Increasing Number of Drip Irrigation Lines for Water Conservation during Strawberry Establishment

ABSTRACT

Strawberry transplants in California are planted through narrow holes in raised beds covered with polyethylene mulch. They are irrigated with overhead sprinklers for the first five weeks to leach salts and maintain plant turgidity; most of this sprinkler-applied water runs off the plastic without reaching the plants. At three coastal locations, we compared grower controlled drip irrigation with doubled number of tapes to sprinkler irrigation immediately after planting. Root zone EC and moisture levels were similar in both irrigation systems. Depending on the location, water use was reduced 24 to 78% in drip-only plots compared to sprinkler and runoff was nearly eliminated. Plants in drip-only plots were similar in size and root biomass to those established with sprinkler irrigation. Early yields were similar in the two irrigation systems. Thus, use of the increased number of drip tapes conserves water during strawberry establishment without negative effects on plant performance.

KEYWORDS:

• Salts
• root development
• sprinkler irrigation

Introduction

California produces nearly 90% of the strawberries in the United States (CSC, 2012). One of the main constraints for production is the diminishing availability of water for irrigation during the current multi-year drought. In coastal production fields of California, bare-root strawberry transplants from high elevation nurseries are planted in the fall in raised beds covered with polyethylene mulch with small holes in the mulch (Strand, 2008). These plants begin to develop new roots and are very sensitive to drying and the buildup of salts. Thus, they are typically irrigated with overhead sprinklers for the first 5 weeks to leach salts and maintain plant turgidity. During the remainder of the season, plants are irrigated via drip lines placed 3–7 cm deep and 10–25 cm away from plant rows during bed preparation. Most of the water applied by sprinklers during plant establishment runs off the plastic without reaching the plants, resulting in significant runoff.

This project focused on minimizing or eliminating sprinkler irrigation during the plant establishment phase with reliance on additional drip lines to improve water delivery and salt leaching near the developing root systems. The main objective was to conserve water and minimize or eliminate runoff during strawberry establishment without compromising and possibly improving plant performance.

Materials and methods

The studies were conducted at the three main production regions in California: Ventura (once at Oxnard and twice at Camarillo), Santa Barbara (Santa Maria), and Monterey (Salinas) counties. All treatments were located in large-scale irrigation blocks (0.25–1.5 ha, depending on the region) in grower fields, and irrigation and management were conducted by growers and their irrigators. To aid growers in decision making we provided data on root zone soil moisture, bulk electrical conductivity (ECb), and plant performance during the season. ECb is a field measurement of soil salinity that depends on soil moisture, temperature, and bulk density at the time of measurement, unlike ECe (saturated paste extract) that is standardized and measured in the laboratory.

Production bed dimensions, drip line configurations, and irrigation treatments varied among the locations (Table 1). At all locations, strawberry establishment with either: (a) drip-only irrigation with increased number of drip lines or (b) reduced sprinkler irrigation were compared to (c) standard sprinkler irrigation. At Santa Maria, in treatments with ‘reduced’ sprinkler or drip irrigation the grower estimated 30% reduction in the amount of applied water compared to his ‘regular’ irrigation practices, however, the exact amount of water has not been measured with flow meters. Placement of additional drip lines was accomplished by making two passes over the same bed while offsetting the rollers that delivered drip lines to the whole bed lengths at a previously measured distance from future planting rows (Table 1). This ensured that drip lines would not be damaged by hole-punching knives in preparation for planting and that the drip line emitters delivered water close to the newly developing plant roots. At Camarillo, Oxnard, and Santa Maria, low-flow drip tape (John Deere [T-systems International, Inc., San Diego, CA, USA] or Aquatraxx [Toro Micro-Irrigation, El Cajon, CA, USA], delivering about 4 L/min/100 m) was used when four drip lines were installed per bed, and high flow drip tape (delivering about 7.8 L/min/100 m) when two drip lines were installed per bed (standard practice). Spacing between emitters was always 20 cm. At Salinas, two drip lines with medium flow discharge rates (5.6 L/min/100 m) were installed adjacent to two plant rows spaced approximately 30 cm apart on 132-cm-wide beds. The drip tape was positioned approximately 10 cm on the inside of each plant row at a depth of 3 cm.

Table 1. Experimental locations and irrigation treatments for strawberry establishment in coastal California.

