Evaluating Supplementary Nutrients to Improve Strawberry Fruit Quality and Yield

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ABSTRACT

A field study was conducted in a randomized complete block design over two growing seasons. The objective was to evaluate the effect of supplementary nutrients, in addition to standard fertigation practices, on fruit yield and quality for annual plasticulture strawberry production. First season treatments included i) a root applied secondary macro and micronutrient mix (0-0-0) + foliar applied nutrient (6-4-3) ii) foliar applied nutrient (6-4-3) and iii) nontreated control. Additional treatments in the second season included root applied micronutrient mix alone, and a foliar nutrient application of an OMRI certified cold-processed sweetener. Treatments were replicated four times using ‘Chandler’ strawberry. Plugs in the root applied treatment were lightly dusted with the nutrient powder just before transplanting. Foliar nutrient was first sprayed at 30% bloom and every 7 to 14 days as frequent harvesting began. There were no significant differences among treatments for yield, fruit size, firmness or total soluble solids content.

KEYWORDS:

• Small fruit
• annual hill plasticulture
• micronutrients
• organic
• glucose

Introduction

Strawberries (Fragaria ×ananassa Duchesne ex Rozier) are one of the major high-value crops cultivated in North America with an estimated farm gate value of 2.2 USD and 3.5 USD billion during the past three years (USDA-NASS, 2018). As per the Montreal Protocol Act of 1987, the most effective fumigant combination, methyl bromide (MB) + chloropicrin (Pic), can no longer be used due to phase out of MB as fumigant in 2005, for developed countries which include the U.S. (USEPA, 2017). Since the ban of MB, alternative fumigants to MB + Pic are being utilized, but crop yield and quality have been reduced. Goodhue et al. (2005) estimated that lack of access to MB resulted in crop yield losses of 10 to 15%. Some berry growers have started including foliar nutrient sprays as part of the production practices with the intent to improve yield and quality, fill the void of MB unavailability, and perhaps increase fruit firmness that can improve tolerance to diseases and insects such as Spotted wing drosophila (Drosophila suzukii) (Burrack et al., 2013).

Nutrients play a role in plant growth, development and optimizing crop yield. Uptake of certain nutrients by plants can be hindered due to environmental factors, and because nutrients can interact amongst themselves (May and Pritts, 1993; Medeiros et al., 2015). Foliar nutrition can play a role to increase nutrient content in above-ground plant parts (Patil and Chetan, 2018; Swietlik and Faust, 1984). Strawberries are a perishable fruit with a storage period of less than a week and may benefit from foliar applications of calcium and other nutrients that play a role in fruit quality. Uptake of nutrients by leaves is quicker than by roots (Karp and Starast, 2002). For the secondary macronutrient calcium, the leaves and fruit compete for the absorption of calcium, so the supply of this element in the form of foliar sprays increases its content in fruits (Bieniasz et al., 2012). The objective of this study was to determine if foliar and root applied nutrients, in addition to standard fertigation practices improve crop yield and berry quality.

Materials and Methods

This study was done at Hampton Roads Agricultural Research and Extension Center (AREC), Virginia Beach, VA, USA in the 2015–16 and 2017–18 growing seasons. The soil at the experimental site was 100% tetotum loam (sandy loam, deep, moderately well-drained, parent material: loamy fluvial and marine sediments) with 0 to 2% slope. Soil tests were conducted prior to the beginning of both seasons in early to mid-August by sending representative soil samples to the Virginia Tech. Soil Testing Laboratory (Figures 1 and 2). Study in both seasons was set up as a randomized complete block design with four treatment replicates and using the cultivar ‘Chandler’ in annual hill plasticulture production. ‘Chandler’ is a popular cultivar for annual hill plasticulture production in Virginia and the south Atlantic U.S. (Samtani et al., 2019). Beds were of dimension 70 cm wide and 15 cm high and a 15-mil single drip line with a 30.5-cm emitter spacing was used to irrigate and fertigate the beds (Berry Hill Irrigation, Inc., Buffalo Junction, VA, USA) when needed for the season. Plants were spaced at 36 cm spacing in two staggered rows per bed. There were twenty plants per replicate. In both seasons, floating row cover of 40 g m⁻² thickness (Atmore Industries, Inc., Atmore, AL, USA) was used to protect the berries as needed for winter and spring frost protection.

