Inorganic growing media
Plants naturally grow in the soil, however a wide range of “artificial” media can be used instead. Many ingredients for these media, particularly for container compost mixes, are organic matter. Below we outline some of the inorganic ingredients available to play various roles in plant growing media:
Sand, grit or gravel
These are available in various grades (sizes) so can be tailored to your requirements. They are generally added to increase the density of the growing media (eg into container compost mixes for very tall plants to help weigh down the container) or to make compost more freely draining (although sands can clog up the compost so should be added with caution, particularly fine sands which are only generally used in seed compost mixes). When using sand it is important to use horticultural sand as this will not have a high lime level which can increase the pH of the growing media. The sand, grit or gravel should always have been washed before use to remove any potential contaminants (if purchasing products for horticultural purposes then this is likely to have been done already). Grit or gravel is often added to compost for alpine plants and cacti.
Perlite
This is a volcanic mineral which is crushed then heated to a very high temperature, causing it to expand and form a white, very lightweight, porous aggregate. It is often mixed into container compost mixes to increase aeration (eg in cutting compost) and has the additional benefit that the rough texture of the surfaces retains water better than, for example, gravel or polystyrene balls. This retention of water can also help in peat compost mixes which can be difficult to re-wet if they have been allowed to dry out. The light weight of perlite also makes it useful for containers which you wish to move around from time to time.
Vermiculite
This mineral is similar to mica and has been expanded by twenty times through the water contained within it being rapidly heated and converted to steam. It is used in growing media to improve aeration, water retention and to retain nutrients. It is often used in plant propagation and different grades are available (although most garden centres will only sell one type). If used in long term planting (eg in permanent containers) it tends to break down and go soggy, so perlite would be a better solution in those situations.
Rockwool
This is a material made from spinning crushed and melted rocks into threads, much like the fibreglass material often used in roof insulation. This results in a growing media which is sterile and inert with good aeration and water retention properties. It tends to have a high (alkaline) pH, although this can be overcome by mixing with more acidic materials or by watering with a slightly acidic solution. It is available as granules for providing aeration in compost mixes, but is generally used in slab form as a complete growing media. Nutrients are then provided within the water. Using just rockwool to grow plants is known as ‘aggregate culture’, although it is also often considered to be a type of ‘hydroponics’ (although, strictly speaking, hydroponics are where a water/nutrient solution alone is used and the plants simply suspended in it).
Expanded polystyrene balls or flakes
These will help to increase aeration of compost mixes, however they don’t retain any water and therefore will reduce the overall water retention capacity (they are often used in winter compost mixes where waterlogging could be a problem). The light weight of the polystyrene can prove helpful in compost mixes for large containers which are moved frequently, however it tends to stick to surfaces (eg hands!) and blows away easily in the wind.
