For the sake of animals, people and planet, it’s time to transition to manures we can grow.
Every so often, I discover a project with the potential to turn the way we do gardening on its head – to make it fit for our rollercoaster future and, above all else, help it raise its ecological ante. The Perennial Green Manures Project: An Innovative Approach to Fertilising Cropland is one such project. Yes, it’s a mouthful, and its focus is at a grower and farm scale, but ‘perennial green manures’ is the bit that should grab your eye.
Animal-based manures are primarily derived from cattle, horses and poultry. We acquire these either in raw form – a steaming heap of cow or horse dung – and rot them down before use, or we buy bagged products where someone else has already done the rotting. Chicken manure is widely sold in pelleted form.
These manures are rich in plant foods and help to build soil organic matter. They cost money and require energy and materials for processing, packaging and transport. The availability of raw manure is limited in larger towns and cities, where most gardening takes place, and will diminish over time as our diets transition to more ecologically sound plant-based foods.
Animal manures, unless they are from a certified organic source, could be from intensive factory farming, and are likely to contain the residues of any chemical treatments that livestock have received, which pass through into their faeces. Horse manure still carries the risk of contamination by the persistent ‘pyralid’ weedkillers, found in the hay fed to horses, which can devastate kitchen gardens.
Livestock farming is responsible for increasing emissions of methane, a powerful greenhouse gas found in cattle burps. Slurry from intensive livestock systems frequently pollutes our water courses, and is a source of ammonia and nitrous oxide, both climate-heating gases (these make your nose sting when farmers spread manure and/or slurry).
Green manures are derived from plants we intentionally grow to improve the soil and stock it with plant foods for our crops. Good examples are legumes such as winter tares, which we sow in autumn. They grow slowly over winter, fixing the plant food nitrogen in nodules in their roots (with the help of bacteria). After we’ve either incorporated them into the soil or chopped them up and allowed them to rot down on the surface, the food becomes available to crops during the following spring and summer. Their tissues also decay, adding organic matter and encouraging soil life to flourish.
When we sow or plant a green manure, there are minimal energy and material costs, and we know for certain what we will get, without the worry of receiving potentially contaminated material. The manure-making happens on the spot, out on our own plots, causing no water or air pollution, and negligible impacts on the earth’s climate. Woody perennial green manures (PGMs) also draw carbon dioxide from the air, storing it in their leaves and shoots. Using them to fertilise and build soil helps to lock some of that climate-troublesome carbon safely away.
We’ve already been making good use of a familiar PGM for decades: comfrey. The PGM Project included comfrey in its trials, which were carried out by small-scale horticultural producers in the Dyfi Valley in mid Wales. The project’s main focus was exploring whether PGMs can offer a more ecologically and economically sustainable way of fertilising crops, especially with essential nitrogen; the production of synthetic nitrogen requires an energy-intensive process releasing significant amounts of greenhouse gases, which are helping to stoke climate breakdown.
Other plants in the trial included common alder, gorse, various grasses, red clover and willow, all abundant or easily grown locally, good at scavenging and storing nutrients, and beneficial to wildlife. The PGMs were used either fresh, dried (alder and willow leaves, for example, were collected in summer, dried and separated from the branches, then used as a nitrogen-rich fertiliser the following spring), or milled and turned into pellets (comfrey is already available in pellet form).
The trial was conducted using a range of common horticultural crops, so that comparisons could be made alongside the techniques growers normally use. The results have been extremely encouraging – I urge you to check out the full report – and there is huge potential for further research into PGMs, their practical applications, and how they can help us navigate a tempestuous food-growing future.
Much of the work of the PGM Project chimes with my own, plant-based experimental gardening, and will find a receptive audience among veganic (vegan and organic) gardeners. My own adventures using sappy green wood chips (AKA chipped branch wood) have been a revelation, allowing me to establish much of my garden without the need for any animal manure inputs (North Wales sheep don’t do steaming heaps). I have trialled (albeit anecdotally rather than scientifically) home-grown coppiced willow and hazel, plus local oak and ash, incorporating their chips into my beds, with much success.
I have also used plants to produce mulch for my no-dig beds (mixing-in of the green wood chips was necessary when the beds were first established). In summer, I double the benefits by using leafy, fresh-cut foliage as greenhouse shading, later shredding the sun-crisped branches into mulch. I now turn to plants first for most of my garden’s needs.
One intriguing aspect of the PGM Project was to explore ‘bioservice areas’; these are areas set aside for the growing of PGMs for use on the most fertile and productive areas. You might already have a bioservice patch: think comfrey. Other plants being included in ongoing trials include Jerusalem artichokes, lupins, miscanthus and poplar. This list, in a garden setting, could surely be much longer, and we can probably reimagine ‘bioservice area’ as something more gardener-friendly – how does ‘garden sustainer border’ sound?
We could be adding home-grown, renewable manure, made entirely from plants using the free power of the sun, to our gardens and allotments, without the need to buy and import goodness from dubious, opaque sources. What if the PGMs most suited to garden growing could deliver the maximum benefits for insects and other wild life, while being non-polluting and climate-friendly? Some of them are bound to be beautiful, too.
To add resilience and self-reliance to our gardens, we need to move beyond the comfrey patch (although it’s fabulous to get started with). What’s needed now is for the ethos and curiosity that fuelled the PGM Project to be extended into the gardening world. We need to devise a nationwide citizen science project to identify, trial, test, chop up, shred, spread and evaluate exactly which perennial plants, including woody ones, we can plant to give us the most benefits and the maximum returns in terms of plant-based manure. Comfrey needs some manure-making allies.
We have the garden, allotment, greenhouse and polytunnel test beds – millions of them – standing ready. All we need now, as we face a topsy-turvy future, is to invest in making gardening a truly green and ecologically sustainable pursuit. Home-grown perennial green manures will help us up our ante.
Text and images © John Walker
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