Carbon dioxide (CO2) is fundamental to photosynthesis, the process where plants convert light energy into chemical energy for growth. In typical indoor growing environments, especially those using high-intensity lighting, the natural CO2 concentration (around 400 parts per million, or ppm) often becomes a limiting factor. As plants rapidly consume the available CO2, the concentration drops, slowing their growth rate. Supplementing the air with additional CO2 provides the necessary raw material, allowing plants to utilize more light energy and potentially increasing growth and yield.
Generating CO2 Using Yeast Fermentation
The yeast fermentation method is a simple, cost-effective way to generate a steady, though unregulated, stream of CO2 gas. This process relies on baker’s yeast consuming simple sugars in an anaerobic environment, metabolizing the sugar into ethyl alcohol and carbon dioxide.
To create a basic generator, use a clean two-liter plastic bottle, a cup of granulated sugar, and a quarter to half a teaspoon of active dry yeast. Fill the bottle three-quarters full with lukewarm water (70°F to 80°F or 21°C–27°C) to activate the yeast. After adding the sugar and yeast, seal the cap tightly and use a tube connected to an airlock or bubble counter to direct the gas flow into the growing area.
The reaction begins slowly, often taking several hours before a steady stream of bubbles is observed. The yeast consumes the sugar, producing CO2 until the sugar is depleted or the alcohol concentration becomes too high. To prolong output, some growers mix baking soda into the solution; this acts as a buffer to neutralize acidity that can inhibit yeast activity. A drop in bubble production signals that the yeast is nearing the end of its food source, requiring a fresh mixture.
Generating CO2 Using Chemical Reactions
An alternative method involves a controlled chemical reaction between an acid and a carbonate base. The most common DIY approach uses citric acid (or vinegar) and baking soda (sodium bicarbonate). This method offers a more immediate and consistent output compared to biological fermentation.
A typical setup requires two separate containers connected by tubing: one for the acid mixture and one for the base mixture. The acid container holds citric acid powder dissolved in water, while the base container holds a solution of baking soda and water. The reaction begins when the acid is slowly introduced to the base, triggering the immediate release of carbon dioxide gas.
The chemical reaction involves citric acid reacting with sodium bicarbonate to produce sodium citrate, water, and CO2 gas. This process is highly controllable, especially in systems that allow for the measured, drop-by-drop transfer of the acid solution. Unlike fermentation, the acid-base reaction provides a vigorous burst of CO2 until one reactant is fully consumed. The primary advantage is the ability to start and stop the reaction, offering a degree of control not possible with continuous yeast fermentation.
Safe Distribution and Monitoring
Effective CO2 supplementation requires careful attention to timing and gas distribution. CO2 application must only occur during the light period, as plants utilize carbon dioxide for photosynthesis only when light is available. During the dark cycle, plants switch to respiration, making CO2 supplementation wasteful and potentially detrimental to the environment.
Since carbon dioxide gas is denser than ambient air, it naturally sinks. Therefore, the CO2 generator or release point should be positioned above the plants, allowing the gas to disperse downward and envelop the foliage. Adequate air circulation within the grow area is also necessary to ensure the gas is distributed uniformly throughout the plant canopy.
For optimal growth, CO2 levels are typically maintained between 800 and 1,500 ppm, significantly higher than the atmospheric baseline. Monitoring the concentration with a dedicated CO2 meter is advised when elevated levels are maintained. High concentrations of CO2 can pose a risk to human and pet health; concentrations above 5,000 ppm are considered unsafe for prolonged exposure. Ensure the grow space is well-ventilated before entering for an extended period.