Tozer Seeds leafy salads account manager Alec Roberts explained how biofumigation "is an ancient form of biocontrol" now being boosted by modern seed breeding.
The Surrey seed supplier's Caliente 199 variety is "hottest" due to its high levels of glucosinolates - a class of compounds that brassicas use as a form of self-defence but that have largely been bred out of edible varieties. When acted on by the enzyme myrosinase it gives rise to isothiocyanate, which is believed to control a wide range of soil-borne pests and pathogens, and even to suppress weeds.
"You have to treat it as a crop, fertilising and irrigating it," said Roberts. "At 5-6ft (1.5-2m) high you get 60-80 tonnes per hectare of biomass, which you reduce to a soup and incorporate. You want maximum leaf volume. That's where the good stuff is. You don't want all flowers and pods."
He explained: "It spring-cleans the soil rather than sterilising it as with a conventional soil fumigant. By making it warmer and better-drained, you have also changed the environment away from what the pathogen likes. It will benefit most crops. Spinach, coriander and chard all like going in after this."
But he admitted: "A lot of growers struggle to find a gap in the season for this - and you need the right crop, grown and incorporated correctly, to get the biofumigation effect. Just chucking green manure into the soil won't give you the soil biota. It's not a quick process. You need the ethos, the time and the money. You will see a better effect if you fit it into a 10-year plan."
Tozer customer services manager Charlotte Wheeler added: "Our flower sprouts are still on an upward trend and are doing well even in Russia. Anything to do with kale is doing well. A new variety takes so long to breed, you hope to get 10 years out of it. We will be showing some new trial varieties later this year."
During the field trip, Dr Rob Simmons and Dr Lynda Deeks of the Cranfield Soil & Agrifood Institute showed how soil compaction can be investigated and tackled. "You'll always get compaction in the wheelings, which low-pressure tyres would alleviate to some extent," said Simmons.
Earthcare Technical soil and water consultant Martin Wood added: "The soil here has high P and K as legacy from wheat growing, but the straw was baled so very little organic material has gone back in. Addressing that would improve its water-holding capacity and the soil biology."
Farm manager Les Britten said the stone content of the soil actually helps with moisture retention. The 20ha open herb field supplies R&G Herbs with a range of cut herbs, bound for Waitrose, Ocado, food service and wholesale.
Britten explained that eight tonnes of herbs a week are hand cut from the farm by 18 workers. Switching from imported mint plants to propagation from cuttings has cut costs by two-thirds, he added. "We take two coriander crops a year. More than that and you are asking for trouble."
R&G Herbs general manager Dean Fowler said: "Millets is a fantastic setup we've been in partnership with for eight years. A close grower relationship is paramount to us and we also work with other UK growers to spread the risk."
Describing itself as the oldest and most established commercial grower and supplier of fresh cut culinary herbs in the UK, the firm is this month moving from its West End premises in Surrey to a new facility at Farnborough, 12km away. Company owner Mathew Prestwich said: "This facility will house new state-of-the-art packaging machinery and the building itself has been constructed to the highest possible standards."
Fowler added: "We are seeing double-digit growth with all our customers. Herbs are in a great place at the moment. People are using more and there is more headroom to go."
CN Seeds business development manager and BHTA chairman Charles Seddon said: "The BHTA is not big but it's very diverse because it incorporates industrial producers, fresh producers, importers, packers - so many different facets and species. To have a day that satisfies everyone is never really going to be the case. What we have regularly achieved is having a forum where we can all talk."
He told Horticulture Week: "Labour and crop protection are our main concerns. I have seen automated coriander bunching but it creates as many problems as it solves. But the demand for our product is there."