Aug 282009

Many well-intentioned composters keep adding kitchen scraps to the compost bin and then wonder why they are left with a smelly rotting mess.

The science of composting is all about getting the carbon/nitrogen ratio of the contents correct. Follow the principles and you will create superb compost suitable for enriching soil.

What on earth is the Carbon/Nitrogen Ratio?

Good compost is a balance of different ingredients, but essentially it comes down to the all important, but oft misunderstood, carbon/nitrogen ratio. So, what’s it all about? Well, let’s call carbon “Brown”, and nitrogen “Green”. We need a good balance of brown to green, generally 30 brown to 1 green. Sounds complicated I know… but it’s not.

Brown materials include things like paper (the shredded Sunday paper minus the super glossy magazine bit is perfect), sawdust (from untreated timber only), dried leaves and oaten hay. Green materials are things high in nitrogen, including lawn clippings, fresh manures (cow, sheep and chook only!), vegetable scraps and (shudder) urine.

Now, I reckon the best way to get this right is to add one bucket of “Browns” for every bucket of “Greens”. So, when I lob my bucket of silverbeet into the compost heap, I also whack in a bucket of shredded newspaper (I find the Herald Sun the best… because I quite enjoy ripping it up!). Because of the different make up of these products, it gets the C/N ratio just right!

What ingredients go into good compost?

All compost bins, or heaps need a balance of materials that:

  • are high in nitrogen, such as blood & bone, Dynamic Lifter, chook manure,
  • contain carbon, such as dried leaves or shredded newspapers, and
  • contain both carbon & nitrogen, such as kitchen scraps, pea straw and green garden prunings.

What else helps the composting process?

In addition, the compost heap/bin needs:

  • water, but only enough so that the contents are moist but not wet
  • oxygen, from air, added by regularly turning over the contents of the heap, and
  • warmth, by putting it in a sunny place. (unless the bin tells you not to… well, the bin won’t tell you, but the instructions will!)

A compost bin does not need worms, but if they are present it may function more efficiently. Generally, a well functioning comport bin with an open base will become the happy home to a whole host of delightful squirmy wormy creatures over time, a sign that your heap is a winner!

Can I add…?

  • Meat Scraps and Dairy Products – yes, but they can present issues. Meat scraps and the rest will decompose eventually, but will smell bad and attract pests. I wouldn’t, unless you are using a well functioning, closed bottom compost bin.
  • Fish Bones – yes, but mix them through the heap, rather than leaving them on top. They can smell bad and attract undesirables… use with discretion!
  • Office Paper – no, if it has been bleached or is glossy.
  • Old Tyres – No… but they do make lovely decorative swan planters!
  • Weeds – yes, but be careful. If they are without seed heads, go for it. If they are those awful bulbous things (like oxalis) or spread on runners (like couch and kikuyu) give it a miss!
  • Bird, Dog and Cat Poo – no… there is a significant risk of disease and general ickiness with pet poop, so don’t use it. Don’t even think about adding person poo!
  • Wood ashes from open fires – yes, in small amounts, but be careful if you add your compost to heavy clay soils as the ash may compound the problem. N.B. Never burn treated timbers, or add treated timber products to compost heaps
  • Tree Branches – yes, shredded before adding, unless disease is present.
  • Eucalyptus Leaves – yes, may take a while to decompose so run them over with a lawn mower first.
  • Lawn Clippings – yes, but not in large quantities unless some dry matter is added at the same time, such as dried leaves or shredded newspaper. This helps prevent the clippings becoming a putrid, slimy mess. Remember, twelve buckets of lawn clippings need one bucket of sawdust, or three buckets of shredded newspaper to get the C/N ratio correct!
  • Citrus Fruit – No, these are highly acidic and may take a while to break down, and they can really slow down the composting process.
  • Coffee grounds and tea bags – yes, yes, yes and the tiny staple on the tea-bag will eventually add a bit of iron to your soil.
  • Take-away pizza cardboard containers – tear them up first and they act as carbon matter for the heap.
  • Newspaper – yes, the better shredded the faster they will compost. Avoid the super glossy inserts!
  • Eggshells – yes, they are a wonderful addition, but decompose slowly, so should be crushed prior to popping in the bin.
  • Diseased Plants – no, this isn’t a great idea as it keeps the disease rolling along in your beautiful garden.

Solving common compost problems

Why is my compost:

Left with half decomposed big lumps?

Adding smaller pieces to the bin should ensure that it all decomposes evenly. Avoid avocado seeds, pineapple tops, twigs and other woody items unless they can be crushed or chopped before adding. Always crush eggshells.

Smelly like rotten eggs?

Well, that’s generally the smell of laziness (or anaerobic decomposition)… it means the master of the heap (you) have not turned the heap adequately, and thus there is a serious shortage of air! Turn it now, for the good of the heap! And while you’re at it, whack in some good, dry brown material, like sawdust, straw or leaves. That will soak up any excess water present in the heap.

Crawling with ants and slaters?

The heap is too dry. Add a sprinkling of water or less dry matter. Ants and slaters are not harmful at all but they do indicate that your compost will not decompose rapidly enough.

Developing into biological warfare?

If you get attacked by tiny flies (drosophila) every time you open the lid, rest assured that they are there because they enjoy the contents of your bin, especially if you have been adding fruit peelings, such as apples or kiwi or pineapples. Add a blanket cover to the contents of your bin, such as hessian sacking, carpet felt underlay or the Saturday Age.

Plagued with rats, mice, blowflies or maggots?

Meat scraps or fish bones can be added to the bin but only if it is working efficiently and quickly. They are best avoided since they do encourage vermin, especially over summer. Rats and mice enter the bin by digging underneath, so fasten a piece of chicken wire under the bin before commencing; or get a cat.

Taking so long to do anything?!!!

The carbon/nitrogen ratio needs to be altered. Remember: too wet, add dry matter, such as newspaper. Too dry, add water along with some high in nitrogen compost activator, such as blood & bone or Dynamic Lifter pellets, or chook poo. And don’t forget to regularly turn the heap over!

Uses of compost

Composting reduces the amount of waste we have to send to landfill. It’s also mighty good for the garden. Use compost as a soil conditioner. Compost may not always be beneficial as a straight plant food as the nitrogen content is easily lost, but addition of organic matter such as compost does encourage worm activity in the soil and hence provide fantastic nutrients to the garden.

  4 Responses to “The Science of Composting”

  1. What about lime? Is it good to add it to the compost?

    • Lime can be added to compost if it has very high wet, green matter, this will reduce odours and slow decomposition. Ammonia (nitrogen) will also be given off. When the composting process is complete compost should have a neutral pH. Apart from anaerobic odour control there is no need to add lime to compost

    • Lime can be added to compost if it has very high wet, green matter, this will reduce odours and slow decomposition. Ammonia (nitrogen) will also be given off. When the composting process is complete compost should have a neutral pH. Apart from anaerobic odour control there is no need to add lime to compost

  2. I love your reference to the Herald-Sun!

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