Tensiometer-based irrigation of cut-flower roses
1994 Project Report to the California Cut-flower Commission
Reduction of run-off in greenhouse cut flower crops through automated
irrigation based on soil moisture tension.
Lorence R. Oki, J. Heinrich Lieth and Steve Tjosvold Dept. of
Environmental Horticulture, University of California, Davis, CA 95616-8587
and University of California Cooperative Extension, Watsonville, CA P.I.:
Dr. J. Heinrich Lieth; Cooperators: Loren Oki and Steve Tjosvold Report
date April 17, 1995
Introduction and Background
Agricultural use of water is an important issue in California with
considerable attention being focussed on greenhouse growers. Water use in
greenhouse production is frequently at excessive levels, causing high
amounts of run-off water. While generation of some run-off is generally
unavoidable and needed to prevent build-up of salts, excessive amounts of
run-off represent an inefficient use of the resource and a significant
source of ground water pollution since such run-off tends to be laden with
fertilizer and pesticides. Growers have responded to this by attempting to
make their irrigation systems more efficient. This may be accomplished by
the placement of the applied water (drip irrigation), containment of
effluent (capturing and reuse), or more precise methods of controlling
irrigation (duration and frequency). The project discussed in this report
focusses on developing new tools for growers attempting to improve
irrigation efficiency and reduce run-off in in-ground cut-flower
production. A problem that is common with any irrigation system is
determining when to irrigate and the amount of water to apply. Growers are
generally "in tune" with their crops and may rely on their
observations to decide when to water. Frequently, however, such visual
observation corresponds to levels of water stress that are not optimal for
crop production. Also, the moisture status may change rapidly so that
watering may be required before a grower notices the need. This could be
alleviated through an irrigation system which uses sensors to monitor the
soil moisture status of the crop. Also, when a decision is made to water,
the amounts of water applied are generally excessive. In cut flower
production, Tjosvold and Schulbach (1991), while examining alternative
approaches to irrigation scheduling, found that growers commonly irrigate
too infrequently and that too much water is applied when irrigations are
made. In in-ground production it is very difficult to know when an
adequate amount of water has been delivered. While the amount of excess
water varies from grower to grower it is frequently substantial. A system
that precisely monitors and controls the amount of applied water and the
changing soil moisture status during an irrigation, may reduce the amount
of generated run-off. Sensors, such as tensiometers, may be used to
measure soil moisture content and to make irrigation decisions.
Tensiometers that have been commercially available were designed for use
in field soils and their gauges have been optimized for these soils by
having a wide tension range (0 to 100 kPa). However, previous research has
shown that the optimum tension levels for producing plants in containers
filled with highly-amended media are in the range of 1 to 5 kPa (Kiehl et
al., 1992). Plaut et al. (1976) found that the moisture tension in
cut-flower rose production should be below 6 kPa. Figure 1: The design of
the computer controlled irrigation system for monitoring the moisture
content of the soil in the root zone with a tensiometer. One problem with
the use of field tensiometers in amended media, is their slow response.
Burger and Paul (1987) developed a tensiometer for use in such soils.
