Phacelia and oil radish uncovered as soil saviours

GROWING phacelia or oil radish cover crops in vegetable rotations helps to build and stabilise soils, with a new trial revealing it could safeguard future productivity. 
 
The trial – funded by Agri-Tech Cornwall with land provided by Duchy College - found that phacelia and oil radish outperformed 24 other species and varieties of cover crop, including legumes and oats. 

Ian Boase, vegetable specialist at Elsoms Seeds, and manager of the independently-assessed trial, said: “Soil Organic Matter (SOM) plays a crucial role in improving soil structure, health and capacity for carbon, nitrogen, and water storage. An incorporated residue from cover crop biomass is a very effective way of getting organic matter back into soils between cultivations.”

In the trial, both phacelia and oil radish produced a lot of biomass, with tangible improvements in terms of nitrogen and carbon sequestration, nutrients in green material, and feed for the following crop.

“At >61 days after March drilling, phacelia and oil radish produced 40t/ha and 35t/ha respectively of above ground biomass – each sequestering around 15t/ha of carbon and 500kg/ha of nitrogen,” explains Mr Boase.
 
He estimates that between one third and half (166kg - 250kg/ha) of that nitrogen could be available to the next crop, but further trials are required to quantify this. 
 
“The fixation of plant available nitrogen from these crops is of particular interest,” he says. “Where growers may be inclined to look at legumes like clover for nitrogen fixation, the trial has identified a critical limitation; they simply can’t establish quickly enough within a minimum 12-week rotation break.”
 
Another limitation is predation by pests. “Egyptian clover was the worst affected by predation with 80% damage,” says Mr Boase. “Phacelia and oil radish were less affected, with recorded damage statistically irrelevant.”
 
The trial also considered ground cover and crop height; the former an indicator of weed suppression efficacy. Here, oil radish triumphed, reliably covering ground across all three sowing windows. The latter, influencing means of crop destruction and residue incorporation, proved a more complicated matter for growers, as oil radish exceeds 20cm in height.
 
“Single pass systems are increasingly popular, used to reduce soil disturbance and to keep cover crop costs low,” he said.

More information and feedback from the trail can be found in the January edition of Potato Review. If you don't already receive a copy, you can subscribe here.

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