As we move into the meat of our “field” season, we’ve had a few questions about identifying redoximorphic features in the soil and how they relate to the presence of seasonal or permanent zones of saturation that are considered limiting soil layers. Identifying features related to saturated soil zones sets the baseline for determining separation distance from the bottom of the system infiltration surface to the limiting layer.

These features at one time were referred to as soil mottles. However, this created confusion because soil mottles referred to any occurrence of color differences within a soil horizon giving the horizon a mottled appearance. For identification of saturated conditions, specific kinds of “mottles” needed to be recognized.

Understanding how redoximorphic features form requires some knowledge of the chemistry and microbiology involved. We do not expect you are or need to be chemists or microbiologists, but it is good to know how the features form. The knowledge can help you sort out if the features you identify in the field are due to soil saturation.

PAINTING THE SOIL

In subsoil horizons, iron oxide minerals give the horizons red, yellow, brown or orange coloration. Manganese oxide minerals produce black colors. These mineral oxides naturally coat the surfaces of individual sand, silt and clay particles. Think of it as a coat of paint on the surfaces of the particles. Without this paint, the particles would be gray in color.

Red, yellow, brown and orange colors occur when iron is in its oxidized state — that is, in the presence of oxygen. Black colors occur when manganese is in its oxidized state. These minerals can be chemically reduced in soils under certain conditions. This means that the iron and manganese ions accept electrons from a source other than oxygen, and in soils, this is usually from organic matter being decomposed by bacteria.

When a soil is well-aerated (not saturated), bacteria consume and reduce oxygen in the air-filled pores. When a soil is saturated, the bacteria consume and reduce the oxygen dissolved in the water. When the oxygen is gone, the bacteria continue to break down the organic matter but they also reduce nitrate-nitrogen and the manganese and iron oxides. The chemical reactions occur in sequence, so oxygen goes first, followed by nitrate, manganese and iron.

When iron and manganese are reduced, several things begin to happen. Iron and manganese dissolve in water, soil color changes to gray, and iron and manganese move with the soil water to other parts of the soil horizon or are leached from the soil. The term redoximorphic refers to the reduction and oxidation chemical reactions and the resulting appearance or morphology of the soil horizon.

When the soil becomes unsaturated and aerated again, areas of the soil where the iron and manganese vacated appear gray due to the natural color of the sand, silt and clay particles mentioned above. The areas where iron and manganese have migrated to are red, brown, yellow, orange or black in color. This pattern provides a mottled appearance to the soil horizon and reflects the fact that the soil is saturated for periods of time long enough for the chemical reactions to occur.

While the length of time it takes for the reaction to occur varies from place to place, it implies for our purpose of identifying a limiting soil layer that saturation occurs for a long enough period that it will interfere with acceptance and treatment of septic tank effluent.

EVALUATION

Three major kinds of features can be recognized in the field by an experienced soil scientist or site evaluator. We maintain that installers and service providers with a knowledge of soils in their area can also identify these features. For this reason, we regularly conduct field workshops or activities to evaluate these features.

The three kinds of features are redox concentrations, redox depletions and reduced matrices. 

Redox concentrations are bodies where iron and manganese have accumulated. These accumulations can have several forms including nodules or concretions, soft masses or pore linings. Nodules and concretions are firm to extremely firm, irregularly shaped bodies with diffuse boundaries. Soft masses are just that irregular and soft within the inside (or matrix) of the soil structural units or peds. Pore linings are where the accumulations are along a root channel or crack.

Redox depletions are areas vacated by the mineral oxides either along old root channels or in the matrix of the peds.

Reduced matrices have the low soil colors in place but when removed from the profile and exposed to oxygen will change color as iron and manganese in the soil is oxidized. This type of condition usually means the soil is most often saturated and water movement through the soil is very slow because the iron and manganese have not leached out of the profile.

With this background information on formation and kinds of redoximorphic features, upcoming columns will discuss interpretation of what is seen in the field and relate the interpretation to some of the questions we are most often asked about redoximorphic features.