Fundamentals of Composting

— Written By and last updated by

Decomposition is a vital process that allows nature to reallocate matter into different parts of the biosphere. Composting, or the controlled (and often accelerated) decomposition of organic materials, is a way to observe this process in action. Composting turns yard wastes and kitchen scraps into a rich soil amendment. When incorporated into garden soil, compost adds organic matter that improves soil structure, boosts nutrient content, encourages healthy root development, and suppresses diseases and pests. When applied as a mulch, compost suppresses weeds, conserves moisture, and reduces erosion.

Compost can be made in piles, or for the sake of neatness and to control pile size, in frames or bins. Compostable materials are classified as ‘browns’ or ‘greens’. Browns are carbon-rich materials that provide energy to the microorganisms responsible for decomposition, and provide physical structure to the compost pile. Brown materials include fallen leaves, sawdust, paper, cardboard, twigs, tree bark, straw, etc. Greens are nitrogen–rich materials such as fruit and vegetable scraps, coffee grounds, tea bags, and herbivore animal manure.

The composting process is most efficient when the ratio of carbon to nitrogen containing materials is around 25:1 to 35:1. If the carbon to nitrogen (C:N) ratio is too high, then decomposition is slow; if the C:N ratio is too low, then nitrogen is lost in the form of ammonia gas. For information on the C:N ratio of different materials, consult the NC State University Extension publications at the link below. A good rule of thumb, however, is to combine browns and greens in a 2:1 ratio.

Avoid adding dairy products, meats, eggs, bones, and grease, as these will smell unpleasant and attract rodents and flies. Do not add cat litter or dog waste because they contain parasites harmful to humans. It is better not to add diseased or pest-infested plants, as some might survive the composting process and spread back into your garden.

An ideal compost bin or pile should be about four feet in each dimension to efficiently retain the heat generated in the decomposition process. Piles that are below 131ºF won’t kill human pathogens, and below 145ºF won’t kill most weed seeds. However, piles that are too hot (over 160º F) may kill desirable microbes. Shredding some of the compostable materials increases the surface area and thus decomposition rate, but if particles are too small, the pile compacts and limits aeration. Aeration is also the reason compost piles need to be turned with a shovel or fork on a weekly basis. Mixing encourages oxygenation of the pile, which promotes the aerobic decomposition needed to produce desirable compost. Low oxygen conditions will favor anaerobic microbes that slow the rate of decomposition and produce unpleasant odors as metabolic byproducts.

A compost pile often fails to decompose properly because it is too dry. Add water periodically so that the pile is about as wet as a wrung-out sponge. If it is too wet, add additional dry brown materials (e.g. dry leaves or sawdust). In short, insufficient aeration, too much compaction, and too much water will turn a nice, earthy-smelling compost pile into a stinky pile of rotting plant debris.

For more information on backyard composting and vermicomposting, visit the NC State Extension Composting portal.

N.C. Cooperative Extension, Chatham County Center and the Master Gardener℠ volunteers of Chatham County are offering a Fundamentals of Composting Workshop on Saturday, Nov. 2, 2019, from 10–12:30 p.m. in Pittsboro. For more information, see the Chatham County website.

Matt Jones is Horticulture Agent for North Carolina Cooperative Extension in Chatham County.