Agriculture Reference
In-Depth Information
PE tubes; this method is popular in south-
ern warm countries, most notably along
the Mediterranean.
A special form of distribution is to dis-
solve CO
2
in the irrigation water (0.6-0.8 g
CO
2
l
−1
) by the system called 'carborain'. It
is admitted that this method does not sig-
nificantly improve photosynthesis, but has
other positive effects on root growth and
nutrient absorption (Nederhoff, 1995).
The CO
2
that comes from combustion
gases requires a proper transport pipeline
with aluminium pipes if the temperature is
high or PVC if it is low. The distribution takes
place through PE pipes. A network of PE film
ducts (of 50 mm diameter) drilled with holes
(of 1 mm diameter) every 20-120 cm is a
usual solution, avoiding ducts longer than
40 m and using a recommended pressure at
the beginning of the duct of 750 Pa, by means
of a fan (Hicklenton, 1988). The holes in the
distribution ducts must be more frequent
near the ends than near the beginning.
The CO
2
must be injected near the
plants. In a greenhouse with crops in paired
rows, one small-diameter perforated PE
tube is placed for each double row, either on
the soil, or under the raised gutter (when
available, in the case of soilless culture).
9.2.6
CO
2
control
Under good photosynthesis conditions, the
consumption of CO
2
ranges from 3 to 4 g
CO
2
m
−2
h
−1
(Bordes, 1992), but an important
amount is lost by leakage or by ventilation,
which may be as high as 75%; the average
consumption in sophisticated greenhouses,
being estimated at 8-13 kg CO
2
m
−2
year
−1
(Baille, 1999).
CO
2
may be supplied from dawn until
dusk, but in many cases it is usually lim-
ited for economic reasons to the hours
around noon. In winter, with low PAR
levels (up to 100 W m
−2
) it has been recom-
mended not to exceed 2 g CO
2
m
−2
h
−1
,
whereas during the spring and summer,
with PAR levels from 100 to 400 W m
−2
,
supplies of 2-8 g CO
2
m
−2
h
−1
are recom-
mended (Chaux and Foury, 1994b),
although in each case, the economic rea-
sons to fix the supply must prevail.
If the CO
2
is free of pollutants, the CO
2
levels do not cause problems between 1000
and 2000 ppm (0.1-0.2 kPa) (Langhams and
Tibbitts, 1997).
Under normal crop conditions in a low-
tech greenhouse, measurements of CO
2
depletion in the air have been recorded of
up to 37% (Sánchez-Guerrero
et al
., 1998,
2005, 2008), reaching values of 55% under
extreme conditions (Lorenzo
et al
., 1997c).
The increases in yield due to CO
2
supply
(700 ppm with vents closed, 350 with vents
open) have been of the order of 19-25% in
cucumber and from 10 to 15% in green bean
(Lorenzo
et al
., 1997c; Sánchez-Guerrero
et al
., 1998, 2005, 2008), supplying the CO
2
from the beginning of the morning until 1 or
2 h after noon.
The combined effect of heating and
CO
2
enrichment has allowed for yield
increases in cucumber in an autumn-
winter cycle of the order of 50% (Sánchez-
Guerrero
et al
., 2001), duplicating the
increases obtained by the use of heating
alone. In green bean, the increases are
smaller (Lorenzo
et al
., 1997c). Never-
theless, the most economic use of both
techniques may involve different yield
increases, depending on the different heat-
ing and CO
2
enrichment set points used,
9.2.5
CO
2
balance
The CO
2
balance depends on: (i) the CO
2
sup-
plied; (ii) the CO
2
exchanged with the external
air; (iii) the CO
2
assimilated in photosynthesis;
and (iv) the CO
2
originating in organic matter.
The last item is usually neglected.
The photosynthesis rate ranges from
1 g m
−2
h
−1
of CO
2
, or less under cloudy
weather, to 4-5 g m
−2
h
−1
under good light
and CO
2
conditions, sometimes even reach-
ing 7 g m
−2
h
−1
(Nederhoff, 1984).
As a rule of thumb, a minimum supply
of 4.5 g m
−2
h
−1
CO
2
is recommended, or its
equivalent as natural gas combustion gases
(Van Berkel and Verveer, 1984), to maintain
high levels (up to 1000 vpm CO
2
) in closed
greenhouses and to avoid important CO
2
depletions in ventilated greenhouses. For
economic reasons, the supply must not be
greater than 4.5 g m
−2
h
−1
of CO
2
.
