plant_physiology → vpd
Vapour Pressure Deficit (VPD) describes the difference between how much moisture the air can hold and how much it currently holds.
It represents the drying power of the air, not humidity alone.
VPD integrates: - Temperature - Relative humidity
VPD directly controls: - Transpiration rate - Stomatal behaviour - Nutrient transport - Leaf cooling - Stress signalling
Plants respond to VPD continuously throughout the day.
When VPD is too low: - Transpiration slows - Calcium and boron transport is reduced - Leaves remain wet for longer - Disease pressure increases
Typical symptoms: - Tip burn - Blossom end rot - Weak tissue - Slow growth
When VPD is too high: - Transpiration accelerates - Water loss exceeds uptake - Stomata close to prevent dehydration - Photosynthesis is restricted
Typical symptoms: - Wilting - Leaf scorch - Growth suppression - Stress-induced nutrient imbalance
The optimal VPD range varies by crop and growth stage, but generally: - Too low → nutrient transport limitation - Too high → water stress and stomatal closure
Young plants and fruiting crops have narrower tolerance windows.
VPD does not act alone.
High VPD combined with: - Poor root oxygen - High EC - Sodium stress
…rapidly overwhelms the plant’s ability to supply water.
Many calcium-related disorders are air-driven, not fertiliser-driven.
Managing VPD is often more effective than adjusting nutrient recipes.