The following are definitions from the Glossary of Soil Science Terms published by the Soil Science Society of America:
Mottled zone– A layer that is marked with spots or blotches of different color or shades of color. The pattern of mottling and the size, abundance, and color contrast of the mottles may vary considerably and should be specified in soil description.
Redoximorphic Features– Redoximorphic concentrations, redoximorphic depletions, reduced matrices, and other features indicating the chemical reduction and oxidation of iron and manganese compounds resulting from saturation.
Redox concentrations– Zones of apparent accumulation of Fe-Mn oxides in soils.
Redox depletions– Zones of low chroma (2 or less) where Fe-Mn oxides alone or both Fe-Mn oxides and clay have been stripped out of the soil.
The difference between mottles and redoximorphic features.
Historically, the term “mottles” has been used to identify differences in color patterns in a soil profile, as stated in the first definition above. Those color difference can be a result of something inherent in the parent material that formed the soil, the movement of organisms (worm channels), deposition of heterogenous materials (alluvial deposits), and chemical reactions in the soil (reduction/oxidation) to name just a few. Because of the wide range of origins for the existence of mottling, soil scientists decided to coin a new term to specifically capture mottles that formed as a result of saturated conditions in the soil. That term is “redoximorphic features”. The word redoximorphic stems from “redox” which is short for reduction and oxidation and “morphic” which is short for “morphology”, which is the study of how things form, in this case soils. So the term literally means the formation of reduction and oxidation features. Therefore, a redoximorphic feature is a type of mottle that specifically identifies features created as a result of saturated conditions in the soil. What is really meant when comparing the two terms is the following:
Mottling caused by saturation = redoximorphic features
Why are redoximorphic features important?
Redoximorphic features can help a trained soil scientist identify zones of saturation in a soil profile even when the soil isn’t currently saturated (such as in the summer or during periods of drought). A brief discussion of the chemical processes related to redoximorphic feature formation in soils is provided to explain how this can be accomplished.
What are the coloring agents in soil?
Soils are often dominated by two main coloring agents: organic matter and iron minerals. What the typical person thinks of as “topsoil” is really a mineral soil horizon that gains its color from organic matter. It does not take a lot of organic matter to make a soil look “organic,” even dark “topsoil” might only contain 3-6% organic matter. Therefore, one can see that organic matter is a dominant coloring agent when present. The color of the rest of the soil profile below the topsoil is typically dominated by iron minerals. Often we see colors of red, yellow, brown, orange, etc. in a soil profile. All of these colors are created by different forms of iron minerals present on the soil particles. Knowing that our soils are dominated by iron minerals gives us a unique tool for soil interpretations.
How do redoximorphic features form and what do they mean?
Soils contain a plethora of organisms, including microorganisms (microbes). These microbes break down organic matter in the soil. They accomplish this through cellular respiration. Respiration requires a food source (the carbon from the organic matter) and an electron acceptor (oxygen). A well aerated soil has plenty of oxygen for this process to occur. When a soil becomes saturated (pores filled with water instead of oxygen), the microbes lose their electron acceptor and therefore can’t respire. There are other microbes that can function with an electron acceptor other than oxygen. In soil, there happens to be several potential electron acceptors for microbes to use. However, all electron acceptors are not created equal. There is a preference for the use of electron acceptors, and it generally proceeds like this:
O > N > Mn > Fe > S > C
The above sequence indicates that microbes will use all the oxygen until it is exhausted, then they will use all the nitrogen until it is exhausted (denitrificaiton), then they will utilize manganese, then iron, then sulfur, and finally carbon.
When microbes use iron as an electron acceptor, it changes the form of iron from Fe3+ to Fe2+. This change alters the iron, making a once immobile compound mobile (Fe2+ is able to move in the soil solution). Knowing that our soils are colored by iron on the surface of soil particles, if we are able to transform the iron into a mobile form we could essentially “wash” the iron off the soil particle. This “washing” of iron off of soil particles is what manifests into the formation of redox depletions. We see redox depletions in soils as areas of low chroma color, typically chroma 2 or less and value 4 or more. These types of colors have a gray to white appearance because those are the colors of the underlying soil mineralogy when it is not coated with iron. Once the iron has become mobile, it is transported through the soil in the soil solution. If the soil is consistently wet enough the iron will eventually be flushed from the system completely. More often, the iron moves around locally in the soil horizon until eventually the period of saturation comes to an end. At this point, as water moves out of the soil pore network, air enters. With the air comes oxygen. When oxygen enters the soil, the iron is able to lose its extra electron and become Fe3+ again. This is similar to when an iron object gets wet and then dries, with rust (oxidized iron) resulting. In the soil, areas where iron oxidizes become zones of accumulation of iron, or in soil lexicon, redox concentrations.
It is important to note that it is the absence of oxygen, not just the presence of water that causes redoximorphic features to form. In fact, if a soil is saturated with well oxygenated water (perhaps near an actively flowing spring) redoximorphic features may not form. Additionally, the formation of redoximorphic features can occur in small pockets within the soil where localized conditions favor anaerobic conditions without the presence of an actual water table. This scenario is common when a finer-textured soil horizon overlies a coarser horizon, such as a clayey horizon over a sandier horizon. Without getting into the detailed physics of the soil-water dynamics, one can just think of it as the clay holding onto the water stronger than the sand can pull the water into it. This has to do with particle size and associated pore size among other factors. The take home message is that the presence of redoximorphic features in a soil does not mean that a water table, as we normally conceptualize (whether perched or regional), was present in the soil. It might just signify that the soil was “field saturated” long enough in a few pockets of the soil to cause conditions to be conducive to the formation of redoximorphic features. It is usually the abundance and contrast of the redoximorphic features as well as the physical characteristics of the soil horizons that aids a soil scientist in determining what conditions led to the formation of the redoximorphic features within the soil profile.