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Composts
Garden soils are variable. The soil from one part of the garden may have different characteristics than soil from another part of the garden, dependent on its structure and texture, and so produce variable results. Garden soils may contain pest and disease organisms or seeds from unwanted plants. Garden soils are less pleasant to handle and use.
In times gone by, gardeners would have their own compost formulations. About eighty years ago, the John Innes Institute, named after a London property developer, left his fortune to horticultural improvement by experiment and research. After years of experimentation, workers came up with a range of composts that gave reliable results. These “John Innes Composts” were mixtures of loam (topsoil) that was heat treated, peat, sand and fertilisers designed to achieve optimum water-holding, air content and nutrients for different groups of plants.
Now, newer mixes have been specifically formulated for common purposes. They can be made up of many different materials like crushed bark, peat, sand, pumice, vermiculite, polystyrene beads and fertiliser in various proportions according to their use, many materials unknown eighty years ago. These newer materials are much easier to handle and do not need sterilisation.
It’s not a good idea to use a seed-raising or a cuttings mix for potting, nor is a potting mix suitable for cuttings or seed. Nothing untoward will happen, only, make every effort to get the best results possible. A good mix provides the ideal growing conditions for stage in growth and for specific plants’ needs. They will be light, easy to handle, retain the correct amount of air and moisture and provide adequate ground support and ease of root penetration.
Here are some of the characteristics of commonly used mixes:
Choose the best mix for the job. Read the labels and ask advice.
At garden centres, beginning gardeners can be confused by the range of artificial growing media available. Plants grow well in soil, so why not use that? We use a mix rather than ordinary garden soil for a number of very good reasons.. 
Seed raising mixes are fine and light to provide good contact between the medium and the seed. This makes sure that as much water as possible is able to reach the surface of the seed to help it germinate. There is no need for fertilisers in the mix because the seed provides its own nutrients and seedlings are moved on soon after germination.
Cuttings mixes provide support and contain large amount of sand. As they drain very easily, care must be taken that they do not dry out. Again, fertilisers are not added because cuttings only need to develop roots.
Potting mixes provide a permanent home for plants. They provide root support and contain a medium that retains water easily, air and fertiliser. It is important that there is a great enough volume so that plants can be left for a time without danger of drying out. Usually a pelleted form of fertiliser is used, coated with a skin that breaks down over time to release nutrients. Often, fertility is supplemented by dissolving fertilisers in water.