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The Soil Beneath Your Feet-
Where does it come from?
Soil Formation - an Introduction
Have you ever wondered
how soil was made, how
old it is or how long it
would last? This booklet
gives an introduction into
soil and how soil was
formed.
Are all soils the same?
If you placed two soils from different parts of Scotland side
by side you may notice that they have different colours
and textures. Running the soils through your fingers, you
may find that one soil is gritty and coarse textured whilst
the other one is smooth and pliable. They may have
different colours and may even smell differently.
We have mapped the whole of Scotland (see opposite
page). Each colour represents a different soil and enables
us to show how the soil changes with geography and how
this effects land use.
What is soil?
Although this seems an easy question to ask, it is very hard to given a definitive answer.
Pore channel Air pore Clay layers Organic ma0er Mineral par*cles An easier question to answer is
“what are soils made out of?”
If we were to “deconstruct” soil
into its constituent parts we
would find minerals such as
quartz, water, organic matter
produced from living and dead
plants and living organisms such
small animals such as earthworms and possibly small mites
and insects depending on where
the soil was from.
(a thin section of soil)
Looking at it through a microscope we may also see microorganisms such as bacteria,
fungi as well as protozoa and nemotodes.
Making a very thin slice of soil shows that soil can be very porous with spaces between
the particles. These spaces allow the flow of water and nutrients through soil and allow
gases such as carbon dioxide and methane to exchange with the atmosphere. All these
different elements are combined forming aggregates of particles which give much of
the structure to soil. And although soil may at first appear unchanging, soils are very
dynamic and form an integral part in the cycling of nutrients within the environment.
Given how complex soil is, to study it in detail, we need the combined disciplines of
geology, biology, chemistry and physics.
Soil Formation - an Introduction
Where does soil come from?
At the end of the nineteenth century, Dokuchaev working in Russia, showed that soils do
not form by chance. Instead they are formed by the interplay of five factors:
Parent Material
Time
Topography
Climate
Biology
Although it is convenient to consider each soil factor separately, none of them act
independently although some exert a stronger influence than others.
Parent Materials: In mineral based soils, the starting or parent material
of soil is bedrock. The same type of parent material can give rise to different soils
depending on the nature of the other soil forming factors.
The process
of making
Chemical & Physical Weathering
this type of soil begins
by breaking down or
“weathering” the bedrock
into smaller fragments.
This can be by both
physical means through
frost shattering, and
chemical processes
through the action of
water or acids in water.
Some types of rock are
more susceptible than
others. You can get some idea of the parent material by feeling the texture of the
soil through your fingers. Granite or schist-based soils are generally coarse textured
whilst fine textured soils are from fine grained sandstones or shales.
The amount of weathering can also determine the final type of soil and soil properties.
For example, soils which are freely draining and have poor or low water retention have
probably come from bedrock which was not completely weathered. This is because
the particles which make up the soil are much larger and therefore give larger spaces
between the particles allowing more water to flow through. Under more intense
weathering, minerals may become associated with clays and so the soil becomes better
at retaining water, affecting drainage.
Water changes the chemical properties of the rock by removing or ‘leaching’ many of
the chemicals and minerals which make up the parent material. The amount removed
is determined by the amount of oxygen, organic acids and carbon dioxide dissolved
in rain water with some chemicals more resistant to this ‘chemical weathering’ than
others. High rainfall can lead to an increased removal of sodium and potassium which
would otherwise have remained associated with clays. This produces a slightly acidic soil
common to Scotland. In areas of low rainfall, chemical leaching is limited as well as any
biological activity.
In Scotland, the geology is so varied that there is not one type of bedrock producing all
of Scotland’s soils. Very few of the soils in Scotland are, however, produced by direct
weathering of the bedrock in situ. When the ice advanced south, glaciers removed much
of the soil leaving bare rocks. This material (mostly clays, sands and gravel), originally
formed over many thousands or millions of years, would then be transported (mainly
south or south east) by the glaciers . When the ice retreated, this “glacial till” was left
behind and formed the parent material of many of the new soils of Scotland.
Soil Formation - an Introduction
Biology: In those mineral based soils which have an organic matter component,
the organic matter originates initially from microbes which are able to directly utilise
many of the chemicals which make up the rock and take in directly (“fix”) nitrogen and
carbon (as carbon dioxide) from the atmosphere. They produce organic acids which
helps to dissolve the rock. This is ‘biological weathering’ of the parent material.
