Tensiometer-based irrigation of cut-flower roses

1995 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:

Substantial improvements in rose crop productivity and quality

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: Lorence Oki and Steve Tjosvold

Report date: May 23, 1996

Bookmarks to move around quickly in this report:
Experimental setup Results and Discussion Figure: 1

Tables: 1, 2, 3, 4

Conclusion

Introduction

Irrigation practices in greenhouse cut-flower production are receiving considerable scrutiny throughout the world. Much of this is due to the recognition that excess irrigation is resulting in high levels of nitrates and other fertilizers in the environment and in the drinking water supply. This project focussed on developing methodologies for use in conjunction with current irrigation practices to address this problem in greenhouse cut-flower production.

During experiments with soil moisture tension (SMT)-based irrigation control on potted plants, we discovered that commercially-acceptable container-grown plants could be grown with substantially less water than had previously been thought possible (Lieth and Burger, 1989). With the financial support of the California Association of Nurserymen, we adapted the system for use in commercial potted plant production (Lieth, 1990; Lieth et al., 1990). We demonstrated that use of such systems can lead to significant reductions in amounts of applied fertilizer and water, coupled with 70 to 90% reductions in run-off. In 1994 we began a study to identify how such a system could be adapted to irrigation systems in greenhouse cut-flower production. The resulting system was tested and showed substantial irrigation water savings (Oki, Lieth and Tjosvold, 1995). During 1995 we continued the testing and data collection at one site; this report documents the results of that effort.

Background

The tensiometers that are used in irrigation management of field crops and turf, have generally been ones equipped with mechanical gages. These instruments are designed for use in native field soils and respond very slowly to changes in moisture conditions. Thus their use is generally restricted to determining approximately when irrigation is needed (but not how much or when to stop irrigating). Replacing the conventional porous tip with a high-flow version results in an instrument suitable for conditions where moisture changes can be quite rapid (e.g. greenhouse rooting media; Burger and Paul, 1987).

The use of tensiometers for scheduling irrigation in cut-flowers has been tried in the past (Marsh et al., 1962; Plaut et al., 1976). Tjosvold and Schulbach (1991) ran a study in Watsonville, CA examining alternative approaches to irrigation scheduling. Using conventional tensiometers, they monitored moisture tensions and reported that growers typically irrigate too infrequently and when they do, too much water is usually applied. Their study showed that tensiometers are a useful tool in cut-flower rose production. Although only the field soil tensiometers were available at the time of that study, sensors suitable for highly-amended media are now available commercially.

While the tensiometer equipped with a gage is a powerful tool, it is also suitable for use in automated systems. In such applications, the gage is replaced or augmented with a pressure transducer to allow an electronic controller or computer to sense the moisture tension. Our earlier work with potted plant irrigation showed that optimal production of container-grown plants occurs if tensions are kept between 1 and 5 kPa (Kiehl et al., 1992). In the project presented here, we modified conventional tensiometers, attached them to an automated irrigation system, and used the same set-points to successfully irrigate cut-flower roses. This report documents that the SMT-based irrigation methodology is feasible and that the reduction in water usage and potential reduction in pollution can be substantial, even while productivity and quality are improved.

Experimental setup

In previous work (funded by the California Cut Flower Commission) we adapted the potted plant irrigation system for use in cut-flower production. The resulting 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. The equipment is controlled by a custom-written program running on the computer and works as follows: the computer continually monitors a tensiometer imbedded in the root-zone of the crop. As the crop extracts water from the root-zone, the tension in the medium rises. Once an upper tension set-point is reached, a solenoid valve is activated. While irrigating, the tensiometer is monitored. As water infiltrates the root zone, the moisture tension will drop. Once a lower tension set-point is reached, the valve is shut. Thus, irrigation based on soil moisture tension involves the following information: (1) a high-tension set-point (the level at which the irrigation system is turned on), (2) a low-tension set-point (to determine when to stop irrigating), (3) minimum and maximum on-times (safety limits) and (4) maximum elapsed time without irrigating (in case of sensor failure).

