The key to how SUStee tells you the best timing for watering is the pF value (Potential Force).
The pF value is a highly important scientific indicator for evaluating soil moisture. It expresses the soil’s water retention capacity (suction force) on a logarithmic scale. Using a device called a tensiometer, it is applied in agriculture for determining irrigation timing and in civil engineering for evaluating safe ground design.

• A higher pF value = stronger water retention in the soil = harder for plants to absorb water.
• A lower pF value = weaker water retention in the soil = easier for plants to absorb water.

Regardless of plant type, the range of soil moisture that plants can comfortably utilize is called the “available water range”, which corresponds to pF 1.7–2.3.

SUStee is designed to gradually turn white when the soil moisture reaches around the median of the available water range (about pF 2.0), and to become completely white at about pF 2.3. This indicates that plants are no longer able to absorb sufficient water from the soil.