Threats to soil quality

Soil quality is at risk from a number of threats driven by a range of man-made and natural pressures including climate change, land use change and land management practices.

Human activities have changed the character and quality of our soils over time. We have destroyed protective vegetation cover and have kept soil bare for long periods of time. We also actively add nutrients and pesticides to soils and cover them with housing and infrastructure. All of these activities can impair, or even destroy, the ability of soil to carry out its essential functions.

Once soil is damaged or contaminated it can be extremely difficult, if not impossible, to restore. In some cases, soils can themselves become sources of pollutants. These pollutants can make their way into watercourses, affecting water quality. Good soil management is therefore essential to maintain and improve Scotland’s water quality.

Detailed below are some of the main threats to soil quality.

Climate change

Climate change is a long-term change of weather patterns including temperature, wind and rainfall. Global warming is enhanced by human activities that increase atmospheric concentrations of greenhouse gases that trap the sun’s heat and warm Earth’s surface.

The main greenhouse gases produced in Scotland are:

carbon dioxide from the burning of fossil fuels such as coal and oil;
methane from livestock and livestock manures;
nitrous oxide mainly from the use of nitrogen fertiliser (both organic and inorganic).

A number of soil processes will be accelerated by increasing temperatures and rainfall intensity - both likely to occur in Scotland as a result of global warming.

Higher temperatures will increase mineralisation (loss) of organic matter and result in increased CO₂ release, especially from organic soils, while mineralisation of wetter soils may result in an increase in methane emissions. Warmer, wetter soils are likely to result in increased N₂O emissions from nitrogen-fertilised soils, as these conditions favour denitrification (the conversion of nitrate to nitrogen gas).

Climate change is an overarching driver affecting numerous soil quality issues such as:

a loss of organic matter because of higher decomposition rates (e.g. increased temperature, drying of wetlands);
erosion as a result of more frequent extreme rainfall events;
reduced trafficability as a result of periods of increased soil wetness;
a reduction in soil fertility;
increased and changing pest loads;
a change of vegetation type and an increase in plant growth (both crops and natural vegetation).

Sealing

Sealing is the permanent covering of soil with hard surfaces such as roads and buildings. The impacts that the sealing of soils can have are:

loss of all soil functions;
loss of high quality agricultural land;
loss of natural habitat;
increased flood risk by making run-off more rapid and peak discharge greater;
habitat fragmentation.

Compaction

Compaction is the process by which soil particles are forced closer together reducing soil porosity. This is caused by heavy machinery traffic and, to a lesser extent, by animals trampling on wet soils. The impacts this can have are:

the number and size of pores within soil is reduced, especially larger pores needed to circulate air throughout the soil;
a reduction in the capacity of water that soil can hold;
an increase in anaerobic subsurface conditions, reducing the amount of oxygen available to organisms and increasing the risk of nitrogen dioxide and methane production;
formation of cemented layers;
a reduction of root growth and therefore plant development;
loss of biodiversity as pores become too small to allow soil invertebrates to move through the soil;
an increase in run-off and flooding.

Erosion

Erosion is a naturally occurring process in which soil particles become detached (usually from the soil surface) by wind or water. Erosion rates can be increased as a result of human activities such as the removal of protective vegetation cover by farming, over-grazing, down-hill ploughing and soil compaction, all of which threaten soil quality. The effects that soil erosion can have are:

a reduction in soil fertility due to loss of nutrient-rich topsoil;
loss of carbon stored in the soil;
diffuse pollution of surface watercourses with nutrients and contaminants (e.g. pesticides, fertilisers);
increased flood risk;
increase in sediment in watercourses resulting in the destruction of spawning grounds in rivers and reduced reservoir capacity which may affect aquatic life.

Landslides

Landslides are a mass movement of soil, rock or debris that flow down a slope as a result of gravity. They can occur for a number of reasons; however, in Scotland they are caused mainly by soil saturation after heavy rain fall or snowmelt. The effects that landslides can have are:

exposure of subsoils and bedrock;
burying of soils and vegetation where the slide comes to rest;
potential for death and injury if people are caught up in a landslide;
potential disruption to infrastructure;
introduction of sediment into watercourses that can block the watercourse, lead to damage to aquatic life, reduce drinking water quality and increase flooding potential.

Organic matter decline

Organic matter decline is the loss of organic material from soils and is caused by intensive farming, drainage of carbon-rich soil (e.g. peat and wetlands), soil erosion and climate change. The impacts that a decline in organic matter can have are:

a potential for biodiversity loss as most soil organisms require organic matter as food;a reduction in soil quality for most land uses, particularly agriculture;
carbon loss to the atmosphere, accelerating climatic warming. This is particularly important in Scotland, where soil organic matter content is generally high;
a potential for increased water pollution, as many pollutants (e.g. heavy metals, pesticides) are bound to organic matter;
an increase of dissolved organic carbon concentration in surface water courses, leading to discolouration of drinking water.

Contamination

Contamination occurs when substances are added to soil, causing an increase in concentrations above background or reference levels. Contamination can come from diffuse (e.g. atmospheric deposition, waste to land) or point sources (spills).

Atmospheric deposition

The burning of fossil fuels by industry, households and vehicles releases gaseous emissions of sulphur dioxide and oxides of nitrogen that can travel hundreds of miles in the atmosphere. These gases can be dissolved in rainwater to form sulphuric and nitric acids. These will subsequently be deposited on soil and result in soil acidification which can cause:

reduced drainage water quality due to increased leakage of acidic compounds and toxic elements;
a loss of above and below ground biodiversity, as certain species are unable to survive in acidic soils;
structural damage to soil minerals.

In addition, excess nitrogen deposition can result in soil eutrophication. Intensive agriculture can release high concentrations of ammonia which tends to be deposited close to the source. Soil eutrophication can result in:

loss of biodiversity and changes to vegetation and ecosystems;
the potential for increased microbial activity resulting in more rapid organic matter decomposition and greenhouse gas emission;
the potential for an increase of nutrients in surface water and groundwater.

Waste to land contamination

The application of waste products to land, such as farmyard manure, sewage sludge and organic materials from industrial processes can have many agricultural and environmental benefits if they are well managed and the appropriate amounts of organic waste applied. However, if they are not well managed they may have adverse effects such as:

an accumulation of toxic components of waste in the soil, e.g. trace metals;
a risk of (soil) biodiversity decline;
unhealthy plant growth caused by unbalanced nutrient supply;
the potential for water eutrophication through nutrient transfer from soil;
the potential for increased production of greenhouse gases to the atmosphere.

We inspect and monitor certain industrial and agricultural activities to prevent pollution to Scotland’s land.

Change in soil biodiversity

The over-exploitation of land and soils, land use changes and climate change all have an effect on soil biodiversity, reducing the number and variety of soil species. The impacts that this can have are:

the potential for a reduction of soil functions;
disruption of food supplies and webs;
loss of potential resources for future applications (e.g. biotechnology, drugs).