The Scoop on Redoximorphic Features: Part 2

We continue to explore the nitty-gritty soil science behind identification based on colors and conditions you encounter during excavation

Last month we discussed in detail the conditions needed to form the features in soil referred to as redoximorphic. This month we will provide more information on the appearance and formation of redoximorphic features in soils, how we determine their presence and some interpretations of their presence or absence.

We use the presence or absence of these features to determine depth to a limiting soil condition due to water saturation. Since saturated soil zones are limiting conditions, the presence of these features will set the elevation of the bottom of our soil treatment trenches or beds to provide adequate separation distance to provide treatment.

Remember, we highlighted there are several kinds of features: redox concentrations, redox depletions and reduced matrices.

Areas of concentration are where iron and manganese have accumulated after being reduced when the soil is saturated and then reoxidized when the soil dries out and oxygen reenters the soil pores. Three kinds of concentrations are recognized: nodules and concretions, masses, and pore linings. Nodules and concretions are firm to extremely firm, so difficult to break. Masses are soft bodies within the soil matrix. Pore linings are just that, accumulations within larger-diameter pores in the soil, such as root channels or wormholes.

Understanding depletions 

Areas of depletion include iron depletions and clay depletions. These are areas within the soil where iron, manganese or clay has been stripped out or removed. When using the Munsell color notation, these are areas of low chroma, less than or equal to two and values higher than four across numerous hues. We used to call these “gray mottles,” but as mentioned last month, there are similarly colored soils not indicative of periodic soil saturation; depletions are more accurate depictions related to soil saturation. Clay depletions are areas along ped surfaces or channels where the clay has been removed, leaving a layer of silt. The clay removed is often found coating soil peds at a lower horizon.

Reduced matrices are the interiors of peds that have a low chroma color because the iron in the peds is reduced. When the soil is exposed to air, the color will change, usually within 30 minutes. This indicates areas in the soil that are saturated for long periods of time without much water movement. So if as an installer you are digging a trench and gray soil is being excavated and the clods change color to brighter reds or browns, it indicates you are excavating in an area that should not be used for soil treatment trenches.

This all seems to be very straightforward; but there are a number of exceptions and complications to correctly identifying and interpreting soil colors related to reduced and saturated conditions. There are soils that have matrix “low chroma” colors associated with accumulations of organic matter or calcium carbonate. These are not redoximorphic features. In some cases, soil parent material is gray or low chroma in color. Then it is natural for the soil to exhibit a low chroma color. This condition occurs more often in sandy soils because the individual sand grains are often predominantly quartz, which is gray or white in color.

On the opposite end of the spectrum are soils with parent materials naturally high in iron content. These are very red colored soils due to the high amount of iron. Even though the soils are reduced, the appearance of the gray areas is very hard to identify unless you are used to working in areas where these soils occur. Where we live in northern Minnesota, Wisconsin and the Upper Peninsula of Michigan, there are large areas where these soils occur. Since they are mostly clay in texture, they are slowly permeable and often wet. It is an example of when knowing the soils where you work helps identify potential problems.

At the edges

Areas of accumulation we discussed above are very resistant once they form. While it may only take less than a year to 100 years to form, they can persist for a few hundred years or more. The way to identify this in the field is to look at the accumulations with a magnifying glass, paying close attention to the edges of the concretion or nodule. Edges that are blurry and not sharp indicate the feature is still forming and reflects current-day conditions. Sharp edges mean the conditions under which they were formed no longer exist or they were transported from another location to where they are today.

Even with these problems, identifying and investigating the cause of these soil features is one of the best ways to avoid excavating too close or into soil horizons that are seasonally or periodically saturated, which can cause premature failure of soil treatment trenches and beds.

We always highlight for installers, site evaluators and inspectors that if you see these types of features and are unsure, it should raise a red flag about the area and work should not proceed until questions about system separation from these features is resolved. This is where it is helpful to consult the soil survey for the area.

Look at the interpretations and the soil profile descriptions for evidence of saturated conditions. This is easier these days then ever because soil surveys are available on the web from the Natural Resource Conservation Service. A visit with a soil scientist from your area conservation district can also help you determine whether features you see should be a concern.



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