Lessons from Scotland
Cultivated agriculture in Scotland might appear to be at low risk from soil erosion. Yields are high and soils that support crops, and especially those that have managed grass in the cropping sequence, are usually well structured, containing adequate organic matter. Fields rarely suffer wholesale collapse through ‘sheet’ and ‘gulley’ erosion of the sort shown in photographs from Bennett and Chapline’s bulletin written before the US dust bowl (page 2). Yet the reality is that soils are now at risk not only from long-term or chronic erosion, but from more severe and rapid losses caused by recent, highly invasive practices.
How were things in the past? Looking back at the hard times, those of agricultural deficit and famine, the degradation and loss of soil were rarely blamed directly. Other factors were limiting output – depleted soil fertility, acidity, shallow plough depth, poor drainage, and hard pans preventing root penetration.
Erosion status now
It was not until the 1950s and subsequent decades that self-powered tractors replaced draught animals with the result that plough depth increased, making more of the soil accessible to crops and grass. Plant nutrients such as nitrogen (N) and phosphorus (P) became widely available through industrial process and mining. Yields of crops and grass increased, aided from the 1970s by a range of chemical pesticides.
Yet the shift to repeated, deep soil cultivation in many fields and heavier machinery causing compression, destructuring and slaking of soil [1]. The risk of erosion increased. It’s not on the scale of the dust bowl but is widespread, gradual, intermittent and not always visible.
How bad is it? Quantitative assessments and ‘risk maps’ covering the whole country face the problems of not enough data. Much more on-the-ground information is needed than is available [1, 2]. The main pathways and causes of loss are certainly known. Government and its agencies both here [3, 4] and in Europe [5] are aware of the problem and its potential scale, but there is still much to do before science can accurately estimate losses of soil in tonnes per hectare for the main forms of land used in the country. Yet loss of soil quality and subsequent erosion are arguably the greatest threats to food security here and throughout the world.
Signs
Soil erosion in Scotland’s arable-grass croplands is not always obvious. While wind erosion happens in some places, it’s water that carries most of the eroded soil. Erosion is usually intermittent or episodic, yet the signs are there in rivers, streams, fields and roadsides.
Great rivers such as the Tay (upper left in the panel below) and Findhorn (lower left) are for the most part clean and clear, yet even these suffer ingress of soil. The image upper right is looking down from a steep bank to one of the rivers in the Tay catchment. A stream carrying soil joins the main river and flows leftwards, while the rest of the river, in the upper half of the image, remains clear. After prolonged rain, a field (lower right) disgorges its soil to a road, which is under water, then to other fields and eventually to a river.
Many lowland streams, including those passing through farmland, are clear for much of the time, but may go brown and cloudy after heavy prolonged rain. The stream in the upper left image in the group below runs through part of lowland Perthshire. It is mostly clear, as here, shown by the reflection on its surface of the blue sky. The two images to the right, taken after heavy rain, show soil covering water plants and streamside leaves. The source of the soil in this case was a small sinkhole (lower left) leading from the bottom of the field, under a grassed margin and into the stream.
In the field itself, heavy tillage or compaction from vehicles, removes accumulated organic matter and reduces aeration of soil, and in so doing inhibits the fungi, bacteria and microfauna that work to create and stabilise soil. Soils lose their structure and internal cohesion. They begin to slake and cake.
Cracking even when soils are moist is a common sign of poor structure (upper right in the panel below). Soil is washed down a field to form muddy deposits, deep enough sometimes to half-bury a crop (lower right). From livestock fields, the deposits are a mixture of slaked soil and dung, the one lower left half-blocking a small stream which later empties into a loch. Compacted soil, for example under ‘tramlines’, reduces ingress of water which freezes into a thick layer of ice, while the plants and soil in the less compacted parts of the field are still in contact with the air.
