Decomposition of plant material is nature's way of recycling nutrients. Decomposition is carried out by a range of creatures, such as worms and various insects, and microorganisms including fungi and bacteria. If this process is done in the presence of oxygen, the compost does not smell badly. If air is excluded, the breakdown takes place anaerobically and it smells. If the compost heap is too dry, the rate of decompostion is slowed down. If it is too wet, air is excluded, it is slowed down and it smells.
A compost heap or bin simply faciliates the natural process of decompostion. Any plant material, green or withered, begins decomposition immeditately it is taken off a living plant and this process will take place whether it is in a heap or not. Placing it in a heap helps to keep it moist and to keep it tidy and available for use when rotted.
The key to success is to have a mix of green and brown withered material. This facilitates the entry of air, whereas only green material, particuarly grass mowings, can clump together and exclude air. It is a good idea to keep a heap of withered brown material available for mixing as the green material becomes available. It is also a good idea to have more than one, possibly three or four compost heaps on the go. One can be ready for use, one full and decomposing and one filling.
Turning compost helps the decompostion and evens out the process through the heap of material so that it is not just composted at the centre and not at the cooler sides. Decomposing plant material heats up and this is a good sign as it helps to kill weed seeds and pests. A compost bin helps to keep the heap tidy, and moist, but can often be too small. There is no absolute need for compost additives but they can help to speed up decomposition, as can a shake of general ferlilizer. Soil can be used to cap a compost heap and keep it tidy and moist, but it tends to introduce weed seeds.
There is no great mystery about composting ... it is a natural process that happens anyway, but it can be enhanced.