Composting has a reputation for being fiddly. It isn’t. Almost every failed pile I have ever been asked to look at failed for the same reason, and it is never the reason people expect.
The pile is unbalanced.
Get the carbon and nitrogen roughly right, keep it as damp as a wrung-out sponge, and give it air. That is the whole job. Get it wrong and you end up with a slimy tub of kitchen waste that smells like a bin and never breaks down. I have done both, more than once, and the difference between them is about five minutes of attention a week.
This guide covers what is actually happening inside a pile, the one ratio that decides whether yours works, what to put in and what to keep out, and how to fix the specific problems that stall most home composters.
A heap left alone in a corner will compost eventually. Everything organic does. What a bin buys you is speed, tidiness and control. A pile you manage properly can be finished in 6 to 12 weeks. A neglected open heap takes 6 months to 2 years. That gap is entirely down to management.
What is actually happening in a compost pile
Composting is the breakdown of organic matter by microorganisms, mostly bacteria and fungi, helped along by worms and beetles. Those organisms need the same four things you would need to run a small factory. Food, water, air and warmth.
The warmth is not incidental. As microorganisms work they generate heat, and decomposition runs fastest between 90°F and 140°F (32–60°C). Below 90°F the process slows markedly. Above 140°F most of the organisms you want cannot survive, and past 160°F (71°C) a pile can effectively sterilise itself and kill off the population doing the work.
That narrow band is why an unbalanced pile stalls so predictably. Too little nitrogen and it never heats up. Too much and it overheats, goes anaerobic, and produces the rotten-egg smell that gave home composting its bad name.
The one number that decides whether your pile works
Composting microorganisms use carbon for energy and nitrogen for growth. The balance between the two in your feedstock is the carbon-to-nitrogen ratio, written C:N.
The ideal starting point is about 30 parts carbon to 1 part nitrogen by weight. In practice microbes work perfectly well across a range, and most extension guidance puts the workable band at 25:1 to 40:1. Finished compost lands somewhere around 10:1 to 25:1, which is a useful signal that it is done.
Here is the catch. Almost no single material sits at 30:1. Composting is a mixing job, not a piling job.
| Material | C:N ratio | Type |
|---|---|---|
| Sawdust | 100–500:1 | Brown |
| Cardboard (shredded) | ~250:1 | Brown |
| Newspaper (shredded) | 150–200:1 | Brown |
| Wood chips and bark | 100–500:1 | Brown |
| Straw | 40–100:1 | Brown |
| Autumn leaves | 30–80:1 | Brown |
| Pine needles | 70–80:1 | Brown |
| Coffee grounds | 20:1 | Green |
| Grass clippings | 15–25:1 | Green |
| Fruit and vegetable scraps | 15–20:1 | Green |
| Manure (poultry, fresh) | 10:1 | Green |
You do not need to calculate anything. Two rules of thumb will carry you. By volume, mix two to three parts browns to one part greens. By weight, aim for roughly 30 parts brown to 1 part green. That sounds dramatic until you realise a single bag of grass clippings is a tiny weight of nitrogen-rich material next to a pile of dry leaves.
Watch the difference between volume and weight, because it catches people out. Leaves run about 40:1 and grass clippings about 20:1, so an even 50/50 split of those two by weight lands almost exactly on 30:1. By volume you need far more leaves, because they are bulky and light for their mass.
What to put in, and what to leave out
Greens are soft, moist and nitrogen-rich. Grass clippings, kitchen peelings, fruit and vegetable scraps, coffee grounds, tea leaves, spent annual plants, and fresh manure from herbivores.
Browns are dry and woody, and carbon-rich. Fallen leaves, chopped prunings and hedge trimmings, dead stems, straw and hay, shredded paper, torn-up cardboard with the tape removed, and wood chips.
Two practical warnings matter more than any theory here.
Do not let grass clippings dominate. Clippings are nitrogen-rich and wet, and on their own they compact into an airless, slimy mat that smells awful. If you have a lot of clippings and not enough browns, stack the clippings beside the bin until you have collected enough dry material to mix through properly.
Chop the big stuff. Long stems, prunings and cardboard break down dramatically faster when shredded, because you are multiplying the surface area available to microorganisms. This is the single best speed upgrade available to a home composter and it costs nothing but a pair of secateurs.
What to leave out, and why. Meat, fish, dairy, fats and oils smell as they rot and attract rats, raccoons and dogs. Cat, dog and pig manure can carry pathogens a home pile may not reliably kill. Persistent perennial weeds such as bindweed, ground elder or comfrey are best avoided unless you are confident your pile genuinely reaches the temperatures that destroy them.
One easy mistake: many teabags contain plastic and will not break down. Check before you tip them in.
Choosing and siting a bin
A bin is not strictly necessary but it is tidier, warmer and much easier to manage than an open heap. A good one does four things. It keeps rain out, holds some warmth, drains freely and lets air in.
Size matters far more than style. Build the pile to at least 3 cubic feet. Anything smaller cannot hold enough heat to get going, which is the most common reason a well-made pile sits cold for months. At the other end, piles wider than about 5 feet stop letting air reach the centre. The middle goes anaerobic while the outside does nothing.
