Compost is the closest thing gardening has to a magic ingredient. It improves soil structure, feeds soil biology, retains moisture, suppresses disease, and provides a slow release of nutrients - all at once. And the raw materials are things you're already throwing away. If there's one skill to develop as a gardener, composting is it.
The Two-Bin System
You can compost in a single pile, but a two-bin (or two-bay) system is far more practical. Bin One is your active pile where you keep adding fresh material. When it's full, you stop adding to it and let it finish decomposing - typically 3 to 6 months. Meanwhile, Bin Two becomes your new active pile. By the time Bin Two is full, Bin One is ready to use.
For most households, two bins of around 1 cubic metre (about 3×3×3 feet) each is sufficient. You can build simple wooden bays from pallets, buy plastic compost bins, or use wire mesh cylinders.
The Carbon-to-Nitrogen Ratio
Compost requires two types of organic material, loosely called "browns" (carbon-rich) and "greens" (nitrogen-rich). The microorganisms driving decomposition need both to thrive.
Browns include: dry leaves, cardboard (torn or shredded), straw, paper, wood chips, sawdust from untreated wood, dry plant stems.
Greens include: vegetable and fruit peelings, coffee grounds and filters, tea bags, fresh grass clippings, fresh plant trimmings, hair and nail clippings.
The ideal carbon:nitrogen ratio is around 25 - 30:1 by weight, which roughly translates to about 3 parts brown to 1 part green by volume. In practice, most home composters have too many greens (smelly, slimy pile) or too many browns (slow, dry pile). When in doubt, add more browns - they're usually the limiting factor.
Moisture and Aeration
Your compost pile should feel like a wrung-out sponge - moist but not dripping. Too dry and microbial activity slows; too wet and the pile goes anaerobic, producing the distinctive foul smell of rotting rather than composting.
Oxygen is equally important. A well-aerated pile decomposes much faster. Turn it with a fork every 2 - 4 weeks to introduce air throughout. You don't have to turn it - passive composting still works - but turning can cut your decomposition time from 12 months to 3 - 4 months.
In dry weather, water the pile lightly. In very wet weather, cover it with a tarp or lid to prevent waterlogging and nutrient leaching.
What Speeds Things Up
A few additions accelerate decomposition significantly:
- Chopping or shredding materials - smaller pieces have more surface area for microbes to work on
- Fresh grass clippings - high in nitrogen, they heat up the pile quickly
- Compost activator - commercial activators or a shovelful of finished compost or soil introduces microorganisms
- Urine - high in nitrogen, diluted or applied direct to the pile, this is a traditional composting accelerant that works well
What Not to Compost
Some materials can cause problems and are best kept out of the pile:
- Meat, fish, bones, dairy - attract rats and other pests
- Cooked food - same pest issue
- Diseased plant material - disease pathogens can survive and spread back to the garden
- Persistent herbicide-treated plants or grass - some herbicides (aminopyralid, clopyralid) survive composting and can damage sensitive crops
- Dog or cat faeces - may contain pathogens harmful to humans
- Very thick, woody material that hasn't been chipped - takes too long and slows the rest of the pile
Knowing When It's Ready
Finished compost is dark brown or black, crumbly, and smells earthy - not rotting. You shouldn't be able to identify the original materials. If you can still see vegetable peelings or grass, it needs more time.
Unfinished compost can harm plants by tying up nitrogen as it continues breaking down. If yours isn't quite ready, use it as a mulch on the surface rather than digging it in, and let it finish decomposing in place.
Small Space Solutions
No outdoor space? Worm bins (vermicomposting) work indoors or on a balcony, processing kitchen scraps with red wigglers into extraordinarily rich worm castings. Bokashi fermentation systems also work indoors, fermenting food waste (including meat and dairy) before it goes into soil or an outdoor pile. Both produce excellent amendments and generate far less waste.