When the soil moisture increases, the water can dissolve the ions contained in part of the soil, so that the resistance of the soil decreases, and therefore the output value of the soil moisture sensor becomes greater. When the water content in the soil further increases (in the case of extremes, imagine that you throw some soil into water), the ion concentration in the soil is diluted by excess water, resulting in a decrease in electrical conductivity and an increase in resistance. Therefore, you may find that the soil moisture sensor outputs a smaller value in pure water than in moist soil (although the former has a significantly higher water content) because the ion concentration in the moist soil is higher and the resistance is smaller.
Remarks: The mBuild soil moisture sensor is a resistive one, and therefore it can also be used, to some extend, to detect resistance. We are considering developing some related ports.
Dimensions: 24 × 72 (mm)
Value range: 0–100
Consistency error: ±5%
Operating current: 14 mA