Figure 1. Preplant soil testing report during land preparation for 2015–16 strawberry growing season. Soil nutrient testing was done by Virginia Tech. Soil Testing Laboratory. Rating scale of VH = very high, H = high and SUFF = sufficient.

Figure 2. Preplant soil testing report for 2017–18 strawberry growing season. Soil nutrient testing was done by Virginia Tech. Soil Testing Laboratory. Rating scale of VH = very high, H = high, M = Medium, and SUFF = sufficient.

2015-16 Growing Season

The soil pH at the site was 6.2 (Figure 1). Preplant fertilizer (10-0-20) was added at bed preparation to meet 62 kg N per ha requirement as per soil testing laboratory recommendations. ‘Chandler’ was transplanted onto beds on 30 Sept. 2015. Three treatments included: (i) an untreated control, (ii) a root applied secondary macro and micronutrients mix (Nutriplant® SD 0-0-0, Ada, MI, USA) + foliar applied (Nutriplant® AG 6-4-3, Ada, MI, USA) treatment and (iii) foliar only treatment (Nutriplant® AG 6-4-3). Strawberry plug plant roots in treatment (ii) were dusted on 30 Sept. 2015 as per recommendations of the distribution company (Access Business Group International LLC., MI) i.e., thoroughly coating the plug roots with the nutrient powder, before transplanting strawberries in raised beds. Foliar nutrient was applied for treatments (ii) and (iii) once during 30 to 50% bloom stage on 30 Mar. 2016, and once a week during fruiting at recommended label rate of 585 mL per ha for strawberries using a three nozzle (cone spray) sprayer on a hand-held boom, with Co2 pressurized backpack sprayer. Foliar spraying during harvest continued from 20 April on a weekly basis till 16 June, 2016. Spring fertigation began March-end 2016, alternating on a weekly basis with calcium nitrate (15.5-0-0; YaraLiva Calcinit, Yara North America Inc., Tampa, FL, USA) and potassium nitrate (13.5-0-46.2; Multi-K GG, Haifa North America, Inc., Altamonte Springs, FL, USA) at rate of 5.9 kg of nitrogen per ha per week using a 53 L min⁻¹ Dosatron® injector (Berry Hill Irrigation, Inc., Buffalo Junction, VA, USA).

2017-18 Growing Season

As per soil testing laboratory report and recommendations (Figure 2), preplant nitrogen (34-0-0) at 62 kg per ha and potassium oxide (0-0-60) at 84 kg per ha were applied at the time of bed making. Strawberry ‘Chandler’ were transplanted on 28 Sept., 2017. Additional treatments in this growing season included (iv) root coated secondary macro and micronutrient mix application (Nutriplant® SD 0-0-0, Ada, MI, USA) (v) foliar nutrient applications of an OMRI certified, cold-processed sweetener- BigSweetYield_TM (AgMaxx Inc., Garden City, MO, USA). Foliar nutrient application for Nutriplant® AG 6-4-3 at 585 mL per ha and BigSweetYield at 1.12 kg per ha were made at 30% bloom on 28 Mar., followed by three applications during the harvest season on 1 May, 10 May and 24 May, 2018. Foliar applications were made using a two nozzle, hand-held boom, pressured at 35 psi with CO₂ pressurized backpack sprayer. Plant leaf tissue samples and leaf petiole samples were analyzed by treatment on 11 May and 5 June by the North Carolina Department of Agriculture, Agronomic Services. These plant tissue samples were collected in the field on 7 May and 30 May respectively. Protocol for collecting and submitting plant tissue sample, methodology used for tissue analysis and details on interpreting report are provided on the North Carolina Department of Agriculture and Consumer Services, Agronomic Services-Plant Tissue Analysis website (2019).

