AUTOMATIC SOIL MOISTURE-BASED DRIP IRRIGATION FOR IMPROVING TOMATO PRODUCTION

Transcription

1 Proc. Fla. State Hort. Soc. 116: AUTOMATIC SOIL MOISTURE-BASED DRIP IRRIGATION FOR IMPROVING TOMATO PRODUCTION RAFAEL MUÑOZ-CARPENA 1, HERBERT BRYAN, AND WALDEMAR KLASSEN University of Florida, IFAS Tropical Research and Education Center SW 280th Street Homestead, FL MICHAEL D. DUKES University of Florida, IFAS Agricultural and Biological Engineering Gainesville, FL Additional index words. Lycopersicon esculentum, gypsum block, irrigation, soil moisture monitoring, switching tensiometer, vegetable crops, water conservation Abstract. A low-volume/high frequency (LVHF) soil moisturebased drip irrigation system was tested on a commercial tomato farm in south Florida. Seven irrigation treatments were compared. In the first six treatments, the system was pressurized by means of an electrical pump and a pressure tank, and controlled by an irrigation timer (controller) set to irrigate five times per day. The last treatment consisted of the farm s standard commercial practice where a portable pump was used on a twice weekly manual irrigation schedule. Four of the six LVHF treatments resulted from interfacing in a closed control loop with the irrigation controller two types of soil moisture sensors (switching tensiometers and granular matrix sensors) set at two moisture points (wet: 10 cbar, optimal:15 cbar), i.e., irrigation was allowed to start when the soil moisture measured by the sensor was below the set point. The other two LVHF resulted from the same system with no sensors set with the timer at two irrigation schedules, one to supply 100% of the maximum recommended seasonal crop water needs (12 min per irrigation) and the other to supply 150% of those needs (18 min per irrigation). Results from the six LVHF treatments show that tomato yields were not different from that of the commercial field while conserving water. Switching tensiometers at 15 cbar set point performed the best (up to 73% reduction in water use compared with commercial farm practice, and 50% with respect to the 100% recommended crop water needs treatment). Routine maintenance (refilling and pumping) was critical for reliable operation of the switching tensiometers, especially on the driest treatment of 15 cbar where twice weekly routine maintenance is recommended. Granular matrix sensors behaved erratically and did not improve water savings compared with the 100% recommended crop water needs treatment. Florida tomato growers are at a competitive disadvantage due to off-shore competition from countries such as Mexico The authors thank T. Dispenza, C. Kameko, Y. Li, M. Codallo, T. Olczyk, R. Regalado and Q. Wang for their cooperation with this project. This research was supported by the Florida Agricultural Experiment Station and by grants from the Florida Tomato Committee and the Florida Fruit and Vegetable Association, and approved for publication as Journal Series No. N Pine Island Farms, Inc. contributed with the use of experimental plot and Mr. M. Knight (T-Tape, Inc.) donated the drip tape used for the experiment. The use of trademarks does not imply the endorsement by the University of Florida. 1 Corresponding author. where labor is considerably cheaper than in the United States. Florida growers will be at an even greater disadvantage with the imminent phase-out of methyl bromide in the U.S. but not in developing countries, including Mexico, Florida s biggest competitor in the winter tomato market. Through proper irrigation, average tomato yields in south Florida can be maintained (or increased) while minimizing environmental impacts caused by excess water applied and subsequent chemical leaching. Thus, improving irrigation efficiency can contribute greatly to reducing production costs per carton of tomatoes making south Florida s tomato industry more competitive and sustainable. Efficient and modern irrigation systems today are based on three cornerstones: 1) Low volumehigh frequency, 2) soil moisture sensor based scheduling, and 3) automatic operation (Dukes et al., 2003). Soils with low water holding capacities (sands, gravels) are common in south Florida. These soils present special water management challenges (Muñoz-Carpena, 2002). Traditional irrigation based on low frequency (a few times a week), large volume irrigation usually results in over-irrigation in south Florida soils. With this type of irrigation, a large portion of the applied water percolates quickly to the shallow groundwater, potentially