The term ‘substrate’ is also used and means the same thing. In the UK some people still use the term ‘compost’ in the same context, however a compost is technically the product of a composting operation (eg the compost heap at the bottom of the garden) and can therefore be a misleading word to use. A composted material may be a component of a growing medium but peat is not a product of composting and nor are many other ingredients we use. In Holland the term ‘potting soil’ is often used instead of ‘growing medium’
There are many different ingredients that can be used to make a growing medium; different parts of the world have developed media based on local availability of various raw materials. Such materials can be inorganic (eg rockwool, perlite) or organic (such as peat, bark). Growing media are often formulated from a blend of different raw materials in order to achieve the correct balance of air and water holding capacity for the plants to be grown.
Requirements of a growing medium:
Manufacturers also need growing media to be physically and chemically stable from the time of production until the time of use (this can be many months for retail products). The bulk density (weight) of the ingredients used is also important because this affects transport costs, a major part of the total cost of production and delivery to the end customer.