Water & Nutrient Management:
Soil Moisture-Based Irrigation Systems

Tensiometric Measurement:
Gypsum Block

Working Principle

The electrical resistance between electrodes embedded in a porous medium (block) is proportional to its water content, which is related to the soil water matric potential of the surrounding soil. Electrical resistance reduces as the soil, hence the block, dries.

Description

A gypsum block sensor constitutes an electrochemical cell with a saturated solution of calcium sulfate as electrolyte. The resistance between the block-embedded electrodes is determined by applying a small AC voltage (to prevent block polarization) using a Wheatstone bridge. Since changes to the soil electrical conductivity would affect readings, gypsum is used as a buffer against soil salinity changes (up to a certain level). The inherent problem is that the block dissolves and degrades over time (especially in saline soils) losing its calibration properties. It is recommended that the block pore size distribution match the soil texture being used. The readings are temperature dependent (up to 3% change/°C) and field measured resistance should be corrected for differences between calibration and field temperatures. Some reading devices contain manual or self-compensating features for temperature or the manufacture provides correction charts or equations. Measurement range is 0.3-2.0 bar.

Advantages

• Up to 4 inch measurement cylinder radius
• No maintenance needed
• Simple and inexpensive
• Salinity effects buffered up to 6 dS/m
• Well suited for irrigation where only "full" and "refill" points are required
• Suited to regulated-deficit irrigation

Drawbacks

• Low resolution, limited use in research
• Block cannot be used for measurements around saturation (0-0.3 bar)
• Block properties change with time, because of clay deposition and gypsum dissolution. Degradation speed depends on soil type, amount of >rainfall and irrigation, and also the type of gypsum block used
• Very slow reaction time. It does not work well in sandy soils, where water drains more quickly than the instrument can equilibrate
• Not suitable for swelling soils
• Inaccurate readings due to the block hysteresis (i.e., at a fixed soil water potential, the sensor can display different resistance when wetting than when drying)
• Temperature dependent. If connected to a logging system, another variable and sensor for temperature must be added to the system

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This page was last updated on June 15, 2010.