Download Basic Concepts and Definitons

Basic Concepts and Definitions
Pedology (pedon, ground + logos, science) a collective term used to refer to the field of Soil
Science that deals with soil morphology, soil genesis, soil classification, soil survey, soil
characterization, and soil interpretation. In brief each sub-field can be described in the
following manner.
Soil morphology is the color, physical structure, and chemical and mineralogical properties
of soil material comprising soil horizons. Soil morphology also includes the number, relative
position, and relative thickness of horizons comprising the soil profile and the 3-D ranges
and variability of these properties.
Soil genesis is the science that deals with the formation of soils through various processes
as influenced by the environmental properties climate, organisms, topography or relief,
parent material, and time.
Soil classification is categorization of soils in groups at varying levels of generalization
according to their morphological properties and/or assumed genesis in a manner important
for the objectives of the classification. Many systems have been and are being used. The
main system we will deal with in this course is Soil Taxonomy (Soil Survey Staff, 1999)
developed in the U.S. but widely used worldwide.
Soil survey is the determination of type and pattern of the occurrence of soil bodies on the
landscape and plotting of these soil bodies on maps.
Soil characterization is the determination and quantification of chemical, physical,
mineralogical, and biological properties of soil samples collected from soil horizons.
Soil interpretation refers to the analysis of soil morphological, chemical, physical,
mineralogical, and biological data to infer the suitability, potential use, and limitations of soils
for various uses.
Concepts and Definitions of Soil
Soil has been defined by many groups in a variety of ways. For example, geologists may
consider soil to be the outer part of the earth's crust. Engineers often consider soil to be any
earthy material that can be manipulated and that has certain characteristics important for
construction. Soil is often been defined as a medium for plant growth and many cultures
since at least 4,000 BC have used plant productivity and/or ability to grow certain crops to
define and group soil. Not until 1862 was the soil first defined as a body unto itself, i.e. "soil
is considered to be the product of weathering, formed as the tooth of time incessantly grinds
the solid covering of our planet and decomposes and destroys its solid mass" Fallou (1862).
During the mid to late 1800's, Dokuchaev (Russian scientist) defined soil as a natural
evolutionary body that developed in response to five factors; climate, vegetation, parent
material, topography, and time. Dokuchaev's work along with the work of many scientists
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from around the world have established soil
as an important resource that is worthy of
study as an independent body.
The pedological definition of soil is: the
collection of natural bodies on the
earth's surface containing living matter
and supporting or capable of supporting
plants out-of-doors.
A few key points of this definition include:
the collection, i.e. there is more than one
kind of soil, natural body (potting media is
Granite on Panola Mountain. The hard rock
not soil by this definition), containing living
is not soil. However, the area immediately
matter (deep sediments with no biological
around the pine tree is soil (supports plant
activity cannot be part of the soil), capable
growth out of doors).
of supporting plants out of doors (again, at
the surface and with properties conducive for plant growth).
An additional part of this definition that has been used and incorporates the five factors of
soil formation is: "each soil has a unique morphology resulting from the combination
of climate, living matter, and relief acting upon
earthy parent materials over time."
By this definition, hard rock is not soil. Neither is potting
soil.
The upper limit of soil is the boundary between soil and
air, shallow water, live plants, or plant materials that
have not begun to decompose. Areas are not
considered to have soil if the surface is permanently
covered by water too deep (typically more than about
2.5 m) for the growth of rooted plants. The horizontal
boundaries of soil are areas where the soil grades to
deep water, barren areas, rock, or ice. In some places
the separation between soil and non-soil is so gradual
that clear distinctions cannot be made.
The lower boundary that separates soil from subjacent
non-soil is most difficult to define. Soil consists of the
horizons near the earth's surface that, in contrast to the
underlying parent material, have been altered by the
interactions of climate, relief, and living organisms
over time. Commonly, soil grades at its lower
boundary to hard rock or to earthy materials virtually
devoid of animals, roots, or other marks of biological
Tree growing in large planter. By the
pedological definition, the tree is not
growing in soil.
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activity. The lowest depth of biological activity, however, is difficult to discern and is often
gradual. For practical purposes, the lower boundary of soil is arbitrarily set at 200 cm. In
soils where either biological activity or current pedogenic processes extend to depths
greater than 200 cm, the lower limit of the soil for classification purposes is still 200 cm.
The following are a few concepts of soil genesis that are good to keep in mind during
subsequent discussions.
Water is the driving force for pedogenesis.
Pedogenic processes active in soils today have been operating over time and have varying
degrees of expression over space.
Many soil-forming processes proceed simultaneously, and the properties of the resulting soil
are the result of the balance among the processes. For example, leaching of water-soluble
salts and clay formation are two soil-forming processes that occur in all soils. Salt leaching
is a faster process than formation of clay. In a humid climate, salts will be leached from the
soil long before there is visible evidence that clay is being formed. Thus, a “young” soil may
have been leached of salts, but there is no evidence that clays have formed from primary
minerals. Clays are being formed, but the process has not proceeded to the point that
evidence of its occurrence is observable by ordinary means. Later in the soil’s development,
salts will still not be present, but evidence of clay formation will be observable. Salt leaching
is still occurring, but since all of the salts have previously leached, there is no obvious
evidence that the process is occurring. In arid climates, both processes are occurring, but
rates are slower than those in humid climates because there is less rainfall to drive the
processes. Thus, a soil with age similar to the young soil in the previous discussion may not
be leached of soluble salts. An older soil may be leached of salts, but with no evidence of
clay formation.
Distinctive processes produce distinctive soils.
Five environmental factors, climate, organisms, relief or topography, parent material, and
time, mediate the pedogenic processes within the soil (leaching, mineral weathering, clay
translocation, etc.).
Current soils carry the imprint of a combination of pedogenic processes that have been
active over the period of soil development, i.e. properties of the soil may have been
developed in earlier geologic periods.
A particular site may have had many different soils as one or more of the factors influencing
soil formation changed over time.
There are few old soils (old in a geologic sense). Most are younger than Pleistocene
(2,000,000 ybp) and a great many have formed during the Holocene (12,000 ybp). Changes
in climate and relief over time erode or bury soils and start the process over.
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Clays form in soil from primary minerals in the rock. Clays in sedimentary deposits formed in
a soil before being eroded and deposited.
Other definitions that may be useful:
Regolith: Unconsolidated part of earth's crust
Pedon: The smallest area (volume) that represents the variation in morphological properties
of a particular soil. In most cases it is a volume of soil with an area of about 1 m 2 at the
surface and depth of 2 m (depth is arbitrary; the soil does not necessarily stop at 2 m and
many soils are not 2 m deep).
Soil profile: A vertical section through the soil. Commonly conceived as a plane at right
angles to the surface
Horizon: A layer, approximately parallel to the soil surface, distinguishable from adjacent
layers by a distinctive set of properties produced by soil forming processes.
Solum: The part of the soil that has been affected by pedogenic processes. The solum
does not include the parent material.
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