What is calibration and why does it matter

A soil moisture reading is really a measure of how easily electricity passes through the soil, and different soils behave very differently. Sandy soil, clay and compost all conduct differently, so the same raw number can mean parched in one bed and comfortable in another.

Calibration anchors the scale to your own soil. You show the monitor what dry looks like and what wet looks like, and from then on the percentage means something real for that ground. It is the single step that separates a monitor you can trust from a cheap meter that reads nonsense.