A. Soil profile with potential plinthite.

B. Close-up of lower Bt horizon with suspected plinthite after months of exposure.

C. Weakly cemented plinthite nodules from slake test indicating 18 percent by volume weakly cemented aggregates (plinthite nodules).

D. An 8 x 8-inch section of the Bt horizon selected for photo analysis with indicator marker to be used by AI to determine percent by volume of dark reddish areas..

E. Masked area identified by AI to be used as the basis for area analysis.

F. Based on the color of the marked area, AI determined percent by volume to be about 58 percent (plus or minus 5 percent) or about 53 to 63 percent.

 

The horizon was tested for cementation by slake test. Slake test indicated about 18 percent by volume weakly cemented material (cemented plinthite)

 

Percent by volume plinthite? Is it 18 percent or 58 percent?

This depends on an interpretation of Soil Taxonomy and the criteria to identify plinthite.

 

NOTE: The following text is taken directly from the 2013 Keys to Soil Taxonomy. Additional comments in parentheses are by John Kelley.

 

The identification of plinthite in the field is somewhat subjective because an exact definition including measurable

properties has not been adopted. Therefore, no “required characteristics” are provided (Keys to Soil Taxonomy, p.37.)

 

The following discussion provides general guidance for identifying plinthite and establishing a need for further clarification.

 

GENERAL Physical Characteristics:

1. Plinthite occurs as dark red redox concentrations that typically form platy, polygonal, or reticulate patterns. (Plinthite at the center of the aggregate commonly has a dark red color; however, the outer edges may be brownish or yellowish, especially if the aggregates are cemented.)

 

2. Plinthite changes irreversibly to an ironstone hardpan or to irregular aggregates on exposure to repeated wetting and drying, especially if it is also exposed to heat from the sun. (This is easily confirmed by observing resistant aggregates on the surface of aged roadcuts or ditch banks.)

 

3. Plinthite forms by segregation of iron.

 

4. Plinthite forms in a horizon that is saturated with water for some time during the year. (A water table is essential for the initial formation of plinthite, although the current moisture regime might not be seasonally saturated. Identifying relic plinthite does not require a seasonal water presence.)

 

5. Plinthite formation--initially, iron is segregated in the form of soft, more or less clayey, red or dark red redox concentrations. These concentrations are not considered plinthite unless there has been enough segregation of iron to permit their irreversible hardening on exposure to repeated wetting and drying. (Reddish redox concentrations should not be considered plinthite unless they meet items 2. and 9. Plinthite is the only diagnostic feature that inherently depends on its future behavior once it is removed from its natural condition--indicating it does not require hardening/cementation in place, but only after exposure. If this is the intent, does non-cemented plinthite in its natural environment present any soil limitations, and if not, how does it differ from other reddish redox concentrations? A degree of cementation should be required to distinguish its special characteristics.)

 

6. Plinthite is firm or very firm when the soil moisture content is near field capacity and hard when the moisture content is below the wilting point. (Moist aggregates have a brittle manner of failure and higher bulk density resulting in reduced water and nutrient availability for the soil.)

 

7. Plinthite occurs as discrete bodies larger than 2 mm that can be separated from the matrix. (Does this indicate a manual separation or does it include mechanical separation as by slaking?)

 

8. A moist aggregate of plinthite will withstand moderate rolling between thumb and forefinger and is less than strongly cemented. (This indicates plinthite can be less than strongly cemented, but cementation is not required as long as it can be removed and withstand moderate rolling.)

 

9. Moist or air-dried plinthite will not slake when submerged in water, even with gentle agitation. (This indicates plinthite must be cemented to some degree.)

 

10. Plinthite does not harden irreversibly as a result of a single cycle of drying and rewetting. After a single drying, it will remoisten and then can be dispersed in large part if it is shaken in water with a dispersing agent. (This indicates moist plinthite aggregates are cemented and can only be broken down by using a dispersing agent.)

 

11. Plinthite (in a moist soil) is soft enough to be cut with a spade. After irreversible hardening, it is no longer considered plinthite but becomes ironstone. (As plinthite progressively hardens in the soil over time it may form ironstone nodules or sheets. This process is age/time dependent. At any one point in time the conditions necessary to form ironstone may be lacking or not enough time has passed for this change to have occurred. It is not uncommon in the SE U.S. to have plinthite, ironstone nodules, and ironstone sheets, i.e., petroferric contact in the same pedon.)

 

12. A small amount of plinthite in the soil does not form a continuous phase; that is, the individual redox concentrations or aggregates are not connected with each other. (e.g., Plinthic subgroup). (Plinthite aggregates are root restrictive. For the most part, roots cannot and do not enter plinthite. If roots are present, they are in non-plinthic material or are along the edges between aggregates. It is common for two or more aggregates to connect, creating zones where roots can’t penetrate, alongside areas where roots can grow. In a small amount of plinthite, the zones that restrict roots measure less than 10 cm in width. If these root restrictive zones are horizontally continuous and are more than 10 cm in width, they form a continuous phase and constitute a large amount of plinthite.

 

13. If a large amount of plinthite is present, it may form a continuous phase. Individual aggregates of plinthite in a continuous phase are interconnected, and the spacing of cracks or zones that roots can enter is 10 cm or more. (e.g., Plinthudults Great Group). (This clearly implies cementation. Areas identified by some as plinthite do not necessarily restrict root growth. "Continuous phase" criteria and >50 percent by volume are used in Soil Taxonomy to classify these soils in a Plinthudult Great Group indicating either criterion can be applied for taxonomic placement. Both criteria are subjective, and the perceived failure of either has historically been used to prevent identifying Plinthudults in the U.S. To maintain consistent classification, it would be useful to explicitly state that plinthite must be cemented.

 

Summary:

To maintain consistent correlation and classification, the criteria for identifying and quantifying plinthite should be improved. Using cementation and slake tests to measure volume percent offers a practical method to ensure reliability. If cementation is not required, AI (Artificial Intelligence) analysis is a helpful tool to determine volume by analyzing a representative horizon section. Refer to the image above.

 

This is done by identifying a section of the image that is thought to meet plinthite criteria. A query is then submitted to AI requesting a designated color (i.e., dark reddish areas) be quantified. During this process you will be able to adjust thresholds and widen or narrow zones of consideration. This is a much more reliable method to determine percent by volume than an individual relying on comparisons to the "Percent of Area Covered" charts provided in the Field Book for Describing and Sampling Soils". No matter which method is used, subjectivity is a limitation to accuracy and consistency.

 

Requiring cementation and using slake tests can reduce subjectivity, ensuring a more consistent correlation of these unique and special soil materials.

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