When you look at a potting mix, you will see that it contains at least two materials, and sometimes more:
Bark – aged pine bark is a very common ingredient of potting mixes. Bark mixes are likely to drain well perhaps too well for many plants. Mixes with lots of coarse bark chips are usually cheaper than other mixes. Fine bark chips are preferable for most plants because they hold water better.
Sawdust – coarse, aged sawdust is used for aeration and water retention. It is an inexpensive material, and is often used in cheaper mixes.
Sand – coarse particles are often added to mixes to improve aeration and drainage. It adds stability and weight to the mix. A very sandy mix is likely to drain well and is best used for plants that prefer dry soil (eg. cacti).
Vermiculite – a small, soft flaky mineral that is used to improve water retention. Often used in more expensive mixes.
Perlite – sterile, porous white material used for water retention. Coarse perlite has better aeration than that with finer particles. Often used in more expensive mixes.
Peat – when moist it has a dark-coloured, spongy appearance. Peat is used to retain water in the mix but if it dries out the mix is very difficult to re-wet. Mixes that contain peat are more expensive. Environmental issues are now affecting the amount of peat used in horticulture.
Soil – not commonly used in potting mixes but sometimes used by nurseries that bag their own mixes. Soil is a highly variable ingredient that should only be used in small quantities in the mix.
Other materials – mushroom compost, peanut shells, blended manures and other composted plant materials are sometimes included.
Fertiliser pellets – these appear as small round balls dispersed through the mix. These will release nutrients slowly to the plant roots – usually for up to 3-6 months.
Water-retaining crystals - are very often added to potting mixes which is ideal for those of us who don’t always remember or who are too busy to water the plants. These are usually added to mixes targeted for containers and hanging baskets. They are small transparent pellets of gel which swells as it absorbs water.
Different mixes are needed for different jobs
Indoor container plants – humidity is a requirement of indoor plants. Most indoor plant mixes will be high in materials that will retain moisture and promote humidity around the plants.
Baskets – these are prone to drying out so good basket mixes contain peat or other water-retentive material; frequently may contain water-retaining crystals or gel:
Special mixes – There are a number of special mixes available for specific plant species such as Cacti and Orchids.
When selecting a potting medium consider all of the following:
What plants do you need it for? Will one mix be suitable for all your plants or will you need different mixes for different plants?
What type of pots will you be using – plastic, terracotta or a hanging basket
Do you want a mix that contains fertiliser and water-retaining crystals?
Does it have a government of industry ‘Approved Standards’ logo?
How much are you prepared to spend? Top quality mixes cost more.
If you cannot find exactly what you require, you can always mix the available potting mixes or add ingredients to make your own special purpose mix.
Waxy layers on organic material repel water. This can make some soils repel water when they are dry (eg. a mix containing a lot of peat moss will usually absorb water if wet but if it totally dries out, water may run off the surface and down the sides of the pot leaving the root ball dry).
Water repellence can be reduced by:
Adding coarse sand
Applying wetting agents (ie. synthetic surfactants).
Wetting Agents are similar to soaps, however be aware that dish washing liquids do not work very long as they biodegrade. Many wetting agents on the market become useless after a short period.