The Prolegso project has shed new light on an age-old process: the planting of legume crops such as peas or beans to enrich the ground with nitrogen. Generations of farmers have used this ‘green manure’ to revitalise depleted soils. The benefit for non-leguminous crops is widely recognised, but the implications for the legumes weren’t necessarily obvious.

Gerlinde De Deyn, of Wageningen University in the Netherlands, explored the phenomenon. The outcomes of her experiments indicate that there can be considerable benefits to the legumes as well — if they are matched up with compatible partners. De Deyn is now building on the results of Prolegso to find optimal combinations.

Unearthing the secrets of the soil

De Deyn’s work in Prolegso focused on the obscure menagerie that inhabits the living soil: a bewildering variety of fungi, bacteria and other microorganisms, along with any number of insects and worms. Different plant species affect the microbial communities near their roots in different ways and are affected by them in return.

These interactions create the potential benefits or drawbacks to other plants growing nearby, or to subsequent crops in a rotation scheme. “People have been doing crop rotations for centuries,” De Deyn explains. “The advice has always been to grow very different crops that aren’t affected by each other’s pathogens. However, by doing so, you lose some of the beneficial effects as well.”

There is a concern, she notes: extra nitrogen could give the non-leguminous plants such a boost that they crowd out the legumes. However, observations in the field indicated that this was not necessarily the case.

De Deyn decided to dig deeper. She wanted to know how legumes and other plants could be combined in ways that allow both to thrive.

She was also keen to see if non-leguminous plants could help to protect their legume neighbours, by limiting the number of tiny beasts that like to chew on their roots. In addition, she analysed how the presence or legacy of non-leguminous plants affects the beneficial bacteria and fungi on which various species rely.

It depends

To explore these aspects, De Deyn tested how clover, a leguminous plant, fared in the presence or in the wake of various non-leguminous grassland species, and how this proximity shaped the microbial communities in the clover roots.

Her findings indicate that there can be substantial benefits to the legumes — if the accompanying species are chosen with care. Planting density has emerged as another key consideration.

Putting down roots

De Deyn’s research was backed by a Marie Curie grant. This support was awarded to help her take a promising career to the next level, and it had the desired effect. Following a string of post-doc assignments that had taken her half way around the world and back, it enabled her to pursue longer-term opportunities closer to home, where she is now an associate professor.

De Deyn is currently working out optimal combinations of leguminous and non-leguminous plants to generate maximum benefits for both. It’s a natural way of managing the soil, reducing the need for mineral fertiliser and pesticides.

Whatever the combination, her research indicates that legume plants always do better in living soil than in soil that has been sterilised. This, she says, means that the beneficial organisms are more significant than the pathogenic ones.

“Don’t be afraid of your soil,” De Deyn concludes. “Try to understand it and manage it well, because you can get more out of it than you think.”