The Art and Science of Composting: Debunking a Few Misconceptions

Written by Manu

Above: Toby the compost tumbler in Lyttleton Gardens

Let's face it, composting is a delicate subject. For seasoned composters, composting is a source of joy, satisfaction, but also pride, rigidity and strong belief that you've got it, you know how to compost, you hold 'the key recipe'. For people who are new to composting, it's a source of excitement as well as anxiety, as you constantly look for advice from 'experts' on how to get it 'right', not to mention the successes, failures, disappointments and a myriad other emotions. Ah the roller coaster ride of emotions that is composting!

Now that I am training in soil microbiology with Dr. Elaine Ingham and Dr Carole Anne Rollins, the two incredible powerhouses behind the Soil Foodweb Institute, I feel that I can now share info about what is actually happening biologically in a compost heap. This will help debunk a few misconceptions I have seen and heard.

I will have to break it to you though: there is no set 'perfect recipe.' Are you shattered... or relieved? People who are really good at composting seem to be the ones who know this, and go with the flow with no set recipe but a lot of knowledge on how to observe, adjust, smell, and even taste when things are right.

What goes on in your compost is heavily dependent on outside temperatures, moisture levels, ingredients, native microorganisms and a host of other external and internal factors. There are, however, a couple of things that apply pretty much universally. 

Firstly, whether you are hot composting, tumbling, cold composting or whatever else (except for bokashi), your compost needs OXYGEN to travel through it regularly. You need to aerate your compost regularly, whether it's in a pile or a bin. There are really cool, inexpensive screw-like contraptions you can use for bins these days. The good microbes you want to have in your compost and to apply to your soil are AEROBIC, which means they need oxygen to survive.

Secondly, you need a DIVERSITY of fresh, nitrogen-rich green materials (garden clippings or food scraps, composted manures, coffee grinds, bones) and dry, carbon-rich materials (like straw, dried leaves, wood chips and wood shavings). Remember to keep everything moist. Squeeze it in your hand, it should let out a maximum of one drop of water and feel moist. 

Thirdly, I find ratios a little hard to understand. I prefer Elaine Ingham's bucket method. Lets keep it simple. A good ballpark reference is at least three buckets of dry carbon-rich materials need to be added for every one bucket of nitrogen-rich materials. This applies to hot and cold composting! Many people keep adding food scraps to their bin thinking they will magically turn into compost. Although compost is indeed magical, it needs all players to be present. Otherwise the end product is rotten, putrified organic matter.

The nitrogen-rich materials are what generates bacterial activity which makes the pile heat up. If you have too many nitrogen-rich materials, your heap will heat up too fast too quickly and kill everything. Strong smells of ammonia, vinegar, vomit, poo in your bin or heap indicate that anaerobic microbes have taken over and are producing organic acids that are toxic to plants.

This is by far the most common mistake I have seen in home as well as large scale composts: too many nitrogen-rich materials, not enough carbon rich materials. Aerate it, add loads of dry, carbon rich materials to kickstart the process again in the right direction. Do not add lime as lime is (chemically speaking) a salt, and kills many beneficial organisms.

Hot or cold?

There is a little bit of confusion as to what constitutes 'hot' and 'cold' composting. In reality, all composts heat up to a degree. This heat has nothing to do with heat from the sun or outside temperatures. It is generated by microbial activity. In fact, I personally never put any of my composts in the sun as UV rays from the sun tend to kill beneficial microbes on the surface of your compost, or heat from the sun will 'cook' them in your bin.  

'Hot' composting involves buiding a big heap or bin all at once and letting it heat up in the centre. This heat (which can reach 70 degrees celsius or higher) has nothing to do with heat from the sun or outside temperatures. It is generated by microbial activity. Aerating the heap by turning it as soon as it hits the 65 degree mark, is essential because we do not want it to get too hot (no hotter than 65 degrees celsius), as this will annihilate all the microorganisms you need in your heap. I really suggest getting yourself a compost thermometer to take the guesswork out of the equation. Hot compost heaps tend to be more prone to drying out to water well when you build the heap and check the moisture each time you turn it. Remember a hot compost heap is all made at once i.e. no adding as you go. HOWEVER, if your standard compost bin is heating up, you are indeed hot composting, and it's time to aerate it. 

Cold composting involves adding as you go, which means it should not heat up anywhere near as much as a hot compost. Remember, for every one bucket of food scraps, you need to add three buckets of dry stuff. One cautionary note: anything that was cut when it was green is nitrogen-rich, even if you then let it dry. For example, dried up grass clippings are still a nitrogen rich material. Remember to check the moisture by squeezing it in your hand. Cold composting takes more time, but the end product has a wider diversity of beneficial microorganisms.

What about compost tumblers?

Technically, if you have a large enough tumbler, you can make hot compost (all at once) or cold compost (add as you go). I personally like to cold compost with my tumbler because it makes the cold composting process much faster through more frequent turning and, as I mentioned, the end product is incredibly rich and diverse in beneficial microorganisms.

I hope this helps debunk a few misconceptions and myths on composting and encourages you to delve into its magic!