Controlled Environment Agriculture Facilities’ Atmospheres

Introduction

Controlled environment agriculture (CEA) includes greenhouses, indoor vertical facilities, and production chambers. Regardless of the type of facility, the atmosphere inside a CEA facility can affect many aspects of plant growth and crop quality. The degree of control that can be exercised over the CEA atmosphere will depend upon the type of facility being used and the technology incorporated. There are three general areas related to the atmosphere of CEA facilities that should be considered: carbon dioxide, humidity, and various air pollutants.

Carbon Dioxide

Carbon dioxide (CO₂) serves as the carbon source for photosynthesis; without it, plants could not grow. During photosynthesis, CO₂ and water are combined to form carbohydrates and oxygen. Carbohydrates are compounds such as glucose, sucrose, fructose, and starch.

Carbohydrates serve as both energy sources (e.g., glucose, sucrose, starch) as well as the building blocks (e.g., cellulose, lignin) a plant uses to grow.

Although the outside atmospheric (ambient) CO₂ concentration varies by location and season, it is approximately 430 - 432 parts-per-million (ppm) at sea level. In enclosed CEA facilities, the CO₂ level may be significantly lower. This is because plants remove CO₂ from the atmosphere during the day or during periods of light. In fact, CO₂ levels in CEA facilities have been reported to be as low as 200 ppm. This is problematic because, as CO₂ concentration decreases, photosynthesis, (figure 1). Typically, the more tightly sealed the facility and the less venting occurs, the greater the potential for CO₂ levels to drop significantly below the ambient outside level. As a result, venting even in winter, particularly for tightly sealed greenhouses, may be necessary to replenish the CO₂ level.

In addition to the fact that plants may absorb CO₂ faster than it is replenished from the outside, many plant species have been shown to respond positively through increased photosynthetic rates to CO₂ levels up to 2000 ppm, and most greenhouse crops respond positively to CO₂ levels up to 1500 ppm (figure 1).

This is obviously much higher than even outside ambient CO₂ concentrations. Therefore, in many greenhouse CEA facilities, CO₂ may be the limiting factor for photosynthesis, and increasing CO₂ concentration above ambient levels may increase plant growth and crop quality. Some common responses of greenhouse crops to increased CO₂ concentrations are listed in Table 1.

Table 1. Common Responses of Some Crops to Increased Carbon Dioxide Concentrations in Controlled Environment Agriculture Facilities.

Because of the positive growth responses to increased CO₂ concentrations, many CEA operations have found it beneficial to inject CO₂ into the interior atmosphere (also referred to as CO₂ enrichment) to increase the atmospheric CO₂ concentration inside the crop production space.

In greenhouse-based CEA operations, increased CO₂ levels are effective only during the day when photosynthesis occurs. However, vents must often be opened during the day (especially during summer and in southern climates). If vents are opened more than 5% of capacity or if exhaust fans are turned on, it is difficult to maintain increased CO₂ concentrations with CO₂ enrichment. Therefore, the period during which CO₂ injection can be used effectively may be limited by season, climate, and production systems. Additionally, to gain the maximum effect of elevated CO₂ levels, supplemental lighting, increased temperatures (5oF to 10oF higher depending upon crop), and increased fertility levels are often required. The plant cannot utilize the additional CO₂ provided if other factors, such as light, are limiting.

Injecting CO₂, as well as increasing lighting and temperature, increases production costs. The gain in production and quality must offset these increased costs. In other words, many crops may respond positively to CO₂ injection, but the response or the level of response must pay for increased inputs. In tightly sealed indoor vertical or chamber CEA facilities, CO₂ injection is typically required because little air exchange occurs with the outside atmosphere. Without supplementation, CO₂ levels will drop below even ambient outside CO₂ levels.

There are three primary methods of increasing greenhouse CO₂ concentrations in CEA facilities.

