How cover crops help to protect water quality

Cover crops can help to reduce nitrate leaching into watercourses by up to 90%, suggests a study, with destruction by chemicals proving most effective for optimising nitrogen availability to the following crop – and for reducing weed burdens.

ADAS scientists evaluated the impact of cover crops and different destruction methods on nitrogen release on crops on two farms. The Nitrogen release from Cover Crops (NiCCs) project was funded by Affinity Water and Portsmouth Water.

Trials were undertaken over two cropping seasons on commercial farms in Hertfordshire and West Sussex with two cover mixes and a weedy stubble control. The two cover crops comprised a mix of phacelia and oil radish; and a mix of Japanese oats, buckwheat and phacelia.

After being sown and grown, the two cover crop mixes were then destroyed. A number of different destruction measures were assessed: mechanical rolling on a frost; chopping and incorporating; and destruction by chemicals.

Outcome

Results identified that cover crops reduced nitrate leaching losses by up to 90% compared to the weedy stubble control. Depending on how well the cover crops established and the species mix, soil nitrogen supply to the following spring cereal crop also increased by up to 35kgN/ha.

When it came to the subsequent spring cereal crop, yields following a cover crop were up by 0.2-1.0 t/ha when compared to yields following the weedy stubble control. On destruction, the cover crops released significantly more nitrogen than the no cover/weedy stubble.

Moving to destruction methods, chemicals were found to have more benefits than mechanical methods. Nitrogen mineral release, grain nitrogen offtake, and total crop nitrogen uptake were all higher with chemical destruction.

This led to subsequent improvements in yield – but it remains marginally more cost-efficient for farmers not to grow cover crops, although this could be overcome with financial incentives that reward better water quality, improved soil health and biodiversity.

Crop performance

“Cover crops can capture significant quantities of nitrogen over winter, thereby protecting surface and ground waters,” said ADAS principal soil scientist Anne Bhogal.

“It has been great working with Affinity and Portsmouth Water on this project which has given us new insights on how cover crop destruction methods can impact the subsequent release of this nitrogen and the following crop performance.”

Portsmouth Water, which co-funded the study, said the project showed that cover crops could help improve drinking water quality and supplies. This trial provides additional information that helps toward improving cover crop management and water quality, it added.

Affinity Water catchment manager Danny Coffey said the company was working with farmers to find sustainable solutions which worked for both crop and water production. Trial data would be used to encourage the best outcomes for crops and water quality.

“The NiCC trial has shown that cover crops are a vital measure for preventing nitrate leaching and demonstrate how farmers can potentially manage the destruction of cover crops to best utilise fixed nitrogen for subsequent crops.”

Two water companies help farmers plant cover crops

More than 1000ha of cover crops were sown last year – thanks to a joint scheme between Anglian Water and Severn Trent.

Supporting farmers to improve and care for rivers is also a central commitment of the Get River Positive programme – so collaboration with farmers is vital to Anglian Water’s work protecting and enhancing river health, says the company.

Pledges

The scheme pledges to transform river water quality across the two regions covered by the water companies. It has already supported farmers to protect water quality through its cover crop fund, which saw 1000ha of cover crops planted across eastern England last summer.

Anglian Water catchment advisor Julie Jackson, who is also a farmer, said: “Our region is particularly vulnerable to climate change, as the driest part of the country, and yet we also have a higher than average percentage of land used for agriculture.

“We know the impact of climate change – alongside challenges with costs and commodity prices coming down – is already hitting farmers, and with 75% of land in our region used for agriculture, it’s hugely important that we work together to protect England’s breadbasket.