Over the years growers 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. To
supplement the nutrient released from the materials in the compost,
if any, various slow release organic manures or small dressings of
powdered soluble inorganic fertilizers have been added to the mixtures
to provide the necessary nutrition.
The correct physical and nutritional conditions are vital to successful
growing in a restricted rooting volume. Significant developments
occurred as a result of the work done in the 1930s at the John Innes
Institute, where the importance of ‘sterile’ (pest and disease free), stable
and uniform ingredients was demonstrated. The range of composts that
resulted from this work established the methods of achieving uniform
production and reliable results with a single potting mixture suitable for
a wide range of plant species.
Loam composts, typified by John Innes composts, are based on loam
sterilized to eliminate the soil-borne fungi (see damping off) and insects
that largely caused the unreliable results from traditional composts.
There is a risk of ammonia toxicity developing after sterilization
of soil with pH greater than 6.5 or very high in organic matter (see
nitrogen cycle). Induced nutrient deficiencies are possible in soils
with a pH greater than 6.5 or less than 5.5. Furthermore, loam should
have sufficient clay and organic matter present to give good structural
stability (the original specification identifies ‘turfy clay loam’). Peat and
sand are added to further improve the physical conditions: the peat gives
a high water-holding capacity and the coarse sand ensures free drainage
and therefore good aeration. There are two main John Innes composts,
one for seed sowing and cuttings, the other for potting.
John Innes seed compost
consists of 2 parts loam, 1 part peat and
1 part sand by volume. Well-drained turfy clay loam low in nutrients
and with a pH between 5.8 and 6.5; undecomposed peat graded 3 mm
to 10 mm with a pH between 3.5 and 5.0; and lime-free sand graded
1–3 mm should be used. 1200 g of superphosphate and 600 g of calcium
carbonate (lime) are added to each cubic metre of compost.
John Innes potting (JIP) composts consist
of 7 parts by volume turfy
clay loam, 3 parts peat and 2 parts sand. To allow for the changing
nutritional requirements of a growing plant, the nutrient level is adjusted
by adding appropriate quantities of JI base fertilizer which consists of
2 parts by volume hoof and horn, 2 parts superphosphate and 1 part
potassium sulphate. To prepare JIP 1, 3 kg JI Base fertilizer and 600 g of
calcium carbonate are added to one cubic metre of compost. To prepare
JIP 2 and JIP 3, double and treble fertilizer levels respectively are used. Whilst the standard JI composts are suitable for a wide range of species,
some modification is required for some specialized plants. For example,
calcifuge plants such as Ericas
should be grown in
a JI(S) mix in which sulphur is used instead of calcium carbonate. All
loam-based composts should be made up from components of known
characteristics and according to the specification given. Such composts
are well proven and are relatively easy to manage because of the waterabsorbing
and nutrient-retention properties of the clay present.
These composts are commonly used by amateurs, for valuable
specimens, and for tall plants where pot stability is important; but
loam-based composts have been superseded in horticulture generally by
cheaper alternatives. The main disadvantage of loam-based composts
has always been the difficulty of obtaining suitable quality loam (‘turfy clay loam’), as well as the high costs associated with steam sterilizing.
Furthermore, the loam must be stored dry before use and the composts
are heavy and difficult to handle in large quantities. Many of the loambased
composts currently produced have relatively low loam content and
consequently exhibit few of its advantages.
Loamless or soilless composts
Loamless composts introduced the advantages of a uniform growing
medium, but with components that are lighter, cleaner to handle, cheaper
to prepare and which do not need to be sterilized (unless being used
more than once). Many have low nutrient levels which enable growers
to manipulate plant growth more precisely through nutrition, but the
control of nutrients is more critical, as many components have a low buffering capacity
has until recently been the basis of most
loamless composts. It is used alone or in combination with materials,
such as sand, to produce the required rooting environment. Peats are
derived from partially decomposed plants and their characteristics
depend on the plant species and the conditions in which they are
formed (see Soil organic matter
). Peats vary and respond differently to
herbicides, growth regulators and lime. All peats have a high cation
exchange capacity, which gives them some buffering capacity. The less
decomposed sphagnum peats have a desirable open structure for making
composts and all peats have high water-holding capacities.
Alternatives to peat
Whilst peat remains a popular choice as a compost ingredient, great
efforts are being made to find alternatives in order to preserve the
wetland habitats where peat is harvested. A list of some of the materials
used is given in Table 22.1. Much progress has been made by using
suitably processed bark or coconut fibre in composts.
|Table 22.1 Alternatives to peat
Along with several
other organic sources they are waste-based and recycling them helps
resources. All such alternatives must be free of toxics
and pathogens. Several inorganic materials, such as sand and grit, have
always been used in composts, but there is now a wider choice available.
