Tensiometer-based irrigation of potted
Dr. Heiner Lieth, Environmental Horticulture, University of California, Davis, CA 95616
The objective of this project was to develop a potted plant irrigation system which uses less
water and fertilizer while reducing run-off.
System components: Tensiometer, transducer, amplifier, analog-to-digital converter, control
computer, 24V AC transformer, solid-state relays, drip irrigation system, solenoid valves,
- The tension (suction) inside the tensiometer is the same as within the container medium. The
transducer mounted on the tensiometer translates this into an electrical signal.
- This signal is analyzed by an analog-to-digital converter (ADC). A digital representation is
sent to the control computer.
- The software in the control computer, using rules which have been programmed, decides
whether something needs to be done.
- If water needs to be applied, then a signal is sent to the ADC to turn on the appropriate output
channel. This activates the proper relay, opening the irrigation solenoid.
- The computer continually checks the tension while water flows from the emitter. When the
cut-off tension is reached (or some maximum on-time has elapsed) the solenoid is turned off.
Experiments of the project:
Experiments were run to answer questions such as: Is the system feasible for commercial
potted plant production systems? How many plants can be controlled with one tensiometer?
How much water can be saved? How much fertilizer can be saved?
Experiments were run in commercial greenhouses in California. At each location new
irrigation solenoids and transformers were installed. The system was operational
continuously, around the clock. The tension-based treatment was watered based on the
threshold tensions. The timed treatment irrigated for three minutes per day; once plants were
very large this was increased to five minutes per day.
At all commercial locations the system functioned very well once initial set-up problems were
overcome. In all experiments we had no control over the fertilizer injection system. Despite this,
most crops were remarkably uniform. No problems with salt build-up were found in any of the
Plant sizes in the tension controlled treatments were generally slightly smaller than the other two
treatments. It is likely that lack of control over the fertilizer injection system may have played a
role in this. Despite this, all plants produced in the tension-based treatment were of very good
quality while many of the timed irrigation treatment were too tall and leggy (always requiring
tying with string).
Amounts of water applied, percent saved, and amount of run-off in various experiments in
commercial greenhouse experiments (all numbers are per pot).
|Experiment, year, species,
|Summer 89, Mums 6"
|Fall/Winter 89, Poinsettia 6"
|Winter 89, Mums 6"
|Winter 89, Mums 6"
|Spring 90, Mums 6"
|Summer 90, Gerbera 6"
|Summer 90, Hydrangea 8"
|Fall/Winter 90, Poinsettia 6"
|1 In these experiments run-off from the tension-based irrigation treatments was always
close to zero (average of less than 4 ml per day).|
2 These numbers include data from events where so much water was applied by the grower
that the collection buckets overflowed; i.e. the actual numbers are larger.
Conclusions for using tensiometer-based control:
- High quality potted plant crops were produced with the system.
- Significantly less water was applied in the treatments based on soil moisture tension.
- Run-off can be reduced to almost nothing with this system (depending on drip system
uniformity, size of circuit, length of crop time, etc.)
- Recent tests with in-ground roses suggest that similar advantages can be obtained
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