Biological Weathering
Later, lichen and then algae, feeding off the organic material left by the
microbes, colonise the rock. This eventually forms a thicker layer of
organic material which can support larger plants. The physical action
of the growing plant root accelerates the breakdown
process of the rock. Nutrients made in the leaves
are passed into the soil by the roots to the soil
microorganisms and also when the plant dies.
nemotode
fungi
protozoa
root
through
The
earthworms
euplodes protozoa
bacteria
When sufficient material has accumulated, larger
animals such as earthworms & mites, can burrow
into the soil, intermixing organic and mineral matter
together and creating passages which can help in
penetration and movement of water and nutrients
the soil.
The impact of organisms, particularly bacteria, fungi
& plants, on soil is enormous. Together, and with any
products they produce, such as polysaccharides, they help bind the soil minerals together to form
aggregates which stabilises the soil and can affect
drainage and erosion.
Climate: Climate is a description of the prevailing long-term atmospheric
conditions and is the principal factor in determining the rate of soil formation.
In Britain, the uneven distribution of rainfall, for example, in upland areas
which generally have increased rainfall with increasing altitude compared to
lowland areas, will have more minerals being leached out from soil and increased
waterlogging.
The increased cloud cover leads to decreasing amounts of sunshine and reduced
temperatures which in turn reduces biological activity. This slows the breakdown
of plant debris leading to accumulation of plant matter for example in the form
of peats. Lower temperatures promote frosts which encourage weathering of the
bedrock.
Climate and altitude affects the length of the growing season, the type of plants which
will grow and how much ground cover plants will give and hence the amount of soil is
shaded which affects moisture loss and soil temperature.
It has been suggested that in prehistoric times, settlements in the lowlands were
determined by the type of soil available, whereas in the highlands, soil conditions were
determined by the climate.
Topography: Topography or relief directly affects soil movement and hence soil thickness.
Topographic features, initially formed by geological activity in the Earth’s
crust, are then modified by the action of water, ice and wind. Soils forming
on slopes are more prone to erosion as rainwater and higher wind speed
move material further down the slope. Soils on flat or gently sloping sites tend to be
deeper compared to soils further up the slope. The amount of sunlight reaching the
soil is also determined by its relief which affects the amount of vegetation cover. Soils
with poor natural drainage tend to occur on gently sloping sites.
Time: Time is an important factor in developing a soil with formation not a
continuous process.
Many of the processes involved with breaking down the bedrock need
considerable amounts of time. Soil organic matter may eventually turn
into “humus” imparting a dark brown colour to the soil and is a good source
of nutrients. Soil minerals themselves will also change as specific elements such
potassium are removed at different rates leading to a soil with different chemical
composition.
Soil Formation - an Introduction
How many types of soil are there in Scotland?
37
Thirty-seven different types of soil have been identified in Scotland but when we take
into consideration the different rock types that these are formed from, we find over a
1000 separate soil series. Peat is the most common single type found in Scotland.
Other soils found in Scotland include brown earths, humus iron podzols, iron podzols,
peaty podzols & sub alpine podzols (Spodosol), mineral alluvial soils, non calcareous
gleys, peaty gleys (Gleysol), organic soils and brown rankers and redzinas to name
but a few.
To tell soils apart, the soil scientist will look at the type and sequence of different soil
horizons and their colour (indicative of specific elements such as iron), texture and
distribution of soil particles (e.g. sandy, silty or clayey (these reflect the mineralogy)),
soil structure and size of aggregates, organic matter, chemical (e.g. pH) and physical
properties, where the soil is in relation to the surface (soil horizon) and the associated
bedrock.
Why do you get soil horizons?
If you dig past the top few centimetres you may notice that
the soil changes colour and texture forming distinct layers or
horizons. These can be viewed by creating a soil profile which is
a vertical section through the different horizons. These vary in
depth from a few centimetres to 10’s of metres.
soil profile of podzol
The horizons are formed because some chemicals such as iron
are moved through the soil by water in preference to others.
Those chemicals which readily dissolve in water will move
fastest but in very dry or very wet soils, movement is limited.
Other factors which will affect the formation of a horizon include pH, amount of oxygen, solubility, interactions with other chemicals and microbes.
Soil scientists label the different horizons with letters which denote the type of
material found there and reflect their origin. The letters L, F, H and O are used to
describe different types of organic horizons, A, E, B and C are used for mineral
horizons and R signifies solid rock. Sometimes horizons can have properties of
more than one, for example BC, and we can also add more detail through the use of
subhorizons, for example Ap represents a ploughed A horizon.
