SOIL AND PLANT ANALYSIS (1+1)

4. Measuring soil moisture potential in-situ (field)-Suction method or equilibrium tension method

Field tensiometers measure the tension with which water is held in the soils. They are used in determining the need for irrigation. The tensiometer is a porous cup attached to a glass tube, which is connected to a mercury monometer. The tube and cup are filled with water and cup inserted in the soil. The water flows through the porous cup into the soil until equilibrium is established. These tension readings in monometer, expressed in terms of cm or atmosphere, measures the tension or suction of the soil. If the soil is dry, water moves through the porous cup, setting up a negative tension (or greater is the suction). The tensiometers are more useful in sandy soils than in fine textured soils and can be used only up to a tension of 1 bar. At tensions above one bar the water column is broken and air enters the ceramic cup. Once the air gets entrapped in the tensiometer, the reliability of readings is questionable.

Advantages

• Recommendation for irrigation policy developed with the aid of tensiometers
• Inexpensive and easily constructed
• Works well in the saturated range
• Easy to install and maintain
• Operates for long periods if properly maintained
• Can be adapted to automatic measurement with pressure transducers
• Can be operated in frozen soil with ethylene glycol
• Can be used with positive or negative gauge to read water table elevation and/or soil water tension

Disadvantages

• Limit range of 0 to 0.8 bar not adequate for sandy soil
• Difficult to translate data to volume water content
• Hysteresis
• Requires regular (weekly or daily) maintenance, depending on range of measurements
• Subject to breakage during installation and cultural practices
• Automated systems costly and not electronically stable
• Disturbs soil above measurement point and can allow infiltration of irrigation water or rainfall along its stem.