IRRIGATION FREQUENCY, DRIP TAPE FLOW RATE, AND ONION PERFORMANCE

Clinton C. Shock, Erik Feibert, and Lamont Saunders
Malheur Experiment Station
Oregon State University
Ontario, OR, 2003

Introduction

Onion production with subsurface drip irrigation has proven at the Malheur Experiment Station to be highly productive on sites that are difficult to irrigate. In 1997 and 1998 onions were submitted to five soil water potential treatments using an automated, high frequency irrigation system (Shock et al. 2000a). The soil water potential was maintained relatively constant by applying 0.06 inch of water up to eight times a day, depending on soil water potential readings. The soil water potential at 8-inch depth that resulted in maximum onion yield, grade, and quality after storage was determined to be -20 kPa. An irrigation frequency of up to eight times a day in small increments is not feasible on a commercial scale. Would reducing the irrigation frequency result in lower water use efficiencies and lower onion yield and quality?

The drip tape that has been used at the Malheur Experiment Station has a flow rate of 0.22 gal/min/100 ft of tape. A reduced flow rate could theoretically result in an improved soil wetting pattern and less water lost to deep percolation. An improved soil wetting pattern could result in the onions on the outside row of a double row receiving more uniform soil moisture. New "ultra low flow" drip irrigation tapes with reduced emitter flow rates are being introduced by drip tape manufacturers. This trial tested four irrigation frequencies and two drip tape flow rates for their effect on onion yield and quality.

Materials and Methods

The onions were grown at the Malheur Experiment Station, Ontario, Oregon on an Owyhee silt loam previously planted to wheat. Onion (cv. 'Vaquero', Sunseeds, Morgan Hill, CA) was planted in two double rows, spaced 22 inches apart (center of double row to center of double row) on 44-inch beds on March 17, 2003. The rows in the "double row" were spaced 3 inches apart. Onion was planted at 150,000 seeds/acre. Drip tape (T-tape, T-systems International, San Diego, CA) was laid at 4-inch depth between the two double onion rows on March 28. The distance between the tape and the double row was 11 inches. The drip tape had emitters spaced 12 inches apart and either of two flow rates: low flow (0.22 gal/min/100 ft) and ultra low flow (0.11 gal/min/100 ft).

Immediately after planting the onion rows received 3.7 oz of Lorsban 15G per 1,000 ft of row (0.82 lb ai/acre), and the soil surface was rolled. Onion emergence started on April 7. The trial was irrigated on April 14 with a minisprinkler system (R10 Turbo Rotator, Nelson Irrigation Corp., Walla Walla, WA) for even stand establishment. Risers were spaced 25 ft apart along the flexible polyethylene hose laterals, which were spaced 30 ft apart.

Onion tissue was sampled for nutrient content on June 19. The roots from 25 onion plants taken from plot border rows representative of the field were washed with deionized water and analyzed for nutrient content by Western Labs, Parma, Idaho. The onions in all treatments were fertilized according to the nutrient analyses (Table 1). Fertilizer was applied through the drip tape: ammonium sulfate at 25 lb N/acre on May 30, urea ammonium nitrate solution at 25 lb N/acre on June 5, 16, and 25, and zinc chelate at 0.25 lb Zn/acre and copper chelate at 0.2 lb Cu/acre on June 25.

Roundup at 24 oz/acre was sprayed on March 28. The field had Prowl (1lb ai/acre) broadcast on April 21 for postemergence weed control. Approximately 0.4 inch of water was applied through the minisprinkler system on April 21 to incorporate the Prowl. The field had Buctril at 0.12 lb ai/acre and Poast at 0.4 lb ai/acre applied on April 28. Thrips were controlled with one aerial application of Warrior on June 5 and two aerial applications of Warrior (0.03 lb ai/acre) plus Lannate (0.4 lb ai/acre) on July 16 and August 4.

