As featured in Arable Farming Magazine
Nurturing soil with living roots
by Arable Farming
Cover crops are a fundamental component of a regenerative farming system but the practicalities of getting them to work across all arable systems can be challenging. Alice Dyer hears from three experts seasoned in roots and covers.
A regenerative farming system is ultimately fuelled by growing plants, with sugars from living roots providing a valuable food source for soil organisms and leaf biomass protecting the soil from rain, sun and frosts and overall promoting a healthier soil bustling with beneficial life forms.
But the precise mechanisms that cause roots and covers to allow certain soil biology to thrive are largely unknown, with limited studies undertaken.
However, Marco Fioratti, soil ecologist at the John Innes Centre, is exploring this further because he believes soil organisms can do everything traditional agricultural practices can do and more.
His PhD work, which is sponsored by the Biotechnology and Biological Sciences Research Council and Syngenta, is looking at is looking at the impact cover crops have on soil mesofauna, which are generally considered as invertebrates between 0.1mm and 2mm in size and include species such as soil mites and springtails.
Soil biology “There aren’t many cover crop studies concentrating on soil biology, with this topic only being researched in recent times.
There is research on earthworms but not soil bacterial communities and even less on mesofauna which form the bulk of invertebrate numbers in soil.We’re just starting to scratch the surface of what can be learned by looking at soil biology.”
Mr Fioratti says cover crops make a lot of sense theoretically because they insert more diversity into a monoculture rotation and keep the soil covered throughout the winter months, but reaping the benefits in practice can be tricky.
Early indications from his work, which has been carried out across a range of sites and soil types in East Anglia, suggest certain practices carried out between cover crops can completely offset the benefits of the cover.
“The worst driver of decline in soil biology is mechanical disturbance, much more so than commonly used pesticides or fertiliser applications.”
This means it is essential to consider the cultivation regime used to establish or destroy the cover, says Mr Fioratti.
Intensive cultivations “You can see negative effects on soil fauna populations years after intensive cultivations have occurred.
If having cover crops entails having additional mechanical disturbance to the soil, then the trade-off might actually be negative overall.”
During the study, soil biology was assessed throughout an 18-month period from cover crop establishment to cash crop harvest.
Mr Fioratti found that although biological communities became more diverse at first, cover crop termination and ploughing saw populations drop to the same level as the bare soil control.
He also found some plants can have a negative impact on soil biology.
“The depressing effects of a brassica monoculture on the soil biology are quite evident in the following season when the crop residue is integrated back into the soil.
“The mechanisms are not entirely clear, but I assume it’s to do with isothiocyanates present in brassica plant matter which are useful for [controlling] nematodes, but they have an impact on the whole trophic change [the relative populations of predator and prey through a food chain, which often results in dramatic changes in ecosystem structure and nutrient cycling].
Rooting architecture “Brassica and radish in particular are good for the rooting architecture, but they’re better employed in a mixture.
“If farmers can use cover crops in a no-till context, they will get all the benefits of this system without the drawbacks, but it’s not always easy to achieve.
In the field David Miller, Hampshire
David Miller, the new AHDB Cereals Strategic Farm South host, hopes his new role will create a roadmap for growers looking to go down the regenerative farming route.
He is well seasoned in cover crops now, having grown them for more than 10 years on the Hampshire light loam covered chalk, heavy clay cap and flint soils.
The introduction of cover crops was the start of a journey to improve soil health and produce crops in a more sustainable way and big changes in soil quality, pest pressure and fertiliser requirements have been seen as a result.
Seven years ago, Mr Miller added no-till establishment into the equation and now all land destined for spring cropping, which is around a third, is cover cropped, as well as all fields where there is at least a six-week window between harvest and autumn drilling.
Spring cropped fields are drilled with a mix of eight to 10 species, which includes clovers, lupins, vetch, sunflowers, camelina, phacelia, beans, linseed and buckwheat and costs £25-£30 per hectare, which is recouped through Countryside Stewardship.
He chooses these species based on the biological, physical or nutritional benefits they contribute and the way they incorporate different rooting depths and architecture of roots.
