Compost – Carbon-to-Nitrogen Ratios

A couple of months ago I posted about Compost-What’s In and What’s Out.

However, it’s not enough that you throw your organics, food and yard waste into your compost bin (although it’s a good start!)  You must also be aware of the carbon to nitrogen ratio so as to keep your compost happy, healthy, and not stinky!  For best performance, the compost pile, or more to the point the composting microorganisms, require the correct proportion of carbon for energy and nitrogen for protein production. Scientists (yes, there are compost scientists) have determined that the fastest way to produce fertile, sweet-smelling compost is to maintain a C:N ratio somewhere around 25 to 30 parts carbon to 1 part nitrogen, or 25-30:1. If the C:N ratio is too high (excess carbon), decomposition slows down. If the C:N ratio is too low (excess nitrogen) you will end up with a stinky pile.

Below are the average C:N ratios for some common organic materials found in the compost bin. For our purposes, the materials containing high amounts of carbon are considered "browns," and materials containing high amounts of nitrogen are considered "greens."

Estimated Carbon-to-Nitrogen Ratios
Browns = High Carbon C:N
Ashes, wood 25:1
Cardboard, shredded 350:1
Corn stalks 75:1
Fruit waste 35:1
Leaves 60:1
Newspaper, shredded 175:1
Peanut shells 35:1
Pine needles 80:1
Sawdust 325:1
Straw 75:1
Wood chips 400:1
Greens = High Nitrogen C:N
Alfalfa 12:1
Clover 23:1
Coffee grounds 20:1
Food waste 20:1
Garden waste 30:1
Grass clippings 20:1
Hay 25:1
Manures 15:1
Seaweed 19:1
Vegetable scraps 25:1
Weeds 30:1

Note: Many ingredients used for composting do not have the ideal ratio of 25-30:1. As a result, most must be mixed to create "the perfect compost recipe." High C:N ratios may be lowered by adding grass clippings or manures. Low C:N ratios may be raised by adding paper, dry leaves or wood chips.  Many home gardeners will put up with a slight odor and keep some excess nitrogen in the pile, just to make sure there is always enough around to keep the pile "cooking!" Learn more about building a hot compost pile here.

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