Location and season	Soil type	Irrigation treatment description	Bed configuration
Oxnard 2013–14	Clay loam	‘Sprinkler’: Sprinkler for first 40 days, followed by drip irrigation with three drip lines, placed 6 cm deep and 18 cm away from plant rows	Beds 30 cm high, 168 cm between bed centers to accommodate four plant rows
		‘4 Drip’: Four drip lines, placed 6 cm deep and 6 cm away from plant rows with two supplemental sprinkler irrigations on the 29th and 34th day after planting (DAP)
Salinas, 2013–14	Sandy loam	‘2 Drip’: Two initial sprinkler irrigations followed by drip-only using two drip lines	30 cm high, 132 cm between bed centers to accommodate 2 plant rows
		‘Sprinkler’: Sprinkler for establishment followed by drip starting at 70 DAP
Santa Maria, 2013–14	Sandy loam	‘2 Drip, reduced’: Two drip lines supplemented with reduced sprinkler irrigation	Beds 30 cm high, 168 cm between bed centers to accommodate four plant rows
		‘2 Drip, regular’: Regular sprinkler followed by irrigation two drip lines
		‘4 Drip, reduced’: Four drip lines supplemented with reduced sprinkler
		‘4 Drip, regular’: Four drip lines supplemented with regular overhead irrigation
		In treatments with four drip lines, they were placed 6 cm deep and 6 cm away from plant rows, while in treatments with two lines they were placed 6 cm deep and 18 cm away from plant rows. After sprinkler removal at 60 DAP, all treatments were drip irrigated.
Camarillo, 2009–10	Loam	‘Sprinkler’: Sprinkler irrigation for first 40 days, followed by drip irrigation with three drip lines, placed 6 cm deep and 18 cm away from plant rows	Beds 30 cm high, 168 cm between bed centers to accommodate four plant rows
		‘4 Drip’: Four drip lines, placed 6 cm deep and 6 cm away from plant rows with two supplemental sprinkler irrigations on the 29th and 34th DAP
Camarillo, 2010–11	Loam	‘Sprinkler’: Sprinkler irrigation for first 40 days, followed by drip irrigation with three drip lines, placed 6 cm deep and 18 cm away from plant rows	Beds 30 cm high, 168 cm between bed centers to accommodate four plant rows
		‘4 Drip’: four drip lines, placed 6 cm deep and 6 cm away from plant rows with two supplemental sprinkler irrigations on the 29th and 34th DAP

At all sites beds were prepared using standard practices and covered with low density polyethylene film (Strand, 2008).

Pre-plant soil fumigation did not differ among irrigation treatments. Pic-Clor 60 EC at 330 kg/ha (a mixture of 56.7% chloropicrin and 37.1% 1,3-dichlorpropene) was used at Camarillo and Oxnard, chloropicrin (Tri-Clor) at 380 kg/ha was used at Santa Maria and Salinas.

Bare-root transplants from high elevation nurseries of ‘Eldorado’ strawberry were planted at Camarillo (11 Oct. 2009 and 9 Oct. 2010), ‘1975’ at Oxnard (19 Oct. 2013) and ‘Albion’ at Santa Maria (13 Nov. 2013) and Salinas (13 Nov 2013).

Immediately after planting irrigation was initiated and applied by field irrigators in all treatment blocks to maintain soil moisture tensions between 5 and 10 centibars at the 15-cm depth for the first 6 weeks after planting. Irrigators made decisions to apply water based on readings of soil tensiometers that they have installed in the field blocks, evapotranspiration (ET) rates, and scheduling considerations. Water use in each separately irrigated block was measured with 7.6-cm-diameter flow meters installed at the main delivery lines at all locations except Santa Maria.

At all locations four plots per treatment served as replicates for each irrigation treatment. Each of those plots was randomly selected at planting and contained 20 contiguous strawberry plants.

Soil moisture and bulk soil EC measurements

From the first day after transplanting, we monitored volumetric soil moisture and ECb of soil at 8–10 cm depths in planting holes using Decagon 5TE sensors and data loggers (Decagon Devices, Pullman, WA, USA). At that depth in soil, below and around the plant crowns, new adventitious roots develop. Their rapid growth is critical for plant performance and is sensitive to moisture deficits and salinity. Within each 20-plant plot we collected soil data from the four planting holes across the bed; the data from those four subsamples were averaged prior to analyses. At 30 days after planting (DAP), soil in plant rows in each plot was sampled (composite of 20 soil cores, 0–15 cm) to determine ECe (electrical conductivity of the saturated paste extract) and specific ion concentrations.

