Written by Shaun Dowman, Agricultural Advisor at Affinity Water
Cover crops are very popular at the moment and for good reason. As readers of Direct Driller you will be more likely than most to grow cover crops on your farm and may well be convinced to invest the time and money needed to grow them successfully. Cover crops can do many good things for your soil and the range of species available is almost endless, each one doing something a little different for the biological, chemical and structural health of the soil. Coupled with other regenerative methods such as direct drilling and grazing, cover crops really can be restorative for soils and can form an essential part of a regenerative farming system.
So why are some water companies like Affinity Water jumping on the bandwagon and extolling the virtues of cover crops as well? Talk to a water company Catchment Advisor about cover crops and very soon you will be talking about nitrogen, specifically its soluble form: nitrate. Nitrate is a big issue for some water companies including Affinity Water. Nitrate occurs naturally in water but, as a result of human activity such as agriculture and wastewater effluent, concentrations of nitrate in many rivers and groundwaters are much higher than they should be. This is a problem for both the ecology of freshwater environments, causing algal blooms that can suffocate river systems, and for the supply of drinking water. There is a drinking water standard for nitrate which stands at 50 mg/l and it is the responsibility of water companies to ensure that the drinking water they supply does not contain more nitrate than the permitted standard.
But nutrient management on farms is much better these days, right? Yes, it certainly is. Greater regulation, better understanding of nutritional need, and precision farming have all contributed towards a reduction in nitrate leaching on farms when compared to the postwar period up to the 1980’s when we reached our peak for nitrogen usage in the UK. Unfortunately, as a legacy of the excessive leaching over the past half century, some of our groundwaters today, specifically those in chalk areas, store high concentrations of nitrate. This means for some of these water sources, Affinity Water have had to install nitrate treatment plants, or blend the water with a source that contains lower nitrate or, in the worst-case scenario, we lose the water source altogether. Treatment and blending cost time, money and energy which, when added to the high carbon footprint of producing and using nitrogen fertiliser, is deeply concerning from a climate change perspective.
Autumn and winter are the high-risk periods of the year when it comes to nitrate leaching; residual nitrate left in the soil after harvest is at risk of being lost to groundwater. Even with the best nutrient management planning there will always be some nitrate left in the soil after a cash crop which, if not managed correctly, can leach. One of the best ways of retaining this residual nitrate is to plant a cover crop, especially species such as oil radish or turnip rape which are particularly hungry for nitrate, although any cover crop is better than none. When I walk in a cover crop in the autumn, it always amazes me how well the Brassica cover crop species respond to areas of the field with excess nitrate perhaps from an overlap of the sprayer or on headlands.
You could argue that we should focus our resources into building treatment works for these high nitrate waters but, as I explained earlier, this is energy intensive, comes with an additional carbon footprint and furthermore we are challenged by our regulators, the Environment Agency, to think of and fund other solutions that do not require water treatment. It is much better to invest in catchment solutions that involve working with farmers to encourage practices such as cover crops, to both reduce nitrate loses at source and provide a whole host of other benefits without the concrete and carbon footprint that comes with grey infrastructure. Each cover crop plant is a natural mini water treatment works that actually captures carbon from, rather than adding carbon to, the atmosphere. The aim is to encourage more cover crops in our target catchments to complement our existing water treatment and hopefully delay, or reduce the need altogether, installation of further water treatment works.
So, with all this enthusiasm towards cover crops, we need to put our money where our mouth is, and that is exactly what we’re doing. Since 2019 we have been running a scheme, in collaboration with Cambridge Water, to encourage more cover cropping across a target area that covers several groundwater catchments. We have used the online environmental trading platform EnTrade to run a reverse auction asking farmers to bid their price for growing cover crops. The farmer chooses from a predefined list which species of cover crops to grow and when to drill them. Earlier drilling dates and certain species, such as oil radish, typically retain more nitrogen in the field and prevent it being lost to groundwater. The auction format means farmers have to consider the cost and value of a cover crop and price competitively for their farm business. At the same time they are rewarded for cover crops which optimise nutrient retention in the areas water companies need it. The scheme has worked well and last year we supported over 800 ha of cover crops and estimated that nearly 40 tonnes of nitrogen was retained in fields. We plan to repeat the scheme this year in areas of North Hertfordshire and South Cambridgeshire around Royston.
We are not the only ones at it either. Other water companies such as United Utilities, Southern Water, Yorkshire Water, South-East Water and Portsmouth Water have their own funding schemes for cover crops. Each water company area has its own specifics of geography, geology, soils and surrounding land-use so funding cover crops might not be appropriate for every company; involvement in these schemes will depend on where your farm is located. It is also worth noting that funding for cover crops is also available through Countryside Stewardship and it will be interesting to see what support ELMs will give towards cover cropping in the future.
I have focused on nitrogen, and touched on carbon, but cover crops do so much more for the environment and soil health as well. Covering the soil over winter will slow overland flow of water and reduce soil run-off, subsequently protecting our water courses. They can also help increase infiltration and also benefit overall farm biodiversity. I would argue that few water company investments deliver so many benefits for a such a modest spend.