Should I stop putting citrus and onion in my worm bin?

You can keep them to about a fifth of the weekly feed. The advice to exclude them entirely comes from worm-farming contexts where trays are fed heavily and acidity builds up; at normal household rates, with bedding and eggshell, they cause no measurable harm.

Is it true that you cannot compost anything cooked?

No. Cooked food composts perfectly well; the restriction is about animals and smell, not about the composting. In a sealed bokashi bucket cooked food is the ideal input. In an open heap, bury it deeply in the core.

Kitchen and home waste: what goes in, what stays out

01Ratios at a glance

02The table

Sorted by C:N, lowest first, with the working band shaded
#	material	C:N	water	N %	dose
01	EggshellsCoarse shells take years to break down and do nothing for pH; grind them.	0:1	3%	0.40	All three, ground to powder. 40 g a week in a worm tray.
02	Cooking oil and greaseCoats particles and blocks water and air; a tablespoon can ruin a worm tray.	0:1	0%	0.00	Bokashi in small amounts (soaked into paper); never in a heap or worm tray.
03	Wood ashAlkaline: raises pH and can volatilise nitrogen if overused. Untreated wood only.	0:1	2%	0.00	Maximum 1 kg per cubic metre of pile; never in a worm tray.
04	Meat, fish and bonesAttracts animals within hours in an open heap; fine in a sealed bucket.	5:1	65%	9.00	Bokashi only, or a hot pile buried 25 cm deep.
05	Wine and beer dregsAlcohol kills worms in quantity; a glass of dregs is fine diluted through bedding.	10:1	95%	1.00	All three in small quantities; pour onto browns, not onto worms.
06	Dairy (milk, cheese, yogurt)Rancid dairy is one of the worst smells in a worm tray — keep it out entirely.	12:1	80%	3.00	Bokashi only. Small amounts in a very hot heap are acceptable.
07	Pet hair and human hairHigh nitrogen but slow; spreads through a pile unevenly if added in clumps.	12:1	12%	4.00	Heap or worm tray, chopped and mixed. Up to 3 % of pile volume.
08	Cooked food leftoversOils, salt and meat content are what make leftovers a problem for worms.	15:1	72%	2.80	Bokashi only. Never in a worm tray; in a heap only if buried 25 cm deep.
09	Onion and garlic skinsThe sulphur compounds are antimicrobial in concentration but break down quickly in a pile.	15:1	70%	2.00	Bokashi and heap freely; worm tray in moderation.
10	Vegetable peelings and trimmingsThe backbone of any kitchen waste stream; 1 kg needs 1.5 kg of cardboard in a heap.	17:1	82%	2.30	All three systems. Chop to 3 cm for a worm tray.
11	Coffee grounds with paper filterMoulds after about five days in storage; compost the week you collect them.	20:1	62%	2.10	All three. 1 kg per 1.3 kg of cardboard in a heap.
12	Tea leaves and paper teabagsMany teabags contain polypropylene; if the wet bag feels like plastic mesh it does not belong.	20:1	75%	2.20	All three. Tear bags open for a worm tray.
13	Cereal, bread and pastaMouldy bread is fine in a heap; in a worm tray it can bloom and go sour.	20:1	35%	1.80	Bokashi, or a heap buried in the core. Small amounts in a worm tray.
14	Fruit scraps and coresFruit fly magnets — bury everything, always.	25:1	80%	1.50	All three. Limit citrus in a worm tray to about 20 % of the feed.
15	Vacuum cleaner dustMostly skin cells, dust and fibre; may contain microplastics from carpet.	30:1	10%	1.00	Heap only, in small amounts, mixed through. Not in a worm tray.
16	Citrus peelThe limonene that makes it smell good is what harms worms in quantity.	30:1	78%	1.00	Bokashi freely; heap freely; worm tray at 20 % of feed maximum.
17	Newspaper and junk mailThe standard worm-tray bedding; keeps the tray from going wet and sour.	175:1	8%	0.20	All three as bedding. Remove plastic wrappers and glossy inserts.
18	Paper towels and napkinsA useful free brown; unbleached and unprinted is best.	180:1	8%	0.20	All three, as bedding or browns. Avoid ones soaked in cooking oil.
19	Cardboard egg cartonsAbsorbs moisture fast without matting — excellent for a wet kitchen pile.	200:1	9%	0.20	All three. Tear to 5 cm and mix rather than layer.

Nitrogen percentages are of dry mass and are typical published values, not laboratory results for any particular batch of material.

Cross-section: reference — How the layers sit

03How to use this table

01 Split your kitchen waste into three containers: raw vegetable, cooked food, and paper.
02 Send cooked food, meat, dairy and oils to a sealed bokashi bucket.
03 Send raw vegetable and fruit trimmings to the worm tray, chopped to 3 cm.
04 Keep a separate stack of cardboard and paper for the heap, and never mix oiled paper in.
05 Bury everything: 5 cm in a worm tray, 20-25 cm in a heap core.
06 Keep citrus and onion to about 20 % of what a worm tray receives.
07 Check teabags and cardboard by hand for plastic before they go into any system.

04The three-way split that makes indoor composting work

Most households that give up on indoor composting do so because they tried to put everything through one system. The workable split is: sealed bucket for cooked food, meat, dairy and oils; worm tray for raw vegetable and fruit material plus paper bedding; heap for everything dry and garden-derived. That split exists because the three systems are killed by different things — worms by oils and salts, buckets by dryness, heaps by compaction.

05Burying is the whole technique

Almost every kitchen-waste problem, from fruit flies to rodents to smell, is a surface problem. Material left on the surface of a heap or a worm tray is exposed to air, insects and animals. Material buried 5 cm down in a worm tray or 20-25 cm down in a heap core is not. This is why the entries in this log record the burial depth as carefully as the weight: it is the difference between a system that works quietly and one that becomes a nuisance within a fortnight.

06Salt, oil and the small exclusions

Three categories should stay out of garden compost entirely: cooking oil in quantity, salted water from cooking, and anything containing a lot of salt or preservative. None of them will destroy a pile, but they are cumulative — a heap that receives a litre of salty pasta water a week will, after a year, have noticeably high sodium and a suppressed microbial community. The same applies to citrus and onion in a worm tray: fine in moderation, harmful in concentration.

07What the numbers do not capture

Two rows in this table are decided by materials rather than chemistry. Teabags: if the bag contains polypropylene it will still be intact in the compost two years later, and the only way to tell is to tear a wet one open. Cardboard and paper: the C:N is excellent, but laminated, waxed or plastic-film-coated stock cannot be composted at all. The chemistry says a material is suitable; only your hands can say whether it is what you think it is.

08Questions this batch raised

Should I stop putting citrus and onion in my worm bin?: You can keep them to about a fifth of the weekly feed. The advice to exclude them entirely comes from worm-farming contexts where trays are fed heavily and acidity builds up; at normal household rates, with bedding and eggshell, they cause no measurable harm.
Is it true that you cannot compost anything cooked?: No. Cooked food composts perfectly well; the restriction is about animals and smell, not about the composting. In a sealed bokashi bucket cooked food is the ideal input. In an open heap, bury it deeply in the core.