Plant performance

Plant establishment was estimated as percent of live plants in four 100-m-long bed sections in each treatment. At 60 days after planting, two-dimensional canopies of previously selected 20 plants were measured and canopy areas calculated using the formula for an ellipse (plant length/2 × plant width/2 × π). The same 20 plants were used for weekly fruit harvests and determination of weights of marketable and unmarketable fruit yields during the first 12 harvests of the season.

Four plants in a row across the bed, immediately outside of the plots served as sub-samples for destructive plant measurements (data averaged prior to analyses). The plants were excavated at 60 DAP assuring that all roots were intact in soil monoliths. Above-ground biomass of these plants was clipped at the top of the crown and separated, while soil around the roots systems was washed away in 19-L buckets by gently agitating plants in the water. New (white) roots originating from the old root system and adventitious roots originating from the crowns were separated by hand with razor blades. All root and above-ground biomass samples were air-dried at 70 °C for 7 days and dry weights were determined.

At Oxnard, a composite sample of 50 leaves with petioles per plot was collected and analyzed at 120 DAP to determine nitrate nitrogen, and other nutrient concentrations in plant tissues.

Statistical analyses

Since the treatments differed among sites, the data was analyzed separately for each location/season dataset. Analyses of variance and differences between treatment means were determined using Fisher’s LSD test (P ≥ 0.05). All statistical computations were performed using SAS (2014).

Results and discussion

Soil moisture and bulk soil EC

Field sites in Southern California (Oxnard and Camarillo) tended to have higher ECb values than Central coast sites (Santa Maria and Salinas), but there were no significant differences among treatments at all sites, except for Oxnard. At Oxnard during 2013–14, soil EC levels near developing plant roots were similar between the two establishment methods during 1–7 DAP suggesting that the effectiveness to leach salts was comparable during this critical establishment phase (Table 2). However, after removal of sprinklers 42 DAP, the beds in the ‘Sprinkler’ treatment were irrigated solely with three drip lines placed between plant rows and failed to leach salts as effectively as four drip lines in the treatment that had four drip lines in proximity of the four planted rows. This increase of soil EC with increased distance from irrigation lines was observed in other studies (Caron et al., personal communication). Soil moisture content was similar among treatments at all locations and seasons (Table 2). Thus, irrigation solely with increased number of drip lines or in combination with reduced sprinkler irrigation can adequately supply water and leach salts near the developing root systems during transplant establishment. The 4-line drip configuration would presumably also be effective for leaching salts during the rest of the season when standard sprinkler irrigation is no longer used due to fruit damage, but the potential for salt accumulation in the root zone would rise with increased evapotranspiration rates from larger plants during warmer months.

Table 2. Bulk electrical conductivity (ECb) and volumetric soil moisture in the root zones (8–10 cm depth) determined using a soil salinity sensor (Decagon EC5 at 0–15 cm depth) of irrigation treatments at five locations in coastal California.

Location and season	Treatment	ECb, dS/m							Volumetric soil moisture, m^3/m^3
		DAP^z							DAP
		1	3	7	42	49	53	70	1	3	7	42	49	53	70
Oxnard, 2013–14	4 Drip	0.74	0.66	0.78	0.62b	0.48b	0.5b	0.48b	0.25	0.27	0.27	0.24	0.23	0.20	0.23
	Sprinkler	0.83	0.84	0.94	1.0a	0.93a	0.9a	0.94a	0.27	0.28	0.27	0.27	0.24	0.23	0.24
Standard error		0.23	0.24	0.19	0.3	0.18	0.17	0.19	0.1	0.08	0.04	0.08	0.02	0.09	0.1
Salinas, 2013–14	2 Drip	0.16	0.28	—	0.27	0.33	0.35	0.23	0.18	0.20	—	0.26	0.27	0.12	0.13
	Sprinkler	0.16	0.26	—	0.23	0.22	0.32	0.18	0.19	0.21	—	0.28	0.26	0.12	0.12
Standard error		0.12	0.8	—	0.4	0.3	0.2	0.9	0.2	0.1	—	0.4	0.1	0.11	0.1
Santa Maria, 2013–14	2 Drip, reduced	0.16	0.13	0.15	0.15	0.15	0.1	0.17	0.16	0.13	0.18	0.18	0.17	0.17	0.17
	2 Drip, regular	0.19	0.08	0.14	0.18	0.12	0.1	0.15	0.19	0.17	0.13	0.21	0.17	0.15	0.16
	4 Drip, reduced	0.1	0.08	0.16	0.14	0.17	0.1	0.12	0.11	0.16	0.16	0.17	0.2	0.17	0.18
	4 Drip, regular	0.16	0.08	0.15	0.11	0.15	0.1	0.14	0.17	0.15	0.18	0.21	0.2	0.19	0.19
Standard error		0.06	0.02	0.03	0.09	0.07	0.05	0.06	0.09	0.08	0.08	0.06	0.05	0.06	0.08
Camarillo, 2009–10	4 Drip	0.86	0.6	0.74	0.8	1.0	0.9	0.9	–	–	–	–	–	–	–
	Sprinkler	0.72	0.54	0.45	0.6	0.9	0.8	0.9	–	–	–	–	–	–	–
Standard error		0.19	0.2	0.46	0.4	0.2	0.2	0.3	–	–	–	–	–	–	–
Camarillo, 2010–11	4 Drip	0.39	0.56	0.56	0.5	0.8	0.8	0.4	–	–	–	–	–	–	–
	Sprinkler	0.45	0.43	0.44	0.44	0.6	0.9	0.3	–	–	–	–	–	–	–
Standard error		0.26	0.2	0.2	0.15	0.35	0.3	0.25	–	–	–	–	–	–	–

