Sensoterra offers two units by which the soil moisture measurements recorded by the sensors can be displayed: the volumetric soil moisture content of the soil [expressed in the app as "Volumetric (%)"] and the Sensoterra Index [expressed as "SI"].
The volumetric soil moisture level provides a percent value of the soil moisture content of the soil. It is simply the ratio of water volume to soil volume.
The volumetric soil moisture data provides a precise expression of the volume of water in the soil. A user may decide to select the "Volumetric (%)" option in the app in order to track smaller changes in the soil moisture content of the soil. It is important to note, however, that different soil types have different thresholds for too dry, too wet, limiting soil moisture levels, productive soil moisture levels, field capacity, and permanent wilting points.
These thresholds and their respective percentages can be seen below. The most important of these thresholds are the "too dry," "too wet," and the "plant available water" (or healthy) zones. A download of this graphic is available at the bottom of the article.
The Sensoterra Index focuses on the "too dry," "too wet," and the "plant available water" (or healthy) zones and spreads the three zones over a 0 to 10 scale. The 0 and 10 values represent oven dry soil and complete saturation of the soil, respectively.
The Sensoterra Index allows the user to compare soil moisture levels over different soil types. It makes it easier to quickly see whether a sensor or group of sensors are showing healthy soil moisture values and to act on these values more quickly.
Regardless of the units that you choose to use for the Sensoterra sensors you install, we encourage you to study the scales for different soil types and to use your experience and intuition when setting your own wet and dry thresholds in the app.