For fields destined for winter cropping, faster growing species such as buckwheat, camelina and phacelia are chosen.
More roots have left the topsoil more friable and better draining, with fewer issues with ponding or capping, he says.
“It is also helped by the encouragement of mycorrhizal fungi which produce a glomalin substance which sticks soil particles together but allows water to pass through without washing particles away. We’re finding soils are much more resilient to erosion.” The cover crop captures various nutrients but mainly nitrogen, phosphate and potash.
“Nutrition is also helped by having plants in the ground all year-round – we’ve got plenty of photosynthesis-producing sugars which is transferring carbon back into the soil,” Mr Miller adds.
“On the biological side, we’re providing an overwinter habitat for a lot of invertebrates. It was not until a few years ago that I started to understand how important these are. Pitfall traps show a lot more beneficials, showing we are rebalancing the ecosystem between pest and predator. “Looking at biology beneath the surface, about 85-90% of all a plant’s nutrition acquisition is microbially mediated. This means if you don’t have a living soil, your crops will be at a disadvantage when it comes to sourcing nutrition from the soil.”
Drill and destruct early to avoid pitfalls
Cover crops are a love-hate topic for some farmers who have seen yields fall or land struggle to dry out as a result of their introduction.
But work at Agrovista has shown success can be dictated by two crucial factors, according to head of soil health Chris Martin.
“Number one for success is date of establishment – the earlier the better. For cover crops, it’s all about biomass. The daylight and temperatures in August offer huge potential, but when you get to September it drops off a cliff.”
For those further north, where getting a cover crop in early might not seem viable, there are options available.
“Blowing it into the growing crop could be an option depending on the herbicides used.
You can then drill some black oats directly into that [post-harvest].
A September drilled black oat might look quite moderate above ground, but what it’s doing below the surface can still be very beneficial,” he says.
Unless a cover crop is being used to target a specific problem on-farm, such as black-grass, a diverse mix of four or five complementary species is a good place to start.
“In a perfect world, the more diversity the better, but in practice not all species will be able to grow and it can be quite expensive, in our experience.
Extensive rooting “I would always base the mix on a black oat as my grass species because it’s like an iceberg with really extensive rooting.”
Other choices should depend on cash crops in the rotation.
“I like a legume such as vetch or clover for nitrogen fixing, but one of the problems is in a dry season you may get pea and bean weevil.
“There’s evidence that some vetch and clovers are beneficial in a pea or bean rotation because they will prime the bacteria and help the beans fix nitrogen.
We’ve had lots of beans after vetch cover crops with no problems.
Phacelia “If you haven’t got brassicas in your rotation they are great, but if you grow OSR I wouldn’t include a brassica. If you want a more robust plant, phacelia is good too.”
The other key to success is to destroy the cover crop early in December or January.
This is not only to allow the soil to dry out ahead of drilling, but because the organic acids produced as part of the decomposition process as plants break down can have a negative impact on establishing the next crop.
“Whereas if you destroy the crop eight weeks before drilling, they are out of the system,” Mr Martin adds.
“Immobilisation is also key, because when something breaks down, it needs nitrogen to enable that process, so if you spray off earlier it is already broken down and the nutrients are available for the crop.
If you break the cover crop down on the day, it’s immobilising, rather than mineralising nitrogen.
“That nitrogen won’t be mineralised until six to eight weeks down the line, which is after the critical period for establishment.
This is why having the right species is key.
“A black oat has the root mass to keep the soil stabilised and help the decomposition process and soil biology when there’s nothing technically growing above ground.”
Work at Agrovista’s Lamport AgX trial site, on heavy clay soils, has demonstrated that as long as a cover crop is destroyed in December or January, it has no detrimental effect on the following crop.
For those drilling ‘on the green’, directly into a cover crop, choosing species such as vetches which have a low carbon:nitrogen (C:N) ratio means immobilisation does not occur as quickly because the plants break down much quicker.
“Something like a mustard with a high C:N can take up to a year for that nitrogen to be available, whereas vetches is just a matter of weeks.”