High‐chroma features have not been adequately defined under existing
terminology or classified under existing systems. The terms “masses” as a
subclass of concentrations used in field definitions and “loose infillings” used in
micromorpological classifications come closest but are not fully satisfactory.
Defined descriptively, high‐chroma features have a typical color of 7.5YR 5/8, are
usually less than 1 to 2 mm in diameter, are poorly cemented, and have a sharp
external boundary with the soil matrix. They are found in well‐drained to poorly
drained soils with first‐appearance typically at depths of 50 to 100 cm. A study
was undertaken to more fully characterize and classify high‐chroma features and
to provide more accurate interpretations of feature morphology for applications
in environmental and soil quality, plant nutrition, and soil genesis.
High‐chroma features found within peds having varying degrees of
hydromorphic expression were assigned to classes depending on internal color
and color patterns. Material removed from features, halos, and the soil matrix
was analyzed using a low‐power stereomicroscope, SEM/EDS, TEM/ED, μ‐XRD,
ICP, and stain tests to determine properties and composition.
Four formation hypotheses are proposed: (1) a non‐pedogenic origin,
features having developed from the weathering of an inherited precursor
mineral; (2) a pedogenic origin resulting from the formation and infilling of
vesicles that formed at depth shortly after deglaciation but are no longer actively
forming; (3) a pedogenic origin but features are actively forming; (4) formation
by dissolution of a soluble mineral fragment and subsequent infilling of the resulting void, analogous to the formation of a geode.
Although high‐chroma features might develop by more than one
pathway, a non‐pedogenic origin is favored. Non‐pedogenic hypothesis (1) and
the hybrid geodic hypothesis (4) offer the most efficient explanations for the
presence of silt, iron, and manganese within high‐chroma features. A proposed
weathering sequence based on feature classification and evidence for the
presence of manganese nodules in the till‐source bedrock also support a nonpedogenic
origin. Pedogenic hypotheses require a sequence of events of
uncertain and in some cases seemingly low probability.
Existing classification systems offer little insight into genesis. Most
importantly, given the evidence for a non‐pedogenic origin, high‐chroma
features should not be interpreted or classified as redoximorphic features as the
term is typically used in the field. Although high‐chroma features may result
from alternating periods of oxidation and reduction, when used alone they are
ambiguous indicators of seasonal wetness.
University of Minnesota Ph.D. dissertation. December 2012. Major: Soil Science. Advisor: James C. Bell. 1 computer file (PDF); vi, 303 pages, appendices A-C.
Pribyl, Douglas Wayne.
Characterization and formation of high-chroma features in loamy soils of southern Minnesota.
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