One important tool missing in irrigated agriculture is a reliable, reasonably priced tensiometer. Is METER working toward providing that for growers and researchers?
Great question. Traditionally, a good tensiometer with an accurate pressure transducer costs around $900 per sensor. This is something we’ve been working on. We’ve just released the new TEROS 32 tensiometer which is a lower cost tensiometer. It’s now more comparable to the cost of our TEROS 21 sensor. We think it’s a step in the right direction because we do see the need for a lower cost tensiometer with high accuracy.
Are there water potential case studies for plants and turf available?
There are a lot of case studies about different types of plants, specifically about optimal water potential ranges. There was a paper done by Dr. Sterling Taylor on this topic, and there are also some studies being done by BYU scientists with in situ water release curves in turfgrass. One of our scientists wrote an article (find it here) and gave a webinar (find it here) about some of the turfgrass work being done at BYU.
If you want to measure water potential at a specific rot depth with a tensiometer, how do you know what depth to install the sensor with a long water column?
If you are using a more traditional tensiometer with a long water column, as long as you install the ceramic cup itself at the depth you want to measure, then you can make corrections based on the height of your water column. For example, if your water column is 10 cm high, it’s 1 hPa per 10 cm. Then you just offset your data based on that value.
What sensor might be appropriate in more arid environments when the soil water potential might be very low much of the year?
One of the better sensors for measuring in really dry conditions is a thermocouple psychrometer. The problem is they are not as commercially available and difficult to find. But if you can find one, they are a really useful tool for arid environments.
Can the relationship between soil moisture and water potential in the sensor range of accuracy be used to infer water potential from soil moisture readings in drier conditions?
This is actually a common approach taken by many people. You can try and develop that relationship in situ and infer what the drier condition water potentials are. There are functions available such as different van Genuchten functions to try and fit those data.
Is a tensiometer the only option for container media?
Yes, currently the best option for measuring water potential in container media is a tensiometer. The soilless media holds most of its water at a very high water potential—typically between 0 and -30 kPa. Once they drop below this, they tend to get very dry fast.
Is there some relationship between the potential readings with tensiometers and the volumetric water content?
Yes, the soil moisture release curve would be used to describe this relationship between the water potential readings from a tensiometer and the volumetric water content of the soil. Read more about it here.
What is the physical meaning of a positive readings from a tensiometer?
A positive reading from a tensiometer indicates that the soil is saturated and has a positive pressure head from a water table. As the water table increases above the sensor, the reading will become more positive.
How sensitive is a tensiometer? If the pressure changes only one millimeter, can you measure it?
This sensitivity depends on the sensitivity of the pressure sensor used in the tensiometer. Some tensiometers are only able to resolve pressure changes to 1 hPa which is approximately 10 mm of water. Newer sensors like the TEROS 32 have higher-precision tensiometers which, in theory, are able to resolve down to the mm level. However, they are not calibrated to be accurate down to this level. So you can resolve these changes, but the accuracy might not be as good as a level sensor for a piezometer.
How do sodium and bicarbonates in water affect a tensiometer reading?
Salts in the water do not interact with the measurement from a tensiometer. Because salts and bicarbonates primarily affect the osmotic potential of the soil, the tensiometers will not see this effect because salts are able to pass freely through the permeable ceramic. One piece we don’t have a lot of experience with are precipitates. If the bicarbonates were to precipitate on the ceramic this could potentially be an issue.
Are water potential sensors available that measure at 2" and 5" at the same time?
Currently, there is not a profile-type water potential sensor. The only way would be to place individual sensors at the desired measurement depths. A profile probe could be a powerful tool for this measurement and is something we may approach in the future.
Is there a paper you can refer me to concerning the effects of digging a soil trench on the soil at a site?
I don’t have a specific paper to refer to on this topic. The concern with large trenches is the way it affects water movement through the soil near the sensor. Depending on how the trench is repacked you can wind up with preferential flow paths which will result in faster water migration through the soil profile. For more information on this topic, see our article: "5 Ways Site Disturbance Impacts Your Data."
Which is the best sensor for measuring water potential lower than -1 atmosphere for research purposes?
For water potential below -1 atm (-100 kPa), a solid matrix sensor like the TEROS 21 is going to be more appropriate.