This post is for people with small (or no) yards, or who otherwise can’t or don’t wish to set up an outdoor compost pile, but who would still love to ‘do their bit’ and feed their trees at the same time.

This composting-in-a-box method is not that well-known here, but is a very popular system in Japan, from where it originates. The always-innovative Japanese, who are very big on recycling overall, have created a very efficient way of turning kitchen scraps into compost in literally a cardboard box!

Please note that I have never actually done this myself — while it absolutely fascinates me as a concept, I’ve had zero motivation to actually do it because, well, we have our larger setup in place. But I have seen it demonstrated, and by a local Japanese lady no less! This is very much a ‘back-burner’ (if ever) project for me, but may well be the thing that fires you up, so let’s begin!

The Box

You need, of course, at least one cardboard box! Top flaps are optional, but if present, tuck them into the box.

You can use two boxes, with one fitting snugly inside the other, but one will still work. Something around the size of an archive box, but there are no hard rules as to dimensions. Roughly 30 – 40 cm width and length and 20 – 30 cm height seems common.

The thicker the cardboard, the longer the box will last, as it will break down slowly over time. This is nothing to be concerned about, as you’ll have plenty of advance notice by observing the box when adding food scraps and mixing the contents daily. I suspect that thicker cardboard will be more insulatory too. With the two-box system, the outer box could become the new inner box of a new setup when the original inner box is well past its prime.

Line the bottom with extra cardboard to a depth of at least 2 cm.

Everything must be cardboard so as to enable air to enter. Plastic or metal boxes and linings won’t work.

Place the box on two bricks, or on some kind of platform which keeps the box 5 – 10 cm off the ground and allows air to circulate underneath.

The Cover

If you have an actual cardboard archive box, simply use the lid that comes with it to cover the box. Otherwise cover the box with an old t-shirt, large tea-towel, towel or some other cloth material which allows air flow. (You may need a large rubber band or some string to hold the cover in place and prevent it sagging and touching the contents.)

The Medium

Carbon Source

The ‘recipe’ followed in Japan uses an ingredient not exactly commonplace here — kuntan, or rice husk ash. (A very common waste product in rice-growing countries, and apparently used in Japan as a soil-enricher.) But you could substitute rice husks (available in bulk) or even rice bran (from a supermarket). This will provide the carbon source, or the ‘browns’ for our microbial decomposers.

In theory, any small high surface area-to-volume carbon-rich material should work here. Rolled oats might work, or bran maybe? Psyllium? What else does a ‘health section’ have (I never go in one)? Finely chopped straw or dried grass should work too, if you could be bothered chopping it finely? Dried herbaceous plants such as parsley gone to seed? Dried lemongrass leaves? Dried, small, fern fronds? (All chopped finely, of course!) Really well-shredded cardboard might work too…

My point is, don’t be a stickler just because it’s written down — if you have some unwanted thing lying around that could work then why not give it a try and put it to good use. If I were to ever do this composting method myself it would probably be only to experiment with all these different materials! But I digress!

Moisture Retention and Aeration

You’ll also need coir, readily available from gardening outlets. Coir is a highly-absorbent material and its purpose here is two-fold. One is to keep the composting medium moist rather than wet, as it will readily absorb the liquid by-products from the decomposition process. The other is to increase air porosity, which is especially important in this system as we want to encourage fast aerobic degradation, rather than the smellier, slower anaerobic degradation.

Go for the finer coco-peat grade, also called coir-peat. This grade looks like soil when rehydrated, while coarser grades are more chip-like. All coir products come as heavily compressed dry bricks and must be wetted up for use. Hydrate a block as per the instructions — you’ll end up with way more than you need, but it will store well for later use. (I use various grades of coir for various things, and find that they all store well even when moist.). Or add the excess to garden soil or potting mix. It also makes a great surface cover and weed-suppressant on pot-plants.

Add the coir and rice bran in a 3:2 ratio — whatever a ‘part’ may be that you use as a measure, add three parts coir to two parts rice bran. Do this until the box is about two-thirds to three-quarters full. Mix the contents thoroughly with a trowel or your hand, gloved or otherwise! A handful of this mixture should roll into a ball yet crumble when touched. If drier than this, just add water gradually until the right consistency is reached.

You’re now ready to compost in a box!

Adding Food Scraps

As with the hot compost method, and the carbon source above, it is really important that all scraps be chopped up finely. This increases the surface area and hastens decomposition.

Simply add these food scraps to the top, and, with that same trowel or gloved-or-otherwise-hand from above, mix them thoroughly into the coir mixture. You can add up to 500 g a day apparently. These scraps are the nitrogen source, or the ‘greens’ for microbial decomposers.

Add scraps daily and mix thoroughly. Not much will happen for two or three days, but you should feel the mixture becoming very warm, reaching about 40 °C from that time on. This is a very good sign, and means your choice of carbon has worked, and microbes are very actively feeding, growing and respiring! (It is their respiration which produces the heat.) Continue adding scraps and mixing well daily hereon in.

Environmental Regulation

Temperature

Unlike the larger-scale composting method, the box is not big enough to enable the middle to reach the really high temperatures that encourage thermophiles, but 40 °C is still a commendable achievement! Not only is this a desirable temperature for mesophiles, but their degradative activity will be faster at this temperature than at room temperature.

A drop in temperature in a larger-scale composting system indicates nitrogen depletion. In this box method, where nitrogen is added daily as fresh food scraps, a drop in temperature indicates carbon depletion. Remedy this by mixing in a handful of your carbon source so it can heat up again.

If the box is too hot, even to the point that steam is coming out, simply remove the cover and let the box cool. Place a fly screen over the box if possible, to prevent insect entry.

Moisture

A too-wet medium runs the risk of becoming anaerobic. Add more coir-peat to mop this excess up (you do have plenty left over after all!).

A too-dry mix runs the risk of slowing degradation, as not only do microbes need water just to survive, water is required as the solvent for many degradative reactions that occur outside their cells.

Storage

It is very important that the box not be exposed to rain. Not only will the contents saturate and possibly become anaerobic, you probably won’t have much of a box left either! The Japanese routinely keep their boxes in their apartments — there is no foul smell or problem with pests as the contents have a rich earthy smell from day one owing to the regular aeration they receive. Putrid, rotting smells come from anaerobic conditions.

Useage

In theory you could feed your box forever with a daily input of food scrap ‘greens’ and a regular top up of carbon-rich ‘browns’. At some point though you’ll either run out of room in the box, or run out of box as it breaks down. After about three to four weeks really examine the contents of the box — how much recognisable material is there? Ideally there should be very little as you have a thriving community of mesophiles decomposing material very quickly. At this point, don’t add any further scraps but do continue to mix daily until there is nothing recognisable left. You now have compost!

You can apply this as a top-dressing and slow-release fertiliser to potted plants and trees. Top that layer again with any excess coir-peat which can act as a mulch.

Store any excess compost in a weatherproof container for later use — it will continue to age and improve during this time.

Then begin anew!