Plinthite: Exposed Plinthic horizon over the "brick" layer
The dense, red layer underlying the plinthic zone (Btv) is referred to by local soil scientists as the "brick" due to its brick-like characteristics (area below wavy boundary).
This zone is particularity perplexing. Determining the correct horizon nomenclature requires numerous considerations. B horizons must have evidence of pedogenesis. This would commonly include soil structure, development of plinthite, and/or translocation of clay in the form of clay films or clay bridging or formation of other diagnostic features. Soil structure in this layer (if observable) is very weak and very to extremely coarse. Weak coarse subangular blocky aggregates can rarely be identified and lack clearly formed ped faces. Plinthite is absent or is less than 5 percent (by volume). Ironstone, if present, is most commonly in an elongated cylindrical pattern encompassing old root channels/animal borrowings, rarely as ironstone concretions. Clay films in the traditional form are very rare or absent; however, thick continuous clay flows or coatings greater than 1mm thick are common in old abandoned root channels or along randomly spaced internal vertical cracks. Clay bridging of sand grains is common, but weakly expressed. If pedogenesis has been concluded, the layer may also have fragic soil properties.
Fragic soil properties (FSPs) are principally based on slaking characteristics (non-cemented material) as well as field criteria including evidence of pedogenesis, a rupture resistance class that is firm or firmer, brittle manner of failure, and the ability to restrict the entry of roots. Note: Although thought to characteristically have prismatic structure, a fragipan horizon may be dominantly massive. The properties required for FSPs are common and observable in this layer.
With the striking visual difference between the overlying Btv horizon and the brick layer, is there a lithologic discontinuity? "Not everyone agrees on the degree of change required for a lithologic discontinuity. "No attempt is made to quantify lithologic discontinuities" (KST). Abrupt changes in color that are not the result of pedogenic processes can be used as indicators of a discontinuity.
Although the transition between the plinthic horizon (Btv) and the brick layer (2BCtx) is most commonly abrupt and very striking, it has been observed to be more gradual is other pedons. This is thought to be the result of the intensity of the flow patterns at the time of deposition or subsequent erosion on the base layer prior to secondary deposition. The brick layer is considered to be of a much greater age than the overlying materials in which the contemporary pedon formed. If the abrupt transition is absent, the materials may be of a single origin to the depth observed.
If the exposure is naturally eroding, the brick layer almost always forms a ledge at the contact with the overlying plinthic horizon.
A plinthic horizon contains a significant amount of plinthite. If the horizon constitutes a "continuous phase", zones that roots can enter are more than 10cm apart and plinthite makes up 50 percent or more of the volume of the horizon (proposed). Plinthite (Gr. plinthos, brick) is an iron-rich, humus-poor mixture of clay with quartz and other highly weathered minerals. It commonly occurs as reddish redox concentrations in a layer that has a polygonal (irregular), platy (lenticular), or reticulate (blocky) pattern.
Plinthite irreversibly hardens upon exposure to repeated wetting and drying, especially if exposed to heat from the sun. Other morphologically similar iron-rich materials that do not progressively harden upon repeated wetting and drying are not considered plinthite. The horizon in which plinthite occurs commonly has 2.5 percent (by mass) or more citrate dithionite extractable iron in the fine-earth fraction and a ratio between acid oxalate extractable Fe and citrate-dithionite extractable Fe of less than 0.10.
Soils that classify as Plinthudults have one or more horizons within 150 cm of the mineral soil surface in which plinthite either forms a continuous phase or constitutes one-half or more of the volume.
For more information about a plinthic horizon, visit;
www.researchgate.net/publication/242649722_Rationale_for_...
or;
www.sciencedirect.com/science/article/pii/S00167061220043...
For more information about describing and sampling soils, visit:
www.nrcs.usda.gov/resources/guides-and-instructions/field...
or Chapter 3 of the Soil Survey manual:
www.nrcs.usda.gov/sites/default/files/2022-09/The-Soil-Su...
For additional information on "How to Use the Field Book for Describing and Sampling Soils" (video reference), visit:
www.youtube.com/watch?v=e_hQaXV7MpM
For additional information about soil classification using USDA-NRCS Soil Taxonomy, visit:
www.nrcs.usda.gov/resources/guides-and-instructions/keys-...
or;
www.nrcs.usda.gov/resources/guides-and-instructions/soil-...