carrying with it nutrients and other agrichemicals applied to the soil. In addition, excess water in the root zone from excess irrigation can reduce tomato yields (Wang et al., unpublished). As an alternative to traditional irrigation systems, a low volume of water can be applied frequently (several times a day) to maintain a desired moisture range in the root zone that is optimal for plant growth. Low volume-high frequency irrigation (LVHF) also avoids percolation. For LVHF systems, the target soil moisture is usually set in terms of soil suction or matric potential (expressed in kpa or cbar, 1 kpa = 1 cbar), or volumetric moisture (expressed in % of water volume in a volume of undisturbed soil). Soil suction is related to the amount of energy that has to be exerted by a plant to absorb water from the soil. One other benefit of high frequency irrigation is convenience. Once the system is set up and verified, only weekly observation is required (Dukes et al., 2003). Soil moisture can be determined by direct (soil sampling) and indirect (soil moisture probes) methods. Direct methods of monitoring soil moisture are not used for LVHF irrigation scheduling because they are destructive and labor intensive. Soil moisture probes can be permanently installed in representative points in an agricultural field to provide repeated moisture readings over time that can be used as a guide for irrigation scheduling. Several moisture sensors are commercially available (Muñoz-Carpena et al., 2002; Zazueta and Xin, 1994). They generally can be used for manual readings to guide irrigation scheduling, while some of them can also be interfaced directly with the irrigation controller in a closed loop control system (Zazueta et al., 1994) to automatically irrigate. Special care is needed when using soil moisture devices in coarse soils, especially in gravelly loam soils (Krome and Chekika series) present in south Florida (Muñoz-Carpena, 2002). Most devices require a close contact with the soil matrix that is sometimes difficult to achieve in gravelly soils. In 80 Proc. Fla. State Hort. Soc. 116: 2003.

2 addition, volumetric soil moisture sensors are relatively insensitive to large increases in suction values in these types of soils. Tensiometers are amongst the most widely used suctionbased soil moisture monitoring devices in Florida. They can be used as stand-alone manual instruments or interfaced with the irrigation controller (switching tensiometers) for automatic watering. Smajstrla and Locascio (1996) reported that using switching tensiometers placed at 15 cm depths and set at 10 and 15 kpa tensions in a fine sandy soil in Florida reduced irrigation requirements of tomatoes by 40-50% without reducing yields. Li et al. (1998) showed that tensiometers can also be used successfully for manual scheduling of tomato irrigation in calcareous gravelly soils (Krome series). In their study, optimal irrigation at 10 kpa increased yield, improved fruit quality, and reduced nutrient leaching. Wang et al. (unpublished) studied tomato yields in Krome soils with irrigation scheduled by manual readings from tensiometers. When compared to irrigation at -5 kpa (control), all three of the lower rates (-10, -20 and -30 kpa) significantly improved yields of marketable, large and extra-large-fruit. The highest yield increases were obtained at -30 kpa, which were about 29, 28, and 22% greater than at -5 kpa (control) for yields of marketable, extra-large, and large fruit, respectively. The objective of this work was to evaluate a LVHF irrigation system for tomatoes interfaced with one of two different soil moisture sensor types in a commercial setting by comparing them side by side with other irrigation treatments (two automatic by means of an irrigation controller with two high frequency watering schedules and the other the standard grower s practice with a twice weekly schedule by manually turning a pump). Water savings, crop yields, pros and cons of the system, and technical aspects are presented. Materials and Methods A research and demonstration project was conducted on a commercial tomato farm, Pine Island Farms, Miami, Fla. The experiment was conducted in a 0.6 ha (1.5-acre) experimental plot inside a 16.2 ha (40 acre) commercial tomato field. The soil was Dade fine sand (30 cm overlaying porous limestone bed rock) and the tomatoes in the entire field, including the experimental plot, were grown on beds separated 1.8 m (6 ft), supplied with dual drip irrigation lines under plastic mulch. This