Agriculture Reference
In-Depth Information
the normal consumption is 1.5-2 g CO 2
m −2 h −1 increasing in spring to average val-
ues of 3 g CO 2 m −2 h −1 in the Mediterranean
area (Berninger, 1989).
3.6.3
CO 2 content
The ventilation, photosynthesis and respi-
ration of the plants and the CO 2 generation
in the soil (by root respiration and decom-
position of organic matter), influence the
CO 2 content of the greenhouse air.
At night, the accumulation of CO 2 due
to plant respiration increases the green-
house concentration above the values out-
side (Fig. 3.6). During the daytime, due to
photosynthesis, the CO 2 content decreases
with respect to the outside value (Fig. 3.6).
In a closed greenhouse, on a sunny day, the
CO 2 content may decrease below 200 ppm,
it being a limiting factor for crop production
(Lorenzo et al ., 1997a, b).
The goal of ventilation is to avoid CO 2
depletions of more than 30 ppm, with
respect to the normal atmospheric content
(360 ppm). The ventilation required to
decrease the temperature, which ranges,
at least, from 20-30 renewals h −1 , is usu-
ally sufficient to maintain appropriate
CO 2 levels (Fig. 3.6). In winter, at noon,
3.6.4
Pollutant gases
In greenhouses, pollutant gases (besides
those that originate from plant protection
treatments) are the result of using inappro-
priate fuels (with excess impurities) or
defective burning, when the combustion
gases are injected inside the greenhouse.
Combustion defects mainly generate carbon
monoxide (CO), ethylene (C 2 H 4 ) and nitro-
gen oxides (NO x ). The most usual impuri-
ties (presence of sulfur) generate sulfur
dioxide (SO 2 ) (see Chapter 7).
The plastizicers present in some mate-
rials (e.g. silicone) may also be toxic to the
plants, but their concentration in normal
greenhouses does not generate problems,
which may occur otherwise in small growth
chambers (Langhams and Tibbits, 1997).
600
500
400
300
200
Night
Night
Day
Outside
concentration
Ventilation (%)
100%
100
Solar radiation (W m -2 )
500
400
300
200
100
Medium wind,
cold temperature
No wind,
mild temperature
0
4
8
12
16
20
Time (h)
0
4
8
12
16
Fig. 3.6. evolution of air CO 2 concentrations through two autumn days in a greenhouse, without CO 2
enrichment and a tomato crop, as influenced by ventilation and outside global radiation (adapted from
Wacquant, 2000).
 
 
 
 
 
 
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