These tensiometers responded rapidly to changes in soil moisture tension (SMT)
and were used in research to maintain a constant moisture level in potted
plants (Lieth and Burger, 1989). This work demonstrated that the amounts
of water used in greenhouse potted plant production is excessive and
plants of commercial quality can be grown using substantially less water
than previously thought necessary. Research supported by the California
Association of Nurserymen has developed an adaptation of this system for
commercial use and has shown that potted plants can be grown with
significant reductions in water use (70-90%) without detrimentally
affecting crop quality (Lieth, 1990; Lieth et al., 1990). The
responsiveness of these "high flow" tensiometers is
significantly better than the traditional field tensiometers. While
tensiometers have long been used in making the decision "when"
to apply water, these "high flow" tensiometers are able to also
aid in the decision of "how much" to apply. When water is
applied to the soil, these tensiometers are able to rapidly detect the
addition of water and can stop the water application when a specified
moisture level is reached. The objective of this project was to develop a
complete system for use in cut-flower rose production, to test the system
under commercial conditions and to compare its performance with standard
irrigation procedures. Experimental setup This work, funded by the
California Cut Flower Commission, adapts the system developed for potted
plant production for use in cut flowers. The system consists of
commercially available soil tensiometers fitted with pressure transducers
(to convert pressure into a voltage signal), signal amplifiers, an
analog-digital signal converter, a computer, and the irrigation system
with automatic valves (Fig. 1). The equipment is controlled by a
custom-written program running on the computer. The program requires
setting a high tension set-point (when irrigation is turned on) and a low
tension set-point (when irrigation is turned off). Safety features include
minimum and maximum on-times and a maximum elapsed time between
irrigations to circumvent a failure of sensors. The scenario for soil
moisture tension-based irrigation is: the computer continuously monitors
the tensiometers placed in the root zone of the crop. As water is
extracted from the soil, the tension in the soil and the tensiometer
increases. When the high tension set-point is reached, the automatic valve
is opened. As water is applied, the soil moisture tension decreases and
the valve is closed when the low set-point is attained. The sensors used
are tensiometers from Irrometer Corporation (Riverside, CA) manufactured
with a "high flow" ceramic porous cup. We modified these by
attaching a transducer to the tensiometer using brass fittings. This
adaptation was simplified with the use of off-the-shelf parts and
electronics purchased directly from vendors. The simplicity of the
adaptation and the availability of parts is important since growers will
be able to readily adopt the technology. Tests of the modified
tensiometers has demonstrated that they respond significantly better than
the standard field tensiometers and can be used in controlling automated
irrigation systems. The dynamics of moisture tension and moisture content
were explored using 'Cara Mia' roses growing in ground beds at UC Davis.
Irrigation was by spray emitters mounted on a perimeter system. Four
different tensiometers were inserted in relatively dry soil: 6" and
18" Irrometer tensiometers with a high flow ceramic tip, a 12"
Irrometer tensiometer with a standard tip, and a tensiometer of the type
built by Lieth and Burger (1989) for use with potted plants (U.C.
tensiometer). The Irrometer tensiometers were modified as described above.
Before, during, and after irrigations (ranging in duration from 10 to 40
minutes) moisture tension were recorded with a data logger. Figure 2.
Comparison of responses of tensiometers with different ceramic porous cups
at different depths to changes in soil moisture. The result of this
verified that the irrigation system was performing as designed and that
the modification of the off-the-shelf components was working as planned.
The response for the standard field tensiometer (Fig 2 C) was
substantially slower than the "high-flow" versions (Fig 2 A, B,
and D). The sensors modified with the high flow tips responded to soil
moisture changes rapidly. The rapid drop in tension registered by the
high-flow tensiometer at 10 cm depth indicated that the system would be
able to follow the moisture tension during an irrigation and be used to
determine when adequate amounts of irrigation water had been delivered. It
was also possible to monitor the plants' extraction of water from the root
zone soil using the tensiometers (Fig. 3). The slope of the tension curves
are steeper when the light levels are higher (corresponding to more rapid
removal of water from the soil by the plants). Figure 3: Soil moisture
tension levels monitored by four tensiometers over a three-day period.
Watsonville Nursery, a prominent rose grower (in Watsonville, CA), was
selected as the site for testing the system. This nursery has a reputation
for excellent rose quality and good nursery practices. They have, on
staff, someone who can assist with problems related to soil, water and
plant nutrition. They are attentive to irrigation practices and have, in
the past, worked with field tensiometers. One bay of one greenhouse,
planted with 'Kardinal', was selected as the site for the experimentation.
The plants on one side of the center aisle were irrigated by the grower;
the plants on the other side were irrigated based on moisture tension
using the system we developed and tested at UC Davis. Both sections were
equipped each with three tensiometers. These were the high-flow type with
porous cups at 5, 24, and 40 cm depth. Data were logged electronically at
half-hour intervals. Since the automated irrigation system was to perform
without any interaction, the grower imposed two requirements on us: 1. the
system could only water at times when it would not interfere with other
irrigation and never after 5 p.m. due to potential problems with high
humidity in the late afternoon and early evening. Thus the software was
modified to irrigate only in a specific time-frame. This "irrigation
window" was from 6 a.m. to 7 a.m. each morning. Within this timeframe
the system was set up to irrigate when a tension of 5 kPa was reached and
to then irrigate until either a maximum on-time of 10 minutes had elapsed
or until a tension of 1 kPa was reached. The tensiometer at 24 cm depth
was used as the control point. The particular greenhouse was selected
since it allowed setting up two blocks of almost-identical rose plants.
