Growing in containers - Principles of Horticulture - page 181

importance of ' sterile ' (pest- and disease-free), stable
and uniform ingredients. They incorporated the
developments in plant nutrition which had identified
the problems associated with unbalanced nutrient
supply, resulting in plants being either too 'hard' and
slow growing or too 'soft' and disease prone. The
range of loam-based 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.

boxes and most plants being grown in medium-
sized containers.
X JIP 3 nutrient content makes it suitable for fi nal
potting for the heavy feeding plants and for mature
foliage plants.
Ericaceous alternative . While 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
Rhododendron , Camellias and some heathers should
be grown in a JI(S) mix in which sulphur is used
instead of calcium carbonate to provide a sufficiently
acid medium.

Loam-based composts
Loam composts, typified by John Innes (JI) composts,
are based on loam sterilized to eliminate the water-
borne fungi (see damping off, p. 254) and insect pests.
The loam should have sufficient clay and organic
matter present to give good structural stability; the
original John Innes specification identifies 'turfy clay
loam'. Peat and sand (grit) are added to improve the
physical conditions: the peat giving a high water-
holding capacity and the coarse sand ensuring 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 two parts
loam, one part peat and one part sand by volume.
Well-drained 'turfy clay loam' low in nutrients with
a pH between 5.8 and 6.5, undecomposed peat
graded 3-10 mm with a pH between 3.5 and 5.0,
and lime-free sand graded 1-3 mm should be used.
Furthermore, 1,200 g of superphosphate and 600 g of
calcium carbonate are added to each cubic metre of
compost.
John Innes potting (JIP) composts consist of seven
parts by volume 'turfy clay loam', three parts peat and
two 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 two parts by volume hoof
and horn, two parts superphosphate and one 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 are used. This
provides a comprehensive series of composts for
growing in containers:
X JIP 1 has low nutrient levels but good phosphate
levels for pricking out or potting up seedlings or
cuttings.
X JIP 2 with the higher nutrient level is suitable
for potting plants from JIP 1, potting up most
houseplants, for use in hanging baskets, window

Calcifuge plants are those adapted to grow on
acid soils below pH 5.5.

Loam-based composts are well proven and are
relatively easy to manage because of the water-
absorbing and nutrient-retention properties of the
clay present. This makes them a good choice for
amateurs, those growing valuable specimens and
for tall plants where 'pot stability' is important. Their
main disadvantage has always been the difficulty
in obtaining suitable quality loam ('turfy clay loam')
as well as the high costs associated with sterilizing.
Furthermore, the loam must be stored dry before use
and the composts are heavy and difficult to handle
in large quantities. Many loam-based composts
now made have relatively low clay content and
consequently exhibit few of its advantages. They have
been superseded in horticulture generally by cheaper
and cleaner alternatives.

15

Loamless composts

Loamless ('soilless') composts can provide 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).
However, without loam the control of nutrients,
including micronutrients, is more critical. Most of
the components have low nutrient levels which
manufacturers and growers who make their own
compost are able to exploit because they can
add nutrients accurately for their intended
purpose.
Peat has, until recently, been the basis of most
loamless composts. Peats are derived from partially
decomposed plants and their characteristics depend
on the plant species and the conditions in which