The experimental design was a randomized complete block with four replicates. The onions were submitted to eight treatments consisting of a combination of two drip tape flow rates and four daily irrigation frequency/duration treatments (Table 2). The onions in each plot (four double rows by 50 ft) were submitted to one irrigation frequency and one tape flow rate. The irrigation frequencies were the daily time interval by which the datalogger (CR10, Campbell Scientific, Logan, UT) checked the sensors and made irrigation decisions. Each plot was irrigated independently when the average soil water potential at 8-inch depth in the plot reached -20 kPa. The irrigation durations for each treatment were adjusted so that when irrigated the maximum number of times, all treatments had the capacity to deliver a maximum of 0.48 inch of water per day.

Soil water potential was measured in each plot with four granular matrix sensors (GMS, Watermark Soil Moisture Sensors Model 200SS, Irrometer Co., Riverside, CA) installed at 8-inch depth in the center of the double row. Sensors were calibrated to swp (Shock et al. 1998a). The GMS were connected to the datalogger via five multiplexers (AM 410 multiplexer, Campbell Scientific, Logan, UT). The datalogger read the sensors and recorded the soil water potential every 3 hours. The irrigations were controlled by the datalogger using a controller (SDM CD16AC controller, Campbell Scientific, Logan, UT) connected to solenoid valves in each plot. The pressure in the drip lines was maintained at 10 psi by pressure regulators in each plot. The amount of water applied to each plot was recorded daily at 8:00 a.m. from a water meter installed between the solenoid valve and the drip tape. The automated drip irrigation system was started on May 22. Irrigations were terminated on September 2.

Onion evapotranspiration (Etc)was calculated with a modified Penman equation (Wright 1982) using data collected at the Malheur Experiment Station by an AgriMet weather station. Onion Etc was estimated and recorded from crop emergence on April 7 until the final irrigation.

On September 11 the onions were lifted to field cure. On September 17, onions in the central 40 ft of the middle two double rows in each subplot were topped and bagged. The bags were placed into storage on September 29. The storage shed was managed to maintain an air temperature of approximately 34°F. Onions were graded on December 11.

During grading bulbs, were separated according to quality: bulbs without blemishes (No. 1s), split bulbs (No. 2s), neck rot (bulbs infected with the fungus Botrytis allii in the neck or side), plate rot (bulbs infected with the fungus Fusarium oxysporum), and black mold (bulbs infected with the fungus Aspergillus niger). The No. 1 bulbs were graded according to diameter: small (<2¼ inches), medium (2¼-3 inches), jumbo (3-4 inches), colossal (4-4¼ inches), and supercolossal (>4¼ inches). Bulb counts per 50 lb of supercolossal onions were determined for each plot of every variety by weighing and counting all supercolossal bulbs during grading.

Results

In the analysis of variance, the year effect was significant for total marketable and jumbo onion yields, both being higher in 2002. The yield of the larger bulb size classes was limited in these trials by the high plant population (Shock et al. 2004). While the yields are above the county average, they are in the range achieved by growers using drip irrigation.

There was no interaction between emitter type or irrigation frequency and year, so the results are analyzed and discussed as the average over the 2 years. Averaged over irrigation frequencies, the drip tape with 0.13 gal/hour emitters had significantly higher total yield, marketable yield, and colossal onion yield than the tape with 0.07 gal/hour emitters (Table 2). Averaged over emitter type, the once per day irrigation frequency (0.48 inch of water applied per irrigation) had among the highest total and marketable onion yields. Averaged over emitter type, the once per day irrigation frequency resulted in the highest colossal onion yield.

There was no significant difference in average soil water potential between treatments (Table 2). The standard deviation of the soil water potential increased with decreasing irrigation frequency, reflecting the higher amplitude of soil water potential oscillation around the criteria of -20 kPa (Table 2, Figs. 1 and 2). There was no significant difference in total water applied between treatments, with, on average, 32 and 28 inches applied in 2002 and 2003, respectively. Onion Etc from emergence to the last irrigation totaled 30.2 and 32 inches in 2002 and 2003, respectively. The total amount of water applied includes 2 and 0.52 inches of water applied with the minisprinkler system after emergence, and 0.84 and 1.28 inches of precipitation, in 2002 and 2003, respectively. Water applications to all treatments closely followed Etc during the season (Figs. 3 and 4).

