Aug 282017

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 correct principles and you will create superb compost suitable for enriching soil.

What is the Carbon/Nitrogen Ratio?

Good compost is a balance of different ingredients, but essentially it comes down to the all important, but often 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.

The best way to get this right is to add one bucket of “Browns” for every bucket of “Greens”. So, when adding a bucket of silverbeet add in a bucket of shredded newspaper.  Because of the different make up of these products, it gets the C/N ratio just right!

If there is too much “Brown” the nitrogen in “Greens” will be locked up by microorganisms and be unavailable for subsequent plant growth.

Potassium is also an important constituent of compost.  Although it is much more available to plants than either the carbon or nitrogen, its content can be reduced if the compost heap is open and subject to a lot of rain.  This problem can be overcome by covering the heap as it approaches maturity.

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 or 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 you are using a purchased bin of some type and the instruction warn against it because of the risk of overheating.

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 home to a host of delightful wormy creatures over time, a sign that your heap is doing well.

Can I add…?

  • Meat scraps and dairy products – yes, but they can present issues. They will decompose eventually, but will smell bad and attract pests.  So avoid them 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 have bulbs (such as oxalis) or spread on runners (like couch and kikuyu), they are to avoided.
  • Bird, dog and cat poo – no… there is a significant risk of disease 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, but they may take a while to decompose so run over them 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 as long as the material making the bag is made from all natural materials free of plastic –  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 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 will perpetuate disease in your 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 that the heap has not been turned 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, put 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 put in 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 thick newspaper.

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.

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, pelleted organic fertiliser or chook poo. And don’t forget to regularly turn the heap over!

Uses of compost

Composting reduces the amount of waste we send to landfill. It’s also very 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 encourages worm activity in the soil and provides an excellent environment for soil microorganisms to do their work.

  23 Responses to “The Science of Composting”

  1. is it ok to put scale & psyllid infected leaves into my compost? Note my compost does not get really hot, so I fear I may be spreading scale & psyllids in the compost

    • It is never a good idea to put any diseased material into compost – probably not even if the pile gets moderately warm. It is better to avoid risking multiplication of disease-causing organisms.

      • Thank you for your reply SGA, I will transfer the leaves from the bucket into the wheelie bin
        Regards Jo

  2. Thanks, great article! Would love some more suggestions of good dry, brown matter to balance out the kitchen scraps. We don’t buy newspapers anymore. Living in Canberra, autumn & winter are great for dry leaves but at this time of year that’s a long time away. We’ve just bought a house & starting a new compost bin from scratch (tumbler type). Just at a bit of a loss as to what I can add as brown matter.
    Do grass clippings that have been left in a pile to dry out become brown matter, or are they always green?

    • The following list of items is ‘brown’ matter and will, no doubt trigger more things you can use. Torn up egg cartons and fast food coffee cup holders, shredded non glossy paper, old coir door mats, dried bunches of flowers,torn up cardboard boxes, dry grass as the nitrogen will have mostly exited the grass, non-treated wood shavings from a joinery and torn up old phone books. Tearing up phone books and cardboard boxes is laborious but you could do a little at a time.

  3. Love composting… great article.
    I was wondering why you suggest fresh manures (cow, sheep and chook only!)?

    I add horse manure which Get for free and seems to really heat up the compost quickly.

    Anything wrong with horse manure?

    I also have concerns about newspaper inks – and toxins within.

    • The reason horse manure is not suggested is because horses are frequently wormed and using fresh horse manure containing medication will also kill earthworms. Use horse manure by all means in your compost but let it sit for about three months first.

      These days, inks used in newspapers are soy-based – as well as being environmentally friendly they render colours more acurately. the bleach used in usually hydrogen peroxide which breaks down readily. If you are still concerned, contact your local newspaper and ask what they use.

  4. Are there optimum places for where to place the compost bin in my garden?

    • A few considerations for compost bin siting. It needs to be at a convenient distance from the house to allow addition of compostable household waste and near enough to your vegetable garden for addition of waste and access to the compost. It will need adequate air circulation and room to work around it. A hot spot behind a shed or in full sun is not suitable as the contents will literally ‘cook’ on a hot day. A site with reasonable drainage and partial shade would be ideal. Sounds complicated but it isn’t really

  5. I am soaking gum leaves in black old fashioned style plastic dustbin, & have added enough (a lot) human urine to make the dry gum leaves swim in it. How long should I leave them in the urine before I bag them up to make leaf mould??? & how long will this method take to break down the leaves??? I give it a stir once a week. Would like to see more information on using human urine in the garden regularly, as its high in nitrogen and easily & freely available

    • I have heard of other who tried this and were unsuccessful also, ending up with a slimy, putrescent smelling product. I know of some who use sawdust in a barrel or bucket and add sawdust as they go. When the bucket is full they empty it into the compost. The extra nitrogen in the urine would help balance the nitrogen ‘lock up’ in the sawdust as it is decomposing. Trust this helps. Urine is also good for rhubarb and citrus trees and is listed as a control for ‘scab’ on apples.

