Compost is beloved by avid gardeners everywhere and the process by which it is created is magical. The scientific word for compost is humus and it is created by decomposition. Decomposition is the breaking down of organic matter to humus. Typically, the kinds of organic matter that are added to a compost heap are grass clippings, paper shreddings, cardboard, fruit and veg, twigs, leaves and wood chips, but organic matter is anything that is/was alive.
Humus is a wonder material for plants, often called “black gold”. It is very difficult to describe as its qualities vary depending on the organic matter it was derived from, but it is a dark, spongy substance, full of plant nutrients that improves aeration and water retention when added to soils. So how is it made?
We see many macro organisms such as worms, ants, mites, centipedes and fungi living in our compost heaps and although they play an important role in mixing up organic matter and breaking it into smaller particles, bacteria are the main organisms that do the work in decomposition. There are millions of species of bacteria and between them they are capable of digesting any organic matter.
Bacteria are divided into two groups, aerobic and anaerobic. Aerobic means with oxygen, anaerobic meaning without oxygen. Aerobic bacteria are vital in the composting process, whereas anaerobic bacteria can create toxins harmful to plants. It is important therefore to keep the compost heap aerated to promote aerobic bacteria and discourage anaerobic.
The good bacteria feed on nitrogen and carbon, which are the chemical elements that all organic matter is made from. Soft plant tissues, such as grass clippings, veg peelings and young leaves contain more nitrogen than carbon, whereas woody material such as bark, twigs, wood chip, paper and cardboard are higher in carbon than nitrogen. The aerobic bacteria need a balance of carbon to nitrogen to function properly (ideally around 40% C to 60% N).
In order to make good compost we need to carefully manage the decomposition process. The best way to manage a compost heap is to add layers different organic matter at a ratio of 40% carbon rich material and 60% nitrogen rich material, encouraging aeration by periodically turning/forking over. Carbon rich, woody materials such as shrub prunings are best chopped or shredded into small pieces to increase their surface area and speed up bacterial decomposition.
There is an art to making compost, but if you understand aerobic bacteria and give them what they need you’ll have no problem in producing your own black gold!