Discussion

An explanation for the increased bulb size with the lowest irrigation frequency could be that, since the lowest irrigation frequencies had the highest amplitude of soil water potential oscillation, the onions might have responded to the soil becoming wetter during irrigations than with the lower irrigation frequencies. Our past research has shown that onions will respond to irrigation criteria higher than -20 kPa with increased bulb size (Shock et al. 1998b, Shock et al. 2000b). An irrigation criteria higher than -20 kPa is not recommended on silt loam soils, because of the unpredictability of onion storage quality, which in some years can be low with irrigation criteria higher than -20 kPa.

The results of this study suggest that the drip tape with 0.066 gal/hour emitters should not be recommended for onion production in the Treasure Valley, since onion yield and size were lower and there were no apparent irrigation benefits.

References

Shock, C.C., J.M. Barnum, and M. Seddigh. 1998a. Calibration of Watermark Soil Moisture Sensors for irrigation management. Pages 139-146 in Proceedings of the International Irrigation Show, Irrigation Association, San Diego, CA.

Shock, C.C., E.B.G. Feibert, and L.D. Saunders. 1998b. Onion yield and quality affected by soil water potential as irrigation threshold. HortScience 33:1188-1191.

Shock, C.C., E.B.G. Feibert, and L.D. Saunders. 2000a. Irrigation criteria for drip-irrigated onions. HortScience 35:63-66.

Shock, C.C., E.B.G. Feibert, and L.D. Saunders. 2004. Plant population and nitrogen fertilization for subsurface drip-irrigated onion. HortScience: In Press.

Shock, C.C., J.K. Ishida, E.P. Eldredge, and L.D. Saunders. 2000b. Yield of yellow onion cultivars in eastern Oregon and southwestern Idaho. HortTechnology 10:613-620.

Wright, J.L. 1982. New evapotranspiration crop coefficients. J. Irrig. Drain. Div., ASCE 108:57-74.

Table 1. Onion root nutrient concentrations on June 19, 2003. Malheur Experiment Station, Oregon State University, Ontario, OR.

Nutrient Sufficiency range* Analysis
NO3 (ppm) 6,200 4,251
P (%) 0.32 - 0.70 0.59
K (%) 2.7 - 7.0 4.95
S (%) 0.24 - 1.4 0.61
Ca (%) 0.4 - 1.6 1.69
Mg (%) 0.3 - 0.6 0.41
Zn (ppm) 32 - 100 27
Mn (ppm) 35 - 100 91
Cu (ppm) 8 - 30 8
Fe (ppm) 60 - 250 448
B (ppm) 19 - 80 27
*supplied by Western Labs, Parma, ID.

Table 2. Effect of irrigation frequency and drip tape emitter flow rate on onion yield and size, Malheur Experiment Station, Oregon State University, Ontario, OR.

Emitter flow rate

Irrigation frequency

Irrigation duration

Water applied Avg soil water potential

Total yield

Marketable yield
Per irrigation

Total

Total

Super colossal

Colossal

Jumbo

Medium

gal/h h h (inch) (inch) (kPa) (cwt/acre) (cwt/acre) (cwt/acre) (cwt/acre) (cwt/acre) (cwt/acre)

2002












0.13 3 1 0.06 32.7 -20.3 ! 3.1 1,042 1028 6 239 764 20
0.13 6 2 0.12 32.2 -19.9 ! 3.4 985 970 11 192 738 30
0.13 12 4 0.24 32.6 -20.3 ! 3.5 1,041 1,028 9 192 800 28
0.13 24 8 0.48 31.5 -18.8 ! 4.1 1,052 1,028 16 287 708 16
Avg


32.3 -19.8 1,030 1,014 10 227 753 24












0.066 3 2 0.06 32.6 -19.8 ! 2.9 969 952 13 185 736 18
0.066 6 4 0.12 31.2 -21.2 ! 3.9 994 972 3 190 751 28
0.066 12 8 0.24 32.9 -19.8 ! 4.2 977 958 10 178 746 24
0.066 24 16 0.48 31.5 -19.9 ! 5.4 1,041 1,025 11 213 778 23
Avg


