- Author: Michael D Cahn
- Author: David Chambers
A tensiometer is a very useful tool for monitoring soil moisture status of vegetable and berry crops. Compared to other sensors that often require equipment such as dataloggers or a computer to collect readings, tensiometers can be easily read by irrigators in the field. Also, tensiometer readings are not affected by variations in soil texture, temperature, and salinity and they can operate without electricity (no batteries needed).
What is tension? Tensiometers 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. 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).
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 lose 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.
A dependable tensiometer design We designed and tested a version of a tensiometer in 2018 that was simple to build and provided accurate readings for a material cost of less than $55 . The design improved the ability of the instrument to retain a vacuum at high tensions. Under moderately moist soil conditions the tensiometer usually required refilling with degassed water less than once per month. Even when the soil dried to tensions above the maximum range of the tensiometer (> 80 kPa), these tensiometers continued to hold a vacuum for about two weeks until all of the water in the shaft was depleted.
The following paragraphs describe the materials needed (Fig. 2) and procedures to build a tensiometer. The vendors of the materials are examples of ones that we use, but you may identify different or cheaper sources for these components. By carefully following these instructions, one should be able to build a dependable tensiometer that provides accurate tension readings. An update to this design can also be found in a more recent blog article.
Materials needed:
Ceramic cups
Vender: SoilMoisture Equipment Corporation, Santa Barbara CA (805-964-3525) Part Number 0655X01-B01M3, Dimensions: 0.875 inch OD x 2.75 inch length. Cost: $30.80 ea.
Epoxy (ceramic/plastic)
Vender: SoilMoisture Equipment Corporation, Santa Barbara CA (805-964-3525)
Part Number 0980V004, Description: 4 oz: epoxy and 4 oz hardener. Cost: $106 ea. Note that the epoxy/hardener is a sufficient volume to make several hundred tensiometers.
Vacuum gauge
Vender: Zoro.com/Grainger.com Part Number 4FMK3, Description: ¼ inch MNPT 2 inch diameter test vacuum gauge. Cost: $18.09 ea.
#1 size rubber stopper
Vender: Grainger.com Part Number 8DWU6, model RST1-S, Description: 24 mm neck, bottom diam. = 14 mm. Top diam. = 20 mm. Cost: $18.08 / 52 pieces
Schedule 40 PVC pipe (½ inch diameter) Vender: irrigation supply or hardware store
PVC “T”
Vender: irrigation supply or hardware store, Manufacturer: Spears Inc. Part number 402-072, Description: ½ inch slip x ¼ inch threaded reducing "T."
PVC glue (gray) and purple primer
Vender: irrigation supply or hardware store
Gas pipe thread sealant (white or blue paste type)
Vender: irrigation supply or hardware store
Painters masking tape
Vender: hardware store
Petroleum Jelly (Vasoline)
Vender: pharmacy
Tools needed:
- PVC saw or PVC cutting tool
- Aluminum Oxide grinding stone, Manufacturer: Forney Part Number: A11 60028 Description: 7/8 in [23 mm] diam. x 2 inch [50.8mm] length
- Power hand-held drill
- Miter box
- Pocket knife
Procedures
1. Cut PVC pipe sections in the following lengths
1 foot depth tensiometer: top shaft = 4 inches, bottom shaft = 17 inches
2 foot depth tensiometer: top shaft = 4inches, bottom shaft = 30 inches
It is advisable to cut the bottom shaft about 1-inch longer than indicated above and then carefully cut the lower end of the shaft using the miter box or electric miter saw to assure that it is cut at a 90-degree angle. The ceramic cup will fit crooked on the end of the shaft if the cut deviates from 90 degrees.
- First glue the top shaft and then the bottom shaft to the ½ PVC “T” using the PVC glue. Make sure that you do not glue the end of the bottom shaft that was trimmed to 90 degrees. In a well-ventilated location, apply PVC primer to both the end of the shaft and the inside of the “slip” end of the “T”. Then apply gray PVC glue to both sides, and push the parts together, and hold in place for about 30 seconds to 1 minute. Tip: slightly twist the parts by about 30 degrees immediately after gluing to assure that the parts are secure. Also cover the non-glued areas with painter's tape to prevent the outside from becoming covered with glue.
