Vapor Pressure Deficit (VPD) measures the air’s capacity to hold moisture compared to the amount it currently contains. This difference indicates “how thirsty” the air is, influencing plant health and growth. Monitoring VPD provides a more complete picture of the growing environment than tracking temperature and humidity separately. The measurement is particularly relevant for growers in controlled environments like greenhouses, as it directly governs the rate at which plants lose water through transpiration.
Transpiration is the process where a plant releases water vapor through tiny pores on its leaves called stomata. This process is necessary for plants to cool themselves and to pull nutrients up from the roots. The VPD determines how wide these stomata open and how quickly the water vapor moves out. When VPD is in the optimal range, the stomata are slightly open, allowing for healthy water movement and carbon dioxide intake, which supports photosynthesis.
Essential Components of Vapor Pressure Deficit
Calculating VPD requires understanding the two components that create the deficit: Saturated Vapor Pressure (SVP) and Actual Vapor Pressure (AVP). Both are measures of pressure, typically expressed in kilopascals (kPa). The relationship between these two values defines the air’s drying potential.
Saturated Vapor Pressure (SVP)
SVP represents the maximum amount of water vapor the air can hold at a given temperature. This condition occurs when the air is 100% saturated with water vapor. Temperature is the variable that determines SVP, as warmer air has a greater capacity to hold moisture.
Actual Vapor Pressure (AVP)
AVP is the measurement of the water vapor pressure currently present in the air. This value is directly related to Relative Humidity (RH), which is the percentage of moisture the air holds compared to its maximum capacity (SVP). AVP is mathematically derived by multiplying the SVP by the Relative Humidity, expressed as a decimal.
Step-by-Step Calculation
The calculation of VPD is a three-step process beginning with air temperature and relative humidity measurements.
Step 1: Calculate Saturated Vapor Pressure (SVP)
The first step determines SVP using the air temperature (\(T\)) measured in degrees Celsius. The SVP (in kPa) is approximated using the Tetens formula:
$\(\text{SVP} = 0.611 \times \exp\left(\frac{17.27 \times T}{T + 237.3}\right)\)$
For example, if the air temperature is \(26^\circ \text{C}\), substituting this value yields an SVP of approximately \(3.36 \text{ kPa}\).
Step 2: Calculate Actual Vapor Pressure (AVP)
AVP is calculated using the SVP and the measured Relative Humidity (RH). The RH must be converted from a percentage into a decimal for the calculation. The formula for AVP is:
$\(\text{AVP} = \text{SVP} \times \left(\frac{\text{RH}}{100}\right)\)$
If the measured Relative Humidity is \(60\%\), the AVP is calculated by multiplying the \(3.36 \text{ kPa}\) SVP by \(0.60\), resulting in approximately \(2.02 \text{ kPa}\).
Step 3: Calculate Vapor Pressure Deficit (VPD)
VPD is found by subtracting the AVP from the SVP. This difference quantifies the driving force for water to leave the plant. The final calculation is:
$\(\text{VPD} = \text{SVP} – \text{AVP}\)$
Using the example values, \(3.36 \text{ kPa}\) minus \(2.02 \text{ kPa}\) gives a VPD of \(1.34 \text{ kPa}\).
Practical Application and Optimal Ranges
The calculated VPD value indicates the evaporative demand placed on a plant. Growers use this number to make precise adjustments to the environment to optimize plant functions. A high VPD indicates that the air is dry and is rapidly pulling moisture from the plant.
High VPD causes a plant to lose water through transpiration faster than its roots can absorb it, leading to wilting and stress. Plants respond by partially closing their stomata to conserve water. Stomatal closure reduces the uptake of carbon dioxide, which slows growth and overall productivity.
Conversely, a low VPD signifies that the air is highly saturated with moisture. Low VPD slows the rate of transpiration, hindering the plant’s ability to move nutrients efficiently from the roots to the leaves. If the VPD is too low, high humidity conditions can also encourage the development of fungal diseases and mold on plant surfaces.
For most plants in the vegetative growth phase, the optimal VPD range is between \(0.8 \text{ kPa}\) and \(1.2 \text{ kPa}\). This range balances the need for healthy transpiration and nutrient uptake with the need to prevent excessive water loss.
Optimal VPD Ranges by Growth Stage
For seedlings and clones, a lower VPD, such as \(0.4 \text{ kPa}\) to \(0.8 \text{ kPa}\), is preferred to reduce stress while they establish their root systems. As plants move into the flowering stage, the optimal range may be slightly higher, often between \(1.0 \text{ kPa}\) and \(1.4 \text{ kPa}\), to help prevent mold.