Plinthite: Exposed Plinthic horizon over the "brick" layer
The dense, red layer underlying the plinthic zone (Btv) is referred to by local soil scientists as the "brick" due to its brick-like characteristics (area below wavy boundary).
This zone is particularity perplexing. Determining the correct horizon nomenclature requires numerous considerations. B horizons must have evidence of pedogenesis. This would commonly include soil structure, development of plinthite, and/or translocation of clay in the form of clay films or clay bridging or formation of other diagnostic features. Soil structure in this layer (if observable) is very weak and very to extremely coarse. Weak coarse subangular blocky aggregates can rarely be identified and lack clearly formed ped faces. Plinthite is absent or is less than 5 percent (by volume). Ironstone, if present, is most commonly in an elongated cylindrical pattern encompassing old root channels/animal borrowings, rarely as ironstone concretions. Clay films in the traditional form are very rare or absent; however, thick continuous clay flows or coatings greater than 1mm thick are common in old abandoned root channels or along randomly spaced internal vertical cracks. Clay bridging of sand grains is common, but weakly expressed. If pedogenesis has been concluded, the layer may also have fragic soil properties.
Fragic soil properties (FSPs) are principally based on slaking characteristics (non-cemented material) as well as field criteria including evidence of pedogenesis, a rupture resistance class that is firm or firmer, brittle manner of failure, and the ability to restrict the entry of roots. Note: Although thought to characteristically have prismatic structure, a fragipan horizon may be dominantly massive. The properties required for FSPs are common and observable in this layer.
With the striking visual difference between the overlying Btv horizon and the brick layer, is there a lithologic discontinuity? "Not everyone agrees on the degree of change required for a lithologic discontinuity. "No attempt is made to quantify lithologic discontinuities" (KST). Abrupt changes in color that are not the result of pedogenic processes can be used as indicators of a discontinuity.
Although the transition between the plinthic horizon (Btv) and the brick layer (2BCtx) is most commonly abrupt and very striking, it has been observed to be more gradual is other pedons. This is thought to be the result of the intensity of the flow patterns at the time of deposition or subsequent erosion on the base layer prior to secondary deposition. The brick layer is considered to be of a much greater age than the overlying materials in which the contemporary pedon formed. If the abrupt transition is absent, the materials may be of a single origin to the depth observed.
If the exposure is naturally eroding, the brick layer almost always forms a ledge at the contact with the overlying plinthic horizon.
A plinthic horizon contains a significant amount of plinthite. If the horizon constitutes a "continuous phase", zones that roots can enter are more than 10cm apart and plinthite makes up 50 percent or more of the volume of the horizon (proposed). Plinthite (Gr. plinthos, brick) is an iron-rich, humus-poor mixture of clay with quartz and other highly weathered minerals. It commonly occurs as reddish redox concentrations in a layer that has a polygonal (irregular), platy (lenticular), or reticulate (blocky) pattern.
Plinthite irreversibly hardens upon exposure to repeated wetting and drying, especially if exposed to heat from the sun. Other morphologically similar iron-rich materials that do not progressively harden upon repeated wetting and drying are not considered plinthite. The horizon in which plinthite occurs commonly has 2.5 percent (by mass) or more citrate dithionite extractable iron in the fine-earth fraction and a ratio between acid oxalate extractable Fe and citrate-dithionite extractable Fe of less than 0.10.
Soils that classify as Plinthudults have one or more horizons within 150 cm of the mineral soil surface in which plinthite either forms a continuous phase or constitutes one-half or more of the volume.
For more information about a plinthic horizon, visit;
www.researchgate.net/publication/242649722_Rationale_for_...
or;
www.sciencedirect.com/science/article/pii/S00167061220043...
For more information about describing and sampling soils, visit:
www.nrcs.usda.gov/resources/guides-and-instructions/field...
or Chapter 3 of the Soil Survey manual:
www.nrcs.usda.gov/sites/default/files/2022-09/The-Soil-Su...
For additional information on "How to Use the Field Book for Describing and Sampling Soils" (video reference), visit:
www.youtube.com/watch?v=e_hQaXV7MpM
For additional information about soil classification using USDA-NRCS Soil Taxonomy, visit:
www.nrcs.usda.gov/resources/guides-and-instructions/keys-...
or;
www.nrcs.usda.gov/resources/guides-and-instructions/soil-...