- Slightly bevel the inside of the lower end of the bottom shaft using the handheld drill and grinding stone (Fig. 3). Alternatively, one can use a knife to bevel the end. Whether using the drill or the knife to bevel the inside of the pipe, stop periodically and test fit the ceramic cup. This way you will not remove too much material, and will quickly get a feel for the appropriate amount to remove.
- Use epoxy to glue the ceramic cup to the lower end of the bottom shaft. Protect the ceramic cup during the gluing process by covering the outside with painter's tape (Fig. 4). Check that the ceramic cup fits snuggly into the PVC tube and is aligned straight. If using the epoxy from SoilMoisture equipment epoxy, mix up 1-part epoxy with 1-part hardener. Mix thoroughly. Only a small amount of epoxy is needed to coat the throat of the ceramic cup and the inside of the PVC tube, so it may be best to glue several tensiometers at the same time so that the epoxy is not wasted. One can usually glue no more than 20 to 40 cups at a time becaue the epoxy begins to cure after an hour. Approximately 20 ml of epoxy is needed for 20 tensiometers. The cure time is temperature dependent. Full cure is 8 hours at 77 °F. It is best to allow more time for curing. After gluing, painter's tape can be used to secure the cup to the shaft. Take care when securing the two with the tape to assure that the cup is aligned with the PVC shaft. Let the glue set for at least 24 hours with the tensiometer supported with the cup-end up in a vertical position. Tip: best if parts are glued at temperatures above 65 °F. More hardener may be needed at lower temperatures. Also, it is advisable to first test a small batch of epoxy to assure that the proportion of hardener to epoxy is enough for epoxy to set up hard.
- Coat the ¼ inch male threads of the gauge with pipe thread sealant and hand screw on the vacuum gauge. Tip: do not over tighten or the PVC “T” will crack!
- Fill the tensiometer fully with degassed distilled water. The water can be degassed by boiling it and allowing it to cool.
- Coat the lower end of the rubber stopper with a thin film of petroleum jelly and insert into the top end of the tensiometer with a light twist to firmly seat the stopper (A loose stopper is the main cause for vacuum leaks).
Preparing the tensiometer for testing and field installation
The tensiometer should be filled with degassed water (preferably distilled) before testing. Tap water will work too, but if it is hard water (contains a high concentrations of calcium and carbonate) it could cause precipitates to form inside the ceramic cup. Boiling will expel much of the dissolved air from the water. We do not recommend using a vacuum pump to remove dissolved air from the water. Boiling works best for degassing water. It also helps to soak the ceramic cup end of the tensiometer in water for a few hours so that the pores of the ceramic cup are saturated before testing and/or installation.
Testing the tensiometer for air (vacuum) leaks
After filling the tensiometer with water and sealing it with a rubber stopper, wrap a dry paper towel on the end of the ceramic cup and hold it tightly (Fig. 6). If the tensiometer is filled with degassed water, the tension should quickly increase to about 20 to 30 kPa as the towel absorbs water from the cup. If the gauge does not increase above 0, air is likely leaking into the tensiometer. Check the glue joints and assure that the stopper is tightly in place.
If the tension quickly increases to more than 20 kPa, then leave the tensiometer out in the sun to assure that the tension rises to above 70 to 80 kPa. This may take some time, minutes to hours, depending on the ambient temperature. If the tension does not increase to a high value, then check glue joints and the stopper. Also check that the gauge is securely threaded into the PVC “T.”
