Plinthic soil fragment (Brazil)
This aggregate exhibits iron cementation in more than 85 percent of its volume. The cementation ranges from weakly cemented to indurated. The reddish zones (about 70 to 80%) are firm and brittle and are less than strongly cemented. The blackish zones (about 5%) are more than strongly cemented. Based on the following definitions, this layer is from a plinthic horizon.
A plinthic horizon (WRB system) (from Greek plinthos, brick) is a subsurface horizon that is rich in Fe (in some cases also Mn) (hydr-)oxides and poor in humus. The clay is mostly kaolinitic, with the presence of other products of strong weathering, such as gibbsite. The plinthic horizon usually changes irreversibly to a layer of hard concretions or nodules or a hardpan on exposure to repeated wetting and drying with free access to oxygen.
A plinthic horizon consists of mineral material and:
1. has within ≥ 15% of the volume, single or in combination:
a. discrete concretions and/or nodules that in the moist state are at least firm, with a redder hue or stronger chroma than the surrounding material; or
b. concentrations in platy, polygonal or reticulate patterns that in the moist state are at least firm, with a redder hue or stronger chroma than the surrounding material; and
2. one or more of the following:
a. has ≥ 2.5% (by mass) Fe dith in the fine earth fraction; or
b. has ≥ 10% (by mass) Fe dith in the concretions, nodules or concentrations; or
c. hardens irreversibly after repeated wetting and drying; and
3. has a ratio between Fe ox and Fe dith of < 0.1 in the fine earth fraction; and
4. does not form part of a petroplinthic or pisoplinthic horizon; and
5. has a thickness of ≥ 15 cm.
A plinthic horizon shows prominent concretions or nodules or concentrations in platy, polygonal or reticulate patterns. In a perennially moist soil, many concretions, nodules or concentrations are not hard but firm or very firm and can be cut with a spade. Repeated wetting and drying will generally change them irreversibly to hard concretions or nodules or a hardpan (ironstone), especially if also exposed to heat from the sun, but they do not harden irreversibly as a result of a single cycle of drying and rewetting.
If the concretions, nodules or mottles do not reach 15% of the volume, it may be a ferric horizon.
If the concretions and nodules of the plinthic horizon harden (strongly or more cemented) and reach ≥ 40% of the volume, the plinthic horizon becomes a pisoplinthic horizon.
If it hardens to a continuous sheet, the plinthic horizon becomes a petroplinthic horizon.
For more information about a plinthic horizon, visit;
www.researchgate.net/publication/242649722_Rationale_for_...
For more information about soil classification using the WRB system, visit:
www.fao.org/3/i3794en/I3794en.pdf
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
Plinthic soil fragment (Brazil)
This aggregate exhibits iron cementation in more than 85 percent of its volume. The cementation ranges from weakly cemented to indurated. The reddish zones (about 70 to 80%) are firm and brittle and are less than strongly cemented. The blackish zones (about 5%) are more than strongly cemented. Based on the following definitions, this layer is from a plinthic horizon.
A plinthic horizon (WRB system) (from Greek plinthos, brick) is a subsurface horizon that is rich in Fe (in some cases also Mn) (hydr-)oxides and poor in humus. The clay is mostly kaolinitic, with the presence of other products of strong weathering, such as gibbsite. The plinthic horizon usually changes irreversibly to a layer of hard concretions or nodules or a hardpan on exposure to repeated wetting and drying with free access to oxygen.
A plinthic horizon consists of mineral material and:
1. has within ≥ 15% of the volume, single or in combination:
a. discrete concretions and/or nodules that in the moist state are at least firm, with a redder hue or stronger chroma than the surrounding material; or
b. concentrations in platy, polygonal or reticulate patterns that in the moist state are at least firm, with a redder hue or stronger chroma than the surrounding material; and
2. one or more of the following:
a. has ≥ 2.5% (by mass) Fe dith in the fine earth fraction; or
b. has ≥ 10% (by mass) Fe dith in the concretions, nodules or concentrations; or
c. hardens irreversibly after repeated wetting and drying; and
3. has a ratio between Fe ox and Fe dith of < 0.1 in the fine earth fraction; and
4. does not form part of a petroplinthic or pisoplinthic horizon; and
5. has a thickness of ≥ 15 cm.
A plinthic horizon shows prominent concretions or nodules or concentrations in platy, polygonal or reticulate patterns. In a perennially moist soil, many concretions, nodules or concentrations are not hard but firm or very firm and can be cut with a spade. Repeated wetting and drying will generally change them irreversibly to hard concretions or nodules or a hardpan (ironstone), especially if also exposed to heat from the sun, but they do not harden irreversibly as a result of a single cycle of drying and rewetting.
If the concretions, nodules or mottles do not reach 15% of the volume, it may be a ferric horizon.
If the concretions and nodules of the plinthic horizon harden (strongly or more cemented) and reach ≥ 40% of the volume, the plinthic horizon becomes a pisoplinthic horizon.
If it hardens to a continuous sheet, the plinthic horizon becomes a petroplinthic horizon.
For more information about a plinthic horizon, visit;
www.researchgate.net/publication/242649722_Rationale_for_...
For more information about soil classification using the WRB system, visit:
www.fao.org/3/i3794en/I3794en.pdf
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