What is municipal compost? How can you use it in your container growing? Is it safe? And is it the answer for the urban grower looking for a good quality, sustainable growing medium? This post will try to answer these questions. My next post will answer more of your questions about municipal compost, posted on Twitter (thanks for all these).
Why sourcing compost is a BIG issue (particularly in cities)
The practical reasons
One of the hardest choices to make when building your container garden is: which growing medium to use. There’s quite a bit to consider. Which will perform best? (And performance does vary hugely). Where do you get it from and how do you shift it? (Particularly if you don’t have a car – I broke two bike axles last year by overloading bike panniers with compost!) And of course there’s the cost.
The environmental reasons
Then there are wider issues. Many growing media sold in the UK still contain peat (unless the bag explicitly says ‘peat free’ it probably contains peat). This is a problem because peat extraction releases greenhouse gases and damages precious habitats. A separate issue is that many growing media are transported long distances, with associated emissions and fuel use.
Is municipal compost the solution?
Can municipal compost offer a more sustainable solution? Made from food and green garden waste (like grass and hedge clippings from gardens) it reduces landfill, recycles organic material, and is locally sourced and made. It’s also usually available free or at low cost. But is it any good for growing in containers? And how reliable is it and how safe is it?
I went down to the LondonWaste EcoPark in North London, which produces 12,000 tonnes of compost a year to see it being made and to look for answers.
How is municipal compost made?
Food waste and green waste from gardens – grass and hedge clippings, prunings – is roughly shredded (to a width of 400mm) and then mixed with a small quantity of compost activator (contains micro-organisms and enzymes to promote efficient composting). This mix goes into a large, enclosed tunnel. Fans under the floor, blow air through the waste. The pile heats up as it begins to decompose, and the temperature is carefully monitored with probes. It must reach 60 oC for at least 48 hours so that pathogens and most weed seeds are killed. After a minimum of one week in the first tunnel, it is turned and transferred into a second tunnel, where the process is repeated. This helps ensure that the materials are thoroughly and evenly composted. The compost is then transferred out of the tunnel, mixed with other batches (to help minimise inconsistencies between batches), and left to mature for several weeks. The final step is to sift the compost through a large drum – removing anything larger than 20mm.
The compost is made to the BSI PAS 100:2011 standard. This means the quality has to be regularly tested, including for heavy metals and other chemical residues.
How can you use it in Containers?
The compost from the EcoPark, like most municipal composts, is rich in nutrients. It is sold as a ‘soil conditioner’ rather than a potting compost.
Reusing old compost
It is probably most useful in container growing to help you re-use old compost. Rich in nutrients, it will add fertility. Also, as most municipal composts contain a high proportion of large particles, it may help improve structure. To your old compost, add between 20% and 50% municipal compost, depending on how rich a growing mix you want. As a rough rule of thumb you want a richer mix for fruiting crops like courgettes, less rich for leafy crops.
As a potting mix
To use municipal compost as a potting mix, the official advice is that one part compost should be mixed with three parts of a more inert growing medium like leaf mould or coir. The problem is that neither leaf mould or coir are very easy to get hold of in the city! The good news is that I know several experienced growers who grow in straight municipal compost – and do so successfully. My own (admittedly limited) experiments with neat municipal compost have also been successful (see image below).
So the best advice is probably to try experimenting with your own local municipal compost to see what works and what doesn’t. (NB municipal composts do vary both from region to region and also, slightly, from season to season). If you do try it neat, make sure it has been left to mature for a few weeks first – as it will be too rich when fresh.
And watch the watering carefully: it can be prone to dry out quicker than some composts.
If you’re interested, the nutrient levels in the Eco Park compost are typically (in kg per tonne): nitrogen (N) 13.86, Phosphate (P) 4.81, Potassium (K) 10.36, Magnesium (Mg) 3.48, Sulphur (S) 4.68.
How to get hold of it
Contact your local council to find out how it is distributed in your region. Large quantities (several tonnes) are often distributed free or at low cost. Smaller quantities are often available for sale in bags at local recycling centres.
But arguably the best place to find it in smaller quantities is from local growing projects. If they have received a big delivery, they will often be happy for local residents to bag up some to take home – either free or for a small donation. The challenge is to sniff out a supply local to you – good sources of information include local allotments, growing projects and websites like Transition Towns or Project Dirt in London. In Camden, London, Alara Wholefoods in Camley Street often has a supply it is happy to donate to deserving causes. This is a great service, that also engages more local people in food growing. Its an idea you could replicate if you or your community group have access to space where compost can be dumped and stored until it is collected.
