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Chapter 7- Parent Material influences on
Pedogenesis
Primary influence is on mineralogy
Some minerals more resistant than others
e.g., feldspar vs quartz
e.g., olivine vs quartz
Measuring influence
• Measure the weathering of key minerals
– Compare those in profile to those that are
unweathered (i.e., parent material)
– Look for depletion of elements, etching, or
clay alteration
• Hornblende etching
Mineral weathering
Hypersthene
Augite
More to least resistant
minerals to weathering
Chemical composition of igneous vs
sedimentary lithologies
Chemical composition influence
• Rocks vary in mineralogy
– Changes the chemical composition of the
material
– Changes the resistance to weathering
• Resistant minerals = harder to weather =
thinner or less well developed soils when
compared to those developed in rocks with
less resistant mineralogy and chemical
composition
Influence of texture on soils
Can refer to consolidated or unconsolidated material
consolidated
porosity or fractures are key
fine grained with highly permeable and porous
conditions is preferable
Depth of leaching varies with permeability
and porosity of parent material
Extreme control
• Podzolization or not Podzolization. That is
the question!
– Common in sandy material formed from
crystalline rock
– Not common in glacial till formed from ground up
sedimentary rocks
– Common in soils with ultramafics
– Not common in soils with lesser amounts of
ultramafics
– Sandy (common) clayey (uncommon)
Limestone soils- terra rosa
• Often contain soils that are nothing like the
parent rock
– Four common interpretations
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Residual materials in carbonate host rock
Fluvial or colluvial from higher positions on landscape
Ash sources
Eolian dust sources
– Lab analysis and field observation can assist in
source determination
• Mass balance, chemical signatures, topographic
expression
Ash soils
• More control over soil formation than
any other substance
– So unique that they have their own soil
order!
• Andisols- melanic epipedon
– Often have unique subhorizons and
weathering materials
• Volcanic glass- weathers into clays like
allophane
– Often mistaken for albic subhorizons
– Simple chemical tests often assist in
determining origins
Uniformity
• Parent material is very important for
assessing development
– e.g. PDI relies heavily on parent material values
• Needed to separate pedogenic processes
from sedimentary processes
– Bedding vs horizonation
Numerous ways to mix up
the parent materials
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Frost heave
Shrink swell clays
Colluvial washdown
Bioturbation
Preferential weathering
Dilution by eolian processes
Coarse fractions fine upward due to input from eolian
sources
Dilution by disintegration
• Preferential disintegration of smaller
sized fractions relative to larger
clasts
– Results in more fines being produced as
large stuff gets left behind
• Surface to volume ratio dictates this
River deposits
• Problematic due to episodic variation in
deposition
– Alternating energy of deposition creates
stratified materials
– Must separate strata from pedo processes when
evaluating