Water & Nutrient Management:
Soil Moisture-Based Irrigation Systems

Soil Psychrometer

Working Principle

Under vapor equilibrium conditions, water potential of a porous material is directly related to the vapor pressure of the air surrounding the porous medium. This means that the soil water potential is determined by measuring the RH of a chamber inside a porous cup equilibrated with the soil solution (Campbell and Gardner, 1971).

Description

A soil psychrometer consists of a ceramic shield or screen building an air chamber, where a thermocouple is located. The screen type is recommended for high salinity environments. RH in the air chamber is calculated from the "wet bulb" vs "dry bulb" temperature difference. Measurement range: 0.5-30 bar (less accurate for 10-30 bar range).

Advantages

• High sensitivity
• Scientifically rigorous readings (except in wetter soil conditions)
• Suitable where typical moisture conditions are very dry

Drawbacks

• Not recommended at shallow soil depths, due to high susceptibility to thermal gradient
• Small sensing volume
• Very slow reaction time, because reaching vapor equilibrium takes time
• Low accuracy in the wet range
• Specialized equipment is required for the sensor's excitation and reading

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