Carbon Dioxide (CO₂) is an odorless gas which makes up about 300ppm of our atmosphere, yet dried plant material contains an average of 40% carbon which comes entirely from CO₂. Therefore we need to consider CO₂ to be a major plant nutrient, one that affects growth rate, yield and one that needs to be supplied in adequate quantities if crop growth is going to be maximized.

How is CO₂ Used?

The process of photosynthesis which produces the energy required for plant growth, uses water, CO₂ and solar energy to produce sugars and oxygen - this process can only be carried out in light, so during the day large amounts of CO₂ are absorbed by the plant's leaves and oxygen is given off. At night, with no solar energy, there is no photosynthesis, but the plants are still respiring and the result is CO₂ is given off and oxygen used. As the temperature and light levels increase in the growing environment, the rate of photosynthesis and absorption of CO₂ also increases up to a point where a maximum is reached. Often, we provide sufficient light and heat with the use of modern technology, thinking that growth rates will now be maximized, but in reality the amount of CO₂ may be holding photosynthesis back (it then becomes the limiting factor in growth).

What Happens in the Growing Environment?

Whether you are producing plants in a greenhouse using natural solar energy or an enclosed grow room, a situation can arise where a lack of CO₂ is not only slowing growth, but preventing any photosynthesis from occurring. In a fairly air tight greenhouse, natural CO₂ levels will be highest at dawn- this is the CO₂ which has been given off during the process of dark respiration and that which naturally leaks into the greenhouse. As light levels begin to increase, the plants begin to photosynthesis using the naturally available CO₂. If no ventilation occurs at this stage, the 300- 400ppm of CO₂ which was present in the growing area can all be depleted within a short space of time (less than 2 hours in strong light)- and with this depletion, growth stops. Ventilation replenishes the CO₂ to ambient levels, but if the environment is being heated, ventilation is often not carried out as it would cause excessive heat loss. In this situation, in order to retain photosynthesis levels, either ventilation to replace the CO₂ used by the plants or artificial CO₂ enrichment is required.

CO₂ Enrichment

Not only is it important to prevent CO₂ depletion, but enrichment to levels much greater than atmospheric levels is known to boost plant growth - sometimes by over 40%. The benefits and levels of CO₂ enrichment is crop dependent, but most plants respond well to levels in the range 300 - 1500ppm. Below 200ppm CO₂ begins to limit plant growth, but over 2000ppm CO₂ becomes toxic to pants and above 4000ppm is not so good for people. Often the exact level of enrichment is matched to the amount of incoming light - the greater the light intensity, the more CO₂ plants can use and the higher the growth rate, provided of course other factors such as water, nutrition and temperature are also at optimum levels. Many commercial crops are enriched with CO₂, usually to a level of 1000-1200ppm - growers have to decide whether the cost of the CO₂ they enrich with is 'paid back' with additional yield and increased returns. CO₂ has its most benefit in fruiting crops such as tomatoes, cucumbers and peppers where crop maturity is often advanced, fruit size is larger and yields higher. CO₂ enrichment is also used for green crops such as lettuce to speed up the time to maturity - particularly in winter when growth can be slow.

Supplying CO₂

The two most commonly methods used for CO₂ enrichment of a growing area are burning of hydrocarbon fuels such as natural gas or propane, and compressed, bottled CO₂. There are actually a few other, less practical ways - these are dry ice, fermentation, burning of candles and oil lamps and decomposition of organic matter.

CO₂ generators

Widely available for use in growing areas and less expensive than using bottled CO₂. The major problem with burning fuel to create CO₂ is that heat is produced as a by product - this may be useful under cooler conditions, but not if the environment is already sufficiently warm. As the CO₂ is introduced to the greenhouse, it needs to be thoroughly mixed with use of a circulation fan.

Compressed, bottled CO₂

Safer option for plant enrichment - in that no toxic by-products or additional heat can be produced. Compressed CO₂ comes in cylinders stored under high pressure (1600-2200 psi). Equipment such as a pressure regulator, flow meter and solenoid valve and timer are required to set up this type of enrichment system. CO₂ is injected into the growing area via the pressure regulator and flow meter which is controlled via a solenoid and timer. One pound of compressed CO₂ gas contains about 8.5 cubic feet of CO₂ gas at normal atmosphere pressure. Very small tightly sealed growing areas can use dry ice to provide CO₂ enrichment - this also gives some cooling effect. Dry ice is solid, very cold CO₂ and needs to be stored and handled with care. Dry ice can also 'melt' very rapidly in warm conditions, so may need to be well managed to ensure a continual supply of CO₂ at the correct level. No matter which method of enrichment is used it is important to firstly bring the environment up to the predetermined level and then constantly replenish to this level as the plants absorb the CO₂. The rate of CO₂ absorption will change with plant size, temperatures and light level and this is why constant monitoring of levels in important.

When to Supply

CO₂ only needs to be supplied when there is sufficient light for photosynthesis and should automatically switch off at night or during the 'off cycle' with artificial lighting. For maximum benefit, CO₂ can be supplied right from seeding(cloning) stage. It should then be maintained until harvest of the crop.

Measuring CO₂

There are a number of devices which can be used to measure and monitor CO₂ levels in your growing environment. There are a range of CO₂ sensors available - from simple 'syringe' test kits which allow a grower to take a sample of the air in the growing environment and determine the CO₂ level, to using timed devises and the use of electronic controls and meters which accurately monitor CO₂ levels and display this on an LCD readout.

Summary

CO₂ enrichment to increase plant growth and yields is a well proven method of crop production which can benefit even the smallest grower and is widely used on a commercial scale.