Here are two examples of classic soil profiles
found in Scotland:
Blanket Peats (a Histosol)
20-25% of Scotland is covered in blanket peat and is mainly found in Northern
Scotland. Peat has been used extensively in the past as a fuel, producing whisky and,
more recently as an additive to horticultural compost.
Peat develops when excess rainfall does not drain away. The accumulating plant
material (mainly mosses), becomes waterlogged and anaerobic. Under these
conditions organic material is only slowly broken down and becomes acidic. The
material at the bottom becomes humified and cannot be completely decomposed. As
the soil is too acidic for earthworms, fresh litter does not get mixed in with the mineral
soil elements and layers develop which over time accumulates forming a deep layer,
which can be as much as 12 m deep (average 2 m). This depth has been estimated to
have taken up to 9000 years to form.
Schematic representation of the formation of peat
Soil Formation - an Introduction
Podzol
In some forests, you may think that the trees are growing on a very thin layer
of soil. However you are only seeing the surface. This type of soil is known as
a podzol and may take up to 1000 years to form. In these soils a thin layer of
organic matter, mixed with some mineral matter, lies on top of the soil forming
material, in this case a sand dune. The organic layer accumulates with the
deeper material gradually decomposing. A dark grey layer forms below the top
organic matter; this is the E horizon. Several elements, especially aluminium
and iron are leached out of this layer into the lower layers which may become
hardened. On the surface, mosses, grasses and eventually pine trees can be
found.
Schematic representation of the formation of a podzol
litter
mix of organic
& mineral
litter
decomposing
organic material
litter
homogeneous mixture
of organic and mineral
material
grey leached horizon
accumulation of humus
material
sand dune
sand dune
sand dune
Why are soils different colours?
Soils are different colours because metals, particularly iron, form different types of
oxides which are themselves different colours. These maybe present in different
proportions within a soil horizon. Goethite and haematite are two common iron
oxides which impart a yellow and red colour respectively to soil. When, under wet
and anaerobic (without oxygen) conditions, iron (ferric) oxide is transformed to iron
(ferrous) oxide, the soil becomes a grey-blue colour. When oxygen is reintroduced e.g.
when the soil dries, the iron is converted back to the red ferric forming a colour contrast
in the soil horizon.
How old are Scottish soils?
Although Scotland possesses the oldest rocks in the country (the Lewisian gneiss
(pronounced “nice”) of the Outer Hebrides and Northwest Highlands was formed 2-3
billion years ago), Scottish soils in some respects can be considered to be relatively
young, formed after the last ice age about 10,000 years ago. To put this in perspective,
some soils from Africa and Australia are thought to be around 65-144 Million years old formed in the Cretaceous period when dinosaurs ruled the Earth!
Pangea
10,000 Years
65-144 Million
Soil Formation - an Introduction
A natural resource:
We all tend to take soil for granted. However, given the length of time it takes for soil
to form we must consider it a finite resource. Our soils are at the heart of our primary
production industries. In 2005 we had a gross agriculture output of £1.75 billion and
have in the past held world records for cereal yields. Our soils are able to store vast
amounts of water (40 billion cubic metres, M3) which helps to prevent flooding.
They filter out and breakdown toxic compounds which may otherwise enter into our
ecosystems from polluted rain.
Soil is also an important store of carbon. It has been calculated that there is about
3000 Mt carbon in our soils which is about 60 times more than there is in Scotland’s
plants, including trees (estimated at 50 Mt). Even though Scotland is only one third
of the UK land mass, our soils contain about 50% of the total UK soil carbon store
(about 6000 Mt carbon). Most of the Scottish carbon store is contained in our peat
deposits (53%). Peat has the potential to store about 0.4 Mt carbon, offsetting about
3% of Scotland’s total carbon dioxide emissions from energy production although
the actual figure may be much less than this as many peatlands are in poor condition
due to erosion, drainage, afforestation, over-grazing, pollution and peat cutting.
Agricultural soils, however, could take up an additional 115 Mt carbon which is
equivalent to the CO2 emissions from 22 years’ total carbon dioxide emissions from
the energy sector alone.
Some of Scotland’s most interesting soils underpin our iconic habitats, e.g. Flow Country,
native pine woodland, machair grasslands, and these in turn support rare biodiversity
such as unique plants, insects and fungi. Like most other soils, our soil supports the
greatest diversity of organisms on the planet in the form of microscopic life forms. A
teaspoon of soil (~ one gram) of a typical agricultural or forest soil can contain 10,000
different species of bacteria and fungi with an as yet unexplored potential for useful new
genes and enzymes.
What is the fate of Scotland’s soil?
