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Making compost: getting the mix right

The balance of ‘browns’ (dry, carbon-rich material) to ‘greens’ (moist, nitrogen-rich material) is important for making good compost, no matter what system you use to make it. If you’ve ever had a heap go nasty and smelly, or remain twiggy and stubbornly uncomposted, chances are you got the balance between the two wrong.

For the best results, build your heap with around half and half of browns and greens. Make sure that your browns include some structural material that will hold some air, keep it slightly damp, and you can’t go wrong. If you want to compost a wider range of things, then ‘hot composting’ may be for you: it needs more work, but produces better quality compost in much less time.

‘Greens’ and ‘browns’ – the basis of good compost

Any organic material will rot down eventually. But if you want to make rich, crumbly compost that doesn’t smell bad then there’s a bit more to the process than piling up your rubbish in a heap and hoping for the best. If you want to make compost fairly quickly, without bad smells, and end up with a rich crumbly mixture to supercharge your squash plants, then you have to get your head around the idea of ‘greens’ and ‘browns’.

making compost in multiple chamber bin

multiple chamber bins make storing ingredients easy

Greens
Greens are moist and rich in nitrogen. Most freshly-cut plant material counts as greens;

  • Weeds (but avoid seeds unless you’re hot-composting)
  • Grass clippings
  • Raw vegetable waste from the kitchen
  • Tea bags and coffee grounds
  • Poultry manure and bedding
  • Urine (great for the heap, especially in hot weather)

Browns
Browns are dry and woody and rich in carbon. Ideally they should have some structure so that they keep air spaces in the compost.

  • Hay
  • Straw
  • Hedge clippings and woody prunings
  • Bracken
  • Untreated wood shavings, chippings or shreddings
  • Dried out plant material, such as bean halms

If you run short (and if you’re composting grass clippings you almost certainly will) you can also use these, but they’re not ideal as they compress down and don’t hold so much air;

  • Untreated sawdust
  • Crumpled or shredded paper and cardboard
Do Not Compost! 
  • Dog, cat or human poo (unless you read the Humanure Handbook first)
  • Meat, fish or cooked food of any sort (except in closed hot-composting systems)
  • Bread, rice or pulses (except in closed hot-composting systems)
  • Diseased plant material (except in closed hot-composting systems)
  • Plant material affected by club root or white rot – NEVER AT ALL

Cold composting

When you’re ready to build your heap you can of course mix the browns and greens beforehand, using a barrow or compost tumbler, but it’s hard work and there’s no real need to do it. Instead, build the heap in layers alternating browns with greens. Don’t make the layers too thick, especially for greens which tend to compress into a smelly, slimy mass. Grass clippings are especially prone to do this.

If, like most gardeners, you’re a utility compost maker (i.e. this is simply how you deal with your organic waste) then that’s pretty much it. Layer on the waste as you produce it, making sure the greens and browns balance. If the heap is looking dry sprinkle it with water, and if it’s looking soggy add some extra browns such as crumpled cardboard to soak up the moisture. Keep adding new layers as you produce more material.

As the heap composts it will collapse down on itself and the lower layers will become compost. After a year or so break it down to get the compost, which will be good enough to use on your beds. Start a new heap with the top layers. If you don’t fancy breaking the heap down like this then make sure it’s moist, cover it with a tarpaulin, and leave it for a year to compost fully. Start a new heap in the meantime.

Hot composting

Hot composting produces better quality compost in far less time, and lets you compost items that would attract vermin in a cold heap, such as cooked food waste. It does need more work from you, but if you see compost as a product rather than a way of dealing with waste, you’ll probably feel that producing your own ‘black gold’ is well worth it.

compost making can get very hotThe process is the same as cold composting (above) except that you need to buy or make a suitable bin and fill it all in one go. This means that you need to store ingredients until you’re ready to go, usually as a cold heap of greens and a covered bin of browns. The browns will also need shredding or chopping to make smaller pieces, as there won’t be time for big lumps to break down properly.

You can build the heap in layers just as described above, adding water if needed as you go, but many composters prefer to mix the heap to get it off to a flying start. Within days (sometimes hours) it gets really hot as micro-organisms get to work: perhaps as high as 70°C for a big bin, but you don’t need temperatures this high unless you’re composting with humanure.

After a week or two the heap will have cooled down quite a bit, and lost quite a bit of its volume. Composting has slowed down, but you can give it another boost by turning it and fluffing it up with a garden fork to get air back into it: adjust the moisture level if you think it needs it by adding water or urine (if too dry) or finely shredded browns (if too wet). The temperature will rocket again, but it won’t get as hot as it did the first time.

If you have the time and energy you can mix the heap a third or even fourth time, but the temperature boost is smaller each time. When you decide to stop mixing, cover the heap to keep the rain out and leave it alone to mature. (You might choose to move it at this point to leave your main bin empty for the next batch.) The maturation stage lasts a few months and involves worms, woodlice and other minibeasts, and it’s when you see their numbers start to reduce that the compost is ready to use.

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2 Responses to “Making compost: getting the mix right”

  1. Ann Jones says:

    I am new to growing and have just bought a polytunnel from 1st Tunnels and found your details in the book they sent me.

    I need to make raised beds due to my disability so plan 2 beds 9ft 6″ long one 3ft wide and the other 1ft 6″ wide.
    I plan to buy 6″ of vegetable mix from Quickcrop for the top but can I put raw grass cuttings from the last 12mths with some leaves and wood ash from our wood burner underneath?
    The ratio is probably too green so can I buy easy to add in brown to make it a more healthy mix or will it be ok under the 6″ veg. soil mix? Thank you.

    • Andy McKee says:

      Hi Ann, and welcome.

      You’re right that the mix is too ‘green’, so it depends on how much of it we’re talking about. What you don’t want is a stinky, compacted layer that worms avoid… especially starting out, as your worm count will be relatively low.

      I’d have thought that an inch would be OK, and more than that might be a problem – adding 25% fallen leaves would help a great deal. If you need to buy in browns, a bale of straw (not hay) will do the trick. Again, 25% should be enough.

      As for ash, do be careful – it tends to clump when damp which can lead to pockets of very high pH, and you don’t have heavy rainfall to even things out. Make sure it’s mixed in very well, or better yet add it to the grass when it first goes on your compost heap.

      Good luck!

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