Using tensiometers to make irrigation decisions in greenhouse production.

Heiner Lieth

Environmental Horticulture

University of California



In greenhouse production plants are generally in a rooting medium that is either a very-porous artificial medium or a soil that has been highly amended with various materials. The resulting rooting medium has a high water-holding capacity and allows water to move very quickly. Many media allow excess water to drain away, but usually the grower must be careful to not over water. The tensiometer is a useful tool in judging the moisture status of the rooting medium.

With the availability of tensiometers that are suitable for very porous media, this tools is being tried by many growers. This article deals with some of the issues that you confront when you start to fine-tune your irrigation practices by using this tool.

Tensiometers measure moisture tension or matric potential of a porous medium (such as soil or potting mix). In scientific jargon, "matric potential" is just one concept of a wide range of terms dealing with water status in soils and plants. It is in "pressure units" - positive values indicate pressure, negative values indicate suction or tension. Thus the matric potential of soils or potting mixes generally will negative. The term moisture tension represents the degree of such suction and will be given as a positive number. For example, if the matric potential of some soil is -10 kPa (kilopascal), then the moisture tension is 10 kPa. This is pretty simple but can lead to confusion when scientists and growers get together and start talking about high or low tensions and potentials.

In this article I use only the term "tensions" (rather than matric potential). Thus all the numbers will be positive. The higher the number, the higher the tension, the dryer the condition. The lower the number, the lower the tension, the wetter the conditions.



Moisture tension in pots

Before one can fully understand the issues and the optimal strategy for irrigation based on SMT (soil moisture tension) it is necessary to understand how tension is related to water content and how this varies with depth of the root-zone. Figure 1 shows the relationship between moisture tension and moisture content for the medium UC mix. The general shape for most potting media is similar, but they do vary enough to affect final recommendations for how to irrigate.

Note that this curve illustrates some general points about irrigating plants. The wet condition (i.e right after a thorough irrigation) is represented by tensions near zero. As water is removed from the pot by the plant (or by evaporation) the status progresses along the curve to higher tensions and lower water contents. Note that this particular mix can have a water content of 75%. So if we have a one-liter (1000 ml) container filled with this medium, then this can hold 750 ml of water. But note that a significant portion of this (220 ml or 22% of the total volume) is not available to the plant. Thus despite having a water holding capacity of 75%, this medium can hold 53% available water (i.e. 75% - 22%).

Another thing to note is that much of this available water is exhausted by the time the dry-down has reached 7 kPa. At this point, extraction of a little more water sends the tension over 10 kPa and beyond. At tensions over 10 kPa a plant which is accustomed to fairly moist conditions will start showing signs of wilting. Unless one starts irrigation fairly soon, the plant will be exposed to conditions which are damaging. Thus it is generally wise to irrigate when the tension is around 5 kPa; after that the urgency increases radically with increasing tensions.

It is possible to conclude from the diagram which ranges of tension are best for the plant (1 to 5 kPa) and that tensions above 10 kPa are dangerous for the plant. Figure 1 also indicates how a human might perceive soils at various moisture tensions. It is particularly interesting that at 10 kPa one is still able to sense moisture in the medium, but very little remains available for the plant. Thus using one's sense of touch is not a particularly good indicator as to when plants need to be watered. Thus it is desirable to use an instrument to measure moisture content. While there are numerous different types of soil moisture sensing devices, only one actually measures moisture tension: the tensiometer.



The Tensiometer

A tensiometer is a device that can measure moisture tension in a porous medium. It consists of a tube which has a ceramic tip on one end, and a vacuum gage attached to the other. The composition of the ceramic of the tip dictates how fast water moves in and out of the instrument. Basically, if the instrument will encounter very dry conditions (high tension), then the ceramic needs to be very fine, so that water moves through it very slowly. If the tensiometer will be not encounter dry conditions, then it can be made of more-porous ceramic through which water can move much faster. Traditional tensiometers, designed for use in field soils, have much finer textured-ceramic than ones designed for use in highly amended media or sand, and are therefor much slower to react to changes. Consequently they are not as useful in greenhouse production as tensiometers that are specifically designed for use in very porous media.

The traditional tensiometer comes equipped with a dial-gage. For instruments designed for use in field soils, tensions are anticipated to be as high as 100 kPa. In greenhouse production we never encounter tensions that high, so that it is preferable to have a gage that reads only the range that is likely to be encountered (0 to 30 kPa). It is also possible to replace the gage with a electronic pressure transducer so that electronics can be used to display the moisture tension being registered by the instrument.



