When we think of soil erosion we tend to conjure images of tropical deluges, with soil gouged from exposed hillsides and washed into rivers and out to sea. But soil erosion is a problem for us in the UK.
A series of prolonged wet periods in recent years, not least the torrential rain of last winter, have highlighted vulnerability of UK soils. Around 2.2 million tonnes of top soil are eroded annually in the UK – in winters such as the one that has just passed the loss would have been much greater. This is something which should concern all of us.
A loss to the farm
For farmers losing topsoil is fundamental – soil provides the medium and the nutrients which support primary production. Erosion loss damages soil structure and washes away vital (and expensive to replace) nutrients and organic matter. It can require repeat operations because of lost seed beds or damaged crops. There is a real and substantial cost to the farm.
Developments in agriculture over the last 50 years, such as increase in field size, use of heavier machinery, changes to cropping, and loss of hedgerows have increased the risk of soil erosion. Climate change and predicted increase in frequency of severe weather is likely to magnify the impact.
Erosion leads to sedimentation and contamination of streams and rivers, damaging fisheries and wildlife, and increasing water treatment costs. Although soil sediment enters water bodies through natural erosion, around 70 % of soil sedimentation is estimated to come from agricultural sources.
Erosion losses are greatest from arable land, with late sown cereal crops, potatoes and sugar beet particularly vulnerable. Bare soil surfaces exposed to heavy rain leads to formation of rills or runoff along ‘tram lines’ – the tractor wheel tracks in crops.
On grassland, problems can occur where livestock have direct access to the water’s edge – trampling of river banks by cattle increases bank erosion. Similarly ‘poaching’ in gateways, around cattle feeders and water troughs or as a result of high stocking rates during wet weather, significantly increases runoff and erosion.
Erosion contributes to the turbidity of water (the cloudiness caused by suspended soil particles) affecting the gills of some fish and their ability to feed. Sediment on gravel beds affects spawning of fish and impacts on economically important fresh water fisheries. Many invertebrates are also adversely affected by sedimentation.
An estimated 25 per cent of the phosphates and 50 per cent of nitrates in rivers are from agricultural sources. Faecal indicator organisms (FIO) such as E.coli , associated with manures, can also contaminate water supplies. Enrichment of water bodies with high concentrations of nutrients leads to eutrophication – excessive growth of algae which, as it dies and decomposes, depletes the water of available oxygen, causing the death of fish and other wildlife.
Pollution from agriculture is a major cause of failure of river catchments under the Water Framework Directive.
Integrating trees in to farming systems
Whilst changing agricultural practices can be an important first step in reducing erosion, some residual issues will persist. Targeted tree planting is one of the ways to mitigate runoff and pollution from agriculture.
Where they are present, existing hedgerows and shelter belts may already be helping to reduce impacts. However planting trees and the creation of tree belts with their associated vegetation incorporated into farming systems mitigate pollutants and safeguard water resources. Trees and woodland intercept and capture pollutants, increasing water infiltration rates and slowing the flow of transported sediments.
Pontbren farmers, a group of innovative farmers in mid Wales, have shown what can be done to reduce runoff and support productive farming through knowledge of the land and targeted tree planting and hedgerow restoration. Shelter belts integrated in to the farms to provide shelter for livestock has also dramatically reduced runoff. Water infiltration was 60 times greater within tree shelter belts compared to neighbouring grazed pasture just 3 years after planting. This led to both a reduction in peak stream flows and sediment entering water courses. It also ensures that valuable water, soil and nutrients stay on the farm.
Now is the time to act
There is a real and significant cost now to the whole of society and to the natural environment from soil erosion. The longer term costs of failure to tackle soil erosion are even more significant. We need to take a critical look at the way in which we manage land and the impact of changing farming practices on runoff and soil erosion. We should also be thinking about the ways in which we can integrate non-crop habitat – trees, shelter belts, other habitat – in to farming systems in ways which can support production while also helping to reduce the risk of erosion.
Mike Townsend, senior conservation adviser