Compost Calculator

The two-parts-browns-to-one-part-greens rule is repeated in every gardening book, and it is a reasonable starting point for exactly one pairing: dry autumn leaves and fresh grass clippings, measured loose by volume. Change either material and it stops being reliable.

The reason is that "brown" and "green" are compressing an enormous range into two words. Chicken manure sits at about 10:1. Sawdust sits at about 500:1. Both are ordinary compost inputs, and one is fifty times more carbon-rich than the other. Straw at 80:1 and dry leaves at 60:1 are both browns, and swapping one for the other changes the nitrogen requirement by a third.

The second problem is water, and it is the bigger one. A material’s carbon and nitrogen are in its dry matter, and fresh grass clippings are four fifths water. Thirty pounds of grass on the heap contributes six pounds of dry matter. Forty pounds of dry leaves contributes twenty-six. The barrow-load intuition that the two are comparable is off by more than a factor of four.

Weighing the inputs and working on dry mass removes both problems, and the answer it gives is usually that the pile needs far more nitrogen material than it looks like it should.

Composting is bacteria eating, and the ratio matters because of what they are building.

Micro-organisms use carbon as an energy source and nitrogen to build protein. The proportions they need are roughly thirty parts carbon per part of nitrogen once respiration losses are accounted for — which is why 25 to 30:1 is the target and not some other number.

Above about 35:1 the nitrogen runs out first. The bacteria have plenty of fuel and nothing to build cells with, so populations stay small, decomposition is slow, and the pile never generates enough heat to matter. This is the failure mode behind almost every heap that sits unchanged for a year.

Below about 20:1 the carbon runs out first, and the surplus nitrogen has nowhere to go. It leaves as ammonia — which is both the smell of a badly built pile and the loss of exactly the nutrient the compost was supposed to deliver. A grass-clippings-only heap does this within a day.

Get the ratio right and the population explodes. Temperature climbs to 130–150°F within a few days, thermophilic bacteria take over from the mesophilic ones that started, and the material breaks down in weeks rather than seasons.

A perfect C:N ratio in a pile that is too small, too wet or too dense will still do nothing.

The practical argument for a hot pile rather than a cold heap is sanitation, and it has a number attached to it.

Three consecutive days above 131°F kills most weed seeds and plant pathogens, and it is the basis of the standards applied to commercial compost. A cold heap does not reach it, which is why cold-composted material reliably grows a crop of whatever went into it. Turning matters here too: seed on the outside of the pile never got hot, so it has to be moved to the middle at least once.

Some things do not belong in a garden pile regardless of temperature. Meat, fish, dairy and cooked food attract rodents and go anaerobic. Dog and cat faeces carry parasites that survive ordinary composting. Diseased plant material is a gamble unless you are confident the pile gets properly hot — and most do not, consistently.

Two less obvious ones are worth knowing. Black walnut leaves and bark contain juglone, which persists through composting and suppresses tomatoes, peppers and several other crops. And manure or hay from an unknown source can carry aminopyralid and clopyralid — persistent broadleaf herbicides that survive both the animal and the compost pile, and that will deform or kill tomatoes and beans in the following season. If you bring in manure, test it first by sprouting a few beans in a pot of it.