Agriculture Reference
In-Depth Information
required by phytochrome, making it suit-
able for its use in photoperiodic control
(Nelson, 1985).
•
Greenhouse CO
2
enrichment is usually
done by means of pure CO
2
injection or
CO
2
produced by combustion. The gen-
eration of CO
2
by decomposition of
organic matter, used in the past, is now
very rare. The supply of pure CO
2
is
the most expensive method.
CO
9.4
Summary
•
The air movement in the greenhouse,
•
2
produced by combustion must be
free of harmful gases, so the fuels used
must be 'clean', especially with a low
content of sulfur. The combustion must
be complete, to avoid the formation of
other harmful gases (carbon monoxide,
ethylene).
The gases most commonly used to gen-
besides avoiding temperature stratifica-
tion, is of great importance to the crop,
affecting photosynthesis, transpiration
and water use, and therefore, growth
and yield. An absence of air move-
ment has a negative effect on crop
production.
The optimum values of air velocity
•
•
erate CO
2
are natural gas and the LPGs
(propane and butane). In heating boil-
ers that use natural gas, the combustion
gases of the boiler are normally used as
a source of CO
2
.
CO
in greenhouses are of the order of
0.5-0.7 m s
−1
. To achieve such values,
fans are used that move the interior air,
with flows of 0.01 m
3
s
−1
per square
metre of greenhouse.
Increases in the CO
•
2
supplies to the greenhouse, for eco-
nomic reasons, do not normally exceed
4.5 g m
−2
h
−1
of CO
2
during working
hours.
Greenhouse light regulation allows for
•
2
air content gener-
ate an increase in photosynthesis, with
a subsequent increase in yield, whose
value depends of the CO
2
level and the
climate conditions. Anticipated yield
increases are higher with the joint use
of CO
2
enrichment and heating.
Nevertheless, the profitability in their
use must be determined by the specific
management of the CO
2
enrichment, in
each case.
In low-tech greenhouses, notable deple-
•
altering the length of the day or to inter-
rupt the duration of the night (by use of
darkening screens or artificial light) as
well as to achieve higher light levels to
increase photosynthesis, complement-
ing natural light and lengthening the
duration of the day with artificial light.
The use of shading systems allows for a
reduction of the intensity of solar
radiation.
The increases in the light available
•
tions of CO
2
are observed, in high radia-
tion conditions, frequently exceeding
20-30%.
Ventilation is the cheapest method to
•
•
for the crop can be obtained by means
of cheap techniques, such as the use
of reflecting materials (white mulch
and others) or painting the surface of
the structural elements white. Nor-
mally artificial supplementary light
is not used under Mediterranean
conditions.
The partial reduction of solar radiation
limit CO
2
depletion (due to the plants'
use of CO
2
for photosynthesis) in the
greenhouse air, below the normal levels
(350 ppm).
Nowadays, the maximum appropri-
•
ate level of CO
2
in the air, in practice,
is 1000 ppm for the majority of
crops.
In Mediterranean greenhouses, the
•
•
is practised, mainly, as a means to limit
high temperatures in poorly ventilated
greenhouses. It is normally done by
means of whitening or using shading
screens. The use of permanent shading
(whitening or permanent screens)
generates a continuous decrease of
most efficient carbon enrichment
strategy is to maintain levels of
350 ppm of CO
2
, by injection, when
the vents are open and, when closing
the vents, to raise the level to 600-
700 ppm.
