Carbon dioxide (CO2) is an essential element of photosynthesis required for growth and food production in plants.
Photosynthesis is a process by which CO2 and water are absorbed by plants and then converted in the leaves into sugars, cellulose and starch under the influence of light energy. More photosynthesis means more growth although there are optimal CO2 levels for all plants, as shown in the table below:
Table 1: Optimum concentrations of CO2 for a sample of greenhouse plants
|Product||kg CO2 / m2 / year||Target level (ppm)||Reference|
|Capsicums||10-15||Up to 1000|||
|Lettuce||Approx 1000||, |
|Kohlrabi||Up to 1000|||
|Geraniums||600-800||Test report from Osnabrück University of Applied Sciences|
| Soyez K., Baier D., Fieback K., Koller M., Matthäi M., Reinhold J., Sommerfeldt H. Verfahren zur Kopplung von Kompostierung und Gewächshausproduktion [Procedure for Connecting Composting and Greenhouse Production] Bioplan study, University of Potsdam, 1996 (http://www.gts-oekotech.de/docs/kurzbericht_carboferm.pdf)|
| CryoGas International, CO2 — The Good Greenhouse Gas Article, May 2007|
| Prof. Dr.-Ing. L. Köhler and Dipl. Ing. (FH) F. Lecker Wirkung von CO2 und NOx auf Gewächshauspflanzen [Effect of CO2 and NOx onGreenhouse Plants] Report, Inst. for Horticultural Technology at Weihenstephan University of Applied Sciences in Freising 1997|
Significant increases in plant growth and yield can be obtained in greenhouse-grown plants by increasing the concentration of CO2 from 400 ppm to the optimum level for that commodity.
The following rule of thumb for improved plant growth and yield is applied when calculating the increase of CO2 levels:
From 250 to 350 ppm: 23% more growth
From 350 to 450 ppm: 12% more growth
From 600 to 700 ppm: 4% more growth
From 1000 to 1100 ppm: 1.5% more growth
There are a number of ways of increasing the levels of CO2 in a greenhouse. Linde supply options for CO2 offer a purity above most onsite generation - meaning you are not introducing toxic flue gas - while our application systems ensure consistent application, optimising the addition of CO2. In a closed environment like a greenhouse, this level could fall to about 120-180 ppm (due to the plants consuming CO2) with the result that photosynthesis would no longer take place and the plants would stop growing.
A Worked Example
Let us assume that the air in the greenhouse contains a CO2 level of 400 ppm; that equates to 0.72 g/m3. At an average greenhouse height of 6 m, that amounts to 4.32 g of CO2 per square metre. Now, to increase production, we want to increase the level of CO2 to 1000 ppm. This would equal 1.8 g/m3, or 10.8g/m2 with our average greenhouse height of 6 m. We therefore need to add around 6.5 g per square metre.
CO2 also escapes from the greenhouse, e.g. through ventilation, leaks, and of course also through the plants and crops. This means that an additional 2.3 g of CO2 needs to be added per square metre per hour. In this example, the total CO2 to be added would amount to 88 kg per hour for an entire hectare.
1Linde internal report: Application Report CO2 in Greenhouses
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