Liquid fertigation was done similar to first growing season on a weekly basis starting 28 Mar., 2018 alternating with either calcium nitrate and potassium nitrate from April to mid-May using a Dosatron® injector. Spring fertigation provided 5.9 kg N per ha per week. The study was fertigated with magnesium sulfate to provide 1.7 kg per ha of sulfur on 5 Apr., 2018. Boron at 0.14 kg per ha was applied as a foliar spray on 26 April, 2018, and Zinc at 0.14 kg per ha in form of zinc monohydrate sulfate was applied via drip irrigation on 24 May, 2018. Plantex (20–20-20; BFG Supply, Burton, OH, USA) was applied via drip irrigation on 24 May, 2018 as the plant leaf tissue report of the leaves sampled on 7 May recommended phosphorous application.

Data Collection and Analysis

Crop stand for each replicate was taken each month of the growing season. Overall visual appearance on the vigor and health of all plants in each replicate were evaluated using a scale of 0 (all plants in the replicate are dead) to 10 (all plants in the replicate appear healthy and extremely vigorous) on monthly basis. Strawberry fruits were harvested twice a week beginning 4 April, 2016 to 22 June, 2016 and 20 April, 2018 through 1 June, 2018 and were sorted into marketable and non-marketable fruits at each picking. Nonmarketable fruits included those that were small (less than 10 g), diseased, misshapen or deformed, or overripe or rotten. The weights of marketable and nonmarketable fruits were added to determine total yield. Cumulative berry yield data from each replicate was totaled for all harvests, divided by the number of plants in each replicate, and the yield expressed as marketable and total yield per plant. In the 2015–16 growing season, five marketable fruits per treatment were collected randomly once a week in May and June to determine fruit size and total soluble solids (TSS) content. A digital Vernier caliper (Neiko, Taiwan) was used to measure fruit width at shoulder. Calyces were removed, berries were crushed, and sieved to separate the juice from the pulp. Total soluble solids (°Brix) was measured, using a digital refractometer (MA 871, Milwaukee Instruments Inc., Rocky Mount, NC) at 21°C sample temperature. In 2017–18 growing season, fruit size was measured once per week in May on five fruits per replicate at the fruit shoulder, and the readings were averaged by replicate across the harvest season. Data for fruit firmness were taken on five marketable fruits per replicate using a GS-15 Fruit Texture Analyzer (QA Supplies, Norfolk, VA, USA). An uncut berry was placed on the analyzer holder with the berry shoulder directly below the needle and reading per fruit was recorded using travel speed at 10 mm per sec, a 2 mm compression and a trigger threshold of 0.2 N. The TSS were taken on these same five fruits per replicate and the readings for both parameters were averaged across the harvest season. Data were subject to analysis of variance using statistical analysis software (SAS v. 9.4; SAS Institute Inc., Cary, NC, USA). Mean separation for the yield data in both seasons and the fruit quality parameters in 2017–18 growing season was done using the least significant difference test at alpha = 0.05. The mean separation on fruit quality parameters in 2015–16 growing season was done using the two-sample, paired, t-test.

Results and Discussion

Initial and final crop stand remained unchanged at twenty plants per replicate in the 2015–16 growing season. In the 2017–18 growing season, crop stand remained mostly unchanged at twenty plants per replicate with a few replicates losing a plant along the growing season. This loss of plant could not be attributed to any particular treatment and was mainly observed in early to mid-season due to either poor establishment of the transplanted plug plants or a weak root system, to begin with. Crop plant health rating by replicate when averaged for the 2015–16 growing season was in the 8.0 to 8.5 range and the 2017–18 growing season was in the 8.6 to 9.9 range. Plug plants in 2017–18 growing season looked healthier than the first season. The primary reason for a lower visual rating for a replicate over others was the non-uniform growth between plants from the same replicate, with some plants appearing weaker or small sized than others within the replicate.

Supplementary nutrient treatments did not markedly influence the leaf tissue nutrient concentrations over untreated control. Nutrient index interpretation scale referred in the text below and listed in Table 1 was provided by the North Carolina Department of Agriculture and Consumer Services (2019). For the 7 May, 2018, nitrate-nitrogen was in the low interpretation index for four of the five treatments (Table 1). Phosphorous concentration was in the low interpretation index for all treatments. Sulfur concentration was in the low interpretation index, for all plants treated with supplementary foliar nutrient treatments, however, the sulfur concentration in the untreated control was close to being in the low interpretation index (Table 1). Fertigation with Plantex (20–20-20) through the drip system increased the phosphorous concentration to sufficient interpretation index as indicated in Table 2 for leaves sampled on 30 May, 2018. Toward the last week in May, nitrate-nitrogen concentrations in the leaf petioles had lowered in crop plants regardless of treatment. As indicated in several studies, foliar nitrogen levels can drop during fruit harvest in short-day cultivars (Archbold and MacKown, 1995). Sulfur concentration in the leaf tissues was in low interpretation index during the first tissue sampling but the addition of magnesium sulfate and zinc monohydrate sulfate increased sulfur concentration as indicated in the second tissue sampling date. A total of four fertigations with calcium nitrate during the spring season may have elevated the calcium concentration in the leaf tissues to high in the second sampling period.