Tensiometers in pots



The gage that is mounted on the upper end of the tensiometer registers all the pull that is exerted on it. This includes the weight of the water in the tensiometer as well as the water in the rooting medium. One important (counterintuitive) point is that it is not the total quantity of water that is important, but the length of the "water column". It is a coincidence that each 10 centimeters of water column (i.e. depth) corresponds to 1 kPa of tension. This is a case where using metric units makes things much easier.

In the example in figure 2 the rooting medium in the pot is 13 cm deep; the water column in the tensiometer is 7 cm long and the top of the ceramic tip of the tensiometer is 9 cm above the bottom of the pot. If the pot is at saturation (i.e. you cannot get anymore water to be in the pot without having it drip out the holes in the bottom) then the tension at the bottom of the pot is 0 kPa; at the ceramic tip (9 cm higher up) it is 0.9 kPa; at the meniscus in the tensiometer it will be 1.6 kPa (16 cm above bottom). Thus, the lowest possible reading on the gage of this tensiometer is 1.6 kPa. I.e. this instrument cannot read zero when installed in this pot. An exception to this occurs during an irrigation where it is possible to have readings near zero until excess water has drained away.

It is also important to note where in the root environment the tension is being measured and how the reading at the gage relates to this. For example, for any reading on the gage, one would need to subtract 0.7 kPa (cm length of water in tube divided by 10) to determine the tension at the ceramic tip. Or one would subtract 1.6 kPa to calculate the tension at the bottom of the pot.

Thus if the gage shows a reading of 5.4 kPa, then this means that the bottom of the pot is at 3.8 kPa, while the middle of the pot, probably the center of the root-ball, is at 4.7 kPa.

Since it is the root-zone of the plant that is of interest to us, I recommend placing the ceramic tip of the tensiometer in an area in the root-zone where most of the roots will be. Thus it is the tension at the ceramic tip that is of primary importance. The main take-home message here is that the reading on the gage of the tensiometer needs to be adjusted to get this number: you should always subtract one-tenth of the length of the tensiometer (in cm) from the reading on the gage to obtain the tension that is relevant to irrigation management.

For example, using the case above: if you want to irrigate when the ceramic tip is at 5 kPa, then you will do this when the gage reads 5.7 kPa (i.e. 5.0 + 7/10 because the tensiometer is 7 cm long)



SMT-based irrigation

There are several possible strategies for using tensiometers. The simplest one is to use a high-tension set-point to indicate when an irrigation is needed. The irrigation itself can then be scheduled. One can then track the progress of the irrigation using the tensiometer and stop the irrigation once a desired level of moisture is reached (usually saturation). If the medium is well-drained then this can be safely done using a timer.

It is also possible to use tensiometers as moisture sensors in automated irrigation to use SMT (soil moisture tension) to make irrigation decisions. There are several different types of automated irrigation system. Regardless of the control technology it is always important that the system be capable of distributing water uniformly. If this is not the case then there will always be problems (regardless of whether the system is automated or not). Even if the system is not uniform, one still has to make the decision as to when to irrigate and when to stop irrigation. Tensiometers can always be helpful in this decision-making process.

In an SMT-based irrigation system the procedure is to have one tensiometer to coincide with each irrigation valve. This sensor needs to be in a representative location in the crop being irrigated. Usually this means that a higher-than-average water-using plant will be selected.

The to make it work one needs two set-points:

1. The high-tension set-point, which represents the level of dryness at which irrigation is warranted.

2. A low-tension set-point represents the set-point at which an ongoing irrigation will be stopped.

These set-points are used as follows.

Typically plants will be extracting water from the root zone. As this occurs (usually over hours and days) the tension in the root-zone will gradually rise. Once the high-tension set-point is reached, the irrigation is initiated (in large scale operations this will mean that this particular irrigation circuit is scheduled for irrigation by placing it in an irrigation queue).

During an irrigation, the tensions will drop (usually over seconds or minutes) as water is applied. The rate of water application should be slow enough so that the tensiometer can follow the change in moisture condition. When the low-tension set-point is reached, the irrigation is stopped. There is always some lag in this system so there will be some over-shoot.

If the high-tension set-point is such that the moisture content of the medium is relatively dry, then as new water is applied it may not move very fast laterally (sideways). This situation should be avoided since it will result in dry sections in the root-zone.