setup was identical to the farmer s production system. The following seven irrigation treatments, each with three replicates, were established on beds 183 m (600 ft) long (Fig. 1): t1. Soil tension reading based on switching low-tension tensiometer (model TGA-LT, Irrometer Co., Inc., Riverside, CA) set to irrigate when soil suction reached 10 kpa (T-10 cbar). t2. Soil tension reading based on switching low-tension set to irrigate when soil suction reached 15 kpa (T-15 cbar). t3. Soil tension reading based on granular matrix sensor (Watermark WEM-II, Irrometer) set to irrigate when soil suction reached 10 kpa (WM-10 cbar). t4. Soil tension reading based on granular matrix sensor set to irrigate when soil suction reached 15 kpa (WM-15 cbar). t5. Timer set to apply 100% of the recommended water needs for Miami (Simmone et al., 2001) in five irrigations per day (12 min per irrigation) (Time-100%). t6. Timer based to apply 150% of the recommended water needs for Miami in 5 irrigations per day (18 min per irrigation) (Time-150%). t7. Standard grower s schedule typical in the area (control; high volume-low frequency, 2-3 times per week, 2-3 h per irrigation). Treatments t1-t6 were LVHF. All replications, except those for time-based treatments (t5 and t6 above), were controlled independently by an off-the-shelf irrigation controller by means of a solenoid valve. A water meter and pressure regulator were installed at the water entry point to the drip lines. An electrical pump in line with a pressurized tank maintained pressure in the system. Table 1 provides details on the design of the irrigation system installed in the plot. The soil moisture sensors were wired in closed loop control with the irrigation timer (Zazueta et al., 1994) according to the manufacturer s specifications. With this setup, if sufficient soil water is available (below 10 or 15 cbar), sensors can by-pass irrigation startup signals sent to the electro-valve by the irrigation controller. The signals were sent on the basis of treatment 5 (T-100%), i.e., five times per day for each sensor-based replication. This way a significant volume of water can potentially be saved during periods of reduced plant-water needs, and the moisture kept at optimal levels in the root zone. The sensors were inserted 30 m from the electro-valve, between plants, in the center of the bed. After the beds had been fully prepared, there was a 3- month delay in obtaining the electrical power needed to operate the irrigation system. Tomatoes (cv. Florida 47) were transplanted on 4 Feb and irrigation started 10 d later. Fertilizer and other agrichemical injections (fertigations) were done in accordance with the farmer s schedule by adding extra irrigations off the preset controller schedule and with equal amounts of water and agrichemicals for all treatments. There was a 2-week delay with respect to the commercial field in injecting the systemic insecticide needed to protect plants from transmission of tomato yellow leaf curl virus (TYLCV) by whiteflies. The delay was caused by equipment difficulties. The first tomato harvest was on 21 Apr followed by the second and final harvest 2 weeks later. Water use in each treatment was continuously recorded by a positive displacement water meter equipped with a magnetically actuated reed switch (ABB Water Meters, Inc., Ocala, Fla.) connected to an event datalogger (Onset Computer Corporation, Bourne, Mass.). Weekly readings were also manually taken from the counters in each water meter. Values obtained from replications in each treatment were averaged. Fruit were graded following Florida Tomato Committee standards (Brown, 2000). The fruit were segregated into extra-large, large, medium, and culls after each harvest, and the marketable and total fruit yields were derived through calculation. Data were analyzed by analysis of variance and means were compared using Duncan s Multiple Range Test at the 5% level of significance (SAS Institute, Inc., 1999). Results and Discussion Water use. Results are summarized in Table 2 and Fig. 2. Treatments 1-6 used substantially less water than traditional irrigation in the commercial field (treatment 7). The automated system with switching tensiometers (treatments 1-2) conserved the most water (67-73% reduction in water use). Proc. Fla. State Hort. Soc. 116:

3 Fig. 1. Plot layout showing experimental treatments. The farmer s tomato field used as the control continues on both sides of the plot. See Table 1 for SI units. 82 Proc. Fla. State Hort. Soc. 116: 2003.

