Carbon dioxide enrichment

Carbon dioxide is essential for the growth of all green plants. The normal atmosphere level is currently about 400 parts per million (ppm) and has been rising over the past few years. Research has shown that many crops benefit from higher levels of CO₂. Crops that show the greatest response include geraniums, mums, poinsettias, tomatoes, lettuce and cucumbers. Levels of 700 to 1,300 ppm are now considered acceptable and economic for many crops. The optimum level depends on light intensity, temperature and stage of maturity.

Things to consider

The CO₂ in the air in a tight greenhouse or growth room can be depleted rapidly by actively growing plants. Plant growth ceases when the level gets below 200 ppm. A level above 1,500 ppm is generally not economic.

CO₂ is only effective for plant growth enhancement when there is light. In greenhouses, it is supplied starting early in the morning while it is cool. Once the vents are opened or fans activated, CO₂ addition is usually stopped, as the cost exceeds the benefit.

Supplying CO₂ from the combustion of fuel adds heat. In greenhouses, this is an advantage during the winter, as it reduces the heating bill.

CO₂ gas is heavier than air. Tank CO₂ can be distributed through a small-diameter plastic tube above the plants or small inflated polyethylene tubes located inside the plant canopy near the soil. CO₂ can also be distributed by the air circulation system, such as horizontal air flow (HAF).

The two sources of CO₂ used in commercial greenhouses are compressed liquid gas and combustion of fossil fuels (natural gas and propane). Each has its pros and cons.

Compressed liquid CO₂

For growth rooms and small greenhouse operations, manufactured CO₂ is usually the best choice, as it is convenient to obtain and handle and usually has no contaminants. It is purchased as a compressed gas by the pound and supplied in pressure cylinders from 20 to 100 pounds.

One pound of compressed CO₂ yields about 8.5 cubic feet of gas. It is usually distributed above or below the crop with ¼- to ½-inch plastic tubing with 1-millimeter holes spaced about 12 inches apart. The equipment needed besides the tank includes a pressure regulator to reduce the tank pressure to about 5 psi, a flow meter that adjusts the rate of flow, a solenoid valve that shuts off flow and a timer/controller that activates the solenoid. Hardware includes piping and fittings.

Combustion of propane and natural gas

In larger greenhouse operations, non-vented combustion of propane or natural gas from burners is commonly used. CO₂, heat and water are byproducts of combustion. This can increase humidity to excessive levels if it is not removed. A dehumidifier or ventilation system may be needed.

The heat generated supplements the normal heating system during cold weather but during warm weather would have to be vented or stored in large, insulated water tanks for nighttime use. About 0.8 of a gallon of water is given off when burning 1 gallon of propane, or 1.1 gallons when burning 100 cubic feet of natural gas.

All fuels contain sulfur, some more than others. During combustion, sulfur is converted to sulfur dioxide. If sulfur dioxide leaks into the greenhouse and combines with the moisture in the air, sulfuric acid is formed. A level as low as 0.5 parts per million can cause burn spots when it condenses on the leaf surfaces of some plants. Long-term, low levels of sulfur dioxide may result in flecking and premature leaf drop.

Burning fuel requires oxygen, so makeup air is required. About 1 square inch of air intake is needed for each 2,000 Btu/hour of heater input. A 60,000 Btu/hour generator such as the Johnson Gas CO₂ generator will require a 30-sqaure-inch or 6-inch diameter pipe.

If natural gas is being used, make sure the sulfur content is less than 1 grain (64.86 milligrams) per 100 cubic feet. If you are burning propane, use an H.D. 5 grade. Natural gas will yield about 105 cubic feet. Propane yields about 108 cubic feet of CO₂ per gallon.

In larger operations with boilers for heat, CO₂ is captured by a flue gas condenser for distribution in the greenhouse. The heat generated supplements the normal heating system or during the day is stored in an insulated water tank for distribution at night.

How much CO₂ is needed?

The total CO₂ that should be added is the amount that the crop needs plus the amount lost by air infiltration. The average rate of plant usage is between 0.002 and 0.004 cubic feet/hour minus the square footage of the floor area. Greater amounts may be used on sunny days for crops with large leaf areas, such as tomatoes and cucumbers. For example, for a 30-foot by 128-foot greenhouse with a plant usage rate of 0.003 cubic feet/hour minus square feet, the usage would be about 11.5 cubic feet/hour.

Infiltration loss depends on the tightness of the greenhouse and is calculated by multiplying the greenhouse volume times the air changes/hour times 0.000001 times the desired CO₂ level minus 400. For a 30-foot by 128-foot poly covered greenhouse, the infiltration needs are about 13.8 cubic feet/hour.

There are several methods for measuring CO₂ levels:

With good light levels, low ambient heat and HAF air circulation, as much as 40% additional plant growth can be achieved with supplemental CO₂.