What is a Tensiometer?

Source: Salina Valley Agriculture

Definition – A tensiometer measure soil moisture in units of negative pressure also known as tension.

Tension is a measure of the force that plant roots need to exert to pull water from the soil pores.  Large pores hold water with less force than small pores.  As plants extract moisture from the soil, water is first taken up from the largest pores.  As the soil dries roots need to exert more force to pull water from the smaller pores.  Hence, high tension values mean that the soil is becoming dry.

How do tensiometers work? Tensiometers are filled with water (preferably distilled) that has been degassed by boiling it.  A key component of the tensiometer is a porous ceramic cup which allows water in the shaft of the tensiometer to freely pass into the soil without air bleeding though the small pores in the cup (Fig. 1).  If the soil is not saturated, water will move from inside the cup into the unfilled soil pores.  Because air cannot replace the space vacated by the exiting water, a vacuum develops in the shaft of the tensiometer that can be measured with an accurate gauge.  Water will stop migrating from inside the tensiometer cup into the soil when the internal vacuum pressure of the tensiometer equals the soil tension, or the force needed to pull water from the soil pores.   The vacuum gauge measures tension in units of kPa or cbars, which are equivalent (1 kPa = 1 cbar).

Figure 1. At tensions less than 80 kPa water can freely move through the small pores in the ceramic cup without air entering into the tensiometer.

Interpretation of tension readings Because the tension value provides a sense of how much energy a plant would need to exert to suck water from the soil, tensiometer readings can be easily related to water stress in crops.  At high tension values a plant experiences more water stress and growth slows.   In addition, a tension reading has a similar meaning in terms of water stress whether the soil has a sandy, clay or loam texture. 

Reliability of tensiometers The one Achilles’ heal or weakness of the tensiometer is that if any air leaks into the instrument it will not retain a vacuum and the readings will be unreliable.   There are several brands of commercial tensiometers available.   Some are relatively inexpensive and simple to use, and others are more complex and can be interfaced with dataloggers to provide continuous readings throughout the day. Based on our experience, some of the most popular commercially available tensiometers often leak air and loses vacuum pressure, and in many cases the gauges do not provide accurate readings or are not durable.   The loss of vacuum pressure means that the tensiometers need to be frequently refilled with degassed water.  Also, irrigators may mistake a low reading to indicate that a crop has adequate moisture when in reality the soil may be dry.

Figure 2. Components needed to build a tensiometer:
a. ceramic cup, b. ½ inch PVC pipe for shafts, c. PVC “T”, d. vacuum gauge, and e. rubber stopper.