Tensiometer-based irrigation of potted plants

Dr. Heiner Lieth, Environmental Horticulture, University of California, Davis, CA 95616

Project objective:

The objective of this project was to develop a potted plant irrigation system which uses less water and fertilizer while reducing run-off.

System Design:

System components: Tensiometer, transducer, amplifier, analog-to-digital converter, control computer, 24V AC transformer, solid-state relays, drip irrigation system, solenoid valves, emitters.

System design

System function:

  1. 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.
  2. This signal is analyzed by an analog-to-digital converter (ADC). A digital representation is sent to the control computer.
  3. The software in the control computer, using rules which have been programmed, decides whether something needs to be done.
  4. 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.
  5. 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.

UCDavis tensiometer-based system

Results:

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 experiments.

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, pot size Applied (ml/day) Savings: Percent difference Grower Run-off1

(ml/day)

Grower applied Tension-

based1

Summer 89, Mums 6" 345 265 23% 68
Fall/Winter 89, Poinsettia 6" 109 105 4% 36
Winter 89, Mums 6" >1098 2 225 79% >708 2
Winter 89, Mums 6" 247 165 33% 38
Spring 90, Mums 6" >1001 2 225 78% >724 2
Summer 90, Gerbera 6" 420 238 43% -
Summer 90, Hydrangea 8" 544 422 23% -
Fall/Winter 90, Poinsettia 6" 239 83 65% 127
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:


For more information or feedback contact jhlieth@ucdavis.edu