Root-zone readings describe water in growing media. Air relative humidity describes water vapour relative to saturation. They answer different questions.
What does the instrument measure?
| Reading | Meaning |
|---|---|
| Air RH | Vapour relative to saturation at the air temperature. |
| Soil VWC example | Liquid-water volume divided by soil volume, often expressed as a percentage. |
| Growing-medium water content | A water-content probe indicates how much water is present. |
| Growing-medium suction | A tensiometer indicates suction: how readily roots can extract water from the medium. |
Sources: UMass; UMN; UMass irrigation handout.
Two percentages, different ratios
RH and soil volumetric water content (VWC) have different denominators. Equal displayed percentages do not mean equal water conditions. The soil example also does not establish that every instrument labelled “moisture %” reports VWC.
Supply and atmospheric demand
Media water status concerns the root-side supply. The vapour-pressure difference between the leaf interior and surrounding air drives transpiration. These are connected through the plant, but an air RH reading cannot substitute for measuring the medium.
A dry surface can hide wetter media
A container may look dry on top while remaining wetter deeper down. UMass describes this difference in its hand-watering guidance. Surface appearance, container weight and a moisture reading provide different observations; pot weight alone is not a calibrated water-volume reading.
Identify the reported quantity before interpreting it. Irrigation decisions also depend on the growing medium, environment and plant needs.
Greenhouse humidity control explains the separate air-side moisture balance.
