Cover crops have little practical contribution to make to soil organic matter improvement, stresses Agrii trials manager and agronomist, Dr Syed Shah. However, his recent trials at the AgriiFocus Technology Centre and eight commercial farms show that, selected and managed correctly, they can trap considerable amounts of nitrogen and make a valuable contribution to subsequent spring cropping performance – although whether this will justify their extra cost in most cases remains uncertain.
“You need to introduce around 18 t/ha of carbon into the top 15 cm of a soil to raise its organic matter from 2% and 4%,” he explained. “Our work with a range of cover crops on different soil types shows they can accumulate up to 600-800 kg/ha of carbon in a season. So, even if this is all captured, it would take well over 20 years to achieve the sort of soil organic matter increase we’ve seen from three or four seasons of organic manuring.
“Where winter covers can be valuable, though, is in trapping nitrogen and making it available to the following spring crop. Averaged across nitrogen rates, we have recorded March-sown malting barley yielding almost a tonne/ha more after a mid-August to late January fodder radish cover than a traditional winter fallow in replicated trials on our Marlborough AgriiFocus site.
“Further work with six different species and mixtures on commercial farms across the Southern Counties has shown us they can accumulate from 20-90 kg N/ha, together with useful amounts of phosphate, potash and magnesium.
“These other nutrients are generally not available to the following spring crop, but a reasonable amount of nitrogen can be, depending on the cover,” he pointed out. “Vetches and berseem clover, for instance, will release their N noticeably fasters than crops like oil radish, black oats and phacelia.”
The Agrii work underlines the extent to which drilling date and conditions as well as species mix affect the value of cover cropping, with the best performance coming from those sown in mid-August on loamy sand and sandy clay loam soils and the worst from September sown covers on heavier land.
“To get the most from covers you have to treat them as crops,” insisted Dr Shah. “Unlike the warmer areas of France and the USA where most of the original work on covers was done, our research here and at other sites across the country underlines the fact that they need to be drilled early enough into good seedbeds with the most effective agronomy for reasonable results.
“We’ve also found it vital to remove the covers effectively with glyphosate a good 4-6 weeks ahead of spring crop drilling. Otherwise their ability for rapid nitrogen release can be seriously reduced and the following crop may struggle to take-off.
“Cover crops can act as significant reservoirs of both slugs and flea beetles too. And their ability to take-up but not release phosphate makes it particularly important to provide available P in seedbed to secure the rapid establishment so crucial to success with spring cropping.
“It’s important in our experience that cover crops deliver sufficient benefits to soil workability and infiltration rates as well as nutrient capture and supply if they are to be cost-effective,” he added. “After all, they are likely to add a good £100/ha to growing costs and that buys a lot more fertiliser value than they can ever provide.”