Exploiting green covers offers organic manure solution

GREEN MANURES could offer organic arable growers a profitable alternative to increasingly rare and expensive organic farmyard manure, according to a Duchy College farm in Cornwall.

“We”re having much more difficulty sourcing FYM due to falling stock numbers,” says Paul Harris, manager of Coswinsawsin Farm, Camborne. “And when you consider the cost of putting FYM on is about 9/t, we could be spending our money more wisely.”

Instead, Mr Harris is using green crops to fix nitrogen and condition the soil. “We”ve found Old English vetch the most successful because it fixes the most nitrogen,” he says.

Vetch is normally sown at 100kg/ha after cereals and before brassicas because its rapid establishment helps to smother weeds. “It doesn”t like min-till systems or wet feet, so needs to go into a good, conventionally prepared seed-bed.”

Trefoil has not been so promising, because, despite a successful undersown crop in 2003, seed failed last spring due to low moisture. “It does provide a lot of green matter and fixes some nitrogen, but I don”t really like undersowing crops because it causes harvest difficulties,” says Mr Harris.

On tired fields he uses a two-year ryegrass and red clover ley to improve soil structure. Although competition from weeds resulted in slow establishment last spring, Mr Harris topped the field three times in eight weeks, resulting in an excellent autumn cover.

White sweet clover had similar problems in suppressing weeds last spring, and Mr Harris reckons he should have used a seed rate of 25kg/ha instead of 15kg/ha. “Despite this, it does produce a lot of green mass and is easy to incorporate. The following crop is looking quite promising,” he says.

Phacelia provides rapid ground cover and is sown at 14kg/ha after potatoes to help with weed control before wheat.

Caliente mustard, sown at 10kg/ha, also covered the ground quickly, but has not produced the green matter as expected. And despite assurances to the contrary, Mr Harris is worried about whether it could infect brassicas with club root later in the rotation.

For more information, see www. organicsoilfertility.co.uk olivia.cooper@rbi.co.uk

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