Abstract
The effect of pH and silicon (Si) in the nutrient solution on the vegetative development of 2-year-old blueberry plants (Vaccinium corymbosum L. cv. Ventura) was studied. Two independent experiments were performed on coir fiber (CF) and sand as substrates. In experiment 1, Si was applied in the nutrient solution at a dose of 0.0, 0.3, 0.6, and 1.2 mm. In experiment 2, plants were treated with nutrient solution at pH 4.00, 4.75, 5.50, and 6.25, using two sources of acidification: nitric acid and citric acid. The parameters of plant growth, foliar surface, and stem biomass were measured. With the application of 1.2 mm Si to CF, plant height registered a significant increase of 8%, and shoot dry and fresh biomass increased by 21% and 25%, respectively. The results of experiment 1 indicated that the application of Si benefits the vegetative growth of blueberry plants in CF, but no effect was observed in the sand substrate. In the results of experiment 2, the pH level of 6.25 in CF decreased the dry weight of stems and leaves by 21% and 18%, respectively. A significant increase in the pH range of 4.00 to 5.50 was recorded in both the citric acid and nitric acid treatments, but these significant effects were not found in sand. Citric acid presented a similar behavior to nitric acid, which indicates that it can be a good source of acidification in organic and ecologically friendly crops.
Blueberry cultivation has become important worldwide as a result of its high profitability (Bañados, 2006; Caspersen et al., 2016; Strik and Yarborough, 2005), organoleptic qualities, and benefits to human health (Ames et al., 1993; Kalt and Dufour, 1997; Peng et al., 2014; Prior et al., 1998). According to the Food and Agriculture Organization of the United Nations (FAO, 2017), from 2012 to 2016, the area of blueberry cultivation worldwide increased by 34.14%, from 82,696 ha to 110,928 ha, and a production of 552,505 t was recorded, with the United States and Canada as the main producers. Spain, with 6412 t, is the eighth largest producer and has become one of the most important producers worldwide. The limiting factors for the development and production of blueberry have been known for a long time (Coville, 1910). One of the main factors influencing the development and production of crops is the pH of the soil, the substrate, and the nutrient solution, which can interfere with the availability of nutrients (Adams, 2004; Arnon and Johnson, 1942; Urrestarazu, 2015). It is well known that blueberries develop well in acidic soils with a pH of 4.5 to 5.5 and with low levels of fertility (Korcak, 1988; Mainland, 2012; Retamales and Hancock, 2012). Some authors recommend nutrient solutions with low electrical conductivity (EC) between 0.45 and 1.5 dS·m−1 (Ameri et al., 2012; Bryla et al., 2010; Ingestad, 1973; Machado et al., 2014). However, the optimal EC of the nutrient solution for the cultivation of blueberries in soilless culture has not yet been determined (Voogt et al., 2014).
Adequate management of mineral nutrition and fertigation parameters in crops ensures good growth and high-quality production (Ehret et al., 2014; Keen and Slavich, 2012; Vargas and Bryla, 2015; Xu et al., 2014). However, it is necessary to develop new programs of mineral nutrition and fertigation with other nutrients and soil improvers for blueberries (Bryla and Strik, 2015; Vargas et al., 2015). Silicon is not considered an essential element, according to the traditional criteria of Arnon and Stout (1939), because many plants can complete their cycle in its absence (Marschner, 2011). However, the beneficial effects of Si have been published both in plant protection (Imtiaz et al., 2016; Luyckx et al., 2017; Urrestarazu et al., 2016; Van Bockhaven et al., 2013) and in mineral nutrition (Kaya et al., 2006; Mengel and Kirkby 2000; Zhu and Gong, 2014). During the past few decades (Heine et al., 2005; Sonneveld and Straver, 1994; Sonneveld and Voogt, 2009) and, more recently (Pozo et al., 2015), Si is considered in the nutrient solution for horticultural and ornamental crops. Both Si and citric acid are active substances accepted in organic farming in Europe (DOUE, 2008) and the United States (USDA Organic, 2015). Although blueberry is a clearly acidophilic plant, there is no possibility of using nitric or other inorganic acid in an organic agriculture by soilless culture, so citric acid is a clear alternative to achieve adequate pH levels. However, there is no information on adequate levels in fertigation. In addition, Si has been published as a plant-beneficial element associated with the mitigation of abiotic and biotic stresses (Boldt et al., 2018).
