Rocketing fertiliser prices are making composts increasingly attractive to farmers both as slow-release nutrient sources and soil conditioners.

More composts are becoming available and recent ADAS trials are confirming their value and highlighting their benefits.

Soil scientist Susie Holmes says more than 4m tonnes of segregated wastes are composted each year. “This represents a fourfold increase in the amount of compost produced in the UK since 2000.”

“The benefits of compost in terms of savings on fertilisers are now significant. Based on nitrogen costing about 80p/kg, phosphate 110p/kg, and potassium 60p/kg, a tonne of typical green compost is now worth £7-9/t in terms of its major nutrient content alone.”

Trace elements

The magnesium, sulphur and trace elements it contains may be particularly beneficial on sandy and shallow soils, adds Mrs Holmes.

“Composts containing green plus food wastes tend to have a slightly higher nitrogen content and so may be worth about £10/t. This assumes that nearly all the nutrients will be available to crops over the rotation except for nitrogen.

“Studies so far suggest very little N is available in the first year from green composts.” The figure proposed for DEFRA’s new version of Fertiliser Recommendations book RB209 is 5%, she notes.

“But continued applications of compost to land will raise organic matter levels and the long-term soil nitrogen supply will be increased.”

Farmers cannot afford to ignore the other longer-term benefits of applying compost, she urges.

“Not only does it add to the nutritional status of the soil, especially potassium levels, but its high organic matter content, in particular the high lignin level, helps increase the soil organic matter status.

“This improves the soil’s water holding capacity, nutrient holding characteristics and physical structure, which makes it easier to cultivate and less susceptible to soil wash and erosion.

“Realistically we are talking about five to 10 years before seeing a real benefit in soil structure and quality.”

Research findings

Initial findings from a £3.4m research project commissioned by Composting Research and funded under the Landfill Tax Credit Scheme through WREN Environmental Body are already showing good long-term benefits for farmers who apply compost to their land, notes Mrs Holmes.

The work involves studies at three sites with distinctly different soil types:

  • Gleadthorpe – light sandy soil.
  • Rosemaund – deep silty/medium soil.
  • Boxworth – heavy boulder clay soil.

The experiments started in autumn 2005, on winter wheat, winter oilseed rape and potatoes, all following cereals. They are comparing plots dressed with green waste/food waste compost in their first year only with those receiving compost every year, with and without the recommended rate of N for the site, and others getting bag N only at a range of rates.

The compost is applied at about 30t/ha, providing 250kg/ha of total nitrogen, the typical maximum application rate for land in a nitrate vulnerable zone.

“By adding compost, not only will water holding in lighter soils improve, but the natural structure of the soil will become more stable and heavy land becomes more workable. This helps cut carbon emissions because tillage requirements are reduced.”

Cross-compliance

As part of cross-compliance, and under the Single Payment Scheme, farmers are required to keep their soils in Good Agricultural and Environmental Condition (GAEC).

Adding compost and increasing the level of soil organic matter will help farmers comply,” says Mrs Holmes.

In dry autumns and in years when over-winter soil conditions are unfavourable, compost use has been shown to improve plant establishment.

Compost is of particular benefit for rotations including potatoes or other root vegetables because of its high potassium content and its potential to improve the water-holding capacity of lighter soils, adds Mrs Holmes.

“Signs from our trials also suggest that adding compost makes soil more biologically active and increases earthworm populations. Some trials have indicated that where compost is used wheat is less susceptible to take-all. This may be due to the improved soil conditions helping to produce a better, healthier root system.”

The economics of using compost clearly depend on its source and farm.

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“The cost of transporting compost is significant and must be added to spreading costs, which can be £3-5/t.

“Unless compost complies with the new Quality Protocol PAS* it’s necessary to apply to the Environment Agency for exemption from the Waste Management Regulations (paragraph 7) before using it on agricultural land.

“There’s an annual charge of £546, which covers application to 50ha, plus consultancy fees associated with this. But often the compost producer will apply on behalf of the farmer and bear this cost.

“Under NVZ rules, you can store compost for up to 12 months in a temporary field heap before spreading as long as it’s not within 50m of a borehole or 10m of a watercourse.

“It’s also worth remembering that limitations on access during winter months may have to be considered.”