Perennial green manures – an Innovative Farmers webinar

Resource explained

Perennial green manures (PGM) are trees, shrubs and perennial plants grown in permanent areas to provide nitrogen-rich leaves to fertilise horticultural and arable land. The concept evolved to solve ‘the nitrogen problem’; the environmental impact of nitrogen fertilisers and animal manures versus taking land out of production to grow clover leys with losses of nitrogen from incorporation of the ley (tillage). Could there be an alternative approach of growing trees and shrubs in permanent ‘bioservice’ areas and harvesting the leaves to fertilise the crops? Research at Bangor University (See Sustaining our soils: Getting the balance right) has been followed up on a small scale with trials by a grower group in mid-Wales, where growers chose the PGM to suit their crops and compared results to their standard treatments. This Innovative Farmers webinar provides an update on trials in 2023 and opportunities to get involved in a field lab on the topic.

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Findings & recommendations

  • The Bangor research concluded that PGMs (alder, gunnera and gorse) provide a slow but effective supply of nitrogen, becoming more effective in the second year of applications. Use results  in lower nitrate in soil solution and lower nitrous oxide emissions than from ammonium nitrate fertiliser or clover.
  • Preliminary results of the Ecodyfi trials have been interesting – willow was surprisingly effective on beetroot, but it could be a chance result. Alder/clover additions at 180 kg/N/ha produced good yields of potatoes in a trial, but not as high as the manure treatment of 800 kg/ha!
  • PGMs provide nitrogen as and when needed by the crop. Clo Ward said: “If we can build up information on other nutrient content like phosphorus and potassium, then we could perhaps address other soil issues by carefully adding the right species.”
  • PGMs provide more carbon sequestration and biodiversity benefits than grass/clover fertility-building leys, but leys also provide a break for reducing weeds, pests and diseases.
  • Questions remain. What quantity of perennial green manures and nutrients can be produced per unit area? What are the most effective and low carbon methods to add the material to crop land? How effective can perennial green manures be at fertilising crops on a field scale?

Explore more information and resources on Perennial Green Manures here.

Summary provided by:

Phil Sumption

Edited by:

Janie Caldbeck

Associated Agricology Partner Organisation(s):

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