There is little information regarding either conventional or organic agriculture on the adjustment of pH level or on the use of different acids for the acidification of the rhizosphere. In addition, there is no information on the potential benefits of Si in blueberry cultivation. Therefore, the objectives of this work were to evaluate the effect of the use of Si in the nutrient solution and to compare citric acid, as a source of acidification, to nitric acid in the nutrient solution for the cultivation of blueberry in organic agriculture.
Materials and Methods
Treatments and growth conditions.
Two independent experiments (Expts. 1 and 2) were carried out at the University of Almería, Spain, in a multispan greenhouse with characteristics that have been described by Valera et al. (2016). On 1 Apr. 2017, 2-year-old blueberry plants (Vaccinium corymbosum L. cv. Ventura) were transplanted from 1-L containers to 22-L containers with Pelemix® CF substrate, the physical characteristics of which have been described by Rodríguez et al. (2014), and silica sand with a granulometry of 0.5 to 1.0 mm in diameter. Vegetative development was evaluated on 1 Sept. 2017. The planting density was 2.5 plants/m2.
Expt. 1: Application of silicon in the nutrient solution.
The plants were fertigated with a nutrient solution similar to that proposed by Sonneveld and Straver (1994), as shown in Table 1. The Si treatments were 0.0, 0.3, 0.6, and 1.2 mm. The Si source used was Siliforte® (CAPA Ecosystems®, Valencia, Spain) (CAPA Ecosystems, 2012). The pH of the nutrient solutions was adjusted to 5.5 with dilute nitric acid.
Nutrient solution supplied in potted plants of ‘Ventura’ blueberry.z
Fertigation scheduling.
Each new fertigation was performed when 10% of the readily available water was used up (Rodríguez et al., 2014; Urrestarazu, 2015; Urrestarazu et al., 2017). The pH and EC of the supplied nutrient solution and the drainages were monitored daily using a Crison MM40+ Conductivimeter and pH meter (LPV2500.98.0002; Hach®, Vizcaya, Spain), and the volume (measured in liters) was determined with a graduated cylinder with 100 mL of precision. Based on the irrigation and drainage volumes, the water uptake was estimated according to Pozo et al. (2014).
Parameters of vegetative growth.
Training pruning was performed 180 d after the application of the treatments, which were used to evaluate vegetative development. The parameters of vegetative growth measured were plant height (measured in meters), shoot biomass (leaves and stems), and leaf area (measured in square meters per plant), as measured with an AM350 Area Meter (ADC BioScientific Ltd., Hertfordshire, UK). The plants were divided by their different organs, and stem and leaf fresh weights were obtained. The dry weight was obtained by placing the material in a convection oven (Thermo Scientific Heratherm®, Germany) at 75 °C until constant weight and then measured using an OHAUS Adventurer® Analytical Precision Analytical Balance (model AX 124/E).
Expt. 2: pH of the nutrient solution.
The nutrient solution used was the same as that used in Expt. 1. The pH was adjusted with nitric acid and citric acid manufactured by Nortem® Biotechnology (Cádiz, Spain). The pH values of the solutions after adjustment were 4.00, 4.75, 5.50, and 6.25. For each acid source, an acidification curve was generated, and the increase in EC was determined (Fig. 1). The EC of the nutrient solutions always remained between 1.5 and 1.6 dS·m−1. Fertigation management and growth parameters were performed according to the description for Expt. 1.
pH and electrical conductivity (EC) of a complete nutrient solution as a function of the dose of nitric or citric acid. Means (n = 4).
Citation: HortScience horts 53, 10; 10.21273/HORTSCI13342-18
pH and electrical conductivity (EC) of a complete nutrient solution as a function of the dose of nitric or citric acid. Means (n = 4).
Citation: HortScience horts 53, 10; 10.21273/HORTSCI13342-18
pH and electrical conductivity (EC) of a complete nutrient solution as a function of the dose of nitric or citric acid. Means (n = 4).
Citation: HortScience horts 53, 10; 10.21273/HORTSCI13342-18
Statistical analysis and experimental design.
Each experiment was performed independently using a randomized complete block design, with four repetitions and four plants per repetition for each treatment in CF and sand (Montgomery, 2004). The results were subjected to an analysis of variance and a comparison of means with a Tukey test (P ≤ 0.05), and to linear and quadratic correlations using the statistical package Statgraphics Centurion® X.V.I.I (2018).
