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Normal air has a carbon dioxide concentration of 340 ppm. This is insufficient for producing the best yield. Plants usually require carbon dioxide concentrations of between 600 and 1,000 ppm. Supplementary carbon dioxide is particularly important when using artificial lighting. If the concentration of carbon dioxide is allowed to fall, growth is halted and artificial lighting provides no benefits.
When extra carbon dioxide is added to greenhouse air, the tomato, cucumber and lettuce yield improves by up to 25-30%, or even more. Cut flowers and potted plants also benefit from carbon dioxide fertilization. Carbon dioxide not only increases yield, but also produces an earlier harvest and improves plants’ resistance to disease and pests.
Carnations can bloom up to one month earlier. Lettuce is ready for harvesting weeks earlier, while tomatoes in tomato sprouts can ripen a week sooner than usual. Carbon dioxide fertilization pays off, as clearly indicated by several studies and practical experience.
In greenhouses heated by burning propane, carbon dioxide is obtained as a by-product, but often at the wrong time of day. Carbon dioxide is needed most at midday, when extra heat is seldom needed. The heating system must be able to store this heat, and release it during the cold hours of the day. There is always the risk of incomplete burning, resulting in the formation of NOX, which is very harmful to plants.
If a greenhouse heating system cannot store heat, adding pure carbon dioxide to the greenhouse air is usually the best solution. This also helps to avoid the moisture problems produced by burning propane. One kilogram of propane generates 1.6 kilograms of water vapor, which condenses into liquid water. Such water won’t go away by itself.