All plants had been planted several months earlier and were about 1.5 m
tall at the start, and ready to go into full production. Although it was
not apparent when the experiment was set up, the pinching pattern was
slightly different between the two areas in the greenhouse with the flush
of shoots on the growers plants being about 1 week earlier. Water meters
were installed at each section to monitor the actual amount of irrigation
water applied to each section. The crop sections were measured to
determine amount of area dedicated to each test section. At each harvest,
all the harvested shoots were counted in each section and noted on a tally
sheet by the harvesters. On two occasions the quality to the rose
production was tested by taking one bundle of harvested roses from each
section and measuring the length of each stem. Results and Discussion The
tensiometers at three depths provided a clear understanding of how
tensions change in response to irrigations (Fig 4). Immediately following
irrigation the tension dropped when the wetting front in the soil passed
the tensiometer; with resulting tensions near 0 indicating that the soil
was briefly saturated and then at field capacity. As the plants and
surface evaporation remove the water the tension increases. The resulting
pattern appears as a stair-step pattern with rapid increases in tension
during the day and only minor changes at night (Fig 4 top curve). Figure
4: Moisture changes at three soil depths during irrigation and subsequent
drying The moisture changes much more rapidly near the soil surface than
at lower depths. Soil moisture tension at all depths decreases during the
day, but changes only at the middle depth at night. Diurnal fluctuations
at the 24 cm. level are the most pronounced probably because there are
more roots present here than at the depths of the other sensors. At this
depth, the most rapid increase in tension is during the period of highest
water demand from the plant (afternoon) and decreases in tension occur
during the night as water moves from deeper soils The fact that his system
can track these changes represents a significant improvement over field
tensiometers and is a vital aspect of the functionality of this system.
These sharp peaks became particularly prevalent in the tension-based
irrigation treatment (Fig 5). This had a significant effect on the
operation of the irrigation system since many of these peaks extend
significantly higher than 5 kPa. When the tensions managed to return to a
level below 5 kPa by 6 a.m. the next morning (e.g. on 10/12/94) there was
no irrigation despite the high tensions during the previous day.
Irrigations we done whenever the tensions did not go back below 5 kPa by 6
am (e.g. Fig 5, 10/13 and 10/17). Figure 5: Changes in soil moisture
during irrigation controlled by soil moisture tension measurements. The
arrows indicate irrigations with the duration the valves were turned on
The applied water reaches the shallower depths but the amount is not great
enough to affect the deeper soils. The tensions at 40 cm. fluctuate
diurnally but do not decrease following irrigation as at the shallower
soil depths. This is the desired performance of the irrigation control
system, since the amount of water applied is not excessive and may not
produce run-off. Readings from the water meters show that there was less
water applied, although at greater frequency, to the tension-based
treatment than the grower beds (Table 1). The summary of data shows that
an average of about 300 gallons of water were applied per irrigation per
1000 sq. ft. of greenhouse by the soil moisture tension-based system
compared to about 850 gallons for the grower controlled system. On a per
day basis, this translates to about 50 gallons of water used per day per
1000 sq. ft. for the soil moisture tension-based system versus about 80
gallons for the grower controlled system. Table 1. Summary of irrigations
during the test period Total water applied Number of irrigations Applied
per irrigation (gal/1000sq. ft.) Average water use per day (gal/1000 sq.
