Agriculture Reference
In-Depth Information
The higher production and earliness of
soilless crops is derived from improvements
in the water and nutrients supply, and the
good root oxygenation which results in
good quality products, if properly managed
(Morard, 1995).
The main disadvantages of substrate-
grown crops are: (i) their higher initial cost
(relative to conventional soil cultivation);
(ii) the requirement for highly technical
crop management by the grower; and (iii) the
low buffering capacity of the systems.
Because of the limited volume of the sub-
strate involved, the limited availability of
water and nutrients requires continuous
monitoring, to avoid failures in the contin-
uous water and nutrient supply, which can
have disastrous results. The cost of the sub-
strate cultivation system is, nowadays,
lower than the implementation of an 'enare-
nado' crop.
Another negative aspect of substrate
cultivation is the generation of a high vol-
ume of solution that drains from the sub-
strate, which has to be removed, as well as
the substrate waste, the recycling of which
depends on the type of substrate.
The substrate must be laid out over a
well-levelled surface, with a certain slope
(not higher than 1%, if possible) to facilitate
drainage; otherwise the drainage system
may be simple or complex depending on
the characteristics of the installation. The
separation of the drained solution from the
root zone is important to avoid possible dis-
ease infections (Fig. 10.1).
10.3.4
Characteristics of the substrates
Physical properties
Among the relevant physical characteristics
of the substrate are: (i) porosity; (ii) water
retention and availability; and (iii) air
content.
The total porosity (or total porous
space) is the total volume of the substrate
that is not occupied by organic nor by min-
eral particles; its optimum level is greater
than 85% (Abad and Noguera, 1998). It is
important to distinguish between the cap-
illary pores, which retain water, and the
macropores, which allow for aeration
(Bunt, 1988).
The available or useful water (see
Chapter 11) in substrates, given their physi-
cal and hydraulic characteristics, lies within
very narrow ranges of matrix tension.
Figure 10.2 shows the water retention curve
of a substrate considered as ideal. The
matrix tension values of 1, 5 and 10 cb
define: (i) the readily available water (RAW)
contents (between 1 and 5 cb); (ii) the
reserve water (RW) (between 5 and 10 cb);
and (iii) the not readily available water
(NRAW) (above 10 cb) of the ideal substrate
(Caldevilla and Lozano, 1993).
The aeration capacity is the propor-
tion of the substrate volume that is occu-
pied by air, once saturated and drained,
which usually corresponds to 20-30% in
volume (Abad and Noguera, 1998). The
height of the container or of the substrate
slab has a great influence on the air con-
tent of the substrate, because the capillar-
ity of the water dictates that with low
height the air content is lower, and vice
versa.
10.3.3
Substrate cultivation systems
In substrate cultivation systems the nutrient
solution is supplied in excess from above,
by means of drippers or micro-sprinklers, or
from below the substrate, so that it ascends
by capillarity action. The excess supply of
nutrient solution must be removed by
drainage.
Another method to irrigate is by sub-
irrigation, which is used in pot plants.
In relation to the positioning of the sub-
strate in the greenhouse, it may be done in
several ways among which we can highlight
(see Fig. 10.1): (i) in a ditch, isolated from
the soil by a plastic film, used with heavy
substrates such as sand; (ii) in a gutter, laid
over the soil (or sand, perlite, etc.); (iii) in
slabs of rockwool or coconut fibre pre-
packed in plastic film; (iv) in a plastic bag
filled with substrate and laid over the soil
(or perlite); and (v) in pots or containers.