Option 1. CO₂ burners can be located within the facility and combust natural gas, propane, butane, or kerosene to produce CO₂. Complete combustion of these hydrocarbon fuels results in the production of CO₂ and water. However, if these systems are not properly maintained and operated, they may produce harmful carbon monoxide or ethylene gases that can be injurious to plants and dangerous to people. Additionally, it has been recommended that natural gas used in CO₂ burners contain no more than 0.02% sulfur (w/w), as otherwise sulfur dioxide may be produced. Sulfur dioxide combines with water on plant surfaces to form damaging sulfuric acid. Kerosene should contain no more than 0.06% sulfur. Burners used to produce CO₂ may also produce nitrogen oxides. High concentrations of nitrous oxides can be injurious to plants. However, most researchers have found that if the CO₂ level is maintained within acceptable ranges, nitrous oxide concentrations will generally be within acceptable levels or at least the negative impact of the nitrous oxides will be significantly less than the beneficial effects of increased CO₂ concentrations.

Option 2. CEA operations may burn natural gas to operate boilers used for heating. During daytime operation, the boilers heat water stored in tanks for nighttime heating. The exhaust gases generated by these systems can be passed through a manifold to separate CO₂ from flue gases. The CO₂ is then pumped into the greenhouse during daylight hours, where it can be used for photosynthesis by the plants.

Option 3. CEA operations may also use systems that directly inject CO₂ using compressed CO₂ in tanks or compressed liquefied carbon dioxide.

The optimal CO₂ concentration will depend upon the crop and cultural conditions. However, CO₂ concentrations are typically increased to 700-1,500 ppm for most crops, though the optimal level often varies across crops and even within cultivars of the same crop. If the CO₂ concentration is too high, plant damage may occur. Common CO₂ toxicity symptoms include leaf rolling, foliar chlorosis, and necrosis. For example, above 1500 ppm CO₂, damage was reported on cucumber. On gerbera, CO₂ concentrations of 1600 ppm caused foliar chlorosis and necrosis, and CO₂ concentrations of 2600 and 4500 ppm caused leaf dieback.

When injecting CO₂, a CO₂ sensor is required to prevent crop damage.

When deciding whether to inject CO₂ into the greenhouse, it is important to research the crop(s) of interest to determine the optimal CO₂ concentration, light, temperature, and fertility regimes, as well as the expected plant growth responses. If possible, the application of CO₂ injection at a small scale to test impacts before facility-wide application is recommended. It is also important to determine the cost of increasing the CO₂ concentration, including the costs of the required increases in light, temperature, and fertility. The increased production, reduced cropping time, and/or increased plant quality achieved will need to cover the cost of the increased inputs.

Humidity

Humidity generally refers to the amount of water that is held in the atmosphere. Because of evaporation of water from floors, root substrates, and other surfaces as well as transpiration (loss of water from plant leaves), the relative humidity in a CEA facility is often high, especially when vents are closed, or when limited exchange of inside air with outside air occurs. High humidity promotes the development of certain diseases (e.g., black spot, powdery mildew) as well as various physiological abnormalities (e.g., leaf edge burn in poinsettia, blossom end rot of tomatoes, and tip burn of lettuce caused by a calcium deficiency induced by high relative humidity) in some greenhouse crops. Additionally, high humidity can increase condensation on the inside of the glazing, thus reducing light levels and causing water to drip onto plants. Too low humidity can cause plant stress and reduce photosynthesis and growth rate.

In CEA, we can measure humidity in two ways:

Option 1. Relative humidity. Relative humidity is the ratio of the amount of water held in the air to the maximum amount the air can hold at a given temperature, expressed as a percentage.

Relative humidity is temperature-dependent. When the air is holding a given amount of water, as the temperature increases, the relative humidity will decrease. As the temperature decreases, the relative humidity will increase.

This is despite the fact that the amount of water in the atmosphere did not change, only what the air could hold. Likewise, at different temperatures, the relative humidity can vary even when the air holds the same amount of water. It has been recommended that a relative humidity of 50% to 80% is optimal for CEA crop production. Still, this range of optimal relative humidity can vary greatly among crops, cultivars, and specific growing conditions.

