Agriculture Reference
In-Depth Information
choose a certain type of greenhouse. For
instance, in a region with a tropical humid
climate, where the defence from the rain is
the main objective of the greenhouse (preva-
lence of the umbrella effect), it will be neces-
sary to choose a different type of construction
from that used in a region of semi-desert or
with a Mediterranean climate.
tool for the management of greenhouse
crops (see Chapter 7, section 7.4.6).
The minimum daily requirements of
radiation of these species are estimated at
around 8.5 MJ m
−2
day
−1
(equivalent to
2.34 kWh m
−2
day
−1
) in the three shortest
months of the year (November, December
and January in the northern hemisphere;
May, June and July in the southern hemi-
sphere). This means around 6 h of light day
−1
,
which translates to a minimum of 500-550 h
of light in these three months (Nisen
et al
.,
1988). The duration of the day and the night
is influenced by the geographical latitude
together with the time of the year. The pho-
toperiodic requirements of some crops are
linked to the duration of the night.
Other desirable climate parameters for
these species would be a soil temperature
above 14°C and an ambient relative humid-
ity of 70-90% (Nisen
et al
., 1988).
4.4.2
Climate requirements of
vegetables
The most usual cultivated species in green-
houses are vegetables with medium to
slightly high thermal requirements (tomato,
pepper, cucumber, melon, watermelon,
marrow, green bean, aubergine) with the
aim of extending the production season
beyond the conventional growing season.
Nowadays, in cases of very high product
prices, greenhouse production in geograph-
ical areas that do not have perfectly suitable
climate conditions force a notable and
expensive artificial intervention over the
climate parameters. In any case, the eco-
nomic results will determine where green-
houses are located.
The cited horticultural crops are,
essentially, warm season species, adapted
to average ambient temperatures ranging
from 17 to 28°C, and whose limits we can
establish as between average temperatures
of 12°C (minimum) and 32°C (maximum)
(Nisen
et al
., 1988). They are sensitive to
the cold, and suffer irreversible damage
with frosts. The persistence of tempera-
tures below 10-12°C for several days, as
well as temperatures above 30°C, in the
case of dry air, or higher than 30-35°C in
cases of high air humidity, affect their
productivity (Nisen
et al
., 1988). In the
past, it was accepted that a daily variation
between day and night average tempera-
tures (thermal periodicity, between 5 and
7°C) was necessary for proper physiologi-
cal functioning (Nisen
et al
., 1988).
Currently, it has been stated (Challa and
Bakker, 1995; Erwin and Heins, 1995) that
this day/night temperature difference,
known as DIF, is not a requirement but a
4.4.3
Obtaining the required
climate conditions
The lack of possibilities to practically
increase, at a reasonable cost, the natural
radiation conditions (except in very sophis-
ticated greenhouses and with high value
crops), has made it necessary to design and
locate greenhouses with the aim of maxi-
mizing the interception of solar radiation
during the months of autumn and winter.
Therefore, the natural radiation availability
is a critical limiting factor when consider-
ing the establishment of greenhouses.
Due to the interdependence between
air and soil temperatures (even with less
oscillation inside a greenhouse than out-
side), achieving a suitable air temperature
involves proper soil temperature values as
well.
According to the methodology pro-
posed by the FAO (Nisen
et al
., 1988),
protected cultivation in greenhouses or high
tunnels enables daytime thermal increases
in relation to the outside air, mainly depend-
ing on: (i) the characteristics of the clad-
ding material; (ii) the outside wind
velocity; (iii) the incident solar radiation;