      • Many THANKS for your reply John, checked the bin & noticed there are some small (very small) white maggot like wrigglers on the surface, I figure whatever they are, they hopefully will help to break down the leaves, don’t know how they got in there because the bin is permanently covered except when I give it a stir for a minute or two. Strangely enough it doesn’t smell too bad (although I would not say it is a pleasant smell) maybe there is something in the gum leaves themselves (eucalyptus oil) that helps control the smell, if anything as the weeks have turned into a month it smells less than it did after the first 7 days.

        All thoughts on this topic gratefully read.

        Many thanks All the best Jo

  6. Thank you for a great article I will come back and re read this. I need to do better with my composting!

  7. Tea bags shouldn’t be put in the compost any more, as the vast majority of major brands have microplastics in them. We save our used bags, and once or twice a day rip them open and tip the leaves into the compost minibin in our kitchen, along with the tags.

    • Thank you for raising that point – we could write a whole article on tea bags. Firstly, not all tea bags contain these microplastics, and those that do have it in only small quantities. And the microplastics used must be “food grade” – that is they must not break down and allow their contents to be ingested directly by the consumer under normal usage. The type of plastic used would, therefore, not leach out of the bag when boiling water is added. HOwever, there is some dispute about this.

      There seems to be considerable variation across countries and manufacturers. There have been reports of people finding the net of teabags still intact in their compost heaps. But manufacturers such as Twinings state that their bags are fully compostible. The microplastics should break down slowly in compost heaps, and the average household probably does not use enough teabags for them to cause a problem.

      Perhaps a bigger issue with teabags is the habit of manufacturers of using the lower grade very fine powdered material that remains after the higher grade leaves have been packed for loose leaf preparations. So flavour may be compromised by using bags instead of loose leaf. And we should be aware that totally paper tea bags have been coated with substances to stop them from falling apart and that this material is potentially carcinogenic if ingested.

      Putting the bags in compost appears to be much safer than using the bags for making something to drink in the first place. We would recommend using loose leaf tea either in a pot, or one of those 2-sided perforated ” spoons”

  8. I use a 400L Gedye type bin at home. I haven’t had any issues with odours or rodents. I like the fact I can add citrus peel, bones etc with out causing issues. There is a resident cockroach population but they’re easily managed via stirring with a spiral rod implement and they can’t escape with the lid on. I’m sure the cockroaches perform a useful function in disposing of the cellulose I add via the newspaper lining I have on the 2.5L compost bucket. There are plenty of compost worms present so I don’t really see the point of having a separate worm farm. Once the bin is nearly full I’ll either buy another one or just lift the bin to access the compost from the base. Simples!

  9. Great article. It would be great if you could ‘normalise’ the use of urine in compost heaps (instead of your bracketed comment ‘ shudder’) – it is a sustainable method of reducing water wastage in the house (use of the toilet) and encourages us all to to use our own waste products in a sustainable and normalised way. Many of us wee on the citrus tree out the back, so what’s different about the compost heap? A better source of nitrogen that would otherwise be wasted, and an option to store purchased blood and bone/fertiliser….

    Also read a great article recently about tonnes of citrus peel waste being dumped in a desert type environment, which years later turned into an amazing, abundant and densely revegetated environment. Yes, citrus peels take a while to break down, but in a compost heap, perhaps chopping them up first rather than throwing them in the bin would be more helpeful advice? (I put everything in my various compost bins – all organic matter decomposes in the end!) Just a suggestion…

    • Good points! We are reluctant to completely normalise urine the compost heap because there are many people who do not like the idea and excessive addition (e.g. from a big family) could make the heap too wet.

      of course citrus peel will break down – eventually. Because the process is slower than other compost heap ingredients, adding it should be undertaken with caution

  10. Hi/ Hail Infobots! what are the minimum and ideal thicknesses for nitrogen/ carbon layers? Is mixing their correct ratios all in together as or not as effective? Am in construction phase, want to get this right. Big thanks, its a great site cheers regina

    • Your question is simple but the answer can be complex. The ideal ratio for carbon to nitrogen (called the C:N ratio) is 30:1. This doesn’t mean 30 units of dry matter to 1 unit of Wet/green matter because both will have carbon and nitrogen. Generally 2 parts of green material to 1 part of brown will achieve this. F your heap is heating up and doesn’t stink you have it right. If the heap is wet and smelly it needs more dry matter and if it is dusty or has white fungus threads through it it needs damping and/or more green matter.

    • Your question is simple but the answer can be complex. The ideal ratio for carbon to nitrogen (called the C:N ratio) is 30:1. This doesn’t mean 30 units of dry matter to 1 unit of Wet/green matter because both will have carbon and nitrogen. Generally 2 parts of green material to 1 part of brown will achieve this. If your heap is heating up and doesn’t stink you have it right. If the heap is wet and smelly it needs more dry matter and if it is dusty or has white fungus threads through it it needs damping and/or more green matter.

  11. 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

  12. 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

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