Are soils like M&Ms™? Yes! Typical soil colors are red, brown, yellow, or black. These colors are often not the color of the minerals in the soil but coatings of iron oxides (Fe203, FeOOH, and so on) or organic matter on particles. The minerals beneath are often quartz or feldspar, which are grey.
To see how coating affects soil color, consider red M&Ms™. If you place a few in a sieve, slowly immerse them in water, and gently shake them, the water will turn pink as the red dye is washed off. If you remove and dry them, you can see they are now white.
A similar process occurs in saturated soil when Fe3+ is reduced to Fe2+ due to a microbial mediated redox reaction. If air, which contains oxygen (O2), is in the soil, the soil is aerobic.
4e- + O2 + 4H+ → 2H2O
If all O2 is removed soil becomes anaerobic (saturation occurs).
Denitrification (no color change):
10e- + 12H+ + 2NO3 → N2 + 6H2O
Iron (Manganese) Reduction (soil turns gray):
2e- + 6H+ + Fe2O3 → 2Fe(II) + 3H2O
Why is this reaction important? Soil scientists can identify where the water table is even when the soil is not saturated. Consider the M&Ms™ again. What is the chance that the now-white ones will turn red again? Slim to none. Soil is the same. Once the red (rusty) colors due to iron oxides have been removed and the soil is gray, it is unlikely that the particles will become coated (turn red) again.
This reaction occurs in saturated and reduced or anaerobic conditions. Thus, the presence of gray colors indicates the water table. This helps soil scientists identify wetland or hydric soils, as well as locate suitable soils for septic systems and related land use. The reaction only occurs if there are sufficient numbers of microbes and a food source (carbon) present to cause anaerobic conditions to occur. To better understand, observe iron reduction in the following activity.
You should see little to no changes in the autoclaved sample, as the microbes have been killed by the heat. In the other jars, bubbles will start to form and over time the soil will turn gray. This may take longer in some soils due to the amount of iron, food, and microbes in the soil.