Soil Erosion and Runoff

• Understand your soils and tackle the fields that pose the greatest risk of pollution and flooding
• Dig holes during wetter months to assess the state of the soil structure
• Address compaction issues; ascertaining the depth of compaction will determine which piece of machinery is best suited to tackling the problem. Then dig a hole and check that the machine is removing the compaction
• When planting maize, consider the use of early drilling and early maturing varieties of maize, leading to early harvesting. This gives time to address any potential post-harvest soil problems

Here are some ideas to help keep the soil in the field and avoid the environmental and economic impacts of erosion.

• Use crops to create ground cover– Avoid bare ground in winter
• Avoid capping and compaction – It helps reduce soil run off containing nutrients and pesticides
• Be aware of field contours – Long unbroken slopes can encourage run off. Make use of hedges, grassland, woodland and buffer strips to halt the movement of water and soil
• Cultivation techniqes – Don’t create very fine seedbeds which can cap and slake after heavy rainfall
• Focus on soil OM – Helps keep a stable soil structure which is less likely to be broken down by raindrops
• Consider bunded or tied ridges – Help prevent soil movement in potato crops on slopes but can interfere with harvesting operations
• Consider increasing or creating woodland cover – Helps reduce the loss of top soil and damage to soil fertility.

• Soil Type, structure and condition – Sandy, silty and low organic matter soils are most vulnerable to erosion caused by the movement of water. Sandy and peaty soils are prone to erosion caused by wind
• Crop cover – Bare soil in winter encourages water movement across the soil surface, so make use of cover crops
• Slope – Erosion can occur on any slope
• Compaction – Where rain can’t filtrate, erosion is more likely to occur
• Cultivations – Excessive cultivations create the greatest risk
• Seedbed – Fine tilth and level seedbeds encourage capping
• Cropping – Root crops, maize and vegetables pose the greatest risk
• Straw disposal – Surface and incorporated crop residues will help add soil stability and aid infiltration
• Field drainage – Good drainage prevents the surface accumulation of water

Avoiding bare ground in winter is a key way of mimising erosion, especially when working in challenging soil or weather conditions.  A survey looking into resource management practices undertaken on farms across the south west examined how many farmers are actually planting cover crops to reduce run off and minimise erosion on their farms.

The graph here shows the responses to the farmer survey (2012) looking at cover crop use on south west farms (click on the graph to expand it).

This shows that 60% of surveyed farmers were using cover crops either on a regular or occasional basis. For more information on the importance of keeping soil in the field, please read our section on mitigating diffuse water pollution and reducing runoff.

Cover crops, cultivations and maize

One of the big risk factors with growing maize is harvesting it in less than ideal conditions. This is a real danger period for your soil in terms of not just compaction but also causing conditions which (with the right weather conditions) will lead to high levels of run off and erosion.

If a cover crop cannot be successfully established after maize, using a heavy cultivator to roughen the surface is advised. The whole maize field does not need to be roughened; a few strips running across the field slope will break the momentum of surface runoff flowing downhill. Strips might need to be added to on the uphill side as they lose their effectiveness over winter, but they should never be reworked.

A single roughened buffer area at the bottom of a slope is unlikely to retain its effectiveness as runoff will likely have gained momentum by the time it is reached, overtopping it.

A rough, uncompacted soil surface to allow infiltration of rainwater and reduce the momentum of surface flow is the key, whether it is as a result of a rough seedbed for winter wheat or using a heavy cultivator.

This image shows work that was undertaken by Rothamsted Research North Wyke, funded by the EA that measured the amount of runoff collected from different pre- and post-harvest management treatments. As can be seen, leaving the field with bare stubble caused the greatest volume of runoff, under sowing proved to be quite effective, but using a chisel plough/heavy cultivator was by far the most effective. The cover crop treatment, rye corn, was sown late during inappropriate conditions with an inappropriate type of direct drill. This led to a poor establishment, which resulted in a destabilised soil and increased soil and phosphorus loss. This treatment was implemented in order to demonstrate the negative effect of bad management on a practice, drilling a following crop, which is considered to be good practice for reducing soil erosion

• Historically the main sources of pollution from agriculture were slurry related, either from collapsed slurry stores or slurry running off the fields. These could usually be traced back to a single source and resulted in large fish kills. These problems have diminished thanks to better management, but the Environment Agency evidence shows that there are increasing problems caused by soil washing into rivers
• Sediment runs off fields into watercourses, making it difficult for spawning fish as the gravel beds get covered in sediment
• In the south west, river pollution from sediment is one of the main reasons for failure under the Water Framework Directive

• The Environment Agency are aiming to get 43% of 1,100 watercourses into ‘good’ ecological status and will be working closely with farmers, businesses and water companies to reduce pollution and improve water quality
• More than 90% of maize and arable soils assessed in East Devon showed high or severe levels of poor soil structure causing runoff
• 40% of soils under maize had evidence of soil erosion
• The most damaged soil structure under grassland was that under short term or ley grass (established for less than 5 years) which had been used for silage. About 80% of such sites had high or severe levels of poor soil structure causing runoff
• Permanent grassland used for livestock grazing was the least damaged with typically only 5% of sites causing runoff
• Key work to date has shown that where land is loosened it has reduced runoff, particularly on maize stubble. Maize stubble with loosened soil could absorb 50mm of simulated rainfall an hour with zero runoff (The Environment Agency)

• Farming has changed considerably over the past few years, becoming more intensive
• The South West has more grazing livestock than any other region with over 37% of England’s dairy herd. This poses a significant environmental challenge both from the volumes of slurry produced and the quantity of maize grown for silage
• 44% of all maize grown in England is grown in the south west. Harvesting of maize usually happens during October, leaving a very small window to carry our soil remediation work to get a following crop in. This quite often leads to runoff problems from late drilled crops
• The use of contractors has increased, reducing the farmer’s control over the timing of field operations
• The use of heavier machinery leads to fields and soils becoming compacted
• Changing weather patterns are also making it harder to maintain a good soil condition. More heavy rainfall will increase the risk of run off and flooding and reduce the amount of time farmers can farm without damaging the soil

Source: Adapted from RBS Newsletter article by Phil Shere, Environment Agency, “Improving water quality by reducing runoff and erosion”

Soil quality is largely governed by the amount of organic matter that is in the soil. Soil organic matter content can be altered by management, managing your soil with the organic matter content in mind will be beneficial. Reduced soil organic matter affects soil structure and stability. It also makes the soil more vulnerable to erosion, as soil organic matter helps to bind nutrients and pesticides into the soil.

Residue and trash incorporation can make a big difference to organic matter content. This can be done through drilling a green cover crop post-harvest or using the trash to provide a soil cover. Better soil management will reap rewards in terms of erosion vulnerability. This can include applying nutrients only where they will be required, i.e. in the soil for the crop, rather than being washed off the field where they will pose environmental risks.

Soil that has been eroded by water can travel large distances and can be seen on roads, and in streams and reservoirs. Nutrients that wash off the fields can cause a buildup in watercourses leading to problems with water quality. The main cause of erosion is bare soil during wet seasons. Excessive tillage operations, as well as cultivating when soil moisture content is low, also helps destroy the soil structure and makes soils more susceptible to erosion.

Source: AHDB Cereals and Oilseeds, and British Potato Council, Soil 2 Crop publication: Soil and crop management issues