Composting garden and kitchen scraps is not only a great way to recycle but it is also a great way to put nutrients back into your soil without buying expensive fertiliser. Compost can also be used as a mulch to prevent water loss, keep out weeds and reduce erosion and adding compost to your garden beds brings lovely worms and other good soil organisms.
What to Compost
Anything organic. And I mean anything - almost. Plenty of people will tell you not to add meat, citrus, onions or a whole host of other things but the truth is, all of these things can be composted. Worms don't like really acidic soil which is why you have to be careful with citrus and onion scraps but you can combat acidity by aerating the compost heap as this helps the organic acids break down, reducing the acidity (raising the pH). Meat and fat can attract rodents and other pests but if you bury it properly or use sealable containers you needn't worry.
It was once thought that adding lime to a compost heap would speed it up but this is no longer recommended. Adding lime to your compost can convert ammonium (a liquid containing nitrogen) to ammonia (a gas containing nitrogen) causing the compost to lose valuable nitrogen. This slows the composting process and reduces the amount of nitrogen added to the soil when compost is applied. Also, a sudden rise in pH can kill other compost microorganisms.
What NOT to Add to your Compost
The only organic plant matter you shouldn't add to your compost heap is diseased plant material because you can spread the disease further. If you're gardening organically, don't add anything that has been sprayed with chemicals. All gardeners should also be careful when adding weeds to a compost heap. It must get hot enough to kill the weeds and their seeds if present otherwise you'll be spreading weeds around your garden. I have a special compost heap just for weeds. It is much larger than my normal compost bins and I cover it with black plastic to help boost the temperature.
It's also really important not to add human, cat or dog faeces. Manures such as those from chickens, cow and sheep are fine and even beneficial but the faeces from humans, cats and dogs can contain harmful pathogens that may make you sick.
To make a hot and efficient compost you need to turn it regularly until you start getting plenty of worms and you need to keep it moist but not water logged. You also need to get the carbon to nitrogen ratio (C:N) correct - that is the ratio of 'brown' matter such as dry leaves and straw, to 'green' matter such as fresh grass clippings and food scraps. A C:N ratio of about 25-30:1 is best for the organisms that break down your organic matter in the compost pile. Now this doesn't mean you need 25-30 times more brown material than green material because all organic material has a C:N of >1:1. If you have about two parts green material for every one part brown material (shredded brown material), then this will give you roughly the right ratio. You can use equal quantities of green and brown matter though. These are the ideal ratios but if you have a lot more green material than you do brown material, you can still build a compost heap, it just might decompose (compost) more slowly or start to smell a bit. Too much brown material is worse and it will take significantly longer to break down.
When is my Compost Ready?
If compost is completely finished, it should have a fine crumbly texture. It should look a lot like garden soil (but darker because it's so jam packet with organic matter). You can use compost when it hasn't completely decomposed - especially if you're digging it into garden beds before planting things - but the nutrients will be more readily available to plants if the compost is fully decomposed. Also, if it's not fully decomposed and it contains a lot of carbon, it may temporarily remove nitrogen from the soil, causing a temporary nitrogen deficiency.