“But does it not smell??!!!” This is always, accompanied by a very sceptical face expression, the first question I get when I tell people about my roof-top compost. Straight answer, no – it has absolutely no odour at all. So let’s get that out the way to leave your minds clear for digesting (excuse the pun) the benefits and know-how of urban composting. Most commentary on composting concentrates on what can be done in a garden, but I would like to focus here on what might work for you on a terrace or yard because it does for me.

Composting is an excellent way to recycle organic household and garden waste and you can produce humus and soil that is perfect for a small permaculture project. Over time it will save you some money from a few less trips to the garden centre. From an environmental perspective, it is a great natural alternative to using fertilizers and you will produce less waste that will go to landfill. Organic landfill waste breaks down differently by releasing methane, which is well understood to contribute to climate change. You will also create a self-contained ecosystem that will invite some other beneficial critters into the garden.

There are different types of composting methods, from smaller kitchen type bins using worms to composting ‘tumblers’ that allow quite rapid digestion of the material. I will comment only on the more traditional approach of a composter bin, where the material is top added in layers and removed periodically by a door at the base.

Composting material is classified as either carbon or nitrogen based (brown or green) and a classic recipe consists of approximately two-thirds brown, one-third green. Carbon based materials consist of mostly dried garden waste like branches, stems, leaves, wood ash, and nitrogen of food scraps and green garden clippings. The availability of oxygen in the compost mass is key and the addition of brown materials allows good aeration. The composter bin should have holes on all sides to assist here, so if you are making your own out of [pallet] wood make sure you drill say 1.5cm holes on a 10cm grid all round. We had to use an off-the-shelf plastic bin from the garden centre, but would have preferred to have used some pallet wood. Importantly, there are a few things that shouldn’t go in the compost to avoid the dreaded bad smell and unwanted visitors – particularly meat, bones, or cooked food.

So normally a composter is located on the ground, where liquid from within the bin can leech out into the ground below and also animals can travel up into the compost mass to aid in the breaking down of the material. Nearly all composter bins available have open bases, so are perhaps not initially ideal for use on a roof terrace with the discharge perhaps creating an obvious mess. So we built a wooden base for ours from marine plywood and then lined the top with a heavy duty plastic sheet (salvaged) to create something a bit more self-contained. That works perfectly. As with many sustainable strategies, active participation is required and we try to control the moisture in the compost by ensuring the lid is closed when it is raining (yes, it does rain in Seville) and that there is not a high proportion of green waste from the kitchen put in at once. We started off with a layer of twigs and brown material, which I collected in a nearby park and then alternated the layers to get the biological processes started. The initial digestion period of the material may take a bit longer if it is a bit drier – we had to wait almost nine months before we were able to remove our first big buckets of rich black soil. But wow, the moment I was shovelling out rich black soil, I couldn’t believe my eyes – I had turned garbage into real fertile useable earth! Now we are able to harvest a bit of soil every four to six weeks. With the slower bio-process in mind, we also avoided composting a few things that can take longer to decompose, such as eggshells, avocado skins and citrus fruit peels.

Our bin is now a thriving condo of wildlife – they all moved in by themselves, God knows from where – that seems quite content to munch on what is provided and not to venture out far. Wood lice, centipedes, earwigs moved in quite soon after we started, and are now crucial in helping to aerate the soil and reduce the biomass down. The fruit flies create a plentiful buffet for the wall geckos and sparrows, and we believe that the increased number of bird visitors also helps to reduce the number of mosquitoes and rogue slugs in the process. Whilst the idea of keeping the company of such a host of bug friends may not be instantly appealing to some, this is in essence what happens in any flourishing garden, whether elevated or earthbound. Getting your hands dirty on your own home ‘grown’ truly organic soil can be quite a satisfying project, it is definitely for me. I will admit that when I ‘harvested’ the first soil I was a bit taken back by all the thriving insects in the dark and warmth of the compost, but my initial shuddering quickly turned into fascination about how easy it is for nature to return to an urban environment. Even in the middle of a city, we can try and bring ourselves a bit closer to nature – it is our connection to nature that makes us human. In our urban lives we are often losing this track of this fundamental human trait and composting might be just the thing for you to reconnect! It is really easy and simple to do – I’m a lazy person, so trust me.