32.0 -20.2 995 977 9 192 753 23












Avg over tape types 3
0.06 32.7 -20.0 1,005 990 9 212 750 19
6
0.12 31.8 -20.4 989 971 8 191 744 29
12
0.24 32.7 -20.1 1,009 993 10 185 773 26

24
0.48 31.5 -19.4 1,047 1,027 14 256 738 19

2003












0.13 3 1 0.06 28.5 -18.0 ! 2.7 861 846 17 164 649 17
0.13 6 2 0.12 28.0 -19.4 ! 3.0 880 846 9 211 610 16
0.13 12 4 0.24 27.7 -18.9 ! 3.3 902 894 6 194 677 18
0.13 24 8 0.48 29.2 -17.4 ! 4.5 947 925 26 269 615 15
Avg


28.4 -18.4 897 878 14 209 637 17












0.066 3 2 0.06 26.9 -18.9 ! 2.8 849 834 2 138 673 20
0.066 6 4 0.12 28.8 -18.9 ! 2.4 805 786 16 150 599 22
0.066 12 8 0.24 24.9 -19.6 ! 3.3 940 901 5 186 692 18
0.066 24 16 0.48 31.1 -18.7 ! 4.1 882 859 13 197 630 19
Avg


27.9 -19.0 869 845 9 168 649 20












Avg over tape types 3
0.06 28.0 -18.5 855 840 10 151 661 18
6
0.12 28.3 -19.2 842 816 12 180 605 19
12
0.24 26.8 -19.2 921 897 6 190 684 18

24
0.48 29.9 -18.1 914 892 19 233 622 17
2002-2003 avg










0.13 3 1 0.06 29.9 -18.8 952 937 11 201 706 18
0.13 6 2 0.12 29.8 -19.6 932 908 10 201 674 23
0.13 12 4 0.24 29.8 -19.5 972 961 7 193 738 23
0.13 24 8 0.48 30.0 -17.9 1,000 976 21 278 662 16
Avg


29.9 -18.9 964 946 12 218 695 20












0.066 3 2 0.06 29.8 -19.2 909 893 8 162 705 19
0.066 6 4 0.12 30.0 -19.6 886 866 10 167 664 24
0.066 12 8 0.24 28.9 -19.7 958 930 8 182 719 21
0.066 24 16 0.48 31.3 -19.1 950 930 12 204 693 21
Avg


30.0 -19.4 926 905 9 179 695 21












Avg over tape types 3
0.06 29.8 -19.0 930 915 9 181 705 19
6
0.12 29.9 -19.6 911 889 10 185 669 24
12
0.24 29.5 -19.6 965 945 8 187 729 22

24
0.48 30.5 -18.5 976 955 17 244 676 18
LSD (0.05) Emitter

NS NS 36 34 NS 4 NS NS
LSD (0.05) Water applied NS NS 50 48 9 50 44 1
LSD (0.05) Emitter X Water applied NS NS NS NS NS NS NS NS
LSD (0.05) Emitter X Water appl. X Year NS NS NS NS NS NS NS NS



Figure 1. Soil water potential over time for drip-irrigated onion using a tape flow rate of 0.22 gal/min/100 ft and four irrigation frequencies (time interval used by datalogger for checking sensors and making irrigation decisions). Soil water potential is the average of 16 sensors. Malheur Experiment Station, Oregon State University, Ontario, OR, 2003.




Figure 2. Soil water potential over time for drip-irrigated onion using a tape flow rate of 0.11 gal/min/100 ft and four irrigation frequencies (time interval used by datalogger for checking sensors and making irrigation decisions). Soil water potential is the average of 16 sensors. Malheur Experiment Station, Oregon State University, Ontario, OR, 2003.




Figure 3. Cumulative water applied and Etc over time for drip-irrigated onion using a tape flow rate of 0.22 gal/min/100 ft and four irrigation frequencies. Water applied is the average of four plots. Malheur Experiment Station, Oregon State University, Ontario, OR, 2003.



Figure 4. Cumulative water applied and Etc over time for drip-irrigated onion using a tape flow rate of 0.11 gal/min/100 ft and four irrigation frequencies. Water applied is the average of four plots. Malheur Experiment Station, Oregon State University, Ontario, OR, 2003.