Spraying crops with compost tea could help give plants a natural defence against disease, resulting in lower costs and pushing up long-term yields by up to 27%.

Duchy Originals Future Farming Programme is trialling compost teas on two organic farms and one conventional arable unit across England.

Compost tea is made by mixing dry compost with non-chlorinated water, which is aerated in a large vat to multiply the micro-organisms.

Liz Bowles, the Soil Association’s head of farming, is managing the research project.

She tells Farmers Weekly that the aim of the trials is to determine if compost teas can increase yields and suppress disease, as well as identify the ideal application timing.

See also: Boost yields and cut nitrogen usage by 20% with sewage sludge applications

“Although there is anecdotal evidence out there to suggest that compost teas are beneficial to crops and soils, there is very little research so far into their efficacy.”

In 2013, south Lincolnshire conventional grower Justin Stafford reported yield increases of between 14% and 27%, having applied compost teas to his wheat for five consecutive years.

Popular with gardeners

Compost teas are already popular with gardeners and are also used on golf courses to keep the short grass from dying.

However these are the first large-scale arable farming trials in Britain, funded by The Prince’s Charities.

“We need to take a holistic approach to our soils,” says Ms Bowles.

“It will take a long time to increase soil organic matter so in the meantime you can feed your crops with compost teas and protect the plants with the beneficial micro-organisms.”

Wiltshire farm manager Paul Dovey is in his fourth year of using compost teas on 100ha of organic arable land.

He says that applying compost tea as a guard against disease, along with the wider issue of soil health, got him interested.

“Just seeing the difference between my crops and the neighbouring farm’s convinced me of the crucial role that nutrients play in the soil and that compost teas really help with this.

“We’ve been watching yellow rust come into some of the triticale and I think the micro-organisms in compost tea have made a difference.”

How it works

The theory is that spraying crops with a compost tea smothers the leaves in good microbes that can overpower incoming disease before it gets a grip on the plant.

“It’s a preventative measure rather than a cure,” adds Mr Dovey.

He manages a mixed farm on chalk land and some heavier soils.

Typical rotation consists of a two- or three-year grass clover ley, followed mostly by wheat, oats and spring beans.

Mr Dovey began trialling compost teas using bought-in dry compost. Now he makes his own from a combination of livestock manure, green waste and a small amount of soil.

Once the compost tea has been brewed for 24 hours, Ms Bowles says it is best to apply the liquid the following day for best effect as it contains living organisms.

“Farmers have tried compost teas in the past and said they don’t do anything, but remember you are dealing with a living thing. If you leave the tea for longer than a couple of days after brewing then the microbes will die.

“Likewise, if you spray them through too small a nozzle or at too high a pressure you’ll probably kill the micro-organisms and there won’t much of a benefit left.”

As well as potentially fighting disease, the microbes in compost tea can help feed crops to bolster yield prospects.

Ms Bowles explains that while compost tea itself does not have particularly high nutrient values, applying it at drilling puts microbes in the soil, which digest carbon and release nitrates that are taken up by plant roots.

Over his three-year trial, Mr Dovey says he has seen yields rise by about 10%.

“We are averaging about 4.9t/ha with our winter wheat, which for organic arable is actually really good,” he says.

According to Ms Bowles, there are a growing number of conventional and organic farmers showing interest in compost teas, highlighting the need for proper research.

When it comes to the types of compost growers would benefit most from using, she explains how this is one of the key aims of the research, but it is still too early to tell.

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Case study: Sophie Alexander, Dorset

Dorset grower Sophie Alexander (right) is in her first year of trialling compost teas on 40ha of spring malting barley variety Westminster

She farms 405ha of mostly chalk land 15 miles north of the Jurassic Coast, which was intensively farmed before she took it on in 2011 and moved to an organic model.

“I’m not a soil biologist, but to me it seems there’s logic in the idea that rather than applying fabricated chemicals you can apply microbes, which will assist the soil in becoming more productive,” says Ms Alexander.

“My hope is that it might also accelerate the speed with which that’s done because we’ve not been organic for that long. I see it as assisting the natural immunity of the soil.”

Total set-up cost for Ms Alexander’s compost tea operation at Hemsworth Farm was about £10,000 for one brewer and two large bowsers to store water before it’s used for brewing. She estimates that the equipment should last for about 15 years before needing a replacement.

Although only in her first trial year, she’s certain of a clear relationship between healthy soils and better yielding crops.

“I think that if the soil is helped to reach its optimum health then our yields should reflect that. There’s certainly scope for this farm to achieve higher yields.”