This is a summary of the Bachelor's subject Introduction to Soil Geography, and includes the full content of this course (except practice material). Most of the resources come from the reader SGL 12803.
Chapter 1 – What is soil?
1.1 Soil defined
Definition of soil for soil scientists: “unconsolidated mineral and organic matter that is either
presently or was at one time at the Earth’s surface where it was influenced by the genetic
and environmental factors of parent material, climate, organisms, and topography, all acting
over a period of time to produce a product – soil – with many physical, chemical, biological
and morphological properties that differ from those of the parent material.”
The soil is often referred to as “the Earth’s skin”.
Soil science or pedology is the science of the properties and functions, as well as the devel-
opment and distribution of soils. It deals with the possibilities for soil use and the risks associ-
ated with misuse by humans, as well as the prevention and remediation of soil contamina-
tion and damage.
Pedosphere: the soil itself; it comprises all soils and covers the entire globe. It is a continuous
zone that covers the continents and the beds of water bodies on land and the seabed. It ex-
ists everywhere the lithosphere, atmosphere, biosphere and hydrosphere meet. If one of
these spheres is absent, there is no soil (i.e. on lava streams that are cooling or on glaciers).
Soils develop in unconsolidated parent material. Origin of parent material is bedrock,
weathered under the influence of biota and climate.
Regolith: the unconsolidated material and weathered rock resting upon solid bedrock. The
term regolith is used for both unconsolidated material as for material that has been reconsol-
idated or cemented by subsequent soil-forming processes. It can be thick or thin.
Two types of regolith:
o Regolith that developed in-situ: the underlying bedrock weathers in place.
o Regolith transported from the site of its initial formation by gravity, water, wind or
ice (the regolith material is deposited after transportation).
The CLORPT model
The CLORPT model consists of the 5 major soil-forming factors:
o Climate
o Organisms
o Relief or topography
o Parent material
o Time: any of the other state variables and their interactions can change over time →
soil formation can change over time.
State factor model of soil formation: any soil type (S) or soil property (s) is a function of the 5
beforementioned variables: S = f (CL, O, R, P, T, …).
o The ‘…’ represent the other factors that may be important locally but not universally.
o It is called the CLORPT model or CLORPT equation.
, o A combination of soil properties, each of which determined by a specific relationship
with the variables in the CLORPT equation, makes up an individual soil type.
The CLORPT equation is often simplified by considering situations where a soil property s is
expressed as a function of only one state variable, under conditions where the other state
variables are (nearly) constant. Example: s = f (CL (climate), O, R, P, T, …) as climofunction.
o You can have climofunction, biofunction, topofunction, lithofunction, chronofunc-
tion. Combined functions are also possible, but soon become difficult to study.
Climate and organisms are the more active factors and determine the rate at which chemical
and biological reactions occur in the soil.
Parent material and relief/topography are more passive factors, acted upon by the active
factors; parent material and relief define the initial state for soil development.
Pedon and polypedon
When classifying or mapping soil, we need to distinguish between soil types and discern the
boundaries between soils that are next to each other. We choose soil characteristics to set
boundaries for distinguishing one soil from another. There are seldom sharp boundaries
between individual soils: properties change gradually.
It is necessary to characterize an individual soil in terms of an imaginary three-dimensional
unit: the pedon (1-10 m2). It is the smallest spatial unit of a soil displaying the full range of
properties that are characteristic for a particular soil.
A soil unit in a landscape is a polypedon: a group of very similar pedons. It
is a landscape component large enough to be called a soil individual. Soil
individuals in the world sharing the same suite of soil properties that fall in
a particular range belong to the same soil type.
1.2 The global significance of soils
The global population is rapidly increasing, which also increases the de-
mand for products originating from natural resources. However, the
amount of soil available to meet these demands decreases due to land degradation and urb-
anization → understanding how to better manage soil resources is essential for our survival
on our planet.
In 2015, the United Nations adopted 17 Sustainable Development Goals (SDGs) which aim at
bringing about a global society by 2030. Most of the SDGs have no direct link with soils, but
soils contribute to general ecosystem services: the many life-sustaining benefits we receive
from nature which contribute to environmental and human health and well-being.
Ecosystem services can be grouped into 4 categories:
o Provisioning services provide the material/energy out-
puts from ecosystems. They include food, raw materials,
fresh water and medicinal resources.
o Regulating services are services that ecosystems provide
by acting as regulators, e.g. regulating the climate, qual-
ity of air and water, providing flood and disease control.
o Supporting services underpin almost all other services.
Ecosystems provide living spaces for plants or animals
and maintain a diversity of species of plants and animals.
o Cultural services include the non-material benefits
people obtain from contact with ecosystems. They include aesthetic, spiritual and
psychological benefits.
Ecosystem services contribute to nearly all the land-related SDGs.
, 7 soil functions:
o Biomass production, including agriculture and forestry.
o Storing, filtering and transforming nutrients, substances and water.
o Biodiversity pool, such as habitats, species and genes.
o Physical and cultural environment for human activities.
o Source of raw material.
o Acting as carbon pool.
o Archive of geological and archaeological heritage.