Results and Discussion
The drainage parameters of the resulting pH, EC, and percentage of volume of fertigation and drainage are commonly used in the practical control of soilless culture (Gorbe and Calatayud, 2010; Moya et al., 2017; Rodríguez et al., 2015; Urrestarazu, 2015; Urrestarazu et al., 2008).
Figure 1 shows a graph of the pH and EC values as a function of the acid dose added to a standard nutrient solution with 2.0 mm bicarbonate from the irrigation water. To lower the pH from 6.2 to 4.0, 1.86 to 2.36 mm nitric acid is required, which assumes an increase in the EC of the nutrient solution from 1.34 to 1.38 dS·m−1. However, if these same acid levels are observed, the citric acid curve would require between 0.51 and 1.60 mm, which would increase the EC to 1.17 to 1.24 dS·m−1. This would assume that the EC of the nutrient solution is between 9% and 7% lower when using citric acid than when using nitric acid at a pH level of 6.2 or 4.0, respectively. Consequently, for a plant that is sensitive to salinity, such as blueberries (Machado et al., 2014), citric acid is preferable.
Expt. 1: Application of silicon in the nutrient solution
Fertigation and water uptake parameters.
There were no significant differences in the levels of Si in the nutrient solution the fertigation parameter of drainage, nor were there significant differences between the levels of Si in the nutrient solutions (data not shown). The pH mean values in the drainages were similar for both substrates (Table 2). The percentage of drained volume was less for CF, whereas EC and water uptake were greater for CF. In relation to EC as a fertigation management reference value, a maximum of 1 to 1.5 units (measured in decisiemens per meter) of EC was established for drainages greater than the input values (nutrient solution) (Urrestarazu, 2004, 2015). With this set value, 40% and 30% drainage of the blueberries was observed throughout the crop cycle in CF and sand, respectively. The drainage EC of the obtained data was, on average, 2.13 and 1.93 units greater than the desired fertigation reference value in CF and sand, respectively.
Effects of substrate type on leachate pH and electric conductivity (EC), drainage, and water uptake in potted plants of ‘Ventura’ blueberry.z
Parameters of vegetative growth.
Except for height, all parameters of vegetative growth had greater values in CF than when sand was used as a substrate (Table 3). In sand, no growth parameter registered a clear improvement with the application of Si in the nutrient solution (Fig. 2). The biomass, both fresh and dry, showed a clear and significant improvement in the organic CF substrate, but this was not observed in sand. Biomass improvement of 16% to 20% was reached at a concentration of 1.2 mm Si. The poor response to Si application in the nutrient solution in sand was probably a result of the potential availability of this extraction from the root zone in the silica sand substrate, because Si is not an essential component in organic matter such as CF. Xie and Wu (2009) found that sand affected the growth of blueberry plants negatively, but that mixtures of peat and sawdust increased the number of leaves, the average leaf surface, stem length, and stem diameter. Similarly, Tasa et al. (2012) reported that a mixture of peat and soil provided a greater amount of nutrients than mineral soil, which resulting in better growth and greater production of blueberries. On the other hand, Heiberg and Lunde (2006) found no significant differences in vegetative development when evaluating different types of substrates and their mixtures in greenhouses. Similarly, Pinto et al. (2017) evaluated two types of substrates [commercial substrate Siro® and a mixture of CF, composted pine bark, and perlite (3:2:1 v/v/v)] and the size of the container used for blueberry plants and concluded there was no impact on the growth of blueberry plants. In suboptimal soil conditions with a high pH, it is advisable to use organic substrates to stabilize the pH of the root zone in blueberry cultivation (Schmid et al., 2009) and to improve the physical properties of the soil (e.g., structure, aeration, water-holding capacity) to improve growth and increase plant production (Retamales and Hancock, 2012). The buffer capacity of the organic matter that constitutes the CF was likely the reason that, despite the lower EC and pH, the CF was more suitable for blueberry plants compared with sand.
Effects of substrate type on vegetative growth in potted plants of ‘Ventura’ blueberry.z
Growth response of ‘Ventura’ blueberry to an increasing concentration of Si in the nutrient solution. The plants were grown in pots filled with coir or sand.
Citation: HortScience horts 53, 10; 10.21273/HORTSCI13342-18
Growth response of ‘Ventura’ blueberry to an increasing concentration of Si in the nutrient solution. The plants were grown in pots filled with coir or sand.
