Alternative growing media
In addition to open ground or greenhouse borders, plants may be grown in pots, troughs, bags and other containers where restricted rooting makes more critical demands on the growing medium for air, water and nutrients. Soil is an inappropriate material to use in containers as it tends to collapse when kept wet; try watering a pot full of soil and note that it is not long before the container is only half full of soil. Soil is replaced in this situation by alternative growing media generally called composts. These materials are also called plant substrates, plant growing media, or just 'mixes' or 'media'.
Compost ingredients need to ensure adequate air space after wetting, with a stability to withstand prolonged watering without collapse. The need for the material to have good water-holding capacity depends on the irrigation system to be used. The nutrient content of the soil alternative needs to be allowed for and it is often advantageous to use one that has none as they can be added more precisely. The material should also be 'partially sterile' (free from pest and diseases) and free from toxics. Increasingly, in intensive production, the preferred alternative to growing in the soil is to use hydroponics (see p394). The weaknesses of soil for sportsground construction leads to its replacement with alternatives, e.g. graded sand on golf greens (see p397-8).
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