On siting, warmth speeds decomposition so a sunny spot is technically better. In reality most gardeners tuck the bin out of sight and accept a slightly slower pile. I do the same. What matters more is an open base, so worms and soil organisms can move in and out freely.
Keeping it going
Once the pile is built, three things need your attention. Nothing else does.
Moisture. Squeeze a handful. Your compost should feel like a wrung-out sponge. Too wet and the air spaces fill with water, the pile turns anaerobic and it stinks. Fix that by turning it and adding dry browns such as shredded paper or sawdust. Too dry and microbial activity simply stops. Water it as you turn.
Air. Turning is what admits oxygen. Without it you get anaerobic conditions and an unpleasant smell. A garden fork roughly mixed through the heap is enough. Weekly turning is ideal. Every few months is realistic, and it still works, just slower. Turning earns its keep twice over, because it also lets you feel the moisture and see whether the pile is heating.
Temperature. A working pile runs hot in the middle. A compost thermometer tells you whether to turn, above 140°F, or whether a cold pile is finished rather than stalled. If the pile is properly moist and turning it produces no heat rise, it is probably done.
When something goes wrong
Nearly every compost problem traces back to one of four causes. This table is worth keeping.
| Symptom | Most likely cause | Fix |
|---|---|---|
| Rotten, sour smell | Too wet, not enough oxygen | Turn the pile; mix in dry browns such as shredded paper, sawdust or straw |
| Ammonia smell | Too much nitrogen | Add high-carbon browns and turn, to keep the nitrogen in the pile |
| Pile never heats up | Too small, too dry, or short of nitrogen | Build it past 3 cu ft, water it, and mix in greens or manure |
| Pile over 140°F | Too much nitrogen, or too large | Turn it and reduce the size of the pile |
| Warm only in the centre | Pile too small to insulate itself | Collect more material and rebuild it larger |
| Rats, raccoons, insects | Meat, fish or fatty scraps in the pile | Remove them, bury food scraps in the centre, use a closed bin |
| Very slow, little happening | Particle size too large | Chop, shred or grind the material and rebuild |
How to tell it is finished, and how to use it
Finished compost is dark, crumbly and smells earthy. No recognisable food scraps, no whole leaves. If you can still identify what went in, it is not done.
It should also hold roughly 40% moisture. Squeeze a handful and you want it damp, with no more than a drop or two of liquid. Screen out anything coarse with quarter-inch mesh and put the leftovers back into the next pile, where they reintroduce microorganisms to fresh material.
One warning catches people out. Do not put unfinished compost in the garden. Material that is still actively decomposing ties up soil nitrogen as it breaks down, which leaves your plants short of the very nutrient they need. If your compost is only partly broken down, leave it to finish or add extra nitrogen to compensate.
Use it as a surface mulch to hold moisture and suppress weeds. Fork it lightly into beds to improve structure. Mix it with garden soil and sharp sand to make your own potting mix. Just do not treat it as fertiliser. Home compost is a soil conditioner first, and its nutrients are largely in organic forms that plants cannot take up straight away.
A note from our own garden
The habit that has saved me the most wasted effort is stacking clippings separately until I have enough dry material to mix through them in balance. Two minutes of work. It is the difference between a pile that is ready in weeks and one I have to rebuild.
Second habit: keep a pair of secateurs by the bin. The biggest jump in speed I have ever had came from cutting things up before they went in, not from any gadget I bought.
And if your pile has stalled, do not start over. Diagnose it with the table above, fix the one thing that is wrong, turn it, and give it a fortnight. In thirty years I have not yet met a pile that could not be rescued.
Sources and further reading
- Royal Horticultural Society — Composting advice (green and brown balance, the 25–50% green guidance, what a good bin must do)
- RHS — Four steps to perfect compost, Adrian Thorne, Horticultural Advisor (the 1:2 green-to-brown ratio and the wrung-out sponge test)
- Cornell University — Compost Chemistry and the C/N ratio calculator (30:1 ideal, about 10:1 finished)
- Cornell Cooperative Extension — Compost temperature fact sheet (the 90–140°F working range and sterilisation above 160°F)
- University of Maryland Extension — How to Make Compost at Home (the 25:1–40:1 band, and brown and green material lists)
- Clemson University Home & Garden Information Center — Composting (C:N values and the 3 cu ft minimum pile size)
- University of California Agriculture & Natural Resources — Carbon and Nitrogen in Composting (balancing the ratio by weight)
- UC Cooperative Extension — Composting 101 Troubleshooting Guide
- Kansas State University Research & Extension — Composting Troubleshooting Guide
- University of New Hampshire Cooperative Extension — Home Composting (timelines, screening, finished moisture)
- Oregon State University Extension — Gardening with Compost (temperature range and aeration limits)
- University of Maine Cooperative Extension — Guidelines for Using Manure on Vegetable Gardens (pathogen risk, and which manures to avoid)
Next in this series: what soil is actually made of and why organic matter matters, and why some piles smell and others do not.