Transcript Soil Classification

LECTURE 17
Soil Classification
Recap from yesterday…
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Soil classification: “The ordering of soils into a
hierarchy of classes. The product is an
arrangement or system of classification
designed to express interrelationships of soils
and to serve as a filing system. Broad
groupings are made on the basis of general
characteristics; subdivisions on the basis of
more detailed differences in specific
properties.” – Soil Science Society of South Africa
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Why would we want to classify?
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To enable communication between specialists
In theory construction
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Advancement in science = the ability to make
generalizations and predictive statements
For mapping purposes
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There are many different classification
systems worldwide.
Basic units of classification:
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In many systems, this is the soil profile
which is defined as being a 3-dimensional
soil body with no significant lateral
variation.
In the USA, it is the soil pedon or
polypedon.
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Pedon: “The smallest 3-dimensional portion
of the soil mantle needed to describe and
sample soil in order to represent the nature
and arrangement of its horizons. Rock or
material that is too deep to be of interest to
agricultural soil users mark the lower limits of
the pedon. A group of contiguous, similar
pedons is called a polypedon and is the soil
individual for the purpose of classification.”
–Soil Science Society of South Africa
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Taxonomy: “Classification, especially
according to natural relationships. The
systematic distinguishing, ordering and
naming of type groups within a subject
field.” – Soil Science Society of South Africa
Soil Taxonomic System for
South Africa…
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Has two levels
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To date, 73 forms and 400 families identified
Each form has a defined set of Master
Horizons
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Form
Each form can contain various families
These differentiated on grounds of position, as
well as physical and chemical properties
Other differences within these forms
distinguish soil families
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e.g. signs of wetness, calcareous horizons and
layers
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You should understand the concept behind
the SA system (i.e. the theory of it) and be
able to describe at least 3 of the soil forms in
SA along with their families, and know how
each of the horizons is defined.
Reference = Soil Classification – a Taxonomic
System for SA (blue book) in the Geography
library.
The United States “Soil
Taxonomy” (USDA, 1975)
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Widely used comprising a hierarchy of 6
levels
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Order
Suborder
Great group
Subgroup
Family
Series
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Important concepts:
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Diagnostic surface horizons (epipedons)
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Diagnostic subsurface horizons
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7 recognized
18 recognized
Soil moisture regimes
Soil temperature regimes
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Nomenclature (names) are largely logical
12 orders:
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Alfisols
Andisols (ando – blacksoil)
Aridisols (aridus – dry)
Entisols (recent)
Gelisols (gelid – very cold)
Histosols (histos – tissue)
Inceptisols (inceptum – beginning)
Mollisols (mollis – soft)
Oxisols (oxide)
Spodosols (spodos – wood ash)
Ultisols (ultimus – last)
Vertisols (verto – turn)
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Suborders and great groups – see handout of
Tables 3.6, 3.7 and 3.8 from Brady & Weil.
Subgroups
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More than 1300 recognised
“Typic” = properties typical to a particular great
group.
Other subgroups indicate slight deviations from
the typic subgroup
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Families
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About 8000 recognised.
Within a subgroup, soils fall into a particular family if
they have similar physical and chemical properties
affecting growth of plant roots at a specified depth, e.g.
CEC, texture, mineralogy.
Series
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19000 recognized in the US.
Specific range of soil properties involving kind, thickness
and arrangement of horizons.