Compost is kitchen and garden waste that worms, fungi and bacteria break down into dark, crumbly humus. You can compost in three ways: in an open cold compost heap, in an insulated hot composter or with bokashi, where food waste ferments in an airtight bucket. Cold and hot composting only work with the right mix of green and brown material, while bokashi depends on keeping the air out.
Three methods side by side
Choose a method based on what you want to compost and how soon you need the soil. If you mostly have leaves, grass clippings and old plant tops, a cold heap is enough. Food waste all year round calls for a hot composter or bokashi.
| Feature | Cold compost | Hot compost | Bokashi |
|---|---|---|---|
| What can go in | Garden waste, peelings, coffee grounds and eggshells | Garden waste and food waste, including cooked food | Food waste, including meat, fish, cheese and citrus peel |
| Temperature | Roughly follows the air temperature | 50-70°C once the process is under way | Room temperature, ideally 20°C or more |
| Time to finished compost | 6 months to 2 years | 2-6 months, longer before it is safe for sowing | 2 weeks in the bucket, then 2-4 weeks in the soil |
| Location | Part shade directly on soil, ideally 2 bays | Shade close to the house, sealed against rodents | Bucket indoors, then buried in the soil |
| Cost | Low, boards or pallet collars are enough | Highest, an insulated container | Low, 2 buckets with tight lids and bokashi bran bought as you go |
| What you get | Humus-rich soil improver with moderate nutrients | Nutrient-rich humus in a shorter time | Pre-compost that breaks down in the soil, plus leachate (bokashi tea) to dilute 1:100 |
Before you start putting food waste in a hot composter or a bokashi bucket, check with your local council what applies where you live.
Green and brown in the right proportions
The microorganisms use carbon for energy and nitrogen to build their cells. Decomposition is fastest at a carbon-to-nitrogen ratio (C:N) of about 25 to 1, that is 25 parts carbon to 1 part nitrogen. With more than 30 parts carbon the heap almost stands still. With fewer than 15, the nitrogen escapes as ammonia, and that is the pungent smell from a wet pile of grass clippings.
You do not need to calculate exactly. Sort the material into two groups:
Green and nitrogen-rich: fresh grass clippings at about 15 parts carbon per part nitrogen, vegetable peelings, coffee grounds, plant tops and weeds without seeds.
Brown and carbon-rich: dry autumn leaves at about 50 parts carbon per part nitrogen, straw, finely cut twigs, torn-up egg boxes and sawdust.
The rule: add 1 bucket of green to 1-3 buckets of brown, measured by volume. The more grass clippings and food scraps, the closer to 3 buckets of brown. A sack holding 50 litres of grass clippings therefore needs 50-150 litres of leaves or straw.
Moisture and air: a handful of material should feel like a wrung-out sponge, which corresponds to 40-60 per cent moisture. If the compost smells of ammonia, add more brown. If it is dry and nothing is happening, add green, water it and loosen it with a garden fork.
Bokashi is a pre-compost
What comes out of a bokashi bucket after 2 weeks is not soil. The food scraps have fermented with lactic acid bacteria and look almost as they did when you put them in, just softer and with a sour smell. They only break down into humus once they are in the soil. We have 2 buckets that take turns, so one is fermenting while the other is being filled.
Fill and press down: sprinkle bokashi bran over each layer of food scraps as directed on the pack, press out the air and open the lid no more than 1-2 times a day.
Drain off the leachate: do this every two to three days. Dilute it 1:100 before watering with it, as it damages roots undiluted.
Let the full bucket ferment: leave it for 2 weeks at room temperature. White, fluffy mould is normal. Green or black mould and a rotten smell mean the fermentation has failed.
Bury it or mix it with soil: cover it with at least 10 cm of soil, or mix 1 part bokashi with 2 parts soil. Wait 2-4 weeks before you sow or plant there.
How to tell when the compost is ready
Mature compost is dark brown, crumbles between your fingers and smells like a damp forest floor. You can no longer recognise leaves or peelings. In a hot composter another sign is that the temperature has dropped to that of the surroundings and does not rise again when you turn the material.
A 5 day cress test shows whether the compost is safe for sowing. Fill one cup with your compost and one with bought seed compost. Sprinkle 20 cress seeds in each cup and keep both moist at room temperature. Then count the seedlings and pull up a few to compare the roots. If fewer seeds germinate or the roots are shorter than in the bought seed compost, your compost needs a few more months to mature.
We do the test every spring before the compost goes into our seed compost. Compost that holds back the cress goes on the borders instead, where it is useful as a soil improver.
Compost for borders, raised beds and seed compost
Borders: spread at least 5 cm of compost on top of the soil between November and April. The worms pull it down, so you do not need to dig. Both clay and sandy soils become easier to grow in as the humus content rises, see the article on soil types. Half-finished compost works as a mulch in a no-dig garden, but not where you are going to sow.
Raised beds: a pallet collar bed of 120 x 80 cm is just under one square metre, so 5 cm equals about 50 litres of compost. Work it into the surface 2-3 weeks before sowing. We mix in Florea Biochar at the same time, 1-3 litres per bed, so the soil holds on to moisture and nutrients for longer. Then the bed is ready for seeds for raised beds. How to build up the layers from the bottom is covered in the article on growing in a pallet collar.
Seed compost: this is where immature compost does the most harm. It can contain substances that inhibit germination, and material that is still breaking down locks up nitrogen the seedlings need. We sieve compost that has passed the cress test through a mesh with 1 cm holes and mix no more than 1 part compost with 3 parts bought seed compost. If the plants need more feeding once they have their first true leaves, liquid Florea Fertilizer is easier to dose.
Common mistakes and what to do instead
A whole sack of grass clippings at once: fresh grass packs into a slimy mat that smells of ammonia. Mix it with 1-3 parts leaves or straw, or let it dry for 1-2 days first.
Seeding weeds in the cold heap: weed seeds only die when the temperature stays above 60°C for several days, and an open heap rarely gets that hot. Put seeding weeds in a hot composter, and let couch grass and ground elder dry out completely in the sun before you compost them. Japanese knotweed survives even a hot composter and should not be composted at all.
A heap that is topped up all the time: fresh material keeps landing on top and the heap is never finished. With 2 bays you fill one while the other matures for 6-12 months.
The compost year month by month
March and April: empty the mature bay, do the cress test and spread compost on raised beds and vegetable beds 2-3 weeks before sowing.
May to August: grass clippings provide most of the green material. Layer them with brown and water the compost in dry weeks.
September to November: raked leaves are the brown material of the year. We keep a few sacks of dry leaves by the compost for the mowing in May. From November, mature compost can be spread on the borders.
December to February: below 0°C the cold heap stands still and starts again when it thaws. The hot composter works more slowly and easily gets too wet, so add more dry bulking material than in summer. Fully fermented bokashi can wait outside in an airtight bin until the ground can be dug.
Author: Emma Vogiatzi
Fact-checked by Erik Hoekstra
Last updated 2026-11-04