Planting fertiliser forests for sustainable crop production
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New research is revealing the potential of perennial green manures in improving soil health without contributing to climate change. Clo Ward shares the latest findings on this greener way to grow.
In issue 125 of Clean Slate, we presented ‘The Nitrogen Problem’, explaining how various sustainable methods of supplying nitrogen to crops come with advantages and disadvantages for mitigating climate change and increasing biodiversity. At that time, we were just beginning the Perennial Green Manures project, which has been exploring a possible solution.
What are perennial green manures?
Perennial green manures (PGMs) are made from the foliage of perennial plants, which is harvested and added to the soil to fertilise crops. Many different plants can be used as PGMs, including nitrogen-fixing trees such as alders, other fast-growing trees such as willows, and ground-covering clovers, grasses and comfrey.
Unlike the production of manufactured fertiliser, making nitrogen available to crops via nitrogen-fixing PGMs does not cause carbon dioxide emissions. And unlike traditional green manures grown in rotation, PGMs can be grown on marginal land, making efficient use of farm resources. They still add organic matter to the soil and can be easily applied at any time (fresh, dried or pelleted) to match the crop’s nutrient needs. This is important for good yields and to reduce the build-up of nitrogen compounds in soil, which can be lost as nitrates into rivers or as the powerful greenhouse gas nitrous oxide.
Insights from the perennial green manures trials
Continuing CAT’s long tradition of research and innovation, Petra Weinmann and the Gardens team were one of five sets of growers that took part in PGM trials in 2023. In the CAT allotment, they experimented with fertilising crops with alder and clover leaves that had been collected and dried the previous year. This was compared with a control with no addition, CAT’s own compost, and a mix of half compost and half PGMs.
Petra found that PGMs boosted crop yields above those of the control, but yields were usually lower than when using CAT’s own wonderful compost. Across the trials, however, we found that growers were often adding large amounts of compost or manure containing more nitrogen than we thought the crops would need. Efficient use of nitrogen is important for sustainability, even with organic materials, and it’s possible that use of a wider range of additions that benefit the soil in different ways might help enable this.
Fertilising kale with PGMs
Alongside the PGM project trials, Maria Cooper, a student on CAT’s Sustainable Food and Natural Resources MSc programme, researched PGMs for her dissertation. Maria experimented with fertilising kale plants with leaves of alder, gorse and broom, and found that alder and broom successfully fertilised the kale with yields as good as those from manufactured fertiliser. The kale developed deep tap roots when fertilised with the PGMs, which didn’t appear when they were fed with manufactured fertiliser. You can read more about Maria’s project in our Perennial Green Manures report.
New learning
Key to good nitrogen-use efficiency is to add the material so it will decompose at the right speed to fertilise crops when they need it. It’s easy to add PGMs in specific quantities, but to get the timing just right we need to build up more knowledge on how the different species break down in the soil. Various crops and soils also benefit from different quantities of macronutrients, such as phosphorus and potassium. We found that through careful selection of PGMs with appropriate macronutrient content we could change the soil characteristics to suit the crop; this could enable us to produce higher yields from PGMs and use fewer soil additions overall.
At CAT, though the compost plot gave the highest yields, the plot supplied with half compost and half PGMs also yielded well and may have benefitted from the diversity in the additions. Petra is keen to carry on experimenting with PGMs and says, “If it turns out to be equally effective or better than our compost alone (as I suspect it will), I plan to build PGMs into the regular fertiliser regime in our rotation.”
At the time of writing, the final stage of the PGM project is underway, with five growers planting their own ‘bioservice’ areas designed to benefit their farm ecology as well as supplying appropriate PGMs for their crops. Perennial green manures are very much at the experimental stage, but we’d like more people to join the discussion. Could use of PGMs be scaled up to make a real contribution to sustainable agriculture?
Our report containing results from the trials and ideas for the future can be downloaded at www.dyfibiosphere.wales/perennial-green-manures.
The Perennial Green Manures project was part of the Ecodyfi development trust delivering sustainable community regeneration in the Dyfi Valley and funded by the Co-op Foundation’s Carbon Innovation Fund.
About the Author
Clo Ward has been an organic gardener for 30 years, including a stint as a CAT display gardener from 2007 to 2012. She has a long-term interest in soil health and perennial systems and is a guest lecturer on our Food and Natural Resources MSc course. Clo is one of many environmental gardeners who started out as a CAT garden volunteer learning horticulture with Roger McLennan, benefitting from his expertise, humour and energy.
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