^zDAP = days after planting. In all seasons and locations, except Salinas 2013–14, EC and soil moisture were measured weekly from 1 to 42 DAP but there were no significant differences among treatments according to Fisher’s LSD test (P ≥ 0.05), except for Oxnard 2013–14, where treatment means with different letters indicate significant differences according to Fisher’s LSD test (P ≥ 0.05).

Analyses of specific ions that contribute to salinity also showed that irrigation with increased number of drip lines had comparable concentrations to those in sprinkler irrigation, and in the case of chloride at Oxnard significantly reduced it (Table 3). Specific negative effects of chlorides on strawberry have been shown to be particularly harmful (Daugovish and Faber, 2014). Besides the ion concentrations displayed in Table 3, there were also no significant differences among treatments in concentration of calcium, manganese, potassium, and iron (data not shown).

Table 3. Electrical conductivity (EC saturated paste, ECe) and specific ion concentrations in soil (0–15 cm depth) in irrigation treatments in coastal California at 30 DAP.

Location and season	Treatment	EC, saturated paste, dS/m	Chloride, meq/L	Sodium,meq/L	Sulfate, meq/L
Oxnard, 2013–14	4 Drip	3.7	0.8a^z	7.1	36.3
	Sprinkler	3.9	2.7b	10.0	45.2
Standard error		0.8	0.7	4.2	12.3
Salinas, 2013–14	2 Drip	1.3	1.6	3.0	-
	Sprinkler	0.8	2.5	3.1	-
Standard error		0.6	0.9	0.4	-
Santa Maria, 2013–14	2 Drip, reduced	3.7	2.9	10.0	28
	2 Drip, regular	4.8	4.1	8.0	38.0
	4 Drip, reduced	4.1	2.8	9.0	41.0
	4 Drip, regular	3.8	3.2	9.0	35.2
Standard error		1.9	1.7	1.4	16.4
Camarillo, 2009–10	4 Drip	4.9	4.2	–	–
	Sprinkler	5.4	6.2	–	–
Standard error		0.8	2.7	–	–
Camarillo, 2010–11	4 Drip	5.6	3.6	–	–
	Sprinkler	4.7	5.2	–	–
Standard error		1.8	1.9	–	–

^zTreatment means with different letters at the same location indicate significant differences according to Fisher’s LSD test (P ≥ 0.05).

However, effective leaching of mobile salts in irrigation with additional drip lines has to be done in consideration with potential leaching of nutrients. At Oxnard, plant tissue samples in the ‘4 Drip’ treatment had concentrations of nitrate nitrogen of 670 ppm and manganese 80 ppm compared to 900 ppm and 170 ppm, respectively, in ‘Sprinkler’ treatment that was irrigated with three lines at the time of sampling. While no significant differences in concentrations of other elements in plant tissues were observed (data not shown), nutrient monitoring and adjustments are necessary to assure that irrigation and fertigation with additional drip lines will maintain nutrients in sufficiency range for strawberry plants described by Bottoms et al. (2013).

