The impact of air humidity on plants
the importance in crop cultivation
Relative air humidity exerts a significant influence on plant physiology, particularly through its effect on transpiration. Transpiration or evapo-transpiration from the areal parts of the plants, is responsible for the water movement through a plant and is vital for the plant’s ‘pump system’. This system enables the uptake and distribution of water and essential nutrients throughout the plant. Moreover, the release of water vapour through the stomata enables the plant to cool itself.
High Air Humidity: When air humidity is high and air movement is low, the water vapor released by plants cannot readily disperse. This leads to a reduction in the rate of transpiration.
Low Air Humidity: Conversely, low humidity levels (often coupled with high temperatures) result in a very high transpiration rate. To prevent excessive water loss and dehydration, plants will close their stomata, the small pores on their leaves responsible for gas exchange and water evaporation.
Optimal Air Humidity: Maintaining the correct air humidity and air movement is essential for healthy plant growth. It allows plants to keep the stomata open, facilitating:
Appropriate uptake of water and nutrients.
Cooling through transpiration.
Intake of carbon dioxide (CO2), necessary for photosynthesis.
Stress Conditions: Prolonged exposure to high temperatures and low humidity can induce stress in plants. This stress can manifest as:
Stunted growth.
Nutrient deficiencies.
Increased susceptibility to diseases.
Similarly, consistently low temperatures coupled with high air humidity can also negatively impact plant development, leading to reduced growth rate and increased risk of fungal diseases.
Monitoring Air Humidity
Monitoring of air humidity is the first step towards effective control.
Hygrometers: Reliable hygrometers are essential tools for measuring air humidity. Good quality hygrometers offer accuracy and less frequent calibration.
Wet/Dry Bulb Thermometers: Simple wet and dry bulb thermometers can provide reasonable indication of relative humidity when used in conjunction with appropriate interpretation tables. Having two such thermometers allows for cross-checking their accuracy.
Humidity Deficit: A more precise indicator of air humidity is the humidity deficit. This refers to the difference between the actual amount of water vapor in the air and the maximum amount the air can hold at saturation point. Warmer air has a greater capacity to hold water vapor, thus increasing its potential humidity deficit. Air with a large humidity deficit has a strong dehydrating effect on plants.
Increasing Relative Air Humidity Levels
Various methods can be employed to increase moisture levels in (semi)-controlled environments. Take note that the plants’ own evapo-transpiration primarily maintains humidity levels within the plant canopy.
Manual Sprinkling: The simplest method involves manually sprinkling water using a hand hose, particularly on open spaces like pathways within and around the cultivation area. The amount of water applied can be regulated by the discharge rate and the operator’s walking speed. This method aims to moisten the topsoil while keeping the plants dry and avoiding the formation of mud puddles, relying on the worker’s skill for even application.
Fixed Systems: Fixed humidification systems, such as overhead misters and sprinklers, can face challenges in achieving uniform water distribution. Fewer discharge points may lead to drier areas between them, while more points can result in overlapping and excessively wet zones.
At soil-level. For systems at soil level, it is generally recommended to position misters, foggers, and mini-sprinklers in open spaces (pathways and edges of beds) rather than directly among the plants. These devices can have vertical (blowing mist upwards) or horizontal (umbrella-type, covering a wider area) discharge patterns. The cooling effect of these soil level devices is low, hence size of the water droplets is less relevant. A low discharge rate (e.g., 10 to 20 liters per hour per emitter) is important to avoid overwatering the soil and inadvertently providing additional irrigation water without nutrients. Even with frequent short bursts (e.g., three intervals of 20-30 seconds per hour), this translates to a relatively low water application rate per unit area, with a humidifying effect through evaporation from the soil surface.
Aerial humidification systems provide a more significant cooling effect as water evaporates in the air before it settles on plants and soil. A small droplet size is paramount. These systems are typically operated for very short durations (seconds) but very frequently (6-8 times per hour) to achieve stable humidity and a cooling effect, without making the plants wet. It is crucial to consider airflow patterns within the greenhouse, as moisture can be carried to one side, leaving other zones dry. The misters shall be equipped with a device to avoid dripping after valves are closed.
