The potting "soil" in which container plants grow, is one of the most crucial factors behind the quality of their growth and development. The basic needs of plants are the same whether they grow in garden soil or in pots. But as the conditions created by containers are quite different, the potting soil, more correctly termed the potting medium, has to have different properties from that of ordinary garden soil.
Air and Water
The plants' root zone has to contain a proper balance between moisture on the one hand and air on the other. Garden soil, especially heavy, clay soil, if used in pots, invariably lacks sufficient air that the roots require for breathing. In such anaerobic conditions, the roots rot, causing the plant as a whole to wither and die.
Alternatively, a potting medium composed entirely of sand, would contain plenty of air in the root zone, but as it dries out so rapidly, would be unable to supply enough moisture for the roots to take up. As sand and soil do not mix well, it follows that an artificial potting medium is an essential precondition for obtaining satisfactory results with container plants. The question is, which type of medium is best?
Today, the various potting media available, attempt to create an optimal balance between air and moisture. Mixtures based on peat and processed substances like vermiculite are most commonly sold in nurseries and garden centers. More recently though, landscape professionals are turning to soilless growing media such as perlite, and because of its special properties, using it almost exclusively in pots and containers. There are a number of reasons for this.
Perlite provides for a virtual optimal balance between air and water. This means that it retains moisture for a reasonable period of time after wetting, but at the same time, contains sufficient air too. It is virtually impossible to over water plants that grow in perlite. It is also extremely light in weight, which is often a vital consideration in roof and balcony gardens. Another important property is the fact that it is chemically inert, which reduces the danger of the medium becoming excessively saline.
Environmental Considerations
Environmentally conscious people have become increasingly concerned about the degeneration of natural bog habitats throughout the world, as a direct result of peat extraction, of which the horticultural industry is a major consumer. Perlite can therefore be preferred on these grounds alone, although its extraction and processing also impacts the environment. Furthermore, being chemically inert, the plants are entirely dependent for their nutrition, on a steady supply of chemical fertilizer, thereby adding indirectly to the pollution of water sources.
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