Carbon dioxide is indispensable during photosynthesis, the process by which green plants use light energy to turn CO₂ into water and sugar. CO₂ increases plant productivity by stimulating plant growth and strength. Therefore, greenhouse farmers consider CO₂ to be feedstock for plants.
In most types of greenhouse farming, the photosynthetic performance improves when CO₂ concentrations rise from 340 to 1000 ppm (parts per million).

Ambient air normally contains about 340 ppm CO₂, a concentration at which all plants thrive. When the CO₂ concentration reaches 1 000 ppm, however, the photosynthesis rate increases proportionally, resulting in higher amounts of sugars and carbonhydrates for plants to use. Any culture of plants in full growth, in a closed greenhouse with little or no ventilation, can make the CO₂ concentration drop to below 200 ppm. The drop in photosynthesis that is registered when CO₂ concentrations fall from 340 ppm to 200 ppm is of the same order of magnitude as the photosynthetic increase obtained by enriching the air from 340 to 1300 ppm. (figure 1).
Generally speaking a drop in carbon dioxide concentrations to below the ambient air level has a greater effect than enriching the air in the greenhouse.

In new greenhouses, especially in ones with double-walls and low gas exchange rates, CO₂ concentrations can easily drop to below 340 ppm several times a year, which is detrimental to plant growth. Daytime ventilation can help to slightly increase CO₂ concentrations, but it can never lift them above the 340 ppm level. Enriching the air with CO₂ therefore seems to be the only solution to compensate for this shortage and lift concentrations above the 340 ppm level; the level at which most plants thrive. The maximum CO2 level depends on the type of plant, the light intensity, temperature, and ventilation, the growth stage of the plant, and the productivity factors of the culture.

Since greenhouses are not fully sealed; outside air - which only contains 340 ppm CO₂ - constantly infiltrates into them and completely refreshes the air (approximately every hour). In order to make up for this air dilution and maintain the desired CO₂ concentration at 1,300 PPM, approximately 0.37 kg CO₂ per 100 m² of ground surface is to be added.

Using liquid CO₂ has the advantage that CO₂ is free from impurities, avoiding the risk of damaging the plants through incomplete combustion. It also doesn't produce heat or moisture, and enables you to always apply the correct dosage on the plant cover. Pure liquid CO₂ is delivered to the greenhouse in bulk trailers. Every site must be equipped with a special tank, which can be rented from ACP Belgium. The compressed liquid CO₂ is in liquid form and has to be vaporized. Liquid CO₂ distribution systems are generally fairly simple and easy to install. Most greenhouse farmers use black vinylchloride hoses (PVC) which are perforated at the right length. Those greenhouse farmers who use smaller amounts of CO₂ can buy cylinder CO₂.