Managing Land Sustainably: Growing high risk crops on vulnerable soils

Overview

Preventing soil erosion and runoff from maize and other high-risk crops that can cause pollution and flooding.

Growing high-risk crops, including maize, can cause soil erosion and runoff on high-risk soils.  This can lead to flooding and pollution.  We strongly recommend that these crops are not grown in high-risk locations.

If you still wish to grow maize in high-risk locations, you will need effective measures that can cope with storm events and prolonged wet weather to reduce the risk of flooding and pollution.  If you cannot achieve this then you should not grow high-risk crops such as maize, fodder beet and potatoes.

Introduction

There have been several serious soil erosion incidents following heavy summer thunderstorms and prolonged wet weather, caused by growing maize and other high-risk crops.  This led to increased flooding and pollution in Devon and Cornwall.  Climate change means we are likely to get more thunderstorms and long periods of wet weather.  Soil erosion can create muddy floodwater, which ends up in homes, on roads, drains and in rivers.  It can cost a lot for homeowners to clean this up.

The farming industry has made good progress towards reducing runoff problems from fields left bare, following the maize harvest.  However, climate change means we are now seeing problems both with intense rainfall in the spring and summer months, and with wetter autumn months.  This can make agricultural land on certain soil types difficult to manage.

This information page explains our latest guidance to address this issue.

High-Risk Sites

The high-risk soils of concern include:

• Light sandy soils that are vulnerable to erosion
• Slowly draining soils that lie wet for prolonged periods, e.g. heavy clay soils.

These are defined by using criteria set out by Defra, considering soil type, slope and proximity to watercourses. 

Light soils

High risk light soils contain 18% clay or less, as shown in this soil texture triangle.

When large areas of these soils are bare for a long time, particularly in the spring and early summer, they are vulnerable during thunderstorms and heavy rainfall.  The battering action of rain and hail causes splash erosion.  Some light soils are prone to capping and sealing at the surface with fine soil particles.  This reduces infiltration and increases soil runoff and erosion.  Large, connected blocks of land can generate large volumes of muddy water with serious consequences.

The light soils of the sandstone hills around Exeter, South Hams and Crediton are most prone to capping.  The light soils associated with granite in Cornwall and in the Blackdown Hills are also areas of concern as they are at risk of erosion.

Light Soils Map from Broad Soil Groups by Defra from the National Soil Map © Cranfield University and the Controller of HMSO (2024)

The risk increases further when bare fields are close to or connected to watercourses and nearby roads and property.

Slowly draining soils

These soils include heavy clay soils and slowly draining loamy soils that are either over deep clay or are influenced by groundwater.  These soils are naturally wet near the surface and are at high-risk of compaction and runoff. 

Slowly Draining Soils Map from Broad Soil Groups by Defra from the National Soil Map © Cranfield University and the Controller of HMSO (2024)

These areas include the clay soils in North Devon and Cornwall (purple), and very wet loamy soils over red clay in East Devon and smaller areas on the slates (brown).

High-Risk Crops and Land Use

We are particularly concerned about land where maize is grown.  Maize farming is common across the southwest.  High-risk periods include:

• Heavy summer rain that may fall on bare seedbeds
• Wet autumns that saturate the soil and make harvesting difficult

Other crops and land use include:

• Potatoes, and field vegetables such as swedes and parsnips
• Fodder beet and Energy (sugar) beet
• Other land uses that leave the soil bare

Support

We have consulted the National Farmers Union (NFU) and the Maize Growers Association, and they have given their support for this message. 

An NFU spokesperson said:

“Investing in and protecting soil health is crucial to the nation’s farming systems and is essential to British food production.  Healthy soil delivers in reducing flood risk, supporting wildlife habitats and biodiversity, and the sequestration and storage of carbon.  Any proactive advice that can support farmers and land managers in delivering these objectives is welcomed while also enabling farmers to produce the crops the market requires.  This should be about an enabling, supportive approach recognising the challenges of producing food alongside farmers protecting their crucial asset, soil.”

The Regulations

The Farming Rules for Water state that ‘the land manager must ensure that reasonable precautions are taken to prevent pollution resulting from soil loss caused by land management and cultivation practices.’

We consider that choosing not to grow high-risk crops is a reasonable precaution.

If you wish to continue to grow high-risk crops, you will need to develop site specific measures to prevent soil erosion.  The measures will need to be able to deal with heavy rainfall and periods of prolonged wet weather.

We know that finding agronomic solutions and failsafe measures to prevent soil erosion with high-risk crops is difficult.  Therefore, we recommend that you should avoid growing these in high-risk areas completely unless you can substantially reduce the risk of muddy run off. 

If you continue to grow maize and other high-risk crops in these high-risk locations and a soil erosion and runoff incident occurs, we can take enforcement action against you.

The Devon and Cornwall Land Management Team