Plant performance

At all locations and for all treatments plant establishment was above commercially acceptable 96% levels (Table 4). Lack of differences among treatments indicated that irrigation and salt leaching with drip or a combination of drip and reduced sprinkler was as effective as standard sprinkler irrigation. Early plant canopy development was also similar between the irrigation treatments at all locations, which was also the case for above-ground biomass (Table 4). However, at Salinas new root biomass in the ‘2 Drip’ treatment was 33% greater than with sprinkler irrigation, while the opposite was true at Camarillo in the 2010–11 season for ‘4 Drip’ compared to ‘Sprinkler’. This may be attributed to higher soil EC levels at Camarillo than at Salinas (Tables 2 and 3). At Santa Maria, treatment with four drip lines and regular sprinkler irrigation (‘4 Drip, regular’) had 24% lower new root biomass compared to other treatments. This is possibly due to periods of excessive saturation near root zones during and shortly after irrigation that combined two water delivery methods and was reflected in 10% smaller (but not statistically different) plant canopy size in that treatment compared to others (Table 4). Similar to other measured parameters, early fruit yields did not differ among treatments at all locations except Santa Maria (Table 4). At Santa Maria, plants in the ‘4 Drip, regular’ treatment with the least root biomass (discussed previously) had 44% lower early fruit production compared to the ‘2 Drip, regular’ treatment. None of the reduced drip treatments had significantly different yields compared to the regular irrigation treatments at this site.

Table 4. Plant establishment, canopy size, early fruit production, and water use in irrigation treatments in coastal California.^z

Location and season	Treatment	Plant establishment, %	Plant canopy area, cm^2	New root dry biomass, g/plant	New leaf dry biomass, g/plant	Early fruit yield, g/plant	Water use during 5 weeks after planting, m^3/ha
Oxnard,2013–14	4 Drip	98.6	453	0.68	2.50	529	145
	Sprinkler	98.3	499	0.72	2.31	434	447
Standard error		1.3	93	0.12	0.8	106	—
Salinas, 2013–14	2 Drip	98.6	342	0.44a^y	2.36	340	252
	Sprinkler	98.5	379	0.33b	2.20	345	310
Standard error		1.9	86	0.04	0.28	49	—
Santa Maria, 2013–14	2 Drip, reduced	96.8	436	0.45a	2.05	360ab	—
	2 Drip, regular	97.2	426	0.5a	1.81	450a	—
	4 Drip, reduced	97.6	379	0.56a	2.65	332ab	—
	4 Drip, regular	96.8	372	0.38b	2.14	248b	—
Standard error		2.8	101	0.06	0.9	138	—
Camarillo, 2009–10	4 Drip	98.2	473	0.30	2.25	1012	265
	Sprinkler	98.7	404	0.32	2.19	1196	533
Standard error		1.6	95	0.04	0.3	248	—
Camarillo, 2010–11	4 Drip	98.2	309	0.32b	2.17	759	145
	Sprinkler	98.5	277	0.42a	2.18	713	447
Standard error		2.0	64	0.03	0.19	108	—

^zCanopy size and new root and leaf biomass were evaluated 60 DAP, while early fruit yield was determined as cumulative marketable yield during the first 12 harvests.

^yTreatment means with different letters within the same location are significantly different from each other according to Fisher’s LSD test (P ≥ 0.05) for all measured parameters, except ‘Water use’ that was not analyzed statistically.

Crop water use

Depending on the location, water use during establishment period (5 weeks after planting) was reduced 20% to 78% in drip-only, or reduced sprinkler plots compared to the standard sprinkler irrigation treatment (Table 4). Applied water was not measured at the Santa Maria trial.

Water use during 5 weeks of plant establishment accounts for about 20% of the total water needs for the 36–42-week production season, ranging from 1800 to 2400 m³/ha (Strand, 2008). However, overhead water application during the establishment period is very inefficient as opposed to the following in-season drip irrigation that is scheduled based on plant needs, ET, and soil sensor measurements. In this project, we showed significant improvements in water delivery to plants and minimized off-target losses during this most inefficient period of strawberry irrigation.

Treatments that received no overhead irrigation had dry furrows and no runoff, which minimized off-site water quality impacts, weed germination (data not shown), and also allowed easy field entry for monitoring, a benefit to growers and pest control managers. Even though runoff parameters were not measured in this study, runoff generated from overhead sprinklers can carry soil particles and associated pesticides from the strawberry fields and increase the toxicity of downstream surface water bodies (Amweg et al., 2006).

This project showed that at the main strawberry production regions in California, reliance on drip irrigation with increased number of lines during plant establishment can improve water use efficiency without negative effects on plant performance. Water conservation in irrigated agriculture is critical to maintaining sustainable production during extended periods of drought. Additionally, improved runoff management with use of drip irrigation can help growers comply with strict regulations of pollutants in agricultural water discharge to the environment.

Acknowledgments

We would like to acknowledge grower collaborators in this project: Manzanita Berry Farms, Dole, Reiter Affiliated Companies, and Ito Bros.