# Soil Moisture Measurements Soil moisture sensors can be used for schedule irrigations (i.e. determine when to water the plants); or for calculating soil water deficit to work out how much water to apply. ## Reference levels For irrigation, the most useful measurement is the **soil water tension**, which tells how much effort the plants need to extract water from the soil, independently of the type of soil. These are the reference levels for the [Watermark tensiometer](https://docs.smartcitizen.me/knowledge/soil-water/moisture/Tensiometer.md), according to the manufacturer [^1]: | Soil water tension (centibars) | Interpretation | |:-: |:- | | 0 – 10 | Saturated soil | | 10 – 30 | Soil is adequately wet (except coarse sands, which are beginning to lose water) | | 30 – 60 | Usual range to irrigate (most soils) | | 60 – 100 | Usual range to irrigate in heavy clay soils | | 100 – 200 | Soil is becoming dangerously dry | Capacitive sensors like the [Chirp](https://docs.smartcitizen.me/knowledge/soil-water/moisture/Chirp_Soil_Moisture.md) give a relative value (%) between their dry and wet calibration points, so the right level for irrigation depends on the soil and the crop. A good practice is to note the reading right after a deep irrigation (field capacity) and the reading when plants start to show stress, and use them as the upper and lower references. ## Supported sensors - [Chirp Soil Moisture](https://docs.smartcitizen.me/knowledge/soil-water/moisture/Chirp_Soil_Moisture.md) (Field | soil; Metric | moisture; Connection | external) - [Tensiometer / Irrometer](https://docs.smartcitizen.me/knowledge/soil-water/moisture/Tensiometer.md) (Field | soil; Metric | soil tension; Connection | external) ## References [^1]: IRROMETER Company. “WATERMARK Soil Moisture Sensor - Model 200SS.” https://www.irrometer.com/pdf/403.pdf.