ft.) gallons gal/1000 sq. ft. Grower 9042 12735 15 849 77.2 Test 4989 8179
28 292 49.6 Analysis of stems harvested from both the test and the grower
beds show a dramatic effect on the production of flowers. The number of
flowers harvested from the tensiometer-based treatment was much higher
than those harvestd in the block being controlled by the grower (Tables
2). At the same time the quality was the same (Table 3). Although the
pinching pattern was slightly different in the two blocks, it is not
likely that this had a significant effect on the difference between the
counts from the two blocks. Table 2. Quantity of stems harvested Month
stems harvested by grower stems harvested in test plot total number
number/1000 sq. ft. total number number/1000 sq. ft. September 1171 1649
2500 4098 October 1175 1655 1783 2923 November 72 101 912 1495 Total 2418
3406 5195 8516 Table 3. Lengths (cm) of harvested stems. (Sample size = 50
stems.) Date Grower Test mean stem length std dev mean stem length std dev
Feb 6 62.88 7.19 61.28 7.36 Feb 13 63.56 7.45 60.16 7.52 When combining
the harvest and water use data (Table 4) a very peculiar result emerges:
the number of stems harvested per gallon of water used is very different
in the two areas. The crop irrigated based on moisture tension produced
1.2 stems per gallon of water used compared to 0.4 stems per gallon
resulting from the grower-controlled system. This is a tremendous
difference and we must run additional tests before we will allow ourselves
to believe this result. It is difficult to accept that the tension-based
irrigation system would be 3 times more efficient. Another way to look at
this is that the soil moisture tension-based system produced flowers using
one third of the amount of water and fertilizer. It was hoped that the
amounts of subsurface run-off water could be estimated by using data from
the meters and the deepest (40 cm.) tensiometers. However, as can be seen
in figure 3, the soil at this depth is almost always saturated (Fig 4 and
5), making it impossible to estimate run-off water as expected. Table 4.
Analysis of harvest and water usage data Grower Test Period stems /1000
sq. ft. gal. used /1000 sq. ft. no. stems/gal no. stems /1000 sq. ft. gal.
used /1000 sq. ft. no. stems/gal 9/6- 9/30 1649 3554 0.46 4098 2018 2.03
10/1-10/31 1655 4338 0.38 2923 3836 0.76 11/1-11/16 101 724 0.14 1495 1230
1.22 Total 3406 8615 0.4 8516 7084 1.2 The soil moisture tension-based
system is efficient in controlling irrigation for the production of
cut-flower roses. The tensiometers modified with transducers and high flow
ceramic tips are able to effectively measure soil moisture tensions in the
root zone of plants grown in beds and are capable of responding to rapid
changes in soil moisture. The system, using these modified tensiometers,
is able to continuously monitor the moisture level of the soil and will
not only demand irrigation when the soil requires water, but turn off the
water when an adequate amount has been applied. The environmental benefit
of this system is a more efficient use of water compared to a grower
controlled system which may result in the reduction in run-off water. The
reduction in water use also has economic benefits to the grower in
reducing the cost to produce and use water (pumping, etc.). Additional
costs savings are realized if a liquid feed program is used since the
amount of fertilizer applied is reduced. Literature Cited: Burger, D.W.
and J.L. Paul. 1987. Soil moisture measurements in containers with
solid-state, electronic tensiometers. Hortscience. 22(2):309-310. Kiehl,
P.A., J.H. Lieth, and D.W. Burger. 1992. Growth response of chrysanthemum
to various container medium moisture levels. J. Amer. Soc. Hort. Sci.
114(2):224-229. Lieth, J.H. 1990. Irrigation Technology: Reducing run-off
in greenhouse and nursery production. Proceedings of the Society of
American Florists. Lieth, J.H. and D.W. Burger. 1989. Growth of
chryssanthemum using an irrigation system controlled by soil moisture
tension. J. Amer. Soc. Hort. Sci. 114(2):387-392. Lieth, J., D. Burger, P.
Kiehl, S. Tjosvold, G. Vogel. 1990. Reduce run-off from your potted crops
by watering based on soil moisture. Grower Talks. September 1990, p.24-32.
Plaut, Z., N. Zieslin, and N. Levev. 1976. Effect of different soil
moisture regimes and canopy wetting on 'Baccara' roses. Scientia Hort. 5:
277-285. Tjosvold, S.A. and K.F. Schulbach. 1991. How to reduce water use
and maximize yields in greenhouse roses. Califonia Agriculture. May-June
1991, p. 31-32.
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