Installing tensiometers in the field
Proper installation of a tensiometer in the field will achieve close contact between the ceramic cup and surrounding soil. Using a soil probe with a ½ inch diameter shaft, make a pilot hole to a depth a few inches shallower that the depth of installation (Fig. 7). Make a soil water slurry by thoroughly mixing soil with the water to a pancake batter-like consistency. Add some slurry into the hole and push the tensiometer to the desired depth (Fig. 8). The soil slurry assures that water can freely move between the ceramic cup and the surrounding soil and fills the voids between the hole and tensiometer shaft. Formation of air gaps between the ceramic cup and the soil will lessen the accuracy of tensiometer readings. After two days of equilibration, the tensiometer reading should accurately reflect the tension of the soil.
- Author: Steven A. Tjosvold
When should container plants be irrigated? How much water should be applied? In this post, I describe the first step of irrigation scheduling, “when”, and the next post covers “how much”. First some background describing soil tension, available water, and the moisture release curve.
After irrigation and full drainage (container capacity), the plant removes water at first from the larger soil pores and then eventually from the smaller soil pores. The water is held more tightly by the smaller pores. As the soil becomes even drier, the soil pulls the water so tightly that the plant eventually begins to wilt. The “pull” that soil exerts on water is called soil tension. The amount of water held by the soil between container capacity and the wilting point is called the available water because it is the water that is available for plant uptake. Here's a relatively simple way to determine the available water. In practice, this would be done with several representative plants. Fig 1.
Soil tension can be measured by soil tensiometers. A tensiometer consists of an air tight, water filled tube with a porous ceramic tip at the bottom and vacuum gauge at the top. As soil dries, the soil sucks water from the tube, creating a vacuum, that translates to a measurement at the gauge. The soil tension is a negative pressure and is measured in recognizable units of pressure such as psi, centibars, milibars, and kilopascals. Fig 2.
With respect to the plant, not all available water is easily available water. This can be seen in a moisture release curve. A moisture release curve, expresses the relationship between the amount of water held in the soil and the soil tension. It is determined by irrigating a plant, allowing it to drain to container capacity, weighing it regularly up to the point that the plant wilts, and measuring the corresponding soil tension with a tensiometer. Generally, a curve such as this is obtained. Fig 3.
In a typical container soil, at container capacity, much of the water is not held tightly and is easily available to the plant (steep part of curve, low tension values). As the easily available water is removed, the remaining water is held more tightly (shallow part of curve, high tension values). Plants grow better at the lower soil tensions. Research has shown that plants should be irrigated when they have used half of their available water roughly corresponding to the point at which soil tensions rise rapidly. Fig 4.
In the example above, it would be recommended to irrigate at the point corresponding to about 25 mbar (2.5 kilopascals). At this point, water would be removed from most of the coarser pores. As the remaining available water is removed from finer pores, the tension begins to rise rapidly. In this illustration, wilting occurs at a tension of about 190 mbar (19 kPa). Note that there is still water in the soil (roughly 10% by volume), but it is unavailable to the plant. Other container soils (and field soils) have similar shaped curves.
Tensiometers are not used much in container culture for various reasons. Intuitively an experienced grower or person responsible for irrigation scheduling picks up the pot and feels its weight. Simply, when the pot “feels light”, an irrigation is scheduled. Well here is my recommendation to improve on that by knowing the available water and quantifying the change in weight over time. Fig 5.
First, determine the available water by weighing representative plants at container capacity and again when the plant wilts. The difference is the available water (see Fig 1 again if you need to). Now, monitor the daily change in weight. Start by measuring the weight of the pot just after an irrigation and drainage. Weigh again every day. The change of weight (in grams) during a 24- hour period represents the volume (milliliters) of water removed per day by the plant. (One gram of water equals one mili-liter of water). Irrigation should occur when half of the available water is used. That amount of water is what the plant extracted from the soil and evaporated from the soil surface. You would think that might tell you also how much water to apply to fill the soil completely back up with water. But actually some adjustments are still needed to calculate how much water to apply.
Next: The adjustments for salinity and irrigation uniformity and examples.
Figures and tables adapted from: Management of Container Media by Richard Evans, UC Davis, for the class, ENH 120.