Table 1. Nutrient concentrations from dried leaf tissues and nitrate nitrogen concentration from leaf petioles sampled on 7 May, 2018 (week 8 of bloom and fruit).

Nutrient^a	Untreated control	Nutriplant® SD	Nutriplant® SD + Nutriplant® AG	Nutriplant® AG	BigSweetYield
Nitrogen (%)	3.1	2.8 (L)	2.9 (L)	3.0	3.0 (L)
Nitrate-nitrogen (ppm)	1560.0 (L)	1520.0 (L)	1530.0 (L)	2060.0	1310.0 (L)
Phosphorous (%)	0.2 (L)	0.2 (L)	0.2 (L)	0.2 (L)	0.2 (L)
Potassium (%)	1.5	1.5	1.4	1.5	1.5
Calcium (%)	1.1	1.2	1.1	1.2	1.2
Magnesium (%)	0.3	0.3	0.3	0.3	0.3
Sulfur (%)	0.14	0.13 (L)	0.13 (L)	0.13 (L)	0.12 (L)
Iron (ppm)	77.5	72.2	71.0	70.5	61.1
Mangenese (ppm)	258.0	245.0	273.0	195.0	172.0
Zinc (ppm)	16.1	16.3	16.7	14.7 (L)	16.1
Copper (ppm)	4.9	4.1	4.5	4.9	4.2
Boron (ppm)	31.4	30.5	29.7	31.4	56.0

^a(L) indicates that the nutrient interpretation index value is low. For all other nutrient concentration readings, the index value was in the sufficient range. Further information on interpretation index can be found at http://www.ncagr.gov/agronomi/pdffiles/uplant.pdf

Table 2. Nutrient concentrations from dried leaf tissues and nitrate nitrogen concentration from leaf petioles sampled on 30 May, 2018 (week 11 of bloom and fruit).

Nutrient ^a	Untreated control	Nutriplant® SD	Nutriplant® SD + Nutriplant® AG	Nutriplant® AG	BigSweetYield
Nitrogen (%)	2.6 (L)	2.5 (L)	2.6 (L)	2.6 (L)	2.5 (L)
Nitrate-nitrogen (ppm)	222.0 (L)	186.0 (L)	117.0 (L)	131.0 (L)	114.0 (L)
Phosphorous (%)	0.2	0.3	0.3	0.3	0.3
Potassium (%)	1.3	1.4	1.3	1.3	1.4
Calcium (%)	1.7 (H)	1.4	1.5 (H)	1.6 (H)	1.5 (H)
Magnesium (%)	0.4	0.4	0.4	0.4	0.4
Sulfur (%)	0.2	0.2	0.2	0.2	0.2
Iron (ppm)	75.7	70.8	81.3	72.9	73.2
Mangenese (ppm)	152.0	192.0	180.0	115.0	133.0
Zinc (ppm)	18.4	19.7	18.0	16.4	18.0
Copper (ppm)	4.3	5.1	5.1	5.0	4.6
Boron (ppm)	28.7	28.2	30.1	27.9	26.5

^a(L) indicates that the nutrient interpretation index value is low and (H) indicates high value. For all other nutrient concentration readings, the index value was in the sufficient range.