In the short term, soil is under threat from many sources. Amongst these are: climate
change (e.g. through flooding or drying), use of pesticides, over fertilisation and over
grazing, pollution, depletion of organic matter and erosion. Developments for industry
and urbanisation also have a direct effect. In 2006 an estimated 1200 hectares per year
was lost through housing and road building. This is equivalent to an area the size of
Dunfermline.
In the long-term, soils will change through more weathering and removal of minerals
and chemical elements such as potassium. Eventually, soil particles may be lost through
movement through the soil layers or erosion on land and finally into the sea.
Soil Formation - an Introduction
Soil: The history beneath your feet
Our soil today is a result of our past. Past climate, management, industrial pollution and
human settlement have all influenced the soil of today. Soil can be seen as a record of
our cultural history and often harbours records of the past and protects archaeological
artefacts. So, in some senses soil is history beneath your feet!
The next time you see soil, give a thought as to where it comes from (and not just the
garden!), and what might be living in it!
Further reading
There are many books and web sites which give information about soils and geology.
Here is a selection:
Essential Soil Science: A clear and concise introduction to soil science by M.R. Ashman and G.
Puri (2002) Blackwell, Oxford UK
Soils in the British Isles by L.F. Curtis, F.M Courtney & S. Trudgill. (1976) Longman, London
An Introduction to Soil Science by E.A. Fitzpatrick (1992) Longman, NY.
Geology and landscapes of Scotland by Con Gillen (2003), Terra Publishing, England
Scotland’s Soil Resource – Current State and Threats. Report to Scottish Executive (2006).
http://www.scotland.gov.uk/Publications/2006/09/21115639/0
Opening of Scottish Parliament
Assassination of Senator Robert Kennedy
Outbreak of World War II
Electron microscope
Alexander Fleming and penicillin
Invention of TV
Outbreak of World War I
Forth Bridge opened
Modern Tay Bridge opened
Tay Bridge disaster
James Clerk Maxwell and electromagnetism
Birth of Robert Burns
Jacobite rebellion
Act of Union
van Leeuwenhoek observes microorganisms
Isaac Newton “discovers” gravity
First compound microscope
Michelangelo paints the Sistine Chapel
Da Vinci paints the Mona Lisa
Battle of Bannockburn
Birth of William Wallace
David I builds castle at Edinburgh castle site
Accession of Macbeth, King of Scotland
Viking Raids
Roman occupation of Scotland
Site at Edinburgh castle first occupied
Skara Brae
Knap o’Howar
Scotland becomes inhabited
Scotland reaches its present latitude
Edinburgh castle rock created by Volcanic activity
years ago
3
6
23
32-22
42
51-43
ca. 60
69
71
72
73
75
76
79
80
82
84
95
96
120
123
129
131
140
143
149
173
180
195
251
265
284
303
334
345
356
420
502-498
507
655-644
696
738
ca. 880
970
979-915
ca. 1030
1280
1931-1641
ca. 2350
2600-2900
2900
3000-5000
5000
5500
7500
8000
10000
14000
15000
ca. 25000
44000
2 million
150 million
300 million
340 million
National Soils Inventory re-sampled
Creation of the Macaulay Land Use Research Institute
First National Soils Inventory of Scotland
1/3rd of Scotland Surveyed
Air survey of Scotland (1:10,000)
Arthur Holmes proposes continental drift
Hans Jenny proposes the five soil factors in soil formation
Soil Survey of Scotland
Macaulay Institute created
Soil classification system (USA)
Soil sampling starts at Macaulay Institute for Soil Research
Macaulay Institute for Soil Research founded
Wegener’s Pangea proposed
Darwin examined role of earthworms in soil formation
Dokuchaev proposes that soil is a natural body with its own genesis
James Croll proposed climate is influenced by ocean currents
Louis Agassiz proposes ice age
Lyell’s Principle of Geology published
First Geological map of Southern Scotland produced by W. Smith
Birth of James Hutton father of modern geology
James Ussher calculates age of Earth to be 6000 year old
Gough Map of Great Britain
Shen Kuo: hyphothesis for land formation & climate change
Birth of Abū Alī Sīnā (Avicenna) father of geology.
Aristotle’s “Meterology” theories about the earth sciences
Babylonian map of the world
Clearing of native forest & establishment of blanket peat
Human activities impact on land/soil
Re-advancement of ice in Scotland
End of last ice age/development of agriculture
Formation of modern soils
First map produced; last ice age starts to retreat from Germany
Formation of Clava clay beds near Inverness
Formation of oil & gas in North Sea
Formation of coal
macaulay.ac.uk