Agriculture Reference
In-Depth Information
substrate, with no CEC and a slightly
alkaline pH (that is easily neutralized and
controlled if the slabs are wetted with an
acidic solution before the beginning of the
crop cycle). It has a homogeneous structure
and low density (which eases its transport)
and good porosity (Smith, 1987).
Nowadays, it is one of the most widely
used substrates in Europe, and elsewhere.
A variant of rockwool is fibreglass, which
has a laminar structure (milfoil type) to ease
the lateral diffusion of the water.
Expanded
perlite
is a very light, very
porous and well-aerated material. Several
sizes of perlite are available commercially,
the most popular comprising particles
between 1.5 and 2.5 mm (Morard, 1995).
Its main problem is its mechanical fragil-
ity which over time degrades its good
porosity and aeration characteristics, as
the grains break up increasing the propor-
tion of fine elements. Although initially it
has a basic reaction, such as rockwool,
after some time it becomes chemically
neutral.
The water retention capacity of perlite
depends on its size. Plate 22 shows the vol-
umetric relations of the water content for
different geometrical dispositions of P-2
perlite (grain size of 0-5 mm). In practice,
P-2 perlite bags usually have a diameter of
15-20 cm in cross-section, are 1.2 m in
length and contain 40 l (Caldevilla and
Lozano, 1993). For economic reasons
smaller volumes are also used (33 l).
The
peats
are organic materials origi-
nating from the decomposition of swamp
plants. They are usually free of pathogens,
despite their organic origin, but have the
inconvenience of a high CEC, the ability to
greatly contract when they dry out, and
being very difficult to re-wet.
The blonde peats have good physical
properties and are easy to re-wet, so they are
very convenient for soilless cultivation in
bags. The black peats are more decomposed
than the blonde types and their physical
properties are inferior to those of the blonde
ones. Both peats are commonly used mixed
with very porous substrates, such as poz-
zolana
Classification of substrates
From the point of view of their horticultural
use, substrates can be classified as organic
and inorganic (or mineral). The organic
substrates can be of natural origin (peats) or
synthetic (polyurethane foams), and also
include several by-products of natural
origin (sawdust, coconut fibre, cork resi-
dues). The mineral substrates can be of nat-
ural origin (sand, gravel) or artificially
transformed (rockwool, perlite), including
in this group several industrial by-products
(blast furnace slag).
Criteria for substrate selection
Although the best substrate for cultivation
will vary in each case depending on the spe-
cific use conditions, a good substrate must
have good physical characteristics (with
high capacity of readily available water
retention, enough aeration, low apparent
density, high total porosity and stability of
characteristics and of the structure) as well
good biological and chemical properties
(scarce or no CEC, reduced salinity, slightly
acid pH and biological stability) (Abad and
Noguera, 1998).
In addition, obviously, it must have a
cost (in which transportation is very impor-
tant) that is in line with its performance. It
is also important to consider its availability
in each local market. Finally, it must not be
forgotten that the choice of substrate must
be in agreement with the technological level
of the greenhouse, especially with the capa-
bilities of the fertigation system.
Most common substrates
The most common substrates are rockwool,
peat, coconut fibre (coir) and perlite.
Rockwool
has excellent characteristics
of water retention (Plate 23), being used in
slabs of 7.5 and, sometimes, 10 cm height
(usual dimensions 100 × 20 × 7.5 cm or
100 × 15 × 10 cm). For ease of raising trans-
plants seeds are sown in rockwool cubes.
Rockwool can be considered an inert
or
pumice
(Penningsfeld
and
Kurzmann, 1983; Urban, 1997b).