Citation: HortScience horts 53, 10; 10.21273/HORTSCI13342-18
Growth response of ‘Ventura’ blueberry to an increasing concentration of Si in the nutrient solution. The plants were grown in pots filled with coir or sand.
Citation: HortScience horts 53, 10; 10.21273/HORTSCI13342-18
These data corroborate the growth benefits found by Pozo et al. (2015), who applied 0.65 mm Si in the nutrient solution and found, in addition to increased protection against Botrytis cinerea, a significant increase in the growth parameters of five families of plants grown in CF. On the other hand, Morikawa and Saigusa (2004), using river water containing between 0.66 and 1 mol·m−3 Si for irrigation, found that blueberry plants accumulated 3.1 and 5.4 times more Si than N in young and old leaves, respectively, thus classifying blueberry as an Si accumulator plant. These same authors noted that this accumulation may be a response to stress by heavy metals (Al, Zn, and Mn) and may serve as protection against diseases. In addition, authors such as Miyake and Takahashi (1986) observed benefits on growth, flowering, and production during strawberry cultivation with the application of Si in the nutrient solution.
Expt. 2: pH of the nutrient solution/Parameters of fertigation and water uptake
Effect on pH of drainages.
Comparison of substrate.
At all pH levels, ranging from 4.00 to 6.25, and for all the variables (two substrates and two types of acids), the pH values in the drainages varied without significant differences (pH 5.45–6.15, data not shown). As observed for Expt. 1, and with the exception of the treatment of pH 6.25 and nitric acid, within the same treatment and substrate, the acidity levels were relatively the same in the rhizosphere. This allows us to state that, in general, both acids are adequate to maintain the necessary acidification in the rhizosphere of blueberry plants (Table 4).
Effects of substrate and acid type on leachate pH and electrical conductivity (EC), drainage, and water uptake in potted plants of ‘Ventura’ blueberry.z
Comparison of the type of acid application.
Only for pH 6.25 (greater or close to the optimum pH described for blueberry plants; data not shown) and in CF was a significant difference observed, in favor of using nitric acid with respect to citric acid. Therefore, and in general for this parameter, both acids can be used as a source of acidification.
It is well known that pH variations of the rhizosphere are strongly influenced by the ratio of ammonium nitrogen (N-NH4+) to nitric nitrogen (N-NO3–). In addition, in sand, Merhaut and Darnell (1996, 1995) found that the pH decreased from ≈4.5 to 3.0 when blueberries were supplemented with only N-NH4+ for 140 d but increased to 6.0 when the plants were supplemented with only N-NO3–. Here, we used an average ratio of ammonium to nitric composition similar to the literature in this regard (e.g., Claussen and Lenz, 1999; Crisóstomo-Crisóstomo et al., 2014), and the pH of the drainages compared with the nutrient solution stayed within the desired limits (Urrestarazu, 2004, 2015).
Effect on EC of the drainages.
Comparison of substrate.
There was a significant decrease in EC for citric acid, whereas this difference was not recorded in sand.
There were significant differences in EC for the acid treatments within both substrates and pH levels (data not shown). However, for all levels of treatments, when comparing the recorded ECs for all acid–substrate combinations, greater values, between 7% and 20% more, were recorded in CF than in the sand with nitric acid, and values between 8% and 13% more were recorded with citric acid.
Comparison of the application of the type of acid.
In CF, the application of nitric acid caused a significant increase in EC in lower pH treatments, which could have been the result of the greater EC value resulting from the same pH value being used in the fertigation with nitric acid (Fig. 1). In contrast, EC was not lower in sand when citric acid was applied as a source of acidification, except at a pH of 4.0, probably because it is more rapidly degraded in a better aerated substrate (the aeration capacity of sand is much greater than that of CF).
Considering the two previous points, including the observation of lower average EC values when using citric acid, we could conclude that citric acid is a more favorable agent of acidification for crops that require a very low pH, such as blueberry.
Effect on the volume percentage of drainage.
Within each substrate, there was no clear variation in drainage EC, but the drainage volumes were much higher in sand.
Effect on uptake volume.
Comparison of substrate.
With the use of nitric acid, a clear increase in absorbed water volume was obtained. This was not observed for sand, although there was better absorption at different levels of pH (data not shown).
