 Figure 1. Schematic drawing of the computer-controlled irrigation system for a typical treatment. |
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Robert C. Hansen Ted H. Short C. C. Pasian R. Peter Fynn
A Q-COM computer-control system along with tensiometers was used to monitor and control moisture tension for container-grown mini-roses (Rosa hybrida 'Meidanclar' and 'Meirutral'). Control capability for low-tension (3 to 6 kPa), medium-tension (9 to 12 kPa), and high-tension (15İtoİ18 kPa) treatments was compared under winter-time conditions using 10 cm pots and summer-time conditions using 15 cm pots. Reliable, stable control of moisture tension within 3 to 6 kPa was achieved under both winter-time and summer-time conditions. Moisture tension above 12 kPa was not successfully controlled.
Successful application of automated computer-controlled microirrigation for container-grown nursery plants depends on reliable, low-cost maintenance-free sensors that accurately measure soil-water tension at the root/potting-media interface. Tensiometers have been recognized for eight decades as standard devices for measuring soil moisture levels and have been commercially available for 45 years (Pogue and Kline, 1995). They are valuable because they sense moisture tension directly in the root zone. Ross (1994) describes tensiometers as the best device to indicate how strongly a soil particle is holding water away from a plant root. If ideal upper and lower soil-moisture tension limits can be specified, these sensors can be programmed with the use of appropriate computer software and hardware to cycle irrigation on and off. Optimum growth can occur while water is being conserved. If nutrients are injected into the water as a part of fertigation, then ideal control would provide sufficient nutrients for best growth but no more.
Burger and Paul (1987) successfully developed a solid-state electronic tensiometer for measuring the moisture potential of plants grown in container medias. Lieth and Burger (1989) used these tensiometers as a part of an irrigation system to compare four levels of soil moisture tension on the growth of Chrysanthemum. Abdel-Rahman et al. (1994) measured the effect of three tension levels on the growth of greenhouse tomato plants. Fynn et al. (1992) used soil tensiometers to control irrigation to potted Poinsettias on ebb-and-flood benches.
The purpose of this paper is to describe experiences with tensiometers while irrigating container-grown Rosa hybrida 'Meidanclar' (pink mini-roses) and Rosa hybrida 'Meirutral' (red mini-roses).
A Q-COM computer control system was used to control, monitor, and record soil-water tension data for the research. Experimental Run No. 1 (December 29, 1995, to May 9, 1996) consisted of four treatments, with each treatment composed of three 10 cm pots. Rosa hybrida 'Meidanclar' (pink mini-roses) were potted in Metromix 360 media (The Scotts Co.) using Osmocote 14-14-14 slow-release fertilizer (Sierra Chemical Co.) applied at 2.5 g per pot. A second set of four treatments (Experimental Run No. 2, June 13 to August 15, 1996) consisted of Rosa hybrida 'Meirutral' (red mini-roses) grown in 15 cm pots.
Typical settings for each treatment are shown in Table 1 for Run No. 1 and Table 2 for Run No. 2. Low, medium, and high soil-water tensions were compared as Treatment No. 1, 2, and 3. For these treatments, tension control was designed so each irrigation event could be turned on for a few seconds (pulse time) when the maximum tension setting was reached after which irrigation stopped for approximately five minutes (pause time) to allow the wetting front to disperse both laterally and vertically within the container. This type of control was created in an attempt to avoid wetting the substrate beyond the minimum tension specified for a given treatment (overshooting). Treatment No. 4 was timed to irrigate at 0900 hr. and 1500 hr. for a three-minute duration each. The irrigation schedule for Treatment No. 4 was adjusted occasionally (based on weather conditions and plant growth) to maintain soil-water tension approximately between 2 and 6 kPa to simulate what a grower might do with manual watering.
| Table 1. Experimental Setup for Run No. 1 (Dec. 27, 1995, to May 9, 1996). | ||||
|---|---|---|---|---|
| Pink Mini-Roses | Treatment | |||
| No. 1 | No. 2 | No. 3 | No. 4 | |
| No. of pots 3 | 3 | 3 | 3 | |
| Pot size (cm) | 10 | 10 | 10 | 10 |
| Irrigation control | Tensiometer | Tensiometer | Tensiometer | Timer |
| Tension (Min) (kPa) | 3 | 7 | 15 | 0900 (3 min) |
| Tension (Max) (kPa) | 6 | 10 | 18 | 1500 (3 min) |
| Pause time (min) | 5 | 5 | 5 | NA |
| Pulse time (sec) | 5 | 4 | 3 | NA |
| Table 2. Experimental Setup for Run No. 2 (June 13 to August 15, 1996). | ||||
|---|---|---|---|---|
| Pink Mini-Roses | Treatment | |||
| No. 1 | No. 2 | No. 3 | No. 4 | |
| No. of pots | 3 | 3 | 3 | 3 |
| Pot size (cm) | 15 | 15 | 15 | 15 |
| Irrigation control | Tensiometer | Tensiometer | Tensiometer | Timer |
| Tension (Min) (kPa) | 3 | 9 | 15 | 0900 (5 min) |
| Tension (Max) (kPa) | 6 | 12 | 18 | 1500 (5 min) |
| Pause time (min) | 5 | 5 | 5 | NA |
| Pulse time (sec) | 7 | 7 | 4 | NA |
In addition to collecting and storing tension data for each treatment, 1000 ml Erlenmeyer flasks were used to collect irrigation water from each treatment by using an additional line and emitter to monitor quantity of water applied (see Figure 1). Also, greenhouse temperature and humidity readings were measured, polled, and historical data was stored with the Q-COM system. The plants were randomly placed on a standard greenhouse bench. Dry-bulb temperatures in the house were set to range from 7 to 20 degrees C (45 to 68 degrees F) during Run No. 1 and 18 to 30 degrees C (65 to 86 degrees F) during Run No. 2.