4 Table 1. Irrigation system specifications (see Fig. 1 for layout and English units). Hardware Pump: 1 HP Well tank: 750 L, pressure control m Controller: Rain-Bird ESP-12LX Main line: lay-flat 50 mm Pressure regulator: 14 m (end of main line) 7 m (after valves) Solenoid valves: 24VAC, dia. 13 mm Drip tape: T-TAPE TSX Internal diameter = 16 mm Drip spacing = 0.30 m Nominal flow = 5.6 L/min/100 m Nominal pressure = 5.6 m Lateral length: 183 m (double lines) Working pressure: 7 m Lateral flow: 18.2 L/min (total both lines) Agronomical parameters Max crop needs = 3.5 mm/day Surface per sub-plot = 330 m 2 Max needs/subplot = 1150 L/day Max. time to irrigate 60 min/plot-day Max no. of irrigations/day = 5 Time/irrigation = 12 min/plot The moisture set point (suction above which the irrigation is allowed to start) for the sandy soil, from 10 cbar to 15 cbar, conserved an additional 16% in tensiometers and 7% in granular moisture sensors. Timer-based LVHF with no sensors (treatments 5-6) also conserved water since deep percolation associated with large volume applications was reduced significantly. Although setting the set point from 10 to 15 cbar used 16% less water use with tensiometer, only 5% was saved with the granular moisture sensors. The standard commercial schedule (treatment 7) used about 81% more water than the recommended plant water needs for the area (T-100%, treatment 5). Compared to irrigation based on plant water needs (treatment 5), as shown in the last column of Table 2, the tensiometer-based treatments (treatments 1 and 2) resulted in a substantial decrease in water use (39-51%) while use of the granular matrix sensors (treatments 3 and 4) did so only marginally (2-7%). Crop yields. Tomato yields at the experimental plots (treatments 1-6) were not significantly different (at 5% level) from those of standard grower irrigation treatment (treatment 7), except for the wettest time-based treatment, T-150% (treatment 6; Table 3). Except for that treatment, yields were similar to the Florida average of kg ha -1 (Maynard, 2001) and similar to average yields in Miami-Dade County (39280 kg ha - 1 ). Although not statistically significant, the experimental plot yields were generally smaller than that of the surrounding farm (outside of the treatment area) grower s results. This could be explained in terms of (a) the lower rate of dry fertilizer incorporated into the experimental beds (1,603 kg ha -1 ) than in the commercial beds (1,781 kg ha -1 ), (b) dissipation of fertilizer in the beds during a 3-month delay in transplanting into the prepared experimental beds caused by a delay in obtaining electrical power to operate the irrigation system, and (c) a greater TYLCV incidence observed in the experimental than in the farmer s adjacent fields. The latter was caused by a 2-week delay in injecting the systemic insecticide needed to protect the plants from infection by whiteflies. The wettest treatments (Time-150% and T-10 cbar; treatments 1 and 6, respectively) also yielded fewer large and extra-large fruit. Despite the large reduction in water use in treatments 1-7 there was not a significant impact on fruit yields. It is remarkable that the automatic irrigation system controlled by the switching tensiometer at 15 cbar yielded the highest fruit quality while conserving 73% of the water compared with the standard commercial irrigation practice (treatment 7) and about 50% compared to irrigating based on 100% recommended plant water needs (treatment 5). Assessment of water sensors and treatments. Tensiometers, when subjected to weekly maintenance, performed well and consistently across repetitions for each treatment (data not shown). However, if left unattended for more than 1 week, air entered the tensiometer, breaking the water column. This was more frequent in the driest treatment (15 cbar) where a twice weekly maintenance schedule (Monday and Friday) was adopted. From a practical point of view, it is essential under south Florida field conditions to include routine maintenance of tensiometers. This routine consists of opening the tensiometer, refilling the column, pumping to purge air bubbles, and