Environmental Horticulture, UCDavis, Davis, CA 95616
May 7, 1997
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The purpose of this document is to identify and discuss technical elements regarding tensiometer-based irrigation in a setting where the sensors are able to register changes in moisture tension rapidly (i.e. in seconds, rather than minutes). Conventional field tensiometers installed in field soil are usually too slow to operate this way.
The information below is aimed at greenhouse production of plants in pots or in the ground, where the rooting medium is either a very-porous, artificial medium, or soil that has been highly amended to have higher water-holding capacity than is common in field soils.
Tensiometers deal with moisture tension or matric potential of some porous medium (such as soil or potting mix). In scientific jargon, "martic potential" is just one concept of a wide range of terms dealing with water pressure 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 be negative. The term moisture tension represents the degree of such suction (and is a positive number). For example, if the matric potential of a soil is -10 kPa (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 discussion I will use only the term "tension" (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.
Note that this curve illustrates some general points about irrigating plants. The wet condition (i.e after a thorough irrigation) is for 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) is not available to plants, so that only 530 ml is available water.
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
Note that the diagram shows which ranges of tension are best for the plant (1 to 5 kPa) and that
tensions above 10 kPa are dangerous. The diagram also indicates how a human might percieve
soils at various moisture tensions. Note that at 10 kPa one will still be able to sense moisture in
The traditional tensiometer comes equipped with a dial-gage. The system discussed here assumes that this gage has been replaced (or supplemented) with a transducer which translates the suction inside the tensiometer into an electrical signal which can be calibrated to represent the tension. With this conversion, the tensiometer can be attached to automated control systems.
Calibration is not a simple matter and involves aspects which probably must remain under the domain of the operator. (See Calibration below)
The basic concept is that the weight of the water in the tensiometer pulls on the gage, as does the water in the medium. It is not the quantity of water that affects the measurement, but the length of the "water column". In other wrods, the "pull" on any point in the system is related to the distance that that point is from the bottom of the pot. For each centimeter above the bottom, the tension rises 0.1 kPa. Thus each 10 cm of water column (ie. depth) corresponds to 1 kPa of tension. (This is a case where using metric units makes things much easier).
In the example (Fig 3) the 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 (ie. you cannot get anymore water to be in the pot without having water 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 reading on this tensiometer is 1.6 kPa. I.e. this instrument cannot read zero in this setting.
Similarly, for any reading on the gage, one would subtract 0.7 kPa to determine the tension at the ceramic tip, or 1.6 kPa for the tension at the bottom. For example a reading of 5.4 kPa on the gage means that the bottom of the pot is at 3.8 kPa. Another example: 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.
There are lots of different types of automated irrigation system. One fundamental assumption about this technology is that the system is 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).
But even if the system is not uniform, one still has to make the decision as to when to irrigate and when to stop irrigation. Regardless of the system, tensiometers can always be helpful in this decision-making process.
To make it work one needs two set-points:
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).
Once the irrigation is in progress, 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.
Note that it is possible for an operator 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,...). It may be wise do build some sort of warning system into control software to track this sort of behavior and warn the operator of the potential danger.
Another relatively complicated pattern that should be warned about is if irrigation does not result
in the expected behavior of the tension signal. Such poor behavior could mean some defect in
the electronics related to the transducer or even in the control computer housing.
There are two aspects to calibration when using tensiometers. The first is to assure that the
instruments measures tensions correctly, the second involves 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). It is possible to have water columns of various heights; the difference in height between the meniscus in the tensiometer and the tube represents this water column. This length of water column is converted to tension as indicated above (10 cm = 1 kPa).
Here is the bottom line: whatever tensions are recommended, assume that they are "above saturation" and pertain to a location in the center of the root zone (usually at the porous tip). So the above recommendation is to have the root zone 5 kPa higher than what it is at saturation before watering, and to stop the irrigation when this point is 1 kPa above 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 make sure the pot is sitting level, and wait until water stops dripping from the pot. At this point take a reading. This is your "saturation reading". Your high-tension set-point would be this value plus 5 kPa. This process of identifying the "saturation reading" is the grower calibration.
As an example consider the system diagrammed in Fig 3 (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 tensiometer tube is 7 cm long. As mentioned earlier, at saturation this container reads 1.6 kPa. Thus the high-tension set-point for this instrument in this situation would have to be 6.6 kPa (the recommended 5 kPa plus the "saturation reading of 1.6 kPa). The low-tension setpoint would be 2.6 kPa.
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