Comparison of acid type.
At a low pH (4.00–4.75), both acids showed similar water absorption volumes in CF and sand. However, for the increase at a greater pH, there was a significant difference between the acids, favoring nitric acid in the CF substrate and citric acid in sand.
Effect on vegetative growth.
Effect on height and foliar surface.
There were no significant differences in plant height, neither with the progressive application of acidity (data not shown) nor in the average data (Table 5). The averages were significantly greater, by 20%, for CF compared with sand.
Effects of substrate and acid type on vegetative growth in potted plant of ‘Ventura’ blueberry.z
There were no significant differences in leaf surface area. The average leaf area of CF in relation to sand was also greater by more than 10%.
Effect on biomass.
The average biomass data did not show a clear and significant tendency when nitric acid or citric acid was used in both substrates. The average biomass of CF was greater by more than 10%. The total shoot dry weight recorded a clear and significant or highly significant linear or quadratic R2 value of 0.99 and 0.95 for CF with nitric acid and citric acid, respectively (Fig. 3). There was no clear effect in sand. Nitric acid increase more than 20% the pH level (6.25). In contrast, when applying citric acid, the maximum was achieved at pH 4.00, with a greater increase than at pH 4.75. This increase was 13% when the pH decreased from 6.25 to 4.00, and was 8% when it decreased from pH 4.75 to 4.00. These results allow us to conclude that in these conditions, the optimum application levels are between 4.00 and 4.75. This finding suggests that more specific trials should be conducted for levels ranging from pH 4.00 (or even less) to 4.75. It is suggested that the levels for cv. Ventura and substrate be lowered to those published by other authors, who noted the desirable pH interval was between 4.5 and 5.5 (Korcak, 1988; Mainland, 2012; Retamales and Hancock, 2012). Other authors working with soilless culture, such as Townsend (1971), adjusted the pH of the nutrient solution with sulfuric acid and sodium hydroxide for two cultivars of blueberry and found that the maximum vegetative growth occurred at pH 4.0 and 4.5, but that there was no significant growth at pH 2.5 and 3.0. However, Schmid et al. (2009) used sulfur to acidify pine sawdust to a pH range of 3.8 and 4.2, and observed increases in growth and production of 48% and 55%, respectively. Nevertheless, they obtained negative results when they used citric acid to acidify the irrigation water, probably resulting from volatilization, leaching, or mineralization. In addition, using soilless culture, Rosen et al. (1990), with frequent adjustments of the pH of the nutrient solution, found the maximum vegetative growth and total dry weight at pH 4.5, regardless of the form of nitrogen used (N-NH4+, N-NO3–, or NH4NO3).
Growth response of ‘Ventura’ blueberry to increasing levels of pH in the nutrient solution. The plants were grown in pots filled with coir or sand, and pH was adjusted using nitric or citric acid.
Citation: HortScience horts 53, 10; 10.21273/HORTSCI13342-18
Growth response of ‘Ventura’ blueberry to increasing levels of pH in the nutrient solution. The plants were grown in pots filled with coir or sand, and pH was adjusted using nitric or citric acid.
Citation: HortScience horts 53, 10; 10.21273/HORTSCI13342-18
Growth response of ‘Ventura’ blueberry to increasing levels of pH in the nutrient solution. The plants were grown in pots filled with coir or sand, and pH was adjusted using nitric or citric acid.
Citation: HortScience horts 53, 10; 10.21273/HORTSCI13342-18
Our results in sand were not significant in any case. The statistical dispersion of the data in sand was much greater than in CF. These results can be justified by the recent results published by Payá-Milans et al. (2017), who indicated there are wild species of Vaccinium arboreum that are capable of adapting to a wider range of soil pH. As our data are only from one cultivar, it would be advisable to perform these same studies in several cultivars of blueberry species.
Similar to our results, authors such as Hall et al. (1964) also observed disparities in their results, finding a positive effect on clonal varieties of blueberry in mixtures of substrates at a pH of 4.2 (soil plus sawdust); however, in other mixtures (with compost, peat, and sand), the best results were obtained at pH values ranging from 4.9 to 5.5.
Conclusions
The application of Si benefits the vegetative growth of blueberry plants by between 8% and 25% in CF. In contrast, in sand, there was no clear benefit following the application of Si.
