Agriculture Reference
In-Depth Information
The
barks
are light and well-aerated
substrates, but retain little water. Their C/N
ratio is high and they absorb a lot of N as
they decompose.
Other agricultural and for-
est by-products
(e.g. wood waste, pressed
grape rests) are used in mixes for pots.
Coconut fibre
(coir) has properties
closer to rockwool rather than to forest by-
products, being widely used in the form of
plasticized slabs and in polystyrene con-
tainers for substrate-grown horticultural
crops, although its CEC complicates its
management, in relation to inert materials
(García and Daverede, 1994). There are
important qualitative differences in the
coconut fibre depending on its origin.
The use of
pozzolana (pumice) and vol-
canic gravels
is limited to those areas in
which they are naturally found, such as in
the Canary Islands, where they are known
as 'picon', or in Mexico ('tesontle'). Their
properties vary depending on their origin
and texture.
The
sands and gravels
of siliceous ori-
gin are preferable to those of calcareous
origin. As they are abundant everywhere,
they were used when substrate cultivation
first began to expand. Their main disadvan-
tage is their heavy weight. The most appli-
cable sand sizes range between 0.2 and
2.0 mm, and the best gravel varies from 2 to
5 mm (Urban, 1997b). The gross materials
(gravels) require a high irrigation frequency
due to their low water retention capacity.
Sands and gravels are commonly used in
mixtures for open field pots, because, due to
their weight, they provide the pots with sta-
bility against the wind.
Vermiculite
is an industrial transforma-
tion of mica, and is light, porous, well aer-
ated and with good water retention capacity
(Zuang and Musard, 1986). It is usually
used in mixtures.
Expanded clay
has good physical
characteristics, but its low water retention
capacity, which forces a higher irrigation
frequency, and its high price have
restricted its use to pot crops (Zuang and
Musard, 1986).
Polyurethane foam
is very durable, is
light, inert and recyclable, but it has a low
water retention capacity.
The joint selection of the substrate
and the soilless growing system
The choice of the substrate determines the
system to be used and vice versa, so they
must be considered together.
The simplest cultivation system is the
use of
ditches
(Fig. 10.1), whose large volume
provides greater inertia than other systems.
Scheduling of irrigation (see Chapter 11)
and fertilization can be done by a timer. The
best adapted substrates are sands, gravels,
perlite, pozzolana and volcanic gravels and
barks, alone or mixed with peat.
Gutters
(Fig. 10.1) that are located over
the soil or suspended have a smaller vol-
ume of substrate than the ditches, so irriga-
tion scheduling must be more precise than
with ditches, using a demand tray (see
Chapter 11) or calculated as a function of
the intercepted radiation. The most suitable
substrate
materials
are
perlite,
bark,
expanded clay and pozzolana.
In the
light bags or slabs
(Fig. 10.1) irri-
gation is scheduled by means of a demand
tray or as a function of the accumulated
radiation and the drainage, usually requir-
ing the use of a computer to manage the fer-
tigation. The most usual substrates are
rockwool and coir (coconut fibre) for the
slabs and perlite, blonde peat, expanded
clay, bark and volcanic materials for the
bags. Sometimes, bags and slabs are isolated
placing an expanded polystyrene board
below to ease the runoff of the leachate
(Fig. 10.1) and, possibly, the incorporation
of a root heating pipe.
Closed systems (with recirculation)
With the aim of avoiding contaminating the
aquifers with the drainage solution from
soilless growing systems, systems have been
developed whereby the drainage water is
recirculated. These are also known as closed
systems, and they require good quality
water. In these systems, where irrigation
is coupled with drainage and recirculation
of the excess solution (usually in the order
of 20-30%), the environmental impact is
avoided or minimized but the danger of
propagating diseases with the recirculating
