Crop Steering Environmental Factors: VPD, EC, VWC, Matric Potential

VPD (VAPOR PRESSURE DEFICIT)

Vapor Pressure Deficit (VPD) is a key factor in crop steering, representing the difference in water pressure between the inside of a leaf (assumed to be at 100% relative humidity) and the surrounding air.

Measured in kilopascals (kPa), VPD is determined by the interplay of leaf temperature, air temperature, and air humidity. It directly influences the plant’s transpiration rate and stomatal behavior, which in turn affects water uptake, nutrient transport, and photosynthesis.

In the context of crop steering, manipulating VPD allows growers to influence the plant’s water relations and, consequently, its growth patterns.

Higher VPD values tend to create more generative conditions by increasing transpiration and creating mild water stress, while lower VPD values promote vegetative growth by reducing transpiration and maintaining higher turgor pressure in the plant tissues.

Vapor Pressure Deficit Recommendations - Black: too high or too low of VPD; Dark Red: Borderline VPD; Bright Red: Optimal VPD

Environmental Targets By Growth Stage

Here is an example of one way a grower might adjust the environment for a well-controlled and closely monitored facility with high light intensity. Note that there is no single optimal environmental strategy.

Vegetative Stage:

• VPD: 0.9–1.0 kPa

• Temperature: 85°F (day) / 85°F (night)

• Relative Humidity (RH): 78% (day) / 78% (night)

• CO2: 600 ppm

Stacking Stage:

• VPD: 1.0–1.2 kPa

• Temperature: 85°F (day) / 80°F (night)

• Relative Humidity (RH): 75% (day) / 70% (night)

• CO2: 800–1200 ppm

Bulk/Swell Stage:

• VPD: 1.2–1.3 kPa

• Temperature: 82°F (day) / 70°F (night)

• Relative Humidity (RH): 70% (day) / 55% (night)

• CO2: 1200–1500 ppm

Ripening Stage:

• VPD: 1.2–1.5 kPa

• Temperature: Gradually decreased from 76°F (day) / 65°F (night) to 65°F (day) / 55°F (night)

• Relative Humidity (RH): Gradually decreased from 62% (day) / 50% (night) to 50% (day) / 40% (night)

• CO2: 1000–600 ppm

Note: During the ripening stage, temperature and relative humidity (RH) are gradually decreased to promote proper flower development and coloration.

EC (ELECTRICAL CONDUCTIVITY) & OSMOTIC POTENTIAL

EC management is crucial for crop steering. Here is one example for EC targets for different growth stages with high PPFD in coco coir:

Vegetative Stage:

• Drip EC (Electrical Conductivity) should be maintained at 3.0–3.5 mS/cm.

• Substrate EC should range from 4–6 mS/cm.

Stacking Stage:

• Drip EC remains consistent at 3.0–3.5 mS/cm.

• Substrate EC increases to a range of 5–12 mS/cm.

Bulk/Swell Stage:

• Drip EC is slightly reduced to 2.5–3.0 mS/cm.

• Substrate EC should stay between 4–7 mS/cm.

Ripening Stage:

• Drip EC is further reduced to 1.7–3.0 mS/cm.

• Substrate EC is adjusted to a range of 6–8 mS/cm.

OSMOTIC POTENTIAL & CROP STEERING

Osmotic potential is the force that plants must overcome to absorb water from the substrate, resulting from the difference in solute concentration between the substrate and the roots. In crop steering, manipulating EC directly affects osmotic potential. Higher EC increases osmotic potential, making it harder for plants to uptake water.

This creates a mild stress that can be used to influence growth patterns:

• Lower EC (lower osmotic potential) promotes vegetative growth by allowing easier water uptake.

• Higher EC (higher osmotic potential) promotes generative growth by creating mild water stress, encouraging the plant to focus on reproductive structures.

VWC (VOLUMETRIC WATER CONTENT) & MATRIC POTENTIAL

VWC is another crucial parameter in crop steering. VWC represents the volume of water contained in the substrate. Here is an example of VWC targets for different growth stages in coco coir. Note that the optimal VWC will be highly dependent on substrate characteristics and desired outcome.

Vegetative Stage:

• The maximum volumetric water content (VWC%) is 50%.

• The minimum volumetric water content (VWC%) is 35%.

• The P3 dryback target is 15%.

Stacking Stage:

• The maximum volumetric water content (VWC%) is 45%.

• The minimum volumetric water content (VWC%) is 20%.

• The P3 dryback target is 25%.

Bulk/Swell Stage:

• The maximum volumetric water content (VWC%) is 50%.

• The minimum volumetric water content (VWC%) is 35%.

• The P3 dryback target is 15%.

Ripening Stage:

• The maximum volumetric water content (VWC%) is 45%.

• The minimum volumetric water content (VWC%) is 20%.

• The P3 dryback target is 25%.

MATRIC POTENTIAL & CROP STEERING

Matric potential is the force that water experiences due to the physical properties of the substrate, such as surface tension and capillary action. It represents how tightly water is held by the substrate particles. In crop steering, understanding and manipulating matric potential is crucial:

• Lower matric potential (wetter substrate) promotes vegetative growth by making water easily available to the plant.

• Higher matric potential (drier substrate) creates mild water stress, promoting generative growth and root development.

The interplay between osmotic potential (influenced by EC) and matric potential (influenced by VWC) allows for precise control over plant water relations and growth patterns throughout the cultivation cycle.

By controlling irrigation frequency and volume, growers can manipulate matric potential to influence plant growth and development. This is particularly important in soil based systems, where matric potential plays a more significant role than in hydroponic setups.