In general, the use of citric acid maintains the pH in the rhizosphere at the same level as nitric acid (<6.25). Therefore, citric acid can be an adequate acidifier in organic agriculture and is as suitable as nitric acid in conventional agriculture.
Maximum vegetative development was obtained at pH values ranging from 4.00 to 5.50 in both the nitric acid and citric acid treatments in CF. The results were not conclusive for sand. A pH of 5.50 to 6.25 is not advisable because of the negative effects on growth parameters. Recent and more detailed studies of the optimum pH to use in soilless culture suggest a pH range of 3.50 to 4.50, which is also recommended for other blueberry cultivars.
Literature Cited
Adams, P. 2004 Aspectos de la nutrición mineral en cultivos sin suelo en relación al suelo, p. 81–111. In: M. Urrestarazu (ed.). Tratado de cultivo sin suelo. Mundi-Prensa, Madrid, Spain
Ameri, A., Tehranifar, A., Shoor, M. & Davarynejad, G.H. 2012 Effect of substrate and cultivar on growth characteristic of strawberry in soilless culture system Afr. J. Biotechnol. 11 56 11960 11966
Ames, B.N., Shigenaga, M.K. & Hagen, T.M. 1993 Oxidants, antioxidants, and the degenerative diseases of aging Proc. Natl. Acad. Sci. USA 90 17 7915 7922
Arnon, D.I. & Johnson, C.M. 1942 Influence of hydrogen ion concentration on the growth of higher plants under controlled conditions Plant Physiol. 17 4 525 539
Arnon, D.I. & Stout, P.R. 1939 The essentiality of certain elements in minute quantity for plant with special reference to cooper Plant Physiol. 14 2 709 719
Bañados, M.P. 2006 Blueberry production in South America Acta Hort. 715 165 172
Boldt, J.K., Locke, J.C. & James, E. 2018 Silicon accumulation and distribution in petunia and sunflower grown in a rice hull-amended substrate HortScience 53 698 703
Bryla, D.R., Shireman, A.D. & Machado, R.M.A. 2010 Effects of method and level of nitrogen fertilizer application on soil pH, electrical conductivity, and availability of ammonium and nitrate in blueberry Acta Hort. 868 95 102
Bryla, D.R. & Strik, B.C. 2015 Nutrient requirements, leaf tissue standards, and new options for fertigation of northern highbush blueberry HortTechnology 25 464 470
CAPA Ecosystems 2012 Siliforte. Technical sheet. Plant health. 10 Jan. 2018. <http://insur.es/wp-content/uploads/2015/03/siliforte-Ficha-de-seguridad.pdf>.
Caspersen, S., Svensson, B., Håkansson, T., Winter, C., Khalil, S. & Asp, H. 2016 Blueberry–soil interactions from an organic perspective Scientia Hort. 208 78 91
Claussen, W. & Lenz, F. 1999 Effect of ammonium or nitrate nutrition on net photosynthesis, growth, and activity of the enzymes nitrate reductase and glutamine synthetase in blueberry, raspberry and strawberry Plant Soil 208 95 102
Coville, F.V. 1910 Experiments in blueberry culture. USDA 1867–1937, Bulletin of the United States Department of Agriculture, Bureau of Plant Industry. 10 Jan. 2018. <https://archive.org/details/experimentsinblu193covi>.
Crisóstomo Crisóstomo, N.M., Hernández, R.O.A., López, M.J., Manjarrez-Domínguez, C. & Pinedo-Álvarez, A. 2014 Relaciones amonio/nitrato en soluciones nutritivas ácidas y alcalinas para arándano: Ammonium/nitrate ratios in acid and alkaline solutions nutritious blueberry Rev. Mex. Cienc. Agr. 5 3 525 532
DOUE 2008 Commission Regulation (EC) no 889/2008 of 5 Sept. 2008 laying down detailed rules for the implementation of Council Regulation (EC) no 834/2007 on organic production and labelling of organic products with regard to organic production, labelling and control. 23 Feb. 2018. <http://data.europa.eu/eli/reg/2008/889/oj>.
Ehret, D.L., Frey, B., Forge, T., Helmer, T., Bryla, D.R. & Zebarth, B.J. 2014 Effects of nitrogen rate and application method on early production and fruit quality in highbush blueberry Can. J. Plant Sci. 94 7 1165 1179
Food and Agricultural Organization 2017 Blueberry crop statistics. 20 Feb. 2018. <http://www.fao.org/faostat/en/#data/QC>.
