The Science of Composting

Farmers are an innovative bunch, cottoning onto gifts that mother nature sends their way. For as long as humans have been farming and later on, when backyard gardening became pleasurable, composting has served as an important means to improve soil organic content, provide fertilization and water conservation, and to kill off soil pathogens. We have used this biological decomposition process for millennia but did not truly understand how it works, until now.

Composting is an aerobic process – in this case, microbes burning a carbon energy source in the presence of oxygen – and using most any organic matter including food waste, manure, leaves, grass clippings, and in more recent times, municipal bio solids. The result, compost - a light, spongy, soil-like material, carbon dioxide, water and heat, lots of it.

In addition to oxygen, the decomposing microbes need nitrogen, water, carbon, and a source of energy, usually in the form of sugars. Oxygen is provided by turning or aerating the pile. Water is added for moisture, but not for wetness. Nitrogen sources include vegetable and fruit scraps, lawn and landscape trimmings, and manure from grass eaters. Sources of carbon include shredded newspaper and cardboard, leaves, straw and wood shavings. And depending on the source of waste, the bacterial and fungal species are specific to the waste type.

Ancient farmers knew that composting has four distinct stages – we now know these are related to microbiological and temperature phases termed mesophilic, thermophilic, cooling, and maturation. These phases can complete in as little as two months (for a well-managed backyard pile or high-end commercial facility) or up to a year for an ignored backyard pile.  During the four phases of the composting cycle the types of microbes change in succession much like a maturing forest.

The simple beauty of composing is that decomposition of organic wastes will occur no matter how experienced you are or the size of pile or bin you use. As a general rule, the larger the pile the quicker it will heat and maintain high temperatures (typically 55° - 71° C). When forming the pile, it is best to thoroughly mix, not layer, the materials. The pile will reach high temperatures at the core and since the center will cool, the pile should be turned twice a month to ensure continued heating and complete decomposition. A pile 1.5 meters high (or bin 1 cubic meter in size) is large enough to generate sufficient heat for decomposition, yet small enough to allow air movement into the center of the pile.

Recently, a landmark study titled Changes in Bacterial and Fungal Communities across Compost Recipes, Preparation Methods, and Composting Times put a lot of the unknowns about compositing to rest. The year long experiment, based out of a municipal compost facility in Vermont, studied three different compost recipes from four different compost techniques, including vermiculture – worm composting. In order to identify the various microbial players, historically a very difficult proposition in an environment where increasingly high temperatures killed off microbes, this group conducted DNA-gene sequence identification on all microbes and, in particular, bacteria and fungal species. The result was the first ever identification of assisting microbe communities, their preferred host types (wood, manure or grasses), in each of the four composting phases.

To check out this particular study visit:

Changes in bacterial and fungal communities across compost recipes, preparation methods, and composting times

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