The Science of Composting
Pete Scales MAg BSc (Hon)
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, providing fertilization and water
conservation and an effective means to kill off soil pathogens. So, 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 and carbon, 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 is made up of four distinct stages – that we now know 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 back yard pile or high end
commercial facility) or up to a year for an ignored back yard 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 (or not) we are or the size of pile or bin you choose. As a general rule, the larger the pile,
the quicker it will heat and maintain high temperatures (typically 55° and 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
as the center cools, the pile should be turned twice a month to ensure continued heating and
decomposition of the entire. 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
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 see:
Another interest of this author is the processing of organic industrial hemp.