Sources
The causes and sources of soil erosion here are much the same as described by Bennett and Chapline in the 1920s USA (see page 2): livestock destroying or reducing the vegetation cover and heavy tillage leaving soil de-structured and exposed.
The panel of images below show typical causes of soil degradation in livestock farming (from upper left c’wise): cattle reducing the soil to pulp which is readily washed away under heavy rain; tractor lines, which were carefully maintained during the crop’s growth, but ignored at harvest, leading to the criss-crossing of traffic marks, compacting the soil over much of the field; flooded pasture, transporting soil and manure out of the field and into a waterway; and sheep feeding on root crops, the ground sloping, the soil exposed.
Moving to arable fields, the panel below shows (upper left c’wise) blanket killing of field margin and roadside vegetation, a poor practice that removes barriers to surface flow and exposes bare soil when the vegetation dies; very heavy traffic lines in winter over a previously harvested field, the wheel lines forming erosion channels; ground prepared for potato, the soil de-stoned, minced and packed into shape, the surface already cracking; and an area inside a field gate, soil compacted, encouraging pooling (reflecting the evening sky!) and surface runoff from the field – this one covered in fleece – directly to a road.
Remedies
Erosion is an inevitable geological process. Over millions of years, mountains are reduced to hills and the rock washed down to the lowlands or out to sea. But the soil that sustains farming here was formed only recently, just thousands of years ago, and it’s already being lost far too quickly.
And it is not only soil that is lost. Livestock droppings and their constituent bacteria go with it, as do fertilisers and various agrochemicals including sheep dip and arable pesticides. Erosion from upland pasture and lowland arable-grass has its damaging effects throughout a catchment and on to the sea.
The causes of soil degradation and pathways of erosion in agriculture are well defined. Most of them can be eliminated or greatly reduced by applying one or more conservation techniques. They begin in the field by treating soil with the equivalent of bandage and ointment. They continue with barriers at field margins and configurations in the landscape that hold water and encourage re-deposition of eroding soil. Even when natural phenomena overcome the best intentions, as the widespread flooding of early 2020 has shown, soil conservation and water management can still reduce damage then aid rehabilitation when the waters recede. Further articles will show how it can be done.
Of all the threats posed by current land use and climate, soil erosion is one that can’t be blamed on anyone else. It can only be managed here. Is farming fully aware and are government and the people doing enough in support? Let’s go back to HH Bennett’s views before the US Dust Bowl: he concluded that current attempts to check the loss of soil are ‘an infinitesimal part of what should be done’. He could be speaking in Scotland now.
Sources, links
Soil erosion is defined by a large international scientific and practical literature. A very good introduction to the situation in Scotland is the report by Lilley and others [1]. The SEPA web page on Soil [3] gives links to a wide range of soil-related issues.
[1] Soil erosion and compaction in Scottish Soils: adapting to a changing climate. 2018. Authors: Lilly A, Baggaley NJ, Loades, KW, McKenzie, BM and Troldborg, M. Published by climateXchange. https://www.climatexchange.org.uk/research/projects/soil-erosion-and-compaction-in-scottish-soils-adapting-to-a-changing-climate/
[2] Lilly, A. and Baggaley N.J. 2018. Soil erosion risk map of Scotland (partial cover). James Hutton Institute, Aberdeen. Available at the Scotland’s Soils web page https://soils.environment.gov.scot/maps/risk-maps/map-of-soil-erosion-risk-partial-cover/
[3] Scottish Environment Protection Agency (SEPA): Soil (including information on status, regulations, protection, pollution). https://www.sepa.org.uk/environment/land/soil/
[4] For a more general background on soils in Scotland: Scotland’s Soils (includes a range of information, maps and downloads) and The Scottish Soil Framework.
[5] The EU and various European agencies have produced assesments and risk maps of soil eosion. Perhaps begin at Eurostat’s Agri-environmental indicator – soil erosion.
[Later articles on this web site will cover current practices in soil conservation, including those researched and practiced at the James Hutton Institute.]