4. Geology and Soils
Geology17
A hard crystalline core of schists and granites occupies much of central and northern Scotland while the gentle undulations of the Central Lowlands and the Border country were formed on softer sedimentary strata. Underlying geology defines landscapes and exerts a powerful influence on natural systems and the development of soils, land use, industry and infrastructure.
Five broadly defined geographic areas of distinctive landform and soils in Scotland relate to geological conditions, and which influence land cover and resultant cultural landscapes.
The broad form of the landscape in each of these five areas is tens of millions of years old. More recent activity from glaciers and ice sheets, which ended around 10,000 years ago, has created an intricate combination of erosion and deposition landforms. Landscapes continue to evolve, but the varying characteristics of underlying rocks and rock structures are still dominant in the landscape of Scotland.
Based on: Scottish Natural Heritage (1995). The Natural Heritage of Scotland - an Overview. Mackey, E.C. (ed.). Scottish Natural Heritage, Perth.
Reprinted with permission from the Scottish Association of Geography Teachers. From Haynes, V.M. (1995). Scotland’s Landforms: A Review. The Journal of the Scottish Association of Geography Teachers, 24. pp18 - 37.
Soils
Soils represent a dynamic interface between physical, biological and hydrological systems. Soils are an integral part of the landscape, reflecting not only natural processes from which they have been formed, but also the influences of human activities, present and past. The ever-evolving nature of soils is that they are being continually formed and modified, both by natural processes and by human activities.
Advances 2, ‘Who’s Afraid of a Bit of Rain’, discussed soil evolution. Advances 3, ‘What’s Under Your Wellies’, discusses soil as a living system.
Soils are principally composed of finely divided rock material, sand, silt and clay, together with organic matter, air and water. The structure of cracks and granules transforms soil from unconsolidated debris into a dynamic, living environment for micro-organisms, plants and animals. There are many different types of soil, characterised by depth, composition and appearance, with differing physical and biological properties.
The soils of Scotland can be broadly divided into four main groups: peats, gleys, podzols and brown forest soils. None develops in isolation and there are a great many variations.
Based on: Scottish Natural Heritage (1995). The Natural Heritage of Scotland - an Overview. Mackey, E.C. (ed.). Scottish Natural Heritage, Perth.
Over much of the Western Isles, northern Scotland and the highlands, wet and cold conditions suppress biological activity. Plant debris accumulates faster than it can be decomposed, producing expanses of peat.
At lower elevations, with lower rainfall and higher temperature, micro-organism activity increases sufficiently to allow further soil development, although still with surface organic accumulation. Gleys develop on fine-textured parent material with poor drainage, resulting in intermittent or semi-permanent waterlogging. Under anaerobic conditions (where air is excluded), iron and other compounds in the soil are chemically reduced due to the lack of oxygen, and this is exhibited as mottled grey, blue and green colours in the subsurface horizons, with rusty brown colours where oxygen reaches into the soil.
In the drier, warmer east, parent materials are often acidic and coarse grained. Podzols and their derivatives are the dominant soil types. Acidic conditions suppress biological activity, allowing surface organic accumulation. Their coarse sandy texture means that these soils are well-drained and, under acid conditions, mineral nutrients are rapidly leached from the upper horizons. This produces a distinctive profile of abruptly different horizons, with: dark, surface organic accumulation; a coarse-grained, bleached and leached upper horizon; and a dark brown lower horizon where the leached minerals are re-deposited. Separating the leaching and the re-deposition zones is often a dark layer of organic accumulation, composed of material washed down from the surface, or in some instances an 'iron pan' (layer of iron oxide accumulation).
In the south-west of Scotland, peaty and gley-type soils dominate. In the south-east, where conditions are warmer and drier, brown forest soils may develop. These form on different types of parent material but their key soil-forming characteristics are lack of acidity, low rainfall and warm temperatures. Such conditions encourage biological activity and consequently organic matter is efficiently broken down, producing deep, fertile soils, with abundant earthworm activity. They are the soils most favoured for agriculture.
