Agriculture Reference
In-Depth Information
ensure that the crumb structure needed withstands
the constant wetting (see p. 151). After watering
a pot full of almost any soil, it is not long before
it is only half full of soil: the soil collapses as the
larger pores are reduced in size and small pores
predominate. This severely reduces the aeration and
drainage in the root environment. Consequently,
alternative growing media (non-soils) are used in
containers. These are generally called composts, but
also plant substrates, plant growing media, or just
'mixes' or 'media'.
Composts
Figure 15.2
Hanging baskets to attract customers
Materials alone or in combination are prepared and
mixed to achieve a rooting environment that is free
from pests and disease organisms and has constant
air (oxygen) supplies, easily available water and
suitable bulk density for the plant to be grown.
While lightweight mixes are usually advantageous,
'heavier' composts are sometimes formulated to give
'pot stability' for taller specimens. This should be
achieved not by compressing the lightweight compost
but by incorporating denser materials such as sand or
grit. Quick-growing plants are normally the aim and
loosely filling containers with the correct compost
formulation, consolidated with a presser board and
settling it with applications of water, will achieve this
(see p. 130). The addition of nutrients has to take
into account not only the plant requirements but also
the nutrient characteristics of the components used.
Most require the addition of lime (see p. 177) and all
the major nutrients. Loamless composts require trace
element supplements.
Growing in a restricted root volume
Most gardeners have at least some of their plants
in containers usually pots, tubs, troughs or hanging
baskets. For some, their plant growing is confined to
window boxes. Many containers are used for indoor
plants, including quite large ones in conservatories. For
others their interest lies in the production of tomatoes,
cucumbers and peppers in their greenhouses using
'grow bags'. Many non-horticultural businesses make
good use of the colourful displays in containers that
are there to create a pleasant working environment
and also to attract customers (Figure 15.2). Across
the horticultural industry there are the equivalents of
these methods of growing, and many more, but on a
much larger scale.
Growing in containers makes considerable demands
on the growing medium for air, water and nutrients
because rooting is severely restricted. Compare the
volume of containers such as flower pots, hanging
baskets and grow bags with the volume that roots
explore in the soil (see p. 140). When restricting plants
like this, we undertake to ensure that all their water
and nutrient requirements are provided through this
relatively small volume. Consequently, growing in a
restricted root volume brings two main challenges:
X
providing a large amount of nutrient in a small
amount of water without 'scorching' the roots (see
p. 120)
X
meeting the plant's water requirements while
maintaining good gaseous exchange (see p. 147).
There needs to be large pore spaces that do not
collapse over the time the plant is in the container.
Compost
is growing medium used for growing
plants in containers.
Over the years gardeners have added a wide variety
of materials such as leaf mould, pine needles, spent
hops, old mortar, crushed bricks, composted animal
and plant residues, peat, sand and grit to selected
soils to produce a compost with suitable physical
properties. Typically there was a different one for
each type of plant and many head gardeners guarded
the details of their successful mixes. Significant
developments were made as a result of the work
done in the 1930s by Lawrence and Newell at
the John Innes Institute. Before they undertook
their primrose trial, they set out to eliminate all
variation except the cultivars to be investigated. This
included eliminating the variable results from the
composts then being used. They demonstrated the
Limitations of using soils in
containers
Soil is an inappropriate material to use in containers
because most types lack the stability needed to
