Compost is the difference between a garden that costs money every year and one that runs on its own waste. It is also the slowest thing to start, which is why the advice on starting a bed is to build the heap on the same day as the bed, even if you have nothing to put in it yet.
The process is genuinely simple. Almost everything that goes wrong is one of three things: too wet, too dry, or all one kind of material.
Feed the soil not the plant
The idea that most changes how a garden behaves.
Healthy soil is alive. Fungi, bacteria, worms and countless other organisms hold nutrients, build structure, move water and make minerals available to roots. Adding organic matter feeds that system, and the system feeds the plants.
Adding soluble fertiliser feeds the plant directly and does nothing for the soil, so you have to keep doing it, and the structure keeps degrading.
The practical version:
- Add organic matter every year, on top, and let the worms take it down.
- Do not dig it in. Digging destroys the structure and the fungal networks you are building.
- Keep the soil covered, with a crop, a mulch or compost. Bare soil erodes, dries, and loses its life.
- Walk on paths, never on beds. Compaction squeezes out the air that everything living in there needs.
Two or three years of this visibly changes a piece of ground, and it is not gardening folklore. Water soaks in instead of running off, the soil darkens, and it becomes easy to push a hand into.
Greens and browns
Roughly equal by volume, and it does not need to be precise. Two parts brown to one part green also works well.
The commonest failure is too much green. A heap of nothing but grass clippings goes slimy, black, airless and smells of ammonia. The fix is to mix in cardboard, and to do it now rather than later.
The second commonest is too much brown. A heap of nothing but leaves and cardboard just sits there for years. The fix is grass clippings, weeds or a bucket of urine, which is free, sterile in a healthy person, and an excellent nitrogen source diluted about one to ten.
Layer as you go, or better, mix. Alternating thin layers of green and brown gives you an even heap without thinking about it.
Building a heap
- Site it on soil, not concrete, so worms and organisms can get in.
- Somewhere convenient. The heap you walk past is the heap that gets filled. Shade or sun both work; shade dries out less.
- A cubic metre is the useful minimum for a heap that heats. Smaller works and stays cold.
- Two bays are much better than one: one being filled, one rotting down. Three is the traditional arrangement.
- Build it from anything: pallets wired together, chicken wire on posts, a bought bin, or just a pile.
- Cover the top with a lid, a piece of carpet or a sheet, to keep rain off and heat in. Not the sides, which need air.
- Add in layers, mixing green and brown as you go.
- Keep it damp, not wet. Like a wrung-out cloth. Water it in a dry summer.
Chop things small. Everything rots faster with more surface area, and a bit of time with shears or a spade on the pile makes a large difference.
Hot or cold
A cold heap is what most households do, and it is perfectly good. Add material as you have it, leave it, and use it eventually. Do not add perennial weed roots or seeding weeds, because they will survive.
A hot heap needs a large quantity of mixed material all at once, at least a cubic metre, well mixed and damp. It heats to around 55 to 65 degrees within days, and turning it every few days keeps it going by reintroducing air and moving the outside to the middle.
A hot heap is worth doing once a year if you can gather the material, because it deals with weed seed and gives you compost in weeks rather than a year.
If it is not heating: too dry, too brown, too small, or not mixed. Water it, add greens, make it bigger, turn it.
What not to put in
Perennial roots can be drowned in a bucket of water for several weeks first, which kills them and gives you a strong liquid feed. Then the sludge composts safely.
Wood ash is useful in small quantities, is alkaline, and is also what you need for lye, so decide which use it is going to.
When it is ready
Dark, crumbly, sweet-smelling, and you cannot identify what went in. It should smell of woodland floor, not of anything sour or ammoniacal.
- Sieve it if you want a fine seed compost, and throw the coarse bits back on the heap.
- Half-rotten is fine as a mulch on top of a bed, where it will finish in place.
- Use it as a top dressing, a few centimetres over the bed in autumn or winter. Do not dig it in.
- Home compost is not sterile and is not the right thing for seed sowing without sieving and, ideally, without weed seed. Bought seed compost is worth it for propagation and unnecessary for everything else.
Leaf mould
The easiest useful thing in the garden, and it is entirely free.
Leaves rot by fungal action rather than bacterial, which is why they do not heat and why they take longer.
- Gather leaves in autumn. Council street sweepings, your own, a neighbour's.
- Put them in a wire cage, or in bin bags with holes punched in and the top loosely folded.
- Wet them.
- Leave them for a year, or two.
One year gives a rough mulch. Two years gives a fine dark crumbly material that is an excellent soil conditioner and the traditional base for seed compost.
It is a conditioner rather than a feed. It adds structure and water holding, not much nutrition, which is exactly what most soils want most.
Avoid leaves from roadsides with heavy traffic, and evergreen leaves, which take much longer.
Liquid feeds for nothing
For hungry crops in containers and mid-season, both of these cost nothing.
Comfrey. Grow a clump of Bocking 14, which does not set seed and will not spread. Cut it several times a season, pack it into a container with a hole in the bottom and a lid, and collect the black liquid that drains out. Dilute roughly one to fifteen. It is high in potassium, which is what tomatoes, beans and fruiting crops want.
Nettles. Same method, or steep in water for a few weeks. Higher in nitrogen, so good for leafy growth early in the season.
Both smell appalling. Put the container somewhere downwind and keep the lid on.
Urine, diluted one to ten, is a genuinely good nitrogen feed for the compost heap and for leafy crops, and it is free and sterile from a healthy person. Do not use it on anything about to be harvested, and do not use it if anybody in the household is on medication that would be excreted.
Knowing your soil
Five minutes that explains a lot about what will and will not do well.
Texture: take a handful of moist soil and squeeze it.
- Falls apart, feels gritty: sandy. Drains fast, warms early, loses nutrients, needs organic matter and water.
- Rolls into a ball, feels smooth or sticky: clay. Holds water and nutrients, cold and slow in spring, cracks in drought, needs organic matter.
- Rolls but crumbles, feels silky: loam. What everybody wants.
The answer for all three is the same: add organic matter every year. It opens up clay and it gives sand something to hold onto.
pH matters less than most books imply for most UK gardens, and a cheap test kit is worth doing once.
- Most vegetables want pH 6.0 to 7.0.
- Brassicas prefer the upper end, and liming can help against clubroot.
- Potatoes tolerate more acid and scab is worse on limed ground, which is why potatoes traditionally go before the lime in a rotation.
Do not lime and manure in the same season, which wastes both.