Human influence
The exploitation of soils in Scotland, which has included the enrichment and improvement of otherwise poor soils, has taken place over several thousand years. The value of soil has long been recognised but it is largely in recent decades that concerns have been raised about the capacity of soil to sustain intensive forms of land use. Soil can be lost due to urban expansion, or modified over large areas by open-cast mining. Mechanised agriculture and the planting of large tracts of land for commercial forestry may exacerbate soil erosion, while industrial development has been associated with pollution and acidification.
Acidification18
Areas of high rainfall and low temperature, with freely draining soils of low base status, will acidify naturally. Large areas of Scotland therefore have naturally acidic soils. Soil processes which contribute naturally to acidification include the leaching of base cations (particularly calcium) and the release of organic acids during organic matter decomposition. Human influence can accelerate this process, for instance by the removal of base cations through cropping, the application of acidifying fertilisers, or coniferous afforestation. Lime, essentially calcium carbonate or calcium hydroxide, is a base-rich neutralising agent which has traditionally been utilised to combat excessive acidity on agricultural land.
The term ‘acid rain’ is loosely used to describe the deposition of acidic compounds in rain, mist or snow. Acidifying pollutants, notably from air-borne compounds of sulphur and nitrogen, are formed from the combustion of fossil fuels (notably coal and oil) – mainly from large industrial combustion plants, power stations, and motor vehicles.
Acidification can lead to enhanced leaching of soil base cations (plant nutrients) and thus to nutrient deficiencies. Aluminium and heavy metals can become mobilised in soil solution, causing damage to roots and mycorrhizae (beneficial fungi associated with plant roots), and so inhibit the uptake of water and nutrients. As well as damaging plant life, acidification of soils will, in time, lead to acidification of surface waters and so also harm aquatic life.
The buffering capacities (ability to neutralise acids) of soils in Great Britain have been determined by chemical analysis and experimentation on plant growth, giving rise to the definition of soil critical loads. Critical loads specify threshold levels of pollutant or acidifying agents beyond which ecological damage will occur.
The impact of acid deposition on any given soil will depend upon its composition. Soils which are high in calcium, magnesium and weatherable soil minerals can readily neutralise acidic inputs. Such soils can tolerate large depositions without showing adverse effects – thus their critical loads are ‘high’. In contrast, soils which have naturally low levels of neutralising substances can become quickly depleted, with resulting acidification. Their critical loads are correspondingly ‘low’. The rate and extent of freshwater acidification are determined by the acid neutralising capacity of the soils and rocks of the surrounding catchments.
The mapping of critical loads for soils, when compared with actual or projected pollutant deposition, determines areas of 'critical load exceedence', delimiting areas where damaging impacts on vegetation are predicted to arise.
This has allowed abatement action to be defined in order to reduce harmful emissions to levels at which critical loads should not be widely exceeded in the future.
CURRICULUM CONNECTIONS
The geological history of Scotland is one of the truly great educational narratives and is a tale which crops up repeatedly throughout the curriculum. A myriad of approaches are in use: a piece of local limestone might be passed around the class; observations exchanged; origins deduced; detective work undertaken; continental drift discovered (again!) – or it may be that illustrated narrative is preferred. Most departments explore this area in their common courses and Standard Grade courses.
The connection between geology and soils is usually approached with less enthusiasm and gusto. This is a pity as it is no less absorbing. Soil is a major natural resource and should be identified as such in stages P7 to S2. At Standard Grade, key ideas 1, 4 & 6 demand a closer look at the origins and development of soils. This interest naturally progresses into a detailed study of the properties of soils and their formation processes at Higher Grade. Advances 2 and 3 have helped to enhance the quality of these lessons and it is hoped that the slides and notes accompanying Advances 4 will prove to be a catalyst to further improvement.
N.B. The concept of critical loads is crucial to quantifying the threat from air pollution and should be introduced into our teaching. Wherever possible, local examples should be referred to. A good rock collection is a must for every geography department. An augur is also an extremely good buy.
- From: Scottish Natural Heritage (1995). The Natural Heritage of Scotland - an Overview. Mackey, E.C. (ed.). Scottish Natural Heritage, Perth.
- Department of the Environment (1991). “Acid Rain” - Critical and Target Loads Maps for the United Kingdom.