recapping. Preferably this should be done at least 1 h before the first daily irrigation set-time or after the last one to give sufficient time for the soil and the tensiometer to equilibrate before the next irrigation. Care should be taken not to break Table 2. Water use at the end of the season and comparison with respect to the commercial field. # Treatment Water applied (mm) Change z from farmer y Change from time - 100% x t1 T-10 cbar % -39% t2 T-15 cbar 91-73% -51% t3 WM-10 cbar % -2% t4 WM-15 cbar % -7% t5 Time - 100% % t6 Time - 150% % +42% t7 Farmer Control % z Treatment - Control Change = Control y Control = Farmer water use. x Control = Time-100%. Proc. Fla. State Hort. Soc. 116:

5 Fig. 2. Average water applied in each automatic irrigation treatment. the tensiometer contact with the surrounding soil by twisting when uncapping for refilling. A de-aerated solution of water boiled for 20 min with a few drops of algicide (unscented household bleach) gives the best results. Two of the tensiometers had to be replaced during the season. One was accidentally perforated when staking the tomatoes and the other one had a faulty seal that made it discharge frequently. The granular matrix sensors performed erratically across repetitions and treatments. The commercial system includes an interface box with a dial on a scale from 1-8 (and an OFF position to by-pass). The same dial setting in the three repetitions of each treatment gave very different soil moisture readings from tensiometers installed just 10 cm from the granular matrix sensor. Also, consecutive steps in the dial scale (from 1 up) did not correspond to linear increases in field soil suction and the granular matrix sensors performed inconsistently across replications and treatments. As a result, although about 50% water savings were observed with respect to the control (commercial farm, treatment 7), no appreciable difference in water savings was found between the 10 and 15 cbar (found equivalent to settings 1 and 3 in the dial scale from limited testing in the laboratory with the farm soils). Table 3. Tomato yield and grades obtained for the total harvest. Total marketable Total extra large Total large Treatment Yield (kg/ha) No. Size (g) Yield (kg/ha) No. Size (g) Yield (kg/ha) No. Size (g) Time-100% a a 168 a a,b a,b,c 242 a a a 160 a,b Time-150% b b 170 a b c 231 a b b 169 a T-10 cbar a,b a 151 b b b,c 221 a a a 155 b,c T-15 cbar a a 161 a,b a,b b,c 222 a a a 153 b,c WM-10 cbar a a 164 a,b a,b b,c 233 a a a 156 a,b,c WM-15 cbar a a 160 a,b a,b a,b 207 a a a 162 a,b Farmer 45243a a 156 a,b a a 211 a a a 144 c a,b,c Different letters depict statistically different means at P Proc. Fla. State Hort. Soc. 116: 2003.

6 Furthermore, when taking as the basis for comparison the Time-100% treatment, only 2-7% water savings were observed. In fact, since the granular matrix sensors were interfaced with the timer pre-set with the same schedule as that for treatment 5, i.e. five irrigations per day of 12 min each, these results mean that they fail to respond to sufficient soil moisture by by-passing irrigations. In addition, two control interface boxes had to be replaced during the season since they stopped working spontaneously. The LVHF time-based treatments (5 and 6) performed well, without requiring any maintenance. Conclusions These results show that when using an automated irrigation system based on feedback from soil moisture sensors, tomato yields were not affected while conserving water (up to 73% reduction in water use when compared with standard commercial practices). Switching tensiometers at 15 cbar performed the best while maintaining yields. A significant reduction in deep percolation and in ensuing chemical transport is expected. Although water savings were obtained with the application of the low volume-high frequency concept (applying water to meet 100% of the crop water needs in small quantities several times per day), these savings were increased when irrigation was automatically controlled with soil moisture sensors. However, not all sensors tested performed the same. Routine maintenance (refilling and pumping) was