Composting at home: hot, cold, bins and bokashi
Turn kitchen scraps and garden waste into rich compost. Greens and browns, which system suits you, keeping it moist in a dry climate, and how to fix a smelly heap.
5 min read 路 Patchmates
Compost is the best thing you can give your soil, and you can make it for free from scraps you'd otherwise throw out. It doesn't need to be complicated. Get the mix roughly right, keep it damp, and nature does the rest.
Hot and cold compost
There are two broad approaches.
Cold compost is the relaxed way. You add materials as you have them and let them break down slowly. It takes six months to a year or more. It doesn't kill weed seeds or diseases, but it's very little work.
Hot compost is built all at once, from a big batch of mixed materials, and turned often. The heap heats up as microbes get busy, and it can be ready in a couple of months. The heat kills many weed seeds and disease organisms. It needs a decent volume, roughly a cubic metre, to hold its heat.
Most home gardeners do something in between, and that's fine.
Greens and browns
Compost needs two kinds of material.
- Greens are rich in nitrogen: fruit and veg scraps, fresh grass clippings, green garden prunings, coffee grounds, fresh manure from plant-eating animals.
- Browns are rich in carbon: dry leaves, straw, shredded paper and cardboard, sawdust from untreated timber, dry plant stems.
As a rule of thumb, aim for roughly 1 part green to 2 to 3 parts brown by volume. You don't need to measure. If it's wet and smelly, add browns. If it's dry and slow, add greens and water.
What goes in, what stays out
| Add | Leave out |
|---|---|
| Fruit and veg scraps | Meat, fish and bones |
| Coffee grounds and tea leaves | Dairy and oily food |
| Eggshells, crushed | Dog and cat poo |
| Leaves, straw, prunings (chopped) | Diseased plants |
| Shredded paper and plain cardboard | Weeds in seed, bulbs and runners |
| Grass clippings, in thin layers | Glossy paper and treated timber |
| Manure from chickens, horses, cows | Ash from treated wood or coal |
Chop things small and they'll break down much faster.
Choosing a system
- Compost bins (the classic plastic cone) suit small yards. They sit on soil so worms can move in. They're easy but hard to turn, so they tend to make cold compost.
- Open heaps or bays suit bigger gardens with lots of garden waste. Two or three bays let you have one filling, one cooking and one ready.
- Tumblers are easy to turn and keep pests out. They work best when filled in batches, and can dry out in summer.
- Bokashi is a sealed bucket where food scraps are fermented with an inoculated bran. It takes things normal compost doesn't like, including small amounts of meat and dairy, and it fits in a kitchen. The fermented result isn't finished compost. Bury it in the garden or add it to a compost heap to finish.
Worm farms are another great option for kitchen scraps, especially in units and small spaces.
Keeping it moist in a dry climate
In Adelaide the most common compost problem is that it dries out. A dry heap simply stops.
- Aim for the dampness of a wrung-out sponge.
- Water the heap when you add dry browns, and check it weekly in summer.
- Put bins in part shade if you can.
- Cover open heaps with an old carpet offcut, hessian or a tarp to hold moisture in.
- Soak dry leaves and straw before adding them.
Fixing common problems
| Problem | Likely cause | Fix |
|---|---|---|
| Smells rotten or like ammonia | Too wet, too many greens, not enough air | Add browns, turn it, loosen it up |
| Nothing is happening | Too dry, too many browns, pieces too big | Water, add greens, chop smaller |
| Flies and vinegar flies | Exposed food scraps | Bury scraps in the middle, cover with browns |
| Rats or mice | Food on top, or meat and bread added | Bury scraps, use a bin with a base of fine mesh |
| Weeds sprouting | Seeds went in and the heap didn't get hot | Keep seedy weeds out, or hot compost them |
Turning the heap every week or two adds air and speeds everything up. A garden fork or compost aerator makes it easier.
When it's ready
Finished compost is dark, crumbly and smells like a forest floor. You shouldn't be able to recognise what went in, apart from the odd eggshell or stick. If it's still warm in the middle or smells sour, give it longer.
Sieve out any big lumps and toss them back in for another round. Spread compost a few centimetres thick over beds before planting, dig a little into planting holes, or use it as a top dressing under mulch. Hungry crops like tomatoes, pumpkins and zucchini love it.
Stay safe
Compost and potting mix can carry Legionella longbeachae, a bacterium that can cause a serious lung infection. SA Health advises some simple precautions:
- Wear gloves, and a mask when handling compost, potting mix or bagged manure.
- Open bags away from your face, in a well-ventilated spot outdoors.
- Dampen the mix first to keep dust down.
- Wash your hands well afterwards, even if you wore gloves.
In Patchmates
Log compost jobs like turning and watering as tasks, so you have a reminder when the heap needs attention.