What to Compost and What Never Goes In: A Sorting Table
A backyard pile can take roughly 25–30% of what a US household throws away, according to EPA waste figures on food scraps and yard trimmings. That only pays off if the right things go in. One bag of herbicide-treated grass clippings or a week of meat scraps can cost you a whole season of compost, plus a rat problem. The sorting decision takes seconds per item, and it is the single cheapest thing you control in composting.
Greens, browns, and the one ratio that matters
Every compostable item falls into one of two jobs. Greens are wet and nitrogen-rich: kitchen scraps, fresh clippings, manure. Browns are dry and carbon-rich: dead leaves, straw, shredded cardboard, wood chips. Microbes burn carbon for energy and use nitrogen to build their own cells, and they work fastest when the blend lands around 25–30 parts carbon to 1 part nitrogen.
Because browns are fluffy and light, a bucket of leaves carries far less mass than a bucket of peels, so a volume rule of two or three buckets of browns per bucket of greens gets most home piles close enough. Too many greens and the pile turns slimy and smells of ammonia. Too many browns and it sits for a year doing almost nothing.
The sorting table
Carbon-to-nitrogen figures below are typical published ranges from university extension and Cornell composting tables; individual batches differ.
| Item | Type | Approx. C:N | Verdict | Why |
|---|---|---|---|---|
| Fruit & vegetable scraps | Green | 15–25:1 | Yes | Breaks down in weeks; chop to under 2 in |
| Coffee grounds & paper filters | Green | ~20:1 | Yes | Grounds act as nitrogen despite the brown color |
| Eggshells (crushed) | Neutral | — | Yes | Adds calcium; whole shells last 1–2 years |
| Fresh grass clippings (untreated) | Green | ~15–20:1 | Yes, thin layers | Mats and goes anaerobic past 2–3 in thick |
| Dry leaves | Brown | 40–80:1 | Yes | Shred first; whole oak leaves take 2+ years |
| Straw | Brown | ~80:1 | Yes | Adds air pockets; hay may carry weed seed |
| Plain cardboard, newspaper | Brown | ~350:1 | Yes, shredded | Strip tape and glossy inserts |
| Herbivore manure (horse, rabbit, cow, poultry) | Green | 7–25:1 | Yes | Hot compost it; raw manure needs 120 days before harvest (USDA organic rule) |
| Wood ash (untreated wood) | Additive | — | Sparingly | Raises pH; cap at about 1 lb per cubic yard |
| Bread, rice, pasta | Green | — | Only in closed bins | Draws rodents in open piles |
| Weeds gone to seed | Green | — | Only in hot piles | Seeds survive below about 131°F |
| Meat, fish, bones | — | — | No | Odor, rats, raccoons, pathogens |
| Dairy, oils, grease | — | — | No | Coats material, blocks air, attracts pests |
| Dog & cat waste, used litter | — | — | No | Roundworm and Toxoplasma risk in food beds |
| Diseased or blighted plants | — | — | No | Home piles rarely hold kill temperatures evenly |
| Treated or painted wood, sawdust from it | — | — | No | Preservatives and paint residues persist |
| Coal or charcoal-briquette ash | — | — | No | Sulfur and additives harm soil |
| Black walnut leaves & hulls | — | — | No (for vegetable beds) | Juglone stunts tomatoes and peppers |
| “Compostable” PLA cups and bags | — | — | No | Certified for industrial facilities, not backyard piles |
The gray zone: items that depend on your setup
Several items are fine in one system and a problem in another. Citrus peels and onion skins compost normally in an outdoor pile but irritate red wigglers, so a worm bin should get them in small amounts only. Bread and cooked grains are harmless chemically, yet in an open pile they are a rodent invitation; a sealed tumbler handles them without trouble.
Weeds are the other judgment call. A pile that holds 131°F or higher for at least 3 days while being turned kills most weed seeds and plant pathogens, which is the same threshold the EPA uses for treating biosolids. Most backyard heaps never reach that across their whole volume, especially under 3 × 3 × 3 ft. If you cannot confirm the temperature with a $10–20 compost thermometer, bag seed heads and blighted tomato vines for the trash instead.
What sorting is worth in dollars
A pile shrinks by roughly half as it finishes, so it takes about 2 cubic yards of raw scraps and leaves to end with 1 cubic yard of compost. That yard would cost $35–55 as bulk compost from a landscape supplier, or $160–215 if you bought it as 27 bagged cubic feet. Against a DIY pallet bin (often $0–40) the math works in the first season. Against a $100–300 tumbler, it only works if you would otherwise buy bagged compost; bulk buyers need three or more years to break even.
| Setup | Upfront cost | Finished compost per year | Payback vs bagged | Payback vs bulk |
|---|---|---|---|---|
| Open pile or pallet bin | $0–40 | 1–2 cu yd | Under 1 season | Under 1 season |
| Plastic stationary bin | $40–120 | 0.5–1 cu yd | 1 season | 1–3 years |
| Dual-chamber tumbler | $100–300 | 0.3–0.7 cu yd | 1–2 seasons | 3–8 years |
The hidden cost is a ruined batch. If contaminated clippings or pet waste force you to dump a finished yard, the payback clock restarts, which is why the “no” column matters more than the “yes” column. Bin-by-bin details are in our compost bin and tumbler comparison.
Common mistakes, in numbers
- Accepting a neighbor's clippings or stable manure blind. Clopyralid and aminopyralid herbicides survive composting and can curl bean and tomato leaves at a few parts per billion. Ask what was sprayed in the last 12 months, or do a pea seedling bioassay in a pot for 2–3 weeks before spreading.
- Building a pile smaller than 1 cubic yard. Below roughly 3 ft on each side, a heap loses heat faster than microbes make it and stays at air temperature.
- Greens only. A bin fed nothing but kitchen scraps runs near 15:1, goes wet and sour, and smells within 7–10 days. Keep a bag of dry leaves next to it.
- Letting it dry out or drown. Microbes stall below about 40% moisture and suffocate above 60%. A handful should release a drop or two when squeezed, not a stream.
FAQ
Can I compost meat if I use a hot pile?
Home piles are not a reliable place for it. Even a hot heap has cool edges where meat rots slowly and draws rats and raccoons. Bokashi fermentation or municipal food-waste pickup handles meat and dairy far more safely than an open backyard pile.
Are coffee grounds green or brown?
Green. They look brown, but at about 20:1 carbon to nitrogen they behave like a nitrogen source, so balance each coffee can of grounds with two or three of leaves or shredded cardboard.
Is dog poop compostable?
Not in a pile headed for food beds. Dog and cat waste can carry roundworms and Toxoplasma that a backyard heap will not reliably kill. Extension services that address it at all recommend a separate digester kept away from edible crops.
Can I put grass clippings in compost?
Yes, if the lawn was not treated with a persistent broadleaf herbicide in the past year. Mix them in thin layers with leaves, because a solid 6 in slab of clippings goes anaerobic in days.
Related:
Educational information only. The sorting verdicts and dollar figures summarize public extension and EPA guidance for home compost used in gardens; check your municipality's yard-waste and food-waste rules, and test any compost made from outside manure or clippings before it touches a vegetable bed.