Other control parameters:

When using tensiometers in automated (computerized) control schemes, one should involve the following parameters in addition to the high and low tension set-points.

It is possible for a grower to set things up so that the high tension set-point is relatively high, the low-tension set-point is near saturation, and the max on-time is short. This can then result in situations where the tension does not drop down to the low-tension set-point. There are reasons why an operator might want to do this, but if it is done inadvertently, then it could lead to problems (salt build-up, part of crop drying out,...).



Calibration

There are two aspects to calibration when using tensiometers. The first is to assure that the instruments measures tensions correctly, the second relates to how the instrument is used. The first should be done by the firm that makes the instrument and the control equipment; the second is the responsibility of the grower. Another way to say this is: the vendor is responsible for assuring that the instruments behaves as illustrated in the diagram above, and the grower is responsible for using it correctly (i.e. identifying the proper reading to go with the tension set-point recommendations (provided below).

Calibration is accomplished by attaching a flexible clear tube to the lower portion of the tensiometer. This tube and the tensiometer are filled with clean water (there should be no bubbles). The free end of the tube is lifted up so that the entire contraption form a U. It should be possible to line up the water level in the tensiometer with that in the tube. Once they are lined up, they are completely balanced and reading on the tensiometer should be zero. By lowering the tube the tension imposed on the inside of the tensiometer will be the difference in height between the meniscus in the tensiometer and the tube. This length of water column is converted to tension as indicated above (10 cm = 1 kPa). Water columns of 10 cm and 50 cm should be imposed to verify that the instrument is correctly reading 1 and 5 kPa, respectively.



Set-points

Extensive research has shown that keeping plants in a 1 kPa to 5 kPa range is optimal for plants growing in highly amended media. Thus the high-tension set-point recommendation is 5 kPa; the low-tension set-point is 1 kPa. However, this information can be confusing if you consider that the height of the pot and length of tensiometer both affect tension readings. So what does this mean?

The high-tension set point of 5 kPa refers to the tension at the ceramic tip in the root-zone. It is the tension that indicates that it is time to start an irrigation. Note that if you have a pot that is 60 cm deep (or deeper), then when the bottom of the pot to be at saturation the soil level 50 cm above the bottom is at 5 kPa. I.e. it would always be in need of irrigating. Under such circumstance the medium diagramed in Fig 1 would not be appropriate. It would be necessary to use a medium (perhaps one including a lot of finer particles, perhaps field soil) with a moisture release curve that does not decline as fast between 0 and 10 kPa as UC mix does.

The low-tension set point of 1 kPa is the point where an irrigation can be stopped. In most systems that were tested, this set point (again, referring to the tension at the ceramic tip in the root-zone) prevented a lot of excessive irrigation water from being wasted. Typically the system over-shoots this point so that most pots in the crop will reach a moisture level just above saturation. It should be noted that at moist conditions the hydraulic conductivity of a medium is generally pretty high so that water can move pretty fast. In a deep pot (e.g. 30 cm) it would be possible (and likely) that even during an irrigation the water column is always able to pull on the tensiometer at more than 10 cm. Thus the tensiometer's ceramic will always experience a tension higher than 1 kPa so that the irrigation system would never cut off if a 1-kPa set point is used.

Thus, for the low-tension set-point, make sure that it is set to be at least kPa above the lowest measurable point (saturation). The way you make this work is as follows. You insert the calibrated, water-filled instrument into the root medium in the pot. Now water thoroughly, making sure that all the medium is wetted completely. This may mean applying water serval times at half-hour intervals. The last time wait until water stops dripping from the pot and then take a reading. Adjust this reading for the length of the tensiometer. This is your "saturation reading". Your low-tension set-point would be the higher of this value plus 0.5 kPa, or 1 kPa. This process of identifying the "saturation reading" is the grower calibration.

As an example consider the system diagramed in figure 2 (above). Here the crop would be growing in containers filled with a potting mix (e.g. UC Mix). The tip of the tensiometer is 9 cm above the bottom of the pot and the water column in the tensiometer is 7 cm long. As mentioned earlier, at saturation the gage reads 1.6 kPa. Thus you would stop an irrigation when the gage reads 2.1 kPa (0.5 kPa plus the saturation reading of 1.6 kPa), corresponding to a tension in the rootzone of 1.4 kPa (i.e. 2.1 kPa - 0.7 kPa). The irrigation would be started when the gage reads 5.7 kPa, corresponding to a tension of 5 kPa in the root zone.