We used tensiometers from Irrometer Corporation (Riverside, CA) equipped with a "high flow" ceramic porous cup. These tensiometers were modified by attaching a transducer to the tensiometer. 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 have demonstrated that they respond significantly better than the standard field tensiometers and can be used in controlling automated irrigation systems.

Watsonville Nursery, a rose grower in Watsonville, CA, was 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 use conventional tensiometers to make irrigation decisions.

One house of a greenhouse range planted with the variety '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 opposite side were irrigated based on moisture tension using the system developed and tested at U.C. Davis. Both sections consisted of five beds of plants and three tensiometers were installed in one bed in each section. These were the high-flow type set to monitor the soil moisture tension at depths of 5, 24, and 40 cm.

Since the automated irrigation system was to perform without any interaction, the grower required that the system water only at times when it would not interfere with other irrigation and never after 5:00 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 "irrigation window" from 6:00 a.m. to 7:00 a.m. each morning. Within this time frame, the system was configured to begin irrigation when a soil moisture tension of 5 kPa was reached and continue to irrigate until either a maximum on-time of 10 minutes had elapsed or a tension of 1 kPa was reached. The tensiometer at the 24 cm depth was used as the control point.

Data were logged electronically at half-hour intervals. Air temperature and light levels were also recorded for use in diagnosing problems and to document any unusual stresses. Water meters were installed at each solenoid valve to quantify the total amount of water applied in each treatment. The harvested shoots were counted daily in one bed of each section and noted on a tally sheet by the harvesters.

On seven occasions, the quality of the roses produced was checked by taking samples of flowers harvested from each section and measuring the length of each stem. Six of the observations were taken in the period the test system was controlling the irrigation. The seventh set of data was taken 71 days after the test system was removed and control of irrigation was returned to the grower (August 4, 1995).

Results and Discussion

The tensiometers at three depths provided a clear understanding of how soil moisture tensions change in response to irrigations (Fig. 1). Immediately following irrigation, tension drops when the wetting front in the soil passes the tensiometer, resulting in tensions near zero, indicating that the soil was briefly saturated. Then, tensions rise as the soil moisture attains field capacity. As the plants and surface evaporation remove water, the tension increases. The result is a stair-step pattern with rapid increases in tension during the day and only minor changes at night.

Figure 5
Figure 1: 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 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 the 24 cm 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 this system can track these changes represents a significant improvement over field tensiometers and is a vital aspect of the functionality of this system.

Table 1. Productivity in the grower and test areas (on a "per month" basis)
Period Grower Test
Productivity during test (stems ft-2 month-1)
September 1994 1.60 4.10
October 1994 1.66 2.92
November 1994 0.10 1.50
December 1994 0.00 0.00
January 1995 0.00 0.00
February 1995 1.29 0.82
March 1995 0.00 0.00
April 1995 0.83 1.65
May (to 25th) 1995 3.10 2.95
9/1/94 to

5/25/95

0.92 1.53
Productivity after test (stems ft-2 month-1)
May 26-31 1995 0.80 1.90
June 1995 1.54 2.05
July 1995 0.69 0.67
Aug 1995 2.16 2.61
5/25/95 to

8/31/95

1.08 1.36
9/1/94

to

8/31/95

0.97 1.47

Analysis of the quantity 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 harvested in the section controlled by the grower (Table 1). During the test period the productivity in the tension-based irrigation treatment was 1.53 stems ft-2 month-1 compared with 0.92 stems ft-2 month-1 for the section irrigated by the grower. This pattern held for all periods except February 1995.

Readings from the water meters show that there was less water applied, although at greater frequency, to the tension-based treatment than to the grower beds (Table 2). The summary of data shows that an average of about 0.42 gallons of water was applied per irrigation per ft2 of greenhouse by the soil moisture tension-based system compared to about 0.76 gallons for the grower controlled system. On a per day basis, this translates to about 0.067 gallons of water used per day per ft2. for the soil moisture tension-based system versus about 0.092 gallons for the grower controlled system.

