Going underground: How roots could hold the answers to our soil health problem
From managing residues to applying manures, for years advice around improving soil quality has focused on the surface of soil.
But for soil health expert Joel Williams, it’s what’s happening below ground that should be capturing farmers’ attention if they really want to make a difference to soil organic carbon.
Root matter — often ignored as a simple by-product of surface-protecting cover crops — is being uncovered as the unlikely hero in the battle to improve soil quality.
And by keeping soils covered year-round with living plants, encouraging root growth could be the key to replenishing soils and nurturing them back to health.
“There have been several concrete studies now which indicate that root matter is more important than shoot matter when it comes to improving soil quality,” says Mr Williams of independent consultancy Integrated Soils.
“Research shows that roots are five-times more likely to be converted into stable organic carbon than the equivalent of above-ground carbon.
“That really shifts the focus to below ground, when for so many years we’ve focused on what’s happening on top.”
In practical terms, that means farmers should be taking a fresh look at cover crops, Mr Williams says — perhaps just not for the reasons they have in the past.
“Organic matter above ground does important things like mulching the soil and protecting it from wind erosion and rainfall impact.
“It’s also very important for conserving soil moisture, which is critical when we have dry summers like we had last year. And of course it provides an important habitat for wildlife and food for the earthworm.
“There’s definitely a lot to be said for maintaining litter on the surface, it’s just emerging that it’s not a key driver for building soil organic carbon; it’s root matter that offers more.”
Going back to the roots
While the benefits of root matter may have been proven, new research looking into the impacts of living root exudates on the soil suggest that they have an even greater role in soil health than first thought.
A report published by researchers at Yale University last year compared the impact of root exudates versus raw plant materials in building soil organic carbon.
While it was the first study of its kind, it found that root exudates are between two and 13-times more likely to be stabilised into soil organic matter than plant matter itself.
“From a practical point of view, that shifts the balance from root residues and mulches to possibly suggesting that the number one principal should be maintaining continual living roots in the soil at all times,” says Mr Williams.
“It could be a critical part of the soil, not just for habitat and erosion reasons, but also from a soil organic carbon point of view.”
“Diverse cover does come at a cost, but it is possible to have living cover on a budget.”
For growers who haven’t been convinced by cover crops in the past, Mr Williams says they shouldn’t be put off by previous poor growing experiences.
“One of the criticisms of cover crops is that it’s such a small part of the growing season that not much biomass is produced,” he says.
“But shoot growth follows root growth, so even on a cover crop which is only 3 or 4 inches tall, there will be a valuable amount of carbon underneath, and that’s what’s important.”
When it comes to selecting cover crop varieties, Mr Williams insists that it doesn’t have to be a complicated or expensive decision.
“The general answer is that grasses produce more fibrous root systems than legumes or herbs,” he says.
“However some grasses don’t produce deeper tap roots, so it’s good to have diversity to bring different benefits.
“Diverse cover does come at a cost, but it is possible to have living cover on a budget,” he adds. “Even if it’s just one or two species, it’s a step forward — it doesn’t have to be a fancy cocktail but that said, producers should try and sow as diverse as they can afford.”
Linking seeds to their surroundings
Growers could even use saved seed, Mr Williams says, something which research is also showing could be beneficial to soil microbiomes, as well as farms’ bottom lines.
“There’s an interesting discussion going on about seed saving and the soil microbiome,” he says.
“Research is indicating that the more seeds interact with a microbiome, the more the genes in the variety become linked to it.
“Each year, the more seed farmers save, the more they find they perform better in sync with their specific soil microbiome.
“It goes against our input-dependent model and the teaching that we should buy new seed every year, but there are so many benefits to be had in fine-tuning varieties so they work at their best in a specific place.”
I have seen extraordinary things, particularly with diverse cover crops, where there have been signs of repair within a few weeks.
For farmers keen to embrace cover crops, Mr Williams says benefits can be seen much faster than might be expected.
“There are two schools of thought when it comes to how long it will take to see soils improve,” he says.
“The traditional one says that because it’s taken us years to degrade the soils, then it will be a long, slow process — at least three to five years — to see signs of them replenishing.
“That’s broadly true, but there are exceptions. I have seen extraordinary things, particularly with diverse cover crops, where there have been signs of repair within a few weeks.
“Particularly on land where there has been a monoculture, you can see an incredible interaction between diverse root systems and soils, and see soils restructure very quickly.
“It’s an indicator that we are moving in the right direction and that the soil will react to further treatment, and that should help steer farmers to help them make better decisions about managing their soils.”
A version of this article first appeared in Agronomist and Arable Farmer