Option 2. Vapor Pressure Deficit. Vapor pressure deficit (VPD) represents the difference in water vapor pressure between the air and leaf temperatures and is expressed in kPa (figure 2). Due to cooling from transpiration, the leaf will typically be cooler than the surrounding air. If leaf temperature can’t be measured (e.g., using a thermocouple or infrared sensor), an estimated VPD can be calculated by assuming the leaf temperature is that of the air temperature, and using standard graphs, an approximate VPD can be determined. The higher the VPD, the lower the humidity. Although optimal VPD for CEA crop production can vary by crop and other growing conditions, it has been recommended that a VPD of 0.2 to 2.0 is optimal for most CEA crops.

During the summer, greenhouse vents and exhaust fans can help manage high humidity. However, during the cool months, when vents are closed and exhaust fan use is limited, high humidity can occur inside the greenhouse. To control the humidity, CEA greenhouse managers may periodically increase greenhouse temperatures to saturate the air with water vapor and then vent the warm, saturated air from the greenhouse. Additionally, horizontal airflow fans may be used to circulate air within the greenhouse, which helps to reduce the effective humidity experienced by the plant by reducing the boundary layer around the plant surfaces.

Indoor vertical facilities and chamber facilities may have systems in place to periodically vent high humidity to the outside, or they may use a dehumidifying system to manage humidity.

In some cases, such as propagation houses and seed germination chambers, it may be desirable to increase the humidity level. For propagation, mist, irrigation booms, or fog systems are most often used to increase the humidity and reduce water loss from cuttings by transpiration. If seeds are being germinated, fog or a fine mist may be used. Large water droplets should be avoided as they can cause splashing and may displace the seed. In germination rooms, growth chambers, and vertical farms, fog systems are typically used to increase humidity without applying additional water to the root substrate in plug trays. This allows for a more even moisture level to be maintained and reduces the amount of free water that accumulates on the floor.

Air Pollutants

Various air pollutants and other potentially harmful compounds can occur in the atmosphere of the CEA facility. Carbon monoxide (CO) is dangerous to people. It may be generated by malfunctioning heaters, vehicles, arc welders, and other combustion engine machinery. Unit heaters without internal heat exchangers should be avoided as they may emit CO into the greenhouse if not functioning properly.

Additionally, poorly maintained or improperly vented unit heaters may result in CO entering the facility. A CO concentration of 50 ppm for an eight-hour time-weighted average is OSHA's upper limit for human safety. In comparison, NIOSH recommends 25 ppm as the upper limit for an eight-hour time-weighted average.

In addition to generating CO, ethylene (C₂H₄), a operation). Exhaust fumes should be properly vented outside of the facility. Ripening fruits and vegetables also produce ethylene, so these products should not be stored in coolers, growth chambers, or other enclosed spaces with living plant material, such as seedlings, cuttings, or whole living plants.

If CO₂ burners are used in greenhouses, low- sulfur fuels should be used to avoid producing injurious levels of SO₂.

Herbicides can damage plants even at very low concentrations. Caution should be taken to ensure that herbicides applied outside of the facility do not drift (and are not pulled in by fans) into the facility. Some herbicides are labeled for greenhouse use, and herbicide labels should always be followed. However, even when used in or around greenhouse CEA facilities according to label directions, problems type of volatile organic compound (VOC), may be generated by such devices as malfunctioning heaters, vehicles, arc welders, or other combustion engine machinery. Ethylene gas is a naturally occurring plant hormone, but very low concentrations (as low as 0.05 ppm) of ethylene can cause plant damage. Typical symptoms of exposure to ethylene include epinasty (malformed leaves that curl or corkscrew often in a downward direction), "sleepy" flowers (flowers appear wilted and curled), and abscission or abortion of flowers and fruits.

Sources of ethylene should be avoided and excluded from the CEA production environment. Unit heaters should be regularly checked and properly maintained to ensure they function correctly (follow the manufacturer’s recommendations for proper maintenance and can occur. For example, if herbicides are sprayed onto active heating pipes, the high temperature may cause volatilization of phytotoxic components even though this would not occur under normal application conditions.

Numerous chemicals, including paint and cleaning materials, may release potentially damaging volatile chemicals and should be used in or around CEA facilities with caution.