Composting is Mother Nature’s ultimate recycling process which converts everything that was once living back into soil. We can compost a lot of organic matter in our homes and gardens, and use it to improve the quality of our soil, rather than toss it out into landfill.
For successful composting, it’s important that we use the right mix of materials. The composting process requires materials which contain both carbon (C) and nitrogen (N). The optimum ratio of carbon to nitrogen (C:N) in the final mix of compost materials needs to be between 25:1 – 30:1 parts carbon to nitrogen by weight.
The reason for this is because the composting bacteria require carbon and nitrogen in these proportions as nutrients to construct their bodies as they reproduce and multiply.
The Two Groups of Composting Materials, Greens and Browns
- Materials high in carbon are typically dry, ‘brown’ materials that rot down very slowly, such as sawdust, cardboard, dried leaves, straw, branches and other woody or fibrous materials
- Materials high in nitrogen are typically moist, ‘green’ materials that rot down very quickly, such as lawn/grass clippings, fruit and vegetable scraps, animal manures and green leafy materials.
When composting, there’s no need to worry about carbon-nitrogen ratios, just work by volume, it’s much easier.
When composting, use 1/3 ‘greens’ (nitrogen containing) materials with 2/3 ‘browns’ (dry carbon materials). In other words, add one bucket of nitrogen-rich material to every two buckets of dry carbon-containing material.
For example, 1/3 manure mixed with of any 2/3 dry carbon materials will work well to produce compost.
When composting, lay down alternating thin layers of greens and browns, and the bacteria will do the rest. The materials will break down the fastest at the bottom of the heap where the materials were placed first.
Try to use a wide variety of ingredients in your compost, the more the better, as it creates a richer compost in the end. There are many ingredients that can be composted, that gardeners aren’t aware of, and there are certain ingredients that should never be put into a compost bin.
Here’s a list of what materials should and shouldn’t place in your your compost bin:
What You Can Put Into Your Compost
- Garden prunings (break down faster if mulched or mowed over with lawnmower first)
- Lawn clippings (use thin layers)
- Garden waste
- Mulched woody branches
- Straw mulch materials such as hay, lucerne, pea straw, sugarcane mulch, etc.
- Leaves (break down faster if mulched or mowed over with lawnmower first)
- Pine needles (these break down very slowly and make the compost more acidic, great for acid-loving plants such as blueberries)
- Cut flowers
- Kitchen scraps (fruits and vegetables)
- Breads, cereals, rice, flour
- Coffee grounds and tea bags (not the plastic tea bags)
- Egg shells (crushed)
- Bamboo skewers, wooden chopsticks, toothpicks (chop into smaller pieces with pruning secateurs)
- Vegetarian animal manures (chicken, sheep, cow, rabbit manure, etc)
- Pet hair and feathers
- Cardboard and egg cartons
- Brown paper bags
- Unbleached non-glossy paper
- Saw dust and wood shavings (small amounts, best to leave them to weather outdoors first outside to leach away the oils and darken)
- Wood ashes (small amounts only as a source of potassium, otherwise they make the compost too alkaline)
- Natural fibres such as cotton, wool, leather, etc (only break down in a fast 18-day hot compost system, don’t break down easily in regular compost)
Hot Composting vs Cold Composting
While it is possible to compost things made of natural fabrics, even leather, this should only be attempted when using a fast and hot composting process such as the Berkeley Hot Composting Method.
Hot composting done properly (not being allowed to cool down until it’s finished!) will get very hot, around 55-65 degrees Celsius (131-149 degree Fahrenheit), and stay that hot continuously for over two weeks, which literally cooks everything, and produces very fine compost in 18 days.
Hot composting will effectively destroy disease pathogens (such as powdery mildew on pumpkin leaves), weed seeds, weed roots (such as couch and kikuyu) and weeds which reproduce through root bulbs (such as oxalis). Some farmers who use the hot compost method even put fresh animal roadkill into the middle of their hot compost heaps because they are rich in nitrogen, and they find nothing but clean bones when the compost is ready. Not a good idea for urban areas though!
The slower, cold composting methods take anywhere from three months to a year, will NOT kill disease pathogens or weed seeds and roots, and produce a coarser compost. Being a slower and less efficient process, cold composting is unsuitable for composting natural fibre fabrics.
What You Should Not Put Into Your Compost
- Diseased plants (can cause diseases to spread in the garden)
- Noxious weeds which regrow from cuttings, seeds or roots
- Pet poo from non-vegetarian animals (contain many pathogens which are a health hazard and can cause diseases)
- Fish and meat scraps, fats, or foods containing these (will rot, smell unpleasant and attract pests such as rodents)
- Dairy products (will rot, smell unpleasant and attract pests such as rodents)
- Cooking oil
- Glossy or coated paper, eg. magazines (contain toxic chemicals)
- Bleached paper, eg. office paper (contain toxic chlorine-based chemicals which shouldn’t be composted)
- Sales receipts and thermal paper (contain a mix of toxic chemicals which shouldn’t be composted)
- Sticky labels, such as the ones on produce and packaging (are made of plastic and/or contain toxic chemicals in the glue)
- Vacuum cleaner dust and dryer lint (once recommended, but now contain too many synthetic fibres which don’t break down)
- Ashes from coal/ barbecue briquettes (contain toxic chemicals)
- Treated wood pieces or sawdust (contain highly toxic copper, chromium and arsenic which will contaminate soil)
- Personal hygiene products, such as tissues, tampons (unhygienic, health hazard)
- Plants treated with toxic pesticides and herbicides (will contaminate compost, soil and food with toxic chemicals)
- Large branches (unless they are mulched first)
- Citrus peels and onions (can use in very small amounts, too much and they can kill the compost bacteria)
Should You Add Lime to Your Compost?
Many composting instructions recommend adding lime (gardening lime, limestone, calcium carbonate) to the compost bin, especially if it becomes a bit too acidic and ‘sours’, as the lime ‘sweetens’ it by reducing acidity.
Don’t add lime, as it will make the compost more alkaline, this will slow down the composting process, as compost microorganisms operate best under a pH range of 5.5 to 8, and ideally prefer a neutral to acidic environment, as acidic conditions are favourable for the growth of fungi and the breakdown of lignin in woody materials and cellulose which is the main compound which makes up plant cell walls.
The other problem with making compost more alkaline is that it causes ammonium nitrogen (NH4+) to be lost to the atmosphere as ammonia gas (NH3). This depletes the compost of its nitrogen, a valuable nutrient for plant growth.
Rather than add lime, it’s easier to simply aerate and mix a compost pile that has become anaerobic and too acidic, this will reduce the acidity.
During the composting process, organic acids are produced at the beginning of the composting process, and later in the process become neutralised. Depending on the ingredients used and the composting process used (slow or fast), mature compost will generally have compost a pH between 6 and 8.
By using the right ingredients in a compost pile, in the correct proportions, it’s easy to create an ongoing supply of nutrient rich compost to improve soil structure and fertility. There’s a little bit of an art to composting, so it pays to experiment a little and find what works best!