Physical aspects

Abstract

From the physical viewpoint, plant substrates can be regarded as comprising solid matter interspaced with voids or pores. The substrate has two main physical functions, i.e. to provide the anchorage that enables the plant to support itself, and to regulate the supply of water and oxygen to the roots. The growing of plants in small, shallow containers creates two physical problems. First, the volume of substrate and water available to each plant is small by comparison with the volume available to field crops. Secondly, because the substrate is in a shallow layer, with the base of the container breaking the continuity present in a normal soil profile, a ‘perched’ water table is created (Spomer 1975a). This prevents the normal amount of drainage and makes the media wetter after irrigation than it would otherwise be, thus rendering the plant liable to inadequate aeration after every irrigation. The other extreme condition of a shortage of water can also occur very quickly under conditions of high evapotranspiration.