Agriculture Reference
In-Depth Information
According to Peet (
1999
), the reduction of light intensity is more likely to be a
limiting factor than otherwise; hence a movable shade applied for only a couple
of hours during sunny periods, is a possible solution. Lorenzo et al. (
2004
), for
instance, reported a 10 % increase in marketable yield of tomatoes, when mobile
shade was applied during a couple of hours of intense sunlight in Spain. In addition,
the combination of different measures for different genotypes, and at different plant
growth and development phases has to be emphasized here.
Air Temperature, Air Humidity and Energy
Considerations
In protected cultivation, global solar radiation, which is composed mainly of short
wavelength, and is transmitted through the cover, is absorbed by the greenhouse
structural elements and mostly converted into heat. Heated air cannot be freely ex-
changed into the free atmosphere, and any reflected energy is of a long wavelength
nature, as both atmosphere and greenhouse covers are partially opaque to these
wavelengths, such energy is trapped within the greenhouse, causing the so called
“greenhouse effect.”
Greenhouses
One of the advantages of cultivation in greenhouses is the possibility of controlling
the air temperature through heating or cooling. In cases of high global radiation,
in hot seasons and arid regions, the plant temperature can exceed air temperature
by 5-10°C. For many plants, ventilation does not provide sufficient cooling. In
arid regions, internal humidity should also be increased for crop growth (von Za-
beltitz
2011
). The combined effect of cooling and humidifying the inside air can be
achieved by evaporative cooling, mostly implemented by one of two systems: (i)
fan and pad; and (ii) fogging.
In the fan and pad cooling system, air is sucked by fans installed on one sidewall
of the greenhouse (Fig.
10.6a
). The air entering the greenhouse, which replaced the
sucked air, passes through a wet pad, installed on the opposite sidewall and which is
fed with water by sprinklers (Fig.
10.6b
). The inflow of external, relatively dry air,
through the wet pad, cools down the air and increases its water vapor content. This
is the so-called “negative pressure” system. In the “positive pressure” system, fans
and wet pad are positioned on the same sidewall and push air into the greenhouse
which then leaves through openings on the opposite sidewall.
The main drawback of the fan and pad system is the generation of thermal gra-
dients along the direction of air flow through the greenhouse. This is due to the air
being heated during its flow along the greenhouse section. To minimize this effect
growers tend to shade the downwind half of the greenhouse where the air is al-