Table 2. Summary of irrigations during the test period
Section Total water applied Number of irrigations Average amount at each irrigation Water use per day
gallons gal/ft2 gal/ft2 gal/ft2
Grower 17,302 24.4 32 0.76 0.092
Test 10,928 17.9 43 0.42 0.067

By combining the harvest and water-use data it can be seen that the number of stems irrigated based on moisture tension produced 0.75 stems per gallon of water used compared to 0.33 stems per gallon under the grower-controlled system over the duration of the testing period suggesting that the tension-based irrigation system is 125% more efficient. Another way to look at this is that the soil moisture tension-based system produced flowers using 45% 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 1, the soil at this depth is almost always saturated, making it impossible to estimate run-off water.

Table 3. Analysis of harvest and water usage data
Totals Grower Test
stems/ft2 gal/ft2 stems/gal stems/ft2 gal/ft2 stems/gal
During test 8.10 24.37 0.33 13.45 17.92 0.75
After test 4.55 11.04 0.41 5.70 11.18 0.51
Difference during test: 66% -26% 126%
Difference after test: 25% 1% 24%
The rose quality data were represented by stem-length measurement since that is the criterion used by rose growers for grading and marketing roses. The data show clear evidence that increases in stem length may result from irrigating using the SMT-based system. Measurements taken during the test period (prior to 5/25/95) revealed substantial variation in the sample (Table 4). Yet at each date the mean stem length was larger for the roses coming from the grower block. In most cases this is statistically significant; even for the harvest on 2/16/95 there is good (79%) confidence that there is a difference in length. The differences in stem length range from 1.6 to 8.8 cm. This indicates that growers who implement tension-based automated irrigation may find a significant boost in stems grading to more profitable, longer grades.

Table 4. Rose stem quality: Lengths of harvested stems and t-tests
Date Grower Test Length difference

(cm)

t-test (df=49)
mean

(cm)

std.

dev.

mean

(cm)

std.

dev.

p-statistic level of significance
Feb 6 61.3 7.36 62.9 7.19 1.6 0.274 79%
Feb 13 60.2 7.52 63.6 7.45 3.4 0.025 98%
May 3 61.5 6.96 70.3 8.77 8.8 0.000 >99%
May 10 63.5 5.96 66.6 4.81 3.1 0.005 >99%
May 17 67.7 6.07 71.2 6.18 3.5 0.005 >99%
May 25 63.8 6.57 70.7 7.03 6.9 0.000 >99%
After test:

Aug 4 61.3 6.54 61.2 7.95 -0.1 0.902 37%
The pattern of differences in stem length was no longer apparent 71 days after the SMT-based irrigation control system had been removed (August 4) and irrigation returned to the former (conventional) practice (Table 4, last line). This supports our claim that the SMT-based irrigation control system was responsible for the increase in stem length.

Conclusion

The soil moisture tension-based irrigation 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, call for irrigation when the soil requires water, and turn the water off when an adequate amount has been applied.

It is an important environmental benefit that this system allows for more efficient use of water than a grower-controlled system. In addition to reducing polluting run-off, the reduction in water use also has economic benefits to the grower through reductions in the cost of producing and using water (pumping, etc.). Additional cost savings are realized if a liquid feed program is used since the amount of fertilizer applied is reduced.

There was a significant increase in the number of stems produced per square foot of greenhouse space as a result of the use of the soil moisture tension based irrigation control system. Small, but significant, increases in stem length were also found. Additional testing (replication at other sites) is needed to verify these results.

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 chrysanthemum 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.

Marsh, A.W., L.F. Werenfels, R.H. Sciaroni, 1962. Tensiometer use in carnation irrigation. Unpublished report.

Oki, L.R., J.H. Lieth and S.A.Tjosvold, 1995. Reduction of run-off in greenhouse cut flower crops through automated irrigation based on soil moisture tension. 1994 Project Report to the California Cut-flower Commission. (Also available as: CCFC project (1994-1995) report at http://lieth.ucdavis.edu/research/tens/ccfc/95/)

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. California Agriculture. May-June 1991, p. 31-32.


For more information or feedback contact jhlieth@ucdavis.edu