Figure 1. Schematic drawing of the computer-controlled irrigation system for a typical treatment. |
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An example three-day history of moisture tension measurements for all four treatments is shown in Figure 2 beginning with February 3. As a partial record of Run No. 1, these results were recorded when skies were generally overcast and the inside of the greenhouse was being artificially heated. The best system response for operating precisely within specification limits occurred for Treatment No. 1 (3 to 6 kPa). Control for Treatment No. 2 (7 to 10 kPa) was excellent at the upper specification limit but typically overshot the lower specification limit by 1 or 2 kPa, particularly in the early morning hours and near the end of the day. Similar control is evident for Treatment No. 3, except deviation from the lower specification limit is more pronounced (1 to 4 kPa) during the cooler part of the day and into the night. The graph shows that moisture tension measurements for all three tensiometer-controlled treatments cycled more frequently during mid-day and less frequently or not at all during morning, evening, and night-time hours. The timer-controlled treatment (No. 4) shows expected cycling at 0900 hr. and 1500 hr.
 Figure 2. Three-day history of moisture tension measurements for all four treatments beginning with Feb 3. |
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More detail is evident in Figure 3, where a one-day history of moisture tension measurements is shown. Note that while the timed treatment was preset to cycle two times in 24 hours, the computer-controlled low-tension treatment cycled four times, the medium-tension treatment cycled six times, and the high-tension treatment cycled 10 times. Lieth and Burger (1989) reported the opposite scenario while growing Chrysanthemum in 15 cm pots, i.e., more frequent cycling occurred for two low-tension treatments, and less frequent cycling occurred for two high-tension treatments.
 Figure 3. One-day history of moisture measurements for all four treatments during Feb 4. |
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An example of precision control of water tension within the range of 3 kPa to 6 kPa (low-tension treatment) is shown in Figureİ4 over a 24-hour period beginning at midnight and ending at midnight on July 4. The corresponding dry-bulb temperature history, recorded inside the greenhouse, is also shown. These results were obtained on a bright summer day with fans and evaporative coolers being used to cool the greenhouse. As expected, the graph shows that irrigation frequency increased (cycle time shortened) during mid-day when radiation and temperatures were high and typically decreased (cycle time lengthened) after sundown. The number of cycles were identical to the winter-time run for this treatment.
 Figure 4. One-day history of moisture tension and dry-bulb temperature measurements for Treatment No.1 during July 4. |
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Figure 5 compares tension history for low- and medium-tension results for three days (July 4, 5, and 6) along with dry-bulb temperatures. While precise control was typical for low tension, it did not occur for medium tension, particularly during summer conditions. Note that the specified settings (9 to 12 kPa) for Run No. 2, Treatment No. 2 were 2 kPa higher than for Run No.1, Treatment No. 2 (7 to 10 kPa). The graph shows a tendency for tension values to go above the 12 kPa upper specification limit and below the 9 kPa lower specification limit by 3 kPa or more. During the third day, tension reached 20 kPa in mid-afternoon during maximum radiation. This may have occurred because the seven-second irrigation pulse followed by a five-minute pause did not supply sufficient water to the plants to keep pace with evapotranspiration. Interestingly, this treatment cycled six times on July 4, five times on July 5, and four times on July 6.
 Figure 5. Three-day history of moisture tension and dry-bulb temperature measurements for Treatments Nos. 1 and 2 beginning July 4. |
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Figure 6 shows results for the 15 to 18 kPa tension range along with corresponding dry-bulb temperature history during summer time conditions. Although there were brief periods when reasonable control was maintained, the erratic results shown are characteristic of this treatment during the summer-time run. The system did a poor job of controlling moisture tension for the high-tension treatment.
 Figure 6. Three-day history of moisture tension and dry-bulb temperature measurements for Treatment No. 3 beginning with July 4. |
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Abdel-Rahman, G. M., R. P. Fynn, R. W. McMahon, and T. H. Short. 1994. Effect of Soil Moisture Tension on the Growth of Greenhouse Tomato Plants. Proceedings of the Fifteenth Annual Conference of the Hydroponic Society of America. Akron, Ohio. April 13-17, 1994.
Burger, D. W. and J. L. Paul. 1987. Soil moisture measurements in containers with solid-state, electronic tensiometers. HortScience 22(2):309-310.
Fynn, R. P., H. A. J. Hoitink, and R. W. McMahon. 1992. Use of soil tensiometers for irrigation control on ebb and flood benches. Unpublished research.
Lieth and D. W. Burger. 1989. Growth of Chrysanthemum using an irrigation system controlled by soil moisture tension. J. Amer. Soc. Hort. Sci. 114(3):387-392.
Pogue, W. R. and J. L. Kline. 1995. Watermark moisture sensors - use with ET based scheduling models. Proceedings of the Fifth International Microirrigation Congress, Orlando, FL. April 2-6. pp. 969-974.
Ross, D. S. 1994. Reducing water use under nursery and landscape conditions. In Recycling and Resource Conservation, A Reference Guide for Nursery and Landscape Industries. 21-35. C. W. Heuser Jr. and P. E. Heuser, eds. Harrisburg: Pennsylvania. Nurserymen's Association, Inc.