Hydric Soils Technical Note 4

Hydric Soil Indicator “Insights”.

Send comments to Michael Whited.

Question: I have a soil with layers that meet the color and redoximorphic requirements of several indicators; however, they do not meet any of the thickness requirements. What guidance is there regarding combining layers to meet a hydric soil indicator?

Answer: Although there is no specific guidance written on this, the developers of the Field Indicators are proponents of this approach. If layers/indicators are combined, the combination needs to meet the most stringent depth/thickness requirements of the combined indicators.

For example, the following is representative of an Argialboll profile (a common soil in temporary and seasonal wetlands in the Prairie Pothole Region of Land Resource Region M):

Ap - 0 to 6 inches; black (10YR 2/1) loam.

A - 6 to 8 inches; very dark grey (10YR 3/1) loam; 2% prominent 10YR 5/6 redox concentrations as pore linings on ped surfaces and in root channels.

E - 8 to 12 inches; grayish brown (10YR 5/2) silt loam; 4% prominent 5YR 2/1 redox concentrations as soft masses of black (N 2/) ferromanganese with diffuse boundaries of 10YR 5/6 iron concentrations.

Bt - 12 plus inches; very dark grey (10YR 3/1) clay loam; 1% distinct 10YR 4/4 redox accumulations as pore linings on ped surfaces in the upper part of the horizon.

In this example, the A horizon meets the requirement (except thickness) of indicator F6, Redox Dark Surface. The E horizon meets the requirement (except thickness) of indicator F3, Depleted Matrix. Examining the indicator language, F6 requires a layer 4 inches thick within 12 inches; F3 requires a layer 6 inches thick starting within 10 inches. In this case, the soil has F6 within 12 inches and has F3 starting within 10 (at 8) inches; the combined thickness is 6 inches. Therefore, this soil meets the combined color, depth, and thickness requirements and should be documented as meeting hydric soil indicator(s) F6 and F3 (combined).

The same (almost) profile with the E horizon only being 3 inches thick would not meet a literal interpretation of the indicators because the combined thickness would only be 5 inches. The Bt horizon does not have enough redox. concentrations to qualify. Would this very similar profile be hydric? In this author's opinion, positively yes. The Field Indicators is a guide to help identify and delineate hydric soils. When dealing with the complexities of spatial variability in soils it is not practical to expect that the Field Indicators will be able to cover all the intricacies of mother nature. As the Field Indicators are refined regionally, it is anticipated that these types of scenarios will be described and incorporated.

This is a real life example of how one Great Group of soils can have hydric morphology that is not "captured" in the national document. Are Argialbolls hydric? Data from the Prairie Pothole Region (PPR) has shown that soils with E horizons having redoximorphic concentrations support hydrophytic plant communities and meet the wetland hydrology (and hydric soil criteria). In the PPR, in fact, throughout the U.S., if the E horizon does not have any redoximorphic features, the soils do not support hydrophytic plant communities and do not meet wetland hydrology criteria.

Question: I am concerned that some sodium affected soils that have light-colored E horizons at the surface may be misidentified as hydric using the definition of a depleted matrix that requires colors of 5/1, 6/1, etc., or 6/2, 7/2, etc., with or without redoximorphic concentrations.

Answer: The definition of a depleted matrix clearly states that A, E, and Calcic horizons are not included unless they have 2% or more distinct or prominent redox concentrations. These horizons can have high value and low chroma that are not a result of soil genesis under anaerobic conditions.

Question: Redox features are often more apparent when the soil is dry, why can't we use dry colors?

Answer: The NTCHS is reviewing this issue and may have further guidance in the next edition of "Field Indicators of Hydric Soils in the United States". It is well documented that in many soils redoximorphic features are more easily seen when the soil is dry. This is especially true for soils with dark colored surface layers. For now, only indicator F6 (Redox Dark Surface) allows the use of dry colors. Field scientists are encouraged to use dry colors to help in identification of redoximorphic features. Both moist and dry colors should be documented where this is an issue. Presently, a literal interpretation of the field indicators requires the use of moist colors (except F6).

Question: Why not arrange the indicators in a sequence of wettest to driest?

Answer: There are several reasons for not doing this. The minimum expression of one indicator may overlap with the maximum expression of another; an indicator may occur in the wettest of hydric soils in one region of the country and that same indicator in the driest of hydric soils in another part of the country. In addition, many of the indicators have as much range within the indicator as they do with competing indicators i.e. maximum expression of an indicator is very wet and minimum expression is less wet. Ranking would require much speculation. The intricacies of soil morphological relationship to wetness has been a subject of research for many years; however, the knowledge gained opens the door to even more questions. The Field Indicators are available on the internet and users are encouraged to cut and paste the document any way they find most useful. Soil and other wetland scientists are encouraged to add local information to the user notes. Such things as the most common landscapes/landforms, parent materials, and even particular map units where specific indicators would be expected would be helpful to many users.