The characteristics of the container affect the root environment, as does
the standing-out area. There is an enormous range of containers used to
meet the many different requirements of growing plants (Figure 12.2). Clay pots are porous and water is lost from the walls by evaporation.
Consequently clay pots dry out quicker than plastic pots, especially in
the winter and, although air does not enter through the walls, this can
help improve air-filled porosity. The higher evaporation rate also keeps
the clay pots slightly cooler, which can be beneficial in hot conditions.
Likewise the contents of white plastic pots can be as much as 4°C lower than in other colours. Pots of white or light green plastic can transmit
sufficient light to adversely affect root growth and encourage algal
Biodegradable containers such as those made from paper have become
popular because they can be planted directly. Some materials decompose
more rapidly than others and there can be a temporary ‘lockup’ of
nitrogen, but most peat containers are now manufactured with added
available nitrogen. It is essential that these containers are soaked and
surrounding soil is kept moist after planting or the roots fail to escape
from the dry wall.
The air to water characteristics of the mixture in the container depend
not only on the nature of the contents, but also on the characteristics
of the base on which the container stands. If containers are stood out
on wire mesh or on stones, relatively little water leaves so the oxygen
content remains poor. It is also important to retain contact between
the compost and the standing out material through adequate holes
in the base, whether to help drainage or to ensure the uptake of
water if irrigated from below.
Blocks are made of a suitable compressed growing medium into which
the seed is sown with no container or simply a net of polypropylene.
Aeration tends to be poorer than in pots, but the high surface area helps
make this a successful means of growing some vegetables on a large
scale. One type of block comes in the form of a dry compressed disc
that expands quickly on soaking ready to receive the seed in the shallow
depression in the top surface. This technique has been replaced in large
measure by the use of rockwool blocks, particularly when these are to be
grown on in rockwool modules (see Fig. 22.3).
Increasingly, traditional seedbed, bare-rooted or block transplant
techniques have been replaced by raising a wide variety of plants in
modules. A module is made by adding a loose growing medium mix
to a tray of cells. The cells are variously wedge or pyramid-shaped, so
designed to enable a highly mechanized transplanting process to be
used. Fine, free-flowing mixes of peat, polystyrene or bark are used to
fill the cells, which have large drainage holes and no rim to hold free
water. Roots in the wedge-shaped cells are ‘air-pruned’ as they reach the
edge of the cell, which encourages secondary root development. ‘Plugs’
are mini-modules in which each transplant develops in less than 10 cm³
of growing medium and are used for bedding plants, as well as vegetable
production. The rate of establishment is largely determined by the water
stress experienced by the transplant. Irrigation of the module or plug
is found to be more successful than applying water to the surrounding