|Figure 22.1 Hanging baskets are
both popular for the garden, but
are also used in public areas by
Local Authorities and businesses
make a good impression
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.
The weaknesses of soil for sportsground construction leads to its
replacement with alternatives, e.g. graded sand on golf greens.
Air-filled porosity (AFP)
The importance of supplying water to plants in a restricted root volume
is usually understood, but the difficulties associated with achieving it
whilst maintaining adequate air-filled porosity (AFP)
are less well
appreciated. Roots require oxygen to maintain growth and activity. As
temperatures rise the plant requires more, but the amount of oxygen that
is dissolved in water decreases. Even in cool conditions, the oxygen
that can be extracted from the water provides only a fraction of the
roots requirements. So, unless the plants have special modifications
to transport oxygen through their tissues, as in aquatic plants, there
has to be good gaseous movement through the growing medium.
Many large interconnected pores allow rapid entry of oxygen (see soil
structure). Creating successful physical conditions depends on the use of
components that provide a high proportion of macropores.
It is generally considered that 10–15 per cent AFP is needed for a wide
range of plants. Azaleas and epiphytic orchids require 20 per cent or
more, whereas others, including chrysanthemums, lilies and poinsettia,
tolerate 5–10 per cent AFP and carnations, conifers, geraniums, ivies
and roses can be grown at levels as low as 2 per cent.
Ensuring that a growing medium in a container has adequate air-filled
porosity is made difficult because water does not readily leave the
container unless it is in good contact through its holes with similar-sized
pore spaces as when placed on sand or capillary matting. However, when standing out on gravel or wire the water will cling to the particles
in the container (see surface tension). This can be tested by
fully watering a pot of compost, holding it until it has finished
dripping then touching the compost through a hole; normally a
stream of water will run down your finger. Furthermore, unless stood
out on appropriate material, the lower layers of the compost remain
saturated (i.e. no air) irrespective of the height or width of the container.
This makes it particularly difficult to get good aeration in shallow trays,
modules and blocks. (You may understand this better if you fully wet
a washing sponge and leave it to drain. After water has left the sponge
under the influence of gravity, the lower layers remain saturated.)
|Figure 22.2 Pore spaces in (a) concrete mix (b) and
(c) sand mixes
Very large quantities of water have to be applied to composts over
the course of a season, so the materials chosen must have very good stability
. Fine sand and silt soils
collapse too quickly and reduce the
size of the pore spaces. Even clay
crumbs, unless reinforced with high
humus content, collapse quite quickly.
The sizes of the components used
must be selected carefully to ensure
that they create macropores, but also
so that the gaps between the larger
particles are not subsequently filled in
by smaller particles (‘fines’). This is
most easily achieved by using closely
graded coarse particles. The reverse
is achieved when combining many
different-sized particles, as one would
in mixing concrete, where the object is
to minimize the air spaces as shown in
The water-holding capacity of compost
ingredients varies enormously. Peat
is significantly better than most others. However, the importance of
this depends on how the plants in the compost are to be irrigated.
It is a major consideration if the plants are in small hanging baskets
watered by hand and there are benefits in using absorbent polymers that improve water-holding more than peat alone. Peat
presents a problem if it dries out, because it does not rewet easily unless
treated with wetters. On the other hand, if there is to be a
constant supply of water through one of the many self-watering systems, this water-holding capacity is far less significant and the
emphasis should be on choosing material that is stable and provides the
right air-filled porosity.