C
arbon dioxide (CO 2 ) is an essential requirement for photo-
synthesis and can be somewhat overlooked by newer
growers. Being odorless, invisible, and only a small fraction
of our atmosphere, CO 2 often doesn’t get the same attention
as nutrients, lights, and other plant-growth factors. The use
of CO 2 enrichment to boost yields, quality, and growth rates
under hydroponic production is, however, widely used in
commercial greenhouse horticulture and has an even greater
potential in enclosed growing spaces. While simply pumping
in some additional CO 2 may seem like a straightforward
option, the use of this technology is a little more complex if its
potential is to be maximized and problems minimized.
CO 2 Enrichment
Ambient CO 2 levels in air are a little more than 400 ppm
(or 0.04 per cent by volume), however, plant tissue contains an
average of 45 per cent carbon that comes entirely from CO 2 .
By boosting CO 2 levels surrounding the leaf surface, above
ambient levels, the rate of photosynthesis increases up until
the point where some other factor, such as the speed at which
plant enzymes will work, is reached. Essentially, the transfer
of CO 2 from the surrounding air to the reaction centers in the
leaf chloroplasts depends both on the concentration difference
between the air and these sites, and the intervening biochem-
ical resistance in various leaf tissues. This means that while
CO 2 enrichment will boost photosynthesis, there comes a
point were further increases will not occur and plant damage
becomes a possibility. Determining this optimal level of CO 2
enrichment for a particular plant or stage of growth is where
the application of CO 2 needs some careful thought.
Carbon dioxide enrichment has become more popular in
recent times with hydroponic growers using a range of low-
and high-tech options to boost CO 2 levels. The most common
methods of generating CO 2 include burning hydrocarbon fuels
and the use of compressed, bottled CO 2 . Smaller growers with
a very limited growing space may use dry ice (solid, very cold
CO 2 ) which releases CO 2 as it “melts” under warm conditions.
Fermentation or the decomposition of organic matter
(composting and fungi) are still effective but less accurate
ways of boosting CO 2 levels through natural processes.
Whichever method is used to generate CO 2 , levels should be
regularly monitored, either with a hand held CO 2 meter or as
part of the environmental control system
in the growing area.
“Determining this optimal level
of CO 2 enrichment for a particular plant
or stage of growth is where the application
of CO 2 needs some careful thought.”
Top: Tomatoes, a crop with a high sink strength, benefit the most from
CO 2 enrichment during the fruit development stage. Bottom: CO 2
enrichment is highly beneficial for heavy fruiting crops such as eggplant,
tomato, cucumber, and capsicum.
Enrichment Levels
If CO 2 enrichment is to be applied, then determining the
correct level is as important with this gaseous element as it is
with nutrient levels. The benefits and levels of CO 2 enrichment
is crop dependent, but most plants respond well to levels
in the range of 500–1,500ppm. Below 200ppm, CO 2 begins to
severely limit plant growth, but more than 2,000ppm of CO 2
becomes toxic to many plants. More than 4,000ppm is a risk
to humans. An excess of CO 2 will cause crop damage in the
form of CO 2 toxicity, which is often misdiagnosed as mineral
deficiencies or disease symptoms. Mild CO 2 toxicity can
cause stunting of growth, or leaf-aging type symptoms, while
excessive levels may cause leaf damage such as chlorosis
(yellowing), necrosis (death of leaf tissue), curling and/
or thickening of the leaves. There is much debate over
which level of enrichment is ideal for each crop,
under various different growing conditions,
however, the most economic use of CO 2 is in
enriching crops to above ambient levels, but
not more than 1,200ppm. Most commercial
growers enrich to within the range of
600-800ppm where an increase in
growth and yields of between
20-30 per cent are common.
Maximum Yield
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