VPD: The Drying Power of Air
What VPD really is, why it — not RH — drives transpiration, and the stage-by-stage kPa targets that steer the crop.
Vapor pressure deficit (VPD), measured in kilopascals (kPa), is the drying power of the air: the saturation vapor pressure at the leaf surface minus the actual vapor pressure of the surrounding air. It is the true driver of transpiration — the water stream a plant pulls from its roots and evaporates through its leaves. Relative humidity alone cannot tell you this, because the same RH at two different temperatures produces very different drying power.
Transpiration does three jobs at once: (1) it cools the leaf, (2) it keeps stomata open for CO2 uptake, and (3) it pulls the water-and-nutrient stream up from the roots. Mass-flow nutrients — especially calcium and boron — ride that stream. When transpiration stalls, so does uptake, even when the nutrient is sitting in the feed. VPD is therefore a nutrition control as much as a climate one.
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