Mist blowing fans: Semi-static or mobile overhead mist-blowing fans offer similar characteristics to fixed overhead mist systems but with the added advantage of air circulation. The swivelling motion of the fan can cover a large area (with an outflow reach of up to 30 meters). These fans can be operated for longer periods, and their mobility allows for more selective water distribution.
A simple high-tech mist system for propagation
Sophisticated Systems: The pad and fan system represents a more comprehensive approach to greenhouse air humidification and cooling. Air is drawn through water-saturated pads, where it absorbs moisture and then flows through or over the plants. Even more advanced humidity control systems exist in some modern glass greenhouses, primarily in Europe and other temperate regions. These highly technical systems are generally not suitable for the semi-open greenhouses common in sub-tropical climates.
Regardless of the humidification method used, it is generally advisable to cease operation by 4 or 5 pm to allow the greenhouse environment to dry up before nightfall, thus preventing the build-up of excessively high humidity during the night.
It is beneficial to install more emitters along the periphery of the greenhouse, where humidity tends to be lower due to the influence of side vents.
System Quality: The quality of a humidification system is reflected, among others, in the droplet size it produces and its resistance to clogging. High water pressure at the discharge point contributes to smaller droplet sizes. Some emitters utilize internal centrifugal force to increase water flow speed and reduce droplet size even at relatively lower pressures. High water speed also aids in self-cleaning the emitters; however, using good-quality water and a reliable filter system remains essential to prevent blockages. Good quality water has low levels of suspended particles and other impurities. Reverse Osmosis water is excellent, but it is costly.
Material Considerations: Metal components in humidification systems allow for operation at higher pressures compared to systems made of PVC or polyethene. While high-pressure systems are common in livestock housing, the more cost-effective, lower-pressure systems are typically used in greenhouses due to their larger scale.
System Maintenance: Regular maintenance is crucial for the optimal performance of all humidification systems. This includes:
Regularly checking and unblocking emitters.
Cleaning filters.
Repairing any leakages (also in hand hoses).
Adjusting emitter placement or operation in response to observed wet or dry spots.
Reduction of Humidity
Managing excessively high humidity, which can occur during certain times of the year (rain), at night, and especially in the early morning, is often more challenging than increasing humidity.
Even when the inside and outside air have similar humidity levels, internal air circulation is beneficial. It helps to even out microclimate variations within the greenhouse, both in terms of humidity and temperature. Notably, it can reduce localized areas of 100% humidity within the plant canopy, which can impede transpiration, allowing the plants to restore their evapo-transpiration.
Heating and Ventilation: The most reliable method for reducing humidity is by heating the air, which increases its capacity to hold moisture, thus lowering the relative humidity. However, this is mostly economically not feasible in semi-open greenhouses. A less perfect but more feasible approach involves increasing airflows.
Utilizing Drier/Warmer Outside Air: If the air outside the greenhouse is drier or warmer than the inside air, exchanging it for the inside air will effectively reduce the internal humidity. Outside air entering the greenhouse should be distributed using circulation fans.
Ventilation vs. Circulation: It is important to distinguish between ventilation and circulation. Ventilation involves the exchange of air between the inside and outside of the greenhouse, while circulation refers to the movement of air within the greenhouse.
Natural Ventilation: Natural ventilation can be achieved through side and top vents, utilizing natural air currents. The airflow can be regulated by partially open or closing the vents, especially in windy conditions.
Forced Ventilation: Forced ventilation employs exhaust fans to draw air out of the greenhouse.
Fan Requirements: Depending on the density and stage of the crop, approximately 12 fans per acre can provide adequate air circulation. An airflow rate of 0.5 to 1.0 meter per second over the crop is generally sufficient. In view of power consumption, the fans shall be operated individually or in blocks.