Plants have certain requirements of the medium in which they are growing, whether it is indoors or out. They need a firm anchor­age for their roots, the correct pH level, and a read­ily accessible supply of air, water, and nutrients in an environment free from pests and diseases.

In the case of a medium for indoor use, this means that the potting soil has to be sterile, pH neutral (unless it is for acid-loving plants), firm enough to support the plant, but lightweight enough to hold air and able to supply the roots with water and nutrients, without letting the plant become waterlogged.

There are two main choices of soil mix: loam- based (which contains soil) and loamless (based on peat or a peat-substitute). Loam-based soils hold nutrients and moisture better than loamless ones, and are more stable, but they are heavier, and more suitable for older plants that are likely to be in their pots for some time, or floor-standing specimens that need the extra weight to prevent them being knocked over. Loamless soils are light, clean, and easy to handle, but are inclined to dry out (and dif­ficult to re-wet), and nutrients wash through them fairly quickly.

Soil additions

In order to tailor the soil mix to the plant, there are various substances that can be added to the basic ingredients, including perlite, vermiculite, sand, and grit, all of which serve to allow more air into the soil and speed up drainage.

Other ingredients can be used with the soil mix to cater for certain plants. Crockery shards (stones or pieces of broken clay pot) can be laid in the bot­tom of the empty container before planting to help speed up drainage if the plant will not tolerate wet soil for any length of time. Charcoal is often used in bottle gardens and terraria to absorb waste and keep the soil fresh. Sphagnum moss is another use­ful addition, which can be incorporated into the soil mix or placed over the surface of the soil to retain moisture.

Types of growing medium

Without the right medium, a plant will struggle to grow well. The consistency, pH. and content of the soil all play a part in the plant’s overall health. See the top line, below, for growing mediums, the middle line for drainage additions, and the bottom line for hydroculture mediums.

Growing mediums: peal, coir, moss, coarse sand, silver sand.

Drainage additions: crockery shards, pebbles/grit, perlite, vermiculite, charcoal.

Hydroculture mediums: dry crystals, gel, clay granules, hyrdoleca.