Chapter 2 – Geological processes, parent materials and
landscapes
2.1 Origin and composition of the Earth’s crust
2.1.1 The Earth’s curst
Magma is the liquid rock in the Earth’s mantle between the crust and the outer core. Differ-
ential cooling processes in magma create convection flows in the Earth’s upper mantle.
Composition of magma depends on which minerals are already formed first, and on the com-
position of incorporated re-melted rocks.
In the middle of the main oceans, magma rises into long ridges and spreads out on either
side, forming new oceanic crust. It is 5-10km thick and consists of basalt (dark, iron-rich).
When oceanic crust collides with continental crust (lighter, less dense), the oceanic crust sub-
ducts (sinks), taking with it sediment from the ocean floor → rock and sediment melt → the
magma rises elsewhere along zones of weakness (faults) in crust → igneous rocks.
Continental crust (5-50km thick, depending on the activity of collision) consists of igneous,
sedimentary and metamorphic rocks. Dominant igneous rock type is granite (less iron).
Crystals
2.1.2 Minerals
A mineral is a solid with a specific chemical composition and specific physical properties. Crystal surfaces
Minerals are the building blocks of rocks (rocks are aggregates of minerals).
The five most important rock-forming minerals are:
o Quartz: light-coloured; consists of pure silica (SiO2).
o Feldspar: light-coloured; consists of silica, aluminium, potassium, sodium/calcium.
o Mica: dark-coloured; composed of silica, potassium, magnesium, iron. Fractures Cleavages
o Amphibole: dark-coloured; consists of calcium, sodium, magnesium, iron. (Quartz) (Feldspar)
o Olivine: dark-coloured; consists only of silica, magnesium and iron.
The dark minerals have very long and complex formulas → the bonds are not
everywhere very strong → bonds can break → weaker than light minerals.
Many of these elements are essential for plant growth → released when rocks weather.
Crystalline: regular arrangement of atoms. Atoms are arranged in a fixed, repetitive pattern.
Amorphous: atoms do not have a regular order. Amorphous substances are created when
the atoms do not have enough time to fully organize themselves, e.g. due to fast solidifica-
tion (volcanic rocks).
Crystalline materials are strong and dense (ionic binding), amorphous materials are weaker.
2.1.4 Igneous rocks
Igneous rocks are rocks that arise from direct cooling of the liquid magma. At some point,
cooling magma reaches a temperature at which certain minerals begin to crystallize (de-
pends on mineral: high melting temperature = earlier crystallization).
, For magmas that remain inside the Earth’s crust, cooling proceeds very slowly, causing large
crystals to form (2-10mm). Crystallization process continues until all magma has become
rock. Rocks formed within the crust are plutonic rocks (intrusive rocks).
o Characteristic: all minerals are crystalline and approx. the same size.
o Plutonic rocks: gabbro, diorite, granite.
Magmas that can reach the Earth’s surface cool very quickly (volcanism) → magma has no
time to crystallize → forms an amorphous mass without crystalline structure but it contains
some crystals that have already formed. These first-formed mini crystals are called pheno-
crysts and are an important characteristic of volcanic rocks (extrusive rocks).
o Phenocrysts form before eruption because each mineral has its own melting temper-
ature.
o Volcanic rocks are often porous, because they contain small cavities left behind after
gases escape from the magma during the eruption.
o Volcanic rocks: basalt, andesite, rhyolite.
Plutonic rocks have clearly visible minerals of about the same size and randomly oriented.
Rock is very dense and solid. Volcanic rocks are amorphous masses (no visible minerals) in
which phenocrysts are visible. Cavities are often present, giving it a light weight.
o Colour of volcanic rock is an indicator of the mineral content. Dark colour indicates a
high content of amorphous dark minerals, light colour means quartz and feldspars.
2.1.5 Weathering and erosion of rocks
Physical weathering: the disintegration and dislocation of rock into smaller fragments. It is
most active in very cold or very dry climates.
Chemical weathering: process in which elements react with each other under the influence
of temperature, atmosphere, water and acids (i.e. dissolution of limestone by acid rain). It is
most active in (sub)tropical areas with high temperatures and high amounts of rainfall.
The disintegration of massive rock is accompanied by an increase in the specific surface ex-
posed to weathering, as the volume of many rock fragments has a larger specific surface area
than the same mass or volume of one large rock → more chemical weathering is possible on
these surfaces → rock weakens → disintegrates more easily → chemical weathering etc.
Weathering products can be removed or transported by water, wind, ice and gravity. To-
gether, these processes are referred to as erosion. The resulting degradation products are
called sediments, which are transported and deposited at lower levels in landscape/sea,
forming a new group of rocks: sedimentary rocks.
2.1.6 Sediments and sedimentary rocks
In places where sedimentation is greater than erosion, thick packets of sediment may form.
These layers of loose grain might become consolidated by pressure. Consolidated and uncon-
solidated sediments are both called sedimentary rocks and cover >75% of the land surface.
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