CO2!
© a.k.a. MBR
CO2 becomes a limiting factor for growth when sufficient light, water and fertilizer levels are available as the CO2 is depleted by the plants and must be replaced by venting or supplementation. The atmospheric concentration of CO2 is ~0.035% (~350 parts per million (ppm)), and plants have been shown to have up to a 200% increase in growth at 700 ppm (Taiz & Zeiger, 1991). The accepted value for maximum growth of most crop plants is 1500 ppm CO2, which is far below the 2 percent (20 000 ppm) level which is suitable for human consumption. Long exposure to 5 percent CO2 (50 000 ppm) causes dizziness and other symptoms.
CO2 can be supplied by various means, including injection from compressed tanks of liquid gas, production via combustion, fermentation, chemical reaction and catalytic combination. Tanks of compressed gas and combustion are the two most common methods for CO2 fertilization, with combustion being mainly used in outdoor greenhouses where excess heat can be used. Fuel supplies for combustion include propane and methane. The fermentation method using brewer's yeast can be applied to small grow-rooms, but is messy and smelly, but cheap and effective. The chemical reaction of baking soda (sodium bicarbonate) and acid can also be used. There are new catalytic converters that combine propane with oxygen to produce CO2 and water vapor without a flame. These are currently being marketed as mosquito repellants.
The control of the levels of CO2 available for assimilation in the growing area can be a problem. The amount of fuel required to burn to achieve a desired CO2 level is predictable using molecular weight calculations, but most commercial production units are calibrated and come with timing sheets for the production rates. When using fermentation, it is nearly impossible to predict the exact amount that will be produced and CO2 monitors are recommended, but are currently quite expensive. It is easiest to use compressed tanks of the gas that can be used with flow-meters or gages that are calibrated accordingly, usually in cubic feet per hour (CFH) which can give you a rough estimate of the levels without purchasing a monitor, but hopefully the prices of monitors will drop because they are the most effective method of regulation. When using compressed gas, it is best to replenish the CO2 after venting when necessary, or approximately every 3 hours if the air has not been removed.
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