critical for reliable operation of the switching tensiometers, especially on the driest treatment of 15 cbar (Monday and Friday routine maintenance is advised in this case). Granular matrix sensors behaved somewhat erratically and did not improve water savings with respect to the case where 100% of the recommended plant water needs were applied with a LVHF system set for five daily irrigations (12 min each) with no sensors. Literature Cited Brown, R Florida Tomato Committee Regulatory Bulletin No. 2. Florida Tomato Committee, Orlando. Dukes, M. D., E. H. Simonne, W. E. Davis, D. W. Studstill, and R. Hochmuth Effect of sensor-based high frequency irrigation on bell pepper yield and water use, p In: Proc. 2nd Intl. Conf. Irrigation Drainage, May, Phoenix, AZ. Li, Y, R. Rao, H. Bryan, and T. Olczyk Optimized irrigation schedule to conserve water and reduce nutrient leaching for tomatoes grown on a calcareous gravelly soil. Proc. Fla. State Hort. Soc. 111: Maynard, D. N Yields of vegetables. Chapter 17. In: D.N. Maynard and M. Olson (eds.). Vegetable Production Guide for Florida. Citrus and Vegetable Magazine and Univ. Fla. Coop. Ext. Serv., Inst. Food Agr. Sci., Gainesville. Muñoz-Carpena, R., Y. Li, and T. Olczyk Alternatives for low cost soil moisture monitoring devices for vegetable production in the south Miami-Dade County agricultural area. Fact Sheet ABE 333. Dept. Agr. Biol. Eng., Univ. Fla. Coop. Ext. Serv., Inst. Food Agr. Sci., Gainesville. edis.ifas.ufl.edu/ae230. SAS Institute, Inc SAS 8.01 [Computer software]. SAS Institute, Cary, NC. Simmone, E. H., M. D. Dukes, and D. Z. Haman Principles and practices of irrigation management for vegetables. Chapter 8. In D.N. Maynard and M. Olson (eds.). Vegetable Production Guide for Florida. Citrus and Vegetable Magazine and Univ. Fla. Coop. Ext. Serv., Inst. Food Agr. Sci., Gainesville. Smajstrla, A. G. and S. J. Locascio Tensiometer-controlled drip irrigation scheduling of tomato. Appl. Eng. Agr. 12: Zazueta, F. S., A. G. Smajstrla, and G. A. Clark Irrigation system controllers. Fact Sheet SS-AGE-22. Dept. Agr. Biol. Eng. Dept., Univ. Fla. Coop. Ext. Serv., Inst. Food Agr. Sci., Gainesville. AE077. Zazueta, F. S. and J. Xin Soil moisture sensors. Bulletin 292, Univ. Fla. Coop. Ext. Serv., Inst. Food Agr. Sci., Gainesville. ufl.edu/eh226. Proc. Fla. State Hort. Soc. 116: LENGTH OF IRRIGATION AND SOIL HUMIDITY AS BASIS FOR DELIVERING FUMIGANTS THROUGH DRIP LINES IN FLORIDA SPODOSOLS BIELINSKI M. SANTOS 1, JAMES P. GILREATH, AND TIMOTHY N. MOTIS University of Florida, IFAS Gulf Coast Research and Education Center th Street E. Bradenton, FL Additional index words. chloropicrin, metham sodium, methyl bromide, 1,3-dichloropropene, sandy soils Abstract. Soil fumigant delivery through microirrigation lines has the potential to replace direct soil injection into planting This research was supported by the Florida Agricultural Experiment Station, and approved for publication as Journal Series No. N Corresponding author; bmsantos@yahoo.com. beds. However, wetting coverage in these spodosols must be improved to increase soilborne pest and weed control. Field trials were carried out to determine the impact of soil humidity on the extent of wetting cross-sectional areas through varying irrigation times. Soil humidity contents were: a) 7% moisture (field capacity), and b) 20% (saturation), along with 2, 4, 6, 8, and 10 h of irrigation. Pressed beds had 70 cm tops. Drip lines had emitters spaced 30 cm apart delivering L min -1 m of row at 55 kpa, and two drip lines were buried at 2.5 cm below the surface and 30 cm apart from each other. Water was mixed with a blue marking dye to analyze the water distribution patterns. Beds were opened at the emitters and high-resolution digital pictures were taken for each treatment. Resulting images were adjusted using photographic software and covered areas across the beds were determined. Regression analysis showed significant quadratic equations for both soil moisture situations, with saturated soils obtaining the highest cross section coverage (90 and 94% after 8 and 10 hours). In field ca- Proc. Fla. State Hort. Soc. 116: