Hydroponics and crop growth

Carbon dioxide as a crop growth promoter

Elevated levels of carbon dioxide enhance plant growth. In greenhouses, the growth rate and development of all plants can be improved by controlling carbon dioxide CO₂ concentrations at levels of around 800 ppm. This is approximately twice as much as the natural concentration of CO₂ in natural ambient air. Higher CO₂ concentrations up to 2000 ppm have been used in greenhouses and hydroponics, but each incremental increase in CO₂ levels above 700 ppm has diminishing benefit to the plants. Despite these diminishing returns, some operators control the CO₂ levels at 1000 or 1200 ppm to fully exploit the potential of CO₂ addition.

At high levels, CO₂ can be toxic to humans and bugs. For humans, the short term exposure limit is 3% and the long term 8-hour time-weighted average exposure limit is 0.5% (5000 ppm). For bugs such as white fly, exposure to a CO₂ concentration of 1% for one hour has been reported to be an effective fumigation technique.

Optimised CO₂ levels in greenhouses raise productivity and crop yields considerably, up to 40% during the darkest time of the year. In addition, they also improve the quality of the crop. When the CO₂ level in the greenhouse is optimised, the plants will produce uniform fruit, salads and vegetables of the best quality. So CO₂ can maximise both the crop yield and the sales price for the harvest. This technique is applied to greenhouses using both hydroponic and conventional soil growing techniques.

CO₂ dosing is no substitute for good management: it is essential to maintain suitable growing conditions in the greenhouse so the crops can benefit from the extra carbon dioxide. Effective management of light levels, temperature, air flow through the greenhouse, exhaust fan capacity, air intake location, crop spacing and nutrient supply all contribute to producing healthy, vigorous plants which are ready to benefit from the CO₂.

Dosing of carbon dioxide to the greenhouse can be from a CO₂ generator or from a carbon dioxide supply cylinder or tank. Greenhouse CO₂ generators are often simple LPG burners, which produce CO₂ and heat from combustion of the LPG. Coregas can supply your farm with LPG for this application. However, enhanced flexibility and improved dosing control can be achieved by using carbon dioxide gas, which can be delivered by Coregas either in bulk liquid tanks or gas cylinders.



Carbon dioxide dosing through the growing stages

Elevated carbon dioxide levels in the greenhouse have tremendous benefits for creating more productive crop plants. It is essential to use CO₂ at the right times during the growth cycle for optimum results.

Plants go through several growth stages during their lives and each stage has its own environmental requirements. For example, seedlings need different light levels and fertiliser strengths than established plants. Similarly, extra CO₂ is more beneficial during some growth stages than others. Generally, dosing of CO₂ is most essential during periods of rapid growth, but researchers have also discovered how extra CO₂ early in the life of a plant can also bring benefits months later.

C02 Dosing at various growth stages

Rooted cutting/seedling stage

Adding CO₂ for about two weeks to plants at the seedling, or rooted cutting, stage results in faster early growth and increased final crop yield, even without extra CO₂ during green growth or crop production. The two-week period leading up to transplanting is the most effective time for this CO₂ technique.

Transplanting stage

Adding carbon dioxide during the transplanting stage is not recommended because plants are adjusting to new growing conditions and can make do with ambient CO₂ levels.

Green growth stage

When plants are established in the green growth stage with full light levels, full strength fertilisers, spreading roots and new top growth, it is time to consider adding CO₂.

Long-day crops

Long-day plants, produce their crops during summer, while continuing to put out new leaves and stems. Tomatoes and roses are typical long-day crops which benefit from supplemental CO₂ during the light cycle right through green growth and crop production stages.

Short-day crops

Short-day crops have a transition stage before flower or crop production begins. They produce green growth during spring and summer, and flowers and crop in autumn. For these plants it can be helpful to stop the CO₂ addition two weeks before the green growth stage until crops pass through this transition stage. CO₂ dosing can recommence when the crop begins to produce flowers. This pause will help keep plants bushy and compact while they are preparing for fruiting and avoid 'stretching' problems common to the early transition period.

Carbon dioxide measurement and control

To control the CO₂ level in the greenhouse an NDIR sensor is typically used. The target set point will generally be 800 ppm of CO₂, which is almost two times the amount of CO₂ in ambient air. When the sensor detects a reduced CO₂ level in the greenhouse it will activate the CO₂ dosing system. When the required CO₂ level has been achieved the measured value will rise and the control system will shut off the CO₂ supply.

To calibrate the CO₂ sensor it might be possible to take it out of the greenhouse into clean pure air and run an auto calibration based on the normal CO₂ concentration in ambient air. However, in professional units where the CO₂ sensor is often fixed into the greenhouse control panel and has mains electrical power supply this calibration in ambient air is not practical. A specialty gases calibration gas mixture cylinder containing a certified pre-mixed gas at 410 ppm of CO₂ in a balance of air or nitrogen can be presented to the NDIR CO₂ sensor to simulate and automated calibration in natural air. Alternatively, if the sensor is capable of being calibrated at user-defined levels, a certified calibration gas mixture at 800 ppm CO₂ in a balance of nitrogen can be used to calibrate the sensor close to its measured value.