Most of the inorganic alternatives are made from non-renewable resources (sand, loam, pumice) or consume energy in
their manufacture (plastic foams, polystyrene) or both
(vermiculite, perlite, rockwool).
Where possible an environmentally friendly
alternative is used, but there is considerable debate
about the relative merits of some of those being
used because of the associated energy use in their
manufacture or transport. However, peat is being
replaced successfully by different substitutes, many
only available locally, according to the needs of the
various sectors of the industry.
Sand, grit or gravel
is used in composts, frequently
in combination with peat. They have no effect on the
nutrient properties of composts except by diluting
other materials. They are used to change physical
properties. As sand or gravel is added to lightweight
materials the density of the compost can be increased,
which is important for ballast when tall plants
are grown in plastic pots. Sand is also used as an
inert medium in aggregate culture. Sand should be
introduced with caution because it tends to reduce
the air-filled porosity (AFP) of the final mix. It is
important that the sands used should have low lime
levels; otherwise they may induce a high pH and associated mineral
deficiencies (see trace elements).
has been used as a mulch and soil conditioner for
many years. More recently it has been tried in compost mixtures as a
replacement for peat. There are many different types of bark and they
have different properties. Its problems include the presence of toxics,
overcome by composting, and a tendency to ‘lock-up’ nitrogen (see
carbon to nitrogen ratio), which can be offset by extra nitrogen in the
feed. When composted with sewage sludge, a material suitable as a
plant-growing medium is produced. It is increasingly being incorporated
into growing mixes in the attempt to reduce the use of peat.
However, the great variation of barks, especially when they are from
a mixed source, makes it difficult to incorporate into growing mixes.
Much of the conifer bark tends to be stringy. Consequently the main
role of bark is in mulching. The import of bark is strictly controlled
by the Forestry Commission to prevent the introduction of pests and
diseases. Wood-fibres based on stabilized shredded wood are being used
to increase the air-filled porosity of mixes, but they tend to be dusty and
not easily dispersed in compost mixes. Sawdust and off-cuts from the
chipboard industry are also being tested for use in growing, but there are
problems associated with their stability and fungal growth in the freshly
wastes such as coir
(the dust particles) are proving to be
useful in growing mixes. The material has good water-holding capacity, rewetting and air-filled porosity characteristics. It has a pH between 5
and 6, which makes it suitable for a wide range of plants, but it cannot
replace peat directly in mixes for calcifuges. It has a carbon:nitrogen
ratio of 80:1 which means that allowance has to be made for its tendency
to ‘lock-up’ nitrogen.
|Figure 22.3 Growing media. Top to
is a mineral that is crushed and then expanded by heat to produce
a white, lightweight aggregate (see Figure 22.3). The granules are
porous and the rough surface holds considerably more water than gravel
or polystyrene balls. It tends to be used to improve aeration
of growing media generally and for the rewetting of peat. It
is devoid of nutrients and has no cation exchange capacity.
Graded samples may be used in aggregate culture, but it
tends to be used to add to mixes to improve the uptake of
is a mica-like mineral expanded to twenty
times its original size by rapid conversion to steam of
its water content. The finished product is available in
several grades, all of which produce growing media with
good aeration and water-holding properties (see Figure
22.3). There is a tendency for the honeycomb structure to
break down and go ‘soggy’. Consequently, for long-term
planting, it tends to be used in mixtures with the more
stable peat or perlite. Some vermiculites are alkaline, but
the slightly acid samples are preferred in horticulture.
Vermiculite has a high cation exchange capacity, which
makes it particularly useful for propagation mixes. Most
samples contain some available potassium and magnesium.
is an insulation material derived from a granitelike
rock crushed, melted, and spun into threads. The
resulting slabs of lightweight, spongy, absorbent, inert
and sterile rockwool provide ideal rooting conditions with
high water-holding capacity and good aeration. Shredded
rockwool can be used in compost mixes (see Figure 22.3).
Its pH is high but is easily reduced by watering with a
slightly acid nutrient solution. It is frequently used in
tomato and cucumber production and film-wrapped cubes
are available for plant raising and pot plants. It is necessary to use a
complete nutrient feed (see aggregate culture). It has some buffering
capacity, but this is very low on a volume basis. The main problem
areas lie in calcium and phosphorus supply and the control of pH and
salt concentration. Some rockwool has been formulated with clay to
overcome some of these problems. This increases its cation exchange
properties, making it very suitable for interior landscaping. Rockwool
is also available in water-absorbent and water-repelling forms. Mixtures
of these enable formulators to achieve the right balance between airfilled porosity, water-holding and capillary lift. Rockwool is available
as granules that provide a flexible alternative for those who produce
their own mixes. However, it is most usually supplied as wrapped slabs, cubes, propagation blocks and plugs that are modularized to create a
complete growing system.
is a porous volcanic rock that is prepared for use as a growing
medium by crushing, washing (to remove salt and ‘fines’) and grading.
