This is almost certain to come up – the RHS is clearly very keen for everyone to make compost.
You will need to describe how you start to build compost heap by choosing a site where your heap can rest directly on the soil so that soil organisms can get in to the material. You should also talk about how the site is sheltered so that it does not get too cold or wet.
The material is added in layers of about 15 cm or so (note: all measurements for the exam are required to be in metric), then a thin layer of soil is added for the microorganisms it contains, before another layer of material is added. The heap should be kept covered and can be turned occasionally to aerate it.
The choice of material for composting is important as the ratio is 15-30 : 1 of carbon to nitrogen. High carbon content material includes newspaper, straw, woody prunings, fallen leaves. High nitrogen content material includes grass cuttings, annual weeds, kitchen peelings.
It is best not to use weeds, plants with developed seed heads, diseased material, woody material or too much grass. It should have a ratio of 25 – 50% green material with the rest being well shredded brown material (leaves, twigs, newspaper).
Composting requires air, water and soil organisms. It uses nitrogen provided by the green waste, so composting activators (basically nitrogen fertilizers) can be added if necessary. Water can also be added if the heap seems too dry.
Temperature speeds up the decomposition and a natural heap will get warm. The size of the heap will determine this as a large heap has a smaller surface area relative to a small heap so it will not lose heat as much from the surface. A small heap can be insulated although this might reduce airflow.
Hotbeds
Very victorian this. But it came up in the exam I sat in June 2019 and it may be making a bit of a comeback in some circles.
Fresh manure is built up into a heap and left for a couple of days. It is then turned and water added if it seems a bit dry. After a couple of weeks, it is ready and either made into a heap or placed in a special container with retaining sides. A layer of soil of about 15 cm depth is placed on top of the manure and the plants are planted in to this soil. The manure is simply there to provide warmth as it decomposes. Sometimes a glass coldframe is put on top of the plants as well. [The specialist containers often have the coldframe built in to the top, with sufficient depth to put the manure and the soil in the bottom]. The manure gives off heat for about 2 or 3 months. It can then be dug into a bed or used as a mulch.
Wormeries
These use tiger, brandling and redworms to break down kitchen waste and are generally quite small. It is a container with a plate across it through which water can drain. There is a tap to drain off the liquid from the bottom part of the container.
A bedding layer of 8 cm of coir is placed on the drainage plate and the worms are added. A layer of 8 cm of material to be digested is then added, and a piece of wet newspaper spread on top to keep the worms damp and to exclude light. The material to be digested should be chopped up small and can be vegetables, leaves, bread etc.
The worms digest the material, and any liquid produced drains down to the bottom section and can be used as a fertilizer. Any solid material can be used as compost.
Wormeries need to be kept at 20 to 25 celsius and pH 6 to 8.
Gardening studies 