment wetland is built on hydric soil, it can get to work faster and more efficiently to remove excess nutrients.
If you think an ag tile-treatment wetland could work as part of the tile-drainage system on your Illinois farm, please complete this form.
Mucky stuff
The material in this photo contains more than 50% of undecomposed plant material so it is considered peat. If the plant organic matter was between 20-50%, it would be considered muck.
Jill Kostel leads the project team as TWI's Senior Environmental Engineer and primary designer of Smart Wetlands. She also works to develop new partnerships to help spread constructed wetlands widely in Illinois.
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