Composting in the garden

Composting is decomposition that you steer. Bacteria and fungi start by breaking down soft plant parts. Then larger helpers join in: compost worms, woodlice, springtails and many other soil creatures. They break the material down further and further until it becomes humus – the dark, nutrient-rich part of good soil.

These creatures need two kinds of food. Green material such as vegetable scraps, coffee grounds or fresh grass clippings mainly provides nitrogen – you can think of it as protein for the small creatures. Brown material such as leaves, straw, twigs or cardboard provides carbon – more like the energy. If one or the other is missing, the process runs poorly.

Most of these organisms also need oxygen. If the compost is too wet and dense, air is lacking. Then other microbes that work without oxygen take over, and it starts to rot and smell unpleasant. If it is too dry, everything stalls. A well-mixed, loose compost, on the other hand, hardly smells and can warm up noticeably inside – a sign that the microbes are hard at work.

The site: put the bin on bare soil so worms and other soil creatures can move in from below, ideally in partial shade so it does not dry out. It should be easy to reach – even in winter with a bowl of kitchen scraps in your hand. A layer of coarse twigs at the bottom helps air get in.

The mix: add green and brown in alternating layers or mixed together. Grass clippings only in thin layers, otherwise they mat together into a putrid layer. Chop coarse material – smaller pieces decompose faster. If the compost is too dry, moisten it; if it is wet and smells, add more brown material and cover it.

Turning it occasionally or loosening it with a garden fork lets air in and speeds up decomposition. The compost is ready when it is dark and crumbly, smells earthy and the original materials are barely recognisable. Sieve it and work it into the bed. Do not put immature compost directly on young, delicate plants.

Well suited: raw fruit and vegetable scraps, coffee grounds, tea leaves, eggshells, leaves, grass clippings in thin layers, straw, small twigs, unprinted paper and torn cardboard. The more varied the mix, the better.

Not for an open home compost: meat, fish, bones, dairy products and cooked leftovers – they attract rats and other animals. Also leave out diseased plants and weeds with ripe seeds, because diseases and seeds can survive in the compost and spread later in the bed.

Also off-limits are pet faeces, treated or painted wood and coal ash. They can contain pathogens or pollutants that do not belong in your vegetable garden. Human waste does not belong on a garden compost either.

Compost is full of microorganisms – that is the point, but it also means you wear gloves while working and wash your hands thoroughly afterwards. Turning dry material can raise dust containing fungal spores. Do not breathe it in: do not lean into the dust cloud, and lightly moisten dry compost first.

Anyone with a weakened immune system – for example due to illness or medication – should check with a doctor beforehand whether compost work is suitable. Small children can help with the compost but should not play in it and should always wash their hands afterwards.

Whether and how garden compost is allowed, for example how far it must be from the neighbouring plot, differs between municipalities – check local rules. A well-managed compost hardly smells. If it smells putrid or attracts animals, that is a sign the mix or contents are wrong.

The US Environmental Protection Agency describes home composting with exactly these basic ingredients: green and brown material, water and air. It also lists what does not belong in a home compost. Gardening handbooks add different bin types – open slatted bins, closed insulated bins or simple heaps.

If you have no garden, you can compost with a worm bin. In it, special compost worms process kitchen scraps, even on a balcony or in a basement. A worm bin does need even temperatures and some care, though, so that the worms thrive.

Compost closes a loop: what grows in the garden returns as food for the soil. That saves waste and fertiliser and makes the soil looser and more fertile over the years. For a vegetable garden, home-made compost is one of the most valuable treasures there is.