Where it’s distributed to in North London
The EcoPark in North London distributes 60% of the finished compost to agriculture, 40% to community groups and allotments. The normal delivery size is a whopping 15 tonnes, the smallest is 7 tonnes. Plans are afoot to introduce 1 tonne bags this year. This willl be a boon for smaller growing projects.
The Last Word
I leave the last words to John Walker, who is a enthusiast for peat free composts. See his excellent website, www.earthfriendlygardener.net, for very helpful trials of different peat free composts.
‘My overall feeling is that if you can get municipal compost then it’s worth using it somehow. That might be as a straight soil improver, mixed (after sieving) with leafmould to make your own potting mix, or mixed with bought-in peat free to make a rich mix for containers. I wouldn’t recommend using it on its own in containers, unless you’ve done a test run and found it works (I think all gardeners should be having fun with their own trials, which would also help build a much clearer picture of how these ‘municipal’ materials are doing). Use it to complement other composts/materials because it helps reduce landfill and has a much smaller ‘footprint’ than say peat-based compost.’
He also advises against using it, ‘straight from the bag – its going to get wet and slump and disappoint’. This advice highlights how municipal composts vary from region to region: I’ve used it straight from the bag and haven’t experienced this issue.
So probably the final, final word should be: Experiment!
If you’ve tried using municipal compost in container growing – or experiment this year – it would be valuable to hear about your experiences.
Your questions answered.
Thanks for all your brilliant questions about municipal compost – and thanks to Wendy and Tom at LondonWaste’s EcoPark for answering them! For more info on municipal compost and how it can be used in containers, see my other post: Muncipal Compost: is it a good thing.
At the London Waste Eco Park, the compost is made from about 5% food waste, the rest is green waste (grass clippings, prunings etc). It varies a bit seasonally (less grass clippings in winter!) and from site to site around the country (eg some sites do not use food waste). Is it suitable for organic gardens? I spoke to the Soil Association about this and the answer is yes, probably. The official line is: ‘The Soil Association recognises that the use of green waste compost is compatible with the basic principles of sustainability and can be used as part of an on-farm fertility and soil health programme.’ However, if you want to use municipal compost to gain organic accreditation you’ll need permission from the Soil Association first. Most composts that have been tested to BI PAS100 will pass the Soil Association’s verification process – but the two tests are slightly different so its not 100% guaranteed. Some municipal composts in the UK have been verified independently by the Soil Association – check with your local producer.
I have experienced the same issue, too, Simon. Here’s what they do to minimize it at the Eco Park (it may vary in other places). The food and green waste is first inspected visually. Any loads that are badly contaminated are diverted to landfill. Then, after composting, the material is screened and anything larger than 20 mm is removed. This takes out all the larger impurities (eg plastic bags), although anything smaller that 20 mm will not be removed. I tend to sieve all my municipal compost at home to remove these smaller bits.
See above for the answer to the first part of your question, Vanessa. In answer to the second part: yes, as part of BI PAS 100 Standard (which this and many other municipal composts meet), every 5000 m3 of the compost is tested for both chemical residues and heavy metals.
Nigel, the pH does vary slightly. Usually it is 7.5, but seasonal variations in the green waste collected do affect it. For example, in January, the composting of Christmas trees can make it more acidic – perhaps as low as pH 6.
Yes, Elaine, the compost is independently tested for herbicidies – this is done by sowing tomato seeds in samples of the municipal compost and comparing to tomato seeds in a control compost. The germination rate is compared and the seedling growth is monitored. If the tomato leaves curl, this is an indicator of herbicides. You should be aware, though, that it is impossible to detect herbicides in the green wastes coming into the site. The tests are run once on every 500o m3 of compost, not every bag . So the risk of herbicides being present cannot be removed completely. But as the London Eco Park has passed every herbicide test done to date, this risk is presumably pretty small.
Julieanne, you are doing the right thing! Common weeds like couch grass and bindweed will be killed by the high temperatures in the process. The exception is the pernicious and dangerous weeds like Japanese Knotweed which should not be put out for council collection.