the soil beneath your feet- where does it come from?

The Soil Beneath Your Feet- Where does it come from?

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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.

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 earth-worms and possibly small mites and insects depending on where the soil was from.

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.

(a thin section of soil)

Mineral par*cles

Organic ma0er

Pore channel

Clay layers

Air pore


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:

Although it is convenient to consider each soil factor separately, none of them act independently although some exert a stronger influence than others.

Time

Topography

Climate

Biology

Parent Material

The process of making 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.

Chemical & Physical Weathering

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.

Climate


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.

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.

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

root penetration and movement of water and nutrients through the soil.

The 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.

Biological Weathering

nemotode

protozoa

fungi

earthworms

bacteria

euplodes protozoa

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.

37Thirty-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.

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.

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.

How many types of soil are there in Scotland?


Why do you get soil horizons?

soil profile of podzol

How many types of soil are there in Scotland?

Why do you get soil horizons?

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


litter

mix of organic & mineral

sand dune

decomposing organic material

homogeneous mixture of organic and mineral material

grey leached horizon

accumulation of humus material

litter

sand dune

litter

sand dune

PodzolIn 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

Why are soils different colours?

65-144 Million10,000 Years

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


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.

Further readingThere 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

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!


Opening of Scottish Parliament

Assassination of Senator Robert Kennedy

Outbreak of World War II

Electron microscope

Alexander Fleming and penicillinInvention of TV

Outbreak of World War IForth Bridge opened

Modern Tay Bridge opened

Tay Bridge disaster

James Clerk Maxwell and electromagnetism

Birth of Robert BurnsJacobite rebellion

Act of Unionvan Leeuwenhoek observes microorganisms

Isaac Newton “discovers” gravity

First compound microscopeMichelangelo paints the Sistine Chapel

Da Vinci paints the Mona Lisa

Battle of BannockburnBirth of William Wallace

David I builds castle at Edinburgh castle siteAccession of Macbeth, King of Scotland

Viking RaidsRoman occupation of Scotland

Site at Edinburgh castle first occupied

Skara BraeKnap o’Howar

Scotland becomes inhabited

Scotland reaches its present latitude

Edinburgh castle rock created by Volcanic activity

years ago36

2332-22

4251-43ca. 60

697172737576798082849596120123129131140143149173180195251265284303334345356420

502-498507

655-644696738

ca. 880970

979-915ca. 1030

12801931-1641ca. 2350

2600-29002900

3000-50005000550075008000100001400015000

ca. 2500044000

2 million150 million300 million340 million

National Soils Inventory re-sampled

Creation of the Macaulay Land Use Research InstituteFirst National Soils Inventory of Scotland1/3rd of Scotland SurveyedAir survey of Scotland (1:10,000)Arthur Holmes proposes continental driftHans Jenny proposes the five soil factors in soil formation

Soil Survey of ScotlandMacaulay Institute createdSoil 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 genesisJames Croll proposed climate is influenced by ocean currents

Louis Agassiz proposes ice ageLyell’s Principle of Geology publishedFirst 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 changeBirth of Abū Alī Sīnā (Avicenna) father of geology.

Aristotle’s “Meterology” theories about the earth sciencesBabylonian map of the world

Clearing of native forest & establishment of blanket peat

Human activities impact on land/soilRe-advancement of ice in ScotlandEnd of last ice age/development of agriculture

Formation of modern soilsFirst map produced; last ice age starts to retreat from GermanyFormation of Clava clay beds near Inverness

Formation of oil & gas in North SeaFormation of coal

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the soil beneath your feet- where does it come from?

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