In both seasons, we found no statistical differences among treatments for yield, fruit size or TSS (Tables 3 and 4). Findings from other foliar nutrient studies have been mixed. Foliar applications of calcium nitrate applied twice weekly at 1.5 g/L on ‘Camarosa’ and ‘Oso Grande’ did not increase the fruit yield or weight over unstressed strawberry plants in control treatment that were grown in sand culture in pots. Total soluble solids (TSS) were increased for both cultivars (Kaya et al., 2002). Fruit firmness increased for the pot-grown strawberry cultivars ‘Luna’ and ‘Zanta’ when foliar applications of calcium were made thrice during the growing season: at full flowering, end of flowering and at beginning of fruit set (Bieniasz et al., 2012) but no increase in yield or fruit biomass was observed. Lanauskas et al. (2006) and Vance et al. (2017) also concluded that foliar applications of calcium did not improve fruit yield, size, or firmness on strawberries.

Table 3. Cumulative marketable yield and total yield for the 2015–16 growing season in Virginia Beach, VA. Seasonal average of total soluble solids, fruit size, and fruit firmness.

	Marketable yield	Total yield	Total soluble solids	Fruit size
Treatment	– – – – (g/plant) – – – -		(°Brix)	(mm^2)
Untreated control	179.6	459.4	7.4	29.1
Nutriplant® SD + Nutriplant® AG	252.2	553.1	7.7	29.4
Nutriplant® AG	181.3	456.3	7.6	29.6
Pr>F Treatment	0.1387	0.3537	NS^z	NS

NS = not significant using the two-sample, paired, t-test.

Table 4. Cumulative marketable yield and total yield for the 2017–18 growing season in Virginia Beach, VA. Seasonal average of total soluble solids, fruit size, and fruit firmness.

	Marketable yield	Total yield	Total soluble solids	Fruit size	Fruit firmness
Treatment	– – – – (g/plant) – – – -		(°Brix)	(mm^2)	(kg)
Untreated control	626.6	901.2	7.2	34.5	0.09
Nutriplant® SD	599.7	869.6	7.3	34.2	0.09
Nutriplant® SD + Nutriplant® AG	529.8	803.7	7.1	34.3	0.10
Nutriplant® AG	619.0	905.2	7.1	33.9	0.09
BigSweetYield	586.8	863.2	7.4	33.2	0.10
Pr > F Treatment	0.5410	0.5953	0.5851	0.2734	0.7144

Different cultivars can respond differently to foliar applications of nutrients as genetic differences in uptake and usage of nutrients do exist at all taxonomic levels (Bieniasz et al., 2012; May and Pritts, 1993). Most studies on strawberry crop and foliar nutrients focus on understanding the effects of calcium-based fertilizers since it is an important secondary nutrient that is xylem-translocated and is known to improve fruit quality and storage. The second most studied nutrient is boron whose deficiency could result in malformed fruits resulting in lower fruit quality. Effect of supplementary nutrients on ‘Chandler’ strawberry is almost lacking in the scientific literature. The one study on ‘Chandler’ found preharvest foliar applications of calcium + boron to be useful in increasing marketable yield and fruit quality over untreated control (Singh et al., 2007). However, detailed information is lacking on additional fertilizers that treatments may have received in addition to foliar nutrients. There is less information on foliar application of composite fertilizers (Lanauskas et al., 2006). Our study purpose was to gather data under Virginia climactic conditions with existing products sold in the market and targeted toward use in commercial strawberry production.

Based on our findings with limited treatments and a single genotype study we do not recommend the use of supplementary foliar nutrients to growers without good justification for making these applications. The leaf tissue sampling in spring and early summer will be a more reliable indicator for growers in determining plant nutrient needs. If the crop plant is maintained under unstressed conditions as was the case in our study and backed by the leaf tissue nutrient analysis, there will likely not be a response to foliar nutrient applications. Environmental factors other than nutrients also influence plant growth and development and plants may not always respond to the addition of nutrients (Patil and Chetan, 2018). Further, interactions among nutrients make the situation complex in understanding treatment effects (May and Pritts, 1993).

Acknowledgments

Authors would like to thank Zachary Landis, Ellen Owen, Danyang Liu, John Christman III, Iman Omer, Cassandra Bush, and Hampton Roads Ag. Res. And Extension Center farm crew for their help with field work and data collection.

Additional information

Funding

The project was funded by Virginia Agricultural Council, Nutriplant ®, and BigSweetYield.US. Staff salary and support was provided by Virginia Agricultural Experiment Station (Blacksburg) and the USDA National Institute of Food and Agriculture.