Water erosion is not only a public menace which pollutes waterways, it can wash away your profits too, writes independent soil specialist Colin Stride
What is water erosion?
Water erosion involves the movement of soil particles by the passage of water. About 2.3m tonnes of soil are estimated to erode from UK fields annually, taking with them pesticides and nutrients.
Loss of nutrients and movement and deposition of soil has a direct cost, resulting in variable crop growth and yield loss. The environment also suffers - soil is washed down slopes into roads, lakes and ditches, causing pollution which can lead to algal blooms and reduced fish stocks.
How is erosion caused?
If water cannot pass freely down through the soil, because of compaction, smearing or soil degradation, it runs off down slopes. Steeper and longer slopes mean the speed and power of the water increases, as does its ability to scour and carry huge volumes of soil. Water erosion is now more common with our increasingly intense rainfall patterns through the year.
What soil types and cropping regimes are vulnerable?
All soils and crops are vulnerable to some extent if rainfall is intense and the site is at some risk of soil erosion, due to soil damage, slope and cropping.
Sand and silty soils are most vulnerable to rills and gulleys due to their weak structures. Clay and clay loam soils are less vulnerable, but they can suffer sheet erosion, where water runs off the whole slope, taking a shallow layer of small particulates.
Later-maturing crops and those which need intense cultivation for planting and harvesting are at most risk. These include potatoes, maize, sugar beet and vegetables. Later-sown cereal crops and late grass reseeds are also at risk as insufficient rooting may not protect the soil.
What are the visible signs?
Rills and gulleys are the classic signs of erosion problems. Rills are common on lighter soil types or on steeper slopes. Narrow, shallow channels form where water concentrates as it flows. These often snake down the slope, or down wheel marks and can join or enlarge to form gulleys. Sheet erosion is best observed during rainfall as it is the hardest to detect afterwards.
What can I do to reduce erosion?
The first step is to assess the risk of soils eroding and then plan new ways of farming to protect them. The soil management plan included in the Entry Level Stewardship scheme requires soil erosion risk assessment and contributes a considerable number of points towards the farm total.
The following techniques help reduce erosion:
- Develop a good open soil structure with good organic content
- Maintain green cover to protect the soil surface
- Ensure good drainage and remove compaction
- Avoid cultivating soils when wet to limit damage
- Use well-managed minimum tillage with at least 30% straw cover
- Break up long slopes with a field boundary or wide grass margin
- Place vulnerable crops on low risk sites, and put sloping, sandy sites down to low-risk crops, such as long grass leys
- Drill earlier to ensure good crop rooting
- Avoid fine seed-beds
- Minimise channels down slopes
- The Aqueel or tied ridges are examples of mechanical solutions.
Why has Austin Knowles' system worked?
This is a farm that is typical of many mixed units throughout the UK. No single factor makes it vulnerable, but a problem with erosion can arise if left unchecked. A mixture of time-intensive enterprises, that draw the focus away from soils, further increases risks.
Austin Knowles has identified where the problems are and the underlying issues. He has adopted simple strategies, observed how they have worked and then spread them round the farm. This approach has ensured both the environment and his business have reaped the benefits.
Case study
Austin Knowles - Malvern, Worcestershire
There are those who believe that erosion and agriculture go hand-in-hand - a little bit of run-off is inevitable to get to the fat of the land. Then there are farmers like Austin Knowles, who believe that neither the business nor the environment should suffer.
"There's only one way to guarantee your soils won't end up in the river, and that's to put your whole farm down to grass. But I think you can farm a fully productive unit with the environment in mind," he says. And it is this philosophy he has adopted on the 182ha arable and dairy farm near Malvern he moved to from Somerset in 1998.
Soon after he took over the unit it became apparent erosion was an issue. "We saw a lot of run-off and gulleying - some channels were up to 1ft wide," he recalls.
But no one cause singled his farm out for particular problems - the loam soils are typical for the area mostly clay loams, with some higher risk silty clay loams and silty loams. The farm has slopes, but they are not dramatic, and cropping is a mixture of potatoes, wheat and maize with grass leys.
It was the combination of all three factors - soil type, location and cropping - that was making his soil prone to erosion.
The first step was to address cropping. With maize and potatoes harvested in autumn, this left land bare at a vulnerable time. So Mr Knowles switched to earlier maize and potato varieties, such as Spartacus, Ixxes and Estima. "If you can get ground cleared of maize earlier, you can get good cover quicker from a crop of winter wheat."
In 2003 the farm joined the Countryside Stewardship Scheme, an ideal opportunity to plant grass margins in particularly vulnerable places - on steep slopes and near streams. "This helped enormously - if your soil's going to move and you put a barrier in its way, that'll stop it."
Underlying compaction issues were also addressed through subsoiling before maize. Most recently, a seven-month ley of Westerwold Italian ryegrass has been sown as an alternative to wheat by mid October. The green cover establishes fast and one or two cuts of silage can be taken before the next crop is planted.
Gulleys are now a problem of the past. Fields affected by erosion have dropped from nearly a third of the farm to just 10%, and this year the heavy rainfall caused just small rills in the most vulnerable fields.
Productivity has been the main winner - the crops are less patchy and yields are higher and more consistent. "It does give you fields you can be proud of, and it helps the environment and the image of farming," says Mr Knowles.
Golden Rules
- Use grass margins strategically - these definitely help in high risk areas
- Watch winter cultivations - don't work seedbeds in unsuitable conditions and try to bring drilling dates earlier
- Keep it green - always try to ensure fields have an over-winter green cover