It is most commonly used to grow long-term crops such as carnations in
troughs or polysacks.
balls or flakes provide a very lightweight
inert material, which can be added to soils or composts as a physical
conditioner. It is non-porous and so reduces the water-holding capacity
of the growing medium while increasing its aeration, thus making it
less liable to waterlogging when over-watered. This has made it an
attractive option for winter propagation mixes. However, it is less
popular than it might be because it is easily blown away and sticks to
of several different types are becoming popular for
propagation because of their open porous structure. They are available
as flakes and balls for addition to composts or as cubes into which the
cuttings can be pushed.
has been used with some success. Generally the
main types available, wheat and barley, break down too easily and a
practicable method of stabilizing them has not yet been found. Stable,
friable material has been derived from bean and oil seed rape straws,
although care is needed in mixes because of the high potassium levels.
is very variable soft brown coal formed from compressed
vegetation; often found at the base of the larger peat bogs. The dusts,
‘fines’, have been used as carriers for fertilizers and the more granular
material can be used to replace grit in mixes, often bringing an improved
have the ability to hold vast quantities of water
that is available to plants. However, this is considerably reduced in
practice because water absorption falls as the salt concentration of the
water increases and the release patterns appear to be very similar to that
of some compost ingredients, such as sphagnum moss peats.
Wetters, or non-phytotoxic detergents,
are included in mixes to enable
water to wet dry composts. They reduce the surface tension of the water,
which improves its penetration of the pores. This speeds up the wetting
process and maximizes the water-holding capacity of the materials used.
Wetters should be selected with care because the different types need to
be matched with the peat in the mix and above all must not be harmful
to the plants.
Materials alone or in combination are prepared and mixed to achieve
a rooting environment that is free from pests and disease organisms
and has adequate air-filled porosity, 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 not be achieved
by compressing the lightweight compost, but by incorporating denser
materials such as sand. 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 obtain this. Firming with a rammer reduces the total pore space
whilst increasing the amount of compost and nutrients in the container.
The reduction in air-filled porosity and available water with an increase
in soluble salt concentration leads to slower growing, harder plants (see
The addition of nutrients must take into account not only the plant
requirements, but also the nutrient characteristics of the ingredients
used. Most loamless composts require trace element supplements and
many, including those based on peat, need the addition of all major
nutrients and lime. The Glasshouse Crops Research Institute developed general purpose potting composts
based on a peat/sand mix (see Table
22.2). They contain different combinations of nutrients and consequently
their storage life differs. One of the range of composts has a slow
release phosphate, removing the need for this element in a liquid feed
(see phosphorus). The GCRI seed compost
contains equal parts by
volume of sphagnum peat and fine, lime-free sand. To each cubic metre
of seed compost is added 0.75 kg of superphosphate, 0.4 kg potassium
nitrate and 3.0 kg calcium carbonate. Variations on these mixtures are formulated with alternatives to peat, taking into account their different
properties particularly with regard to their particle size, water-holding
capacity and final air-filled porosity.
|Table 22.2 GCRI composts
It is most important when making up the desired compost formulation
to achieve a uniform product and, commercially, it must be undertaken
with a minimum labour input. The ingredients of the compost must
be as near as possible to the specification for the chosen formulation.
Materials must not be too moist when mixing because it then becomes
impossible to achieve an even distribution of nutrients. There are several
designs of compost mixer
. Continuous mixers are usually employed
by specialist compost mixing firms and require careful supervision
to ensure a satisfactory product. Batch mixers of the ‘concrete mixer’
design are produced for a wide range of capacities to cover most nursery
needs. Many of the bigger mixers have attachments which aid filling.
Emptying equipment is often linked to automatic tray or pot-filling
Ingredients used in loamless composts or growing modules do not
normally require partial sterilization unless they are being reused, but sterilizing equipment
is certainly needed to prepare loams for inclusion
in loam-based composts. Where steam is used it is injected through
perforated pipes on a base plate and rises through the material being
sterilized. In contrast a steam–air mixture injected from the top under an
air-proof covering is forced downwards to escape through a permeable
Storage of prepared composts should be avoided if possible and should
not exceed three weeks if slow release fertilizers are incorporated. If
nitrogen sources in the compost are mineralized, ammonium ions
produced followed by a steady increase in nitrates
changes lead to a rise in compost pH followed by a fall. As nitrates
increase, the salt concentration rises towards harmful levels (see
conductivity). Peat-based composts can become infested during storage
by sciarid flies.