Gorbe, E. & Calatayud, A. 2010 Optimization of nutrition in soilless systems: A review Adv. Bot. Res. 53 193 245
Hall, I.V., Aalders, L.E. & Townsend, L.R. 1964 The effects of soil pH on the mineral composition and growth of the lowbush blueberry Can. J. Plant Sci. 44 5 433 438
Heiberg, N. & Lunde, R. 2006 Effect of growth media on highbush blueberries grown in pots Acta Hort. 715 219 223
Heine, G., Tikum, G. & Horst, W.J. 2005 Silicon nutrition of tomato and bitter gourd with special emphasis on silicon distribution in root fractions J. Plant Nutr. Soil Sci. 168 600 606
Imtiaz, M., Rizwan, M.S., Mushtaq, M.A., Ashraf, M., Shahzad, S.M., Yousaf, B., Saeed, D.A., Rizwan, M., Nawaz, M.A., Mehmood, S. & Tu, S. 2016 Silicon occurrence, uptake, transport and mechanisms of heavy metals, minerals and salinity enhanced tolerance in plants with future prospects: A review J. Environ. Mgt. 183 521 529
Ingestad, T. 1973 Mineral nutrient requirements of Vaccinium vitis idaea and V. myrtillus Physiol. Plant. 29 239 246
Kalt, W. & Dufour, D. 1997 Health functionality of blueberries HortTechnology 7 216 221
Kaya, C., Tuna, L. & Higgs, D. 2006 Effect of silicon on plant growth and mineral nutrition of maize grown under water-stress conditions J. Plant Nutr. 29 8 1469 1480
Keen, B. & Slavich, P. 2012 Comparison of irrigation scheduling strategies for achieving water use efficiency in highbush blueberry New Zeal. J. Crop Hort. 40 1 3 20
Korcak, R.F. 1988 Nutrition of blueberry and other calcifuges Hort. Rev. 10 183 227
Luyckx, M., Hausman, J.F., Lutts, S. & Guerriero, G. 2017 Silicon and plants: Current knowledge and technological perspectives Front. Plant Sci. 8 411
Machado, R.M.A., Bryla, D.R. & Vargas, O. 2014 Effects of salinity induced by ammonium sulfate fertilizer on root and shoot growth of highbush blueberry Acta Hort. 1017 407 414
Mainland, C.M. 2012 Frederick V. Coville, and the history of North American highbush blueberry culture Intl. J. Fruit Sci. 12 4 13
Marschner, P. 2011 Marschner’s mineral nutrition of higher plants. 3rd ed. Academic Press, Elsevier, Amsterdam, Netherlands
Mengel, K. & Kirkby, E.A. 2000 Principles of plant nutrition. International Potash Institute, Bern, Switzerland
Merhaut, D.J. & Darnell, R.L. 1995 Ammonium and nitrate accumulation in containerized southern highbush blueberry plants HortScience 30 1378 1381
Merhaut, D.J. & Darnell, R.L. 1996 Vegetative growth and nitrogen/carbon partitioning in blueberry as influenced by nitrogen fertilization J. Amer. Soc. Hort. Sci. 121 875 879
Miyake, Y. & Takahashi, E. 1986 Effect of silicon on the growth and fruit production of strawberry plants in a solution culture Soil Sci. Plant Nutr. 32 2 321 326
Montgomery, D.C. 2004 Diseño y análisis de experimentos. Wiley, New York, NY
Morikawa, C.K. & Saigusa, M. 2004 Mineral composition and accumulation of silicon in tissues of blueberry (Vaccinum corymbosus cv. Bluecrop) cuttings Plant Soil 258 1 1 8
Moya, C., Oyanedel, E., Verdugo, G., Flores, M.F., Urrestarazu, M. & Álvaro, J.E. 2017 Increased electrical conductivity in nutrient solution management enhances dietary and organoleptic qualities in soilless culture tomato HortScience 52 868 872
Nortem Chem 2015 Nortembio. Citric acid. Technical sheet. 20 Jan. 2018. <http://nortembiotechnology.com/>. <http://nortembio.com/nuestros-productos/acido-citrico>.
Payá-Milans, M., Nunez, G.H., Olmstead, J.W., Rinehart, T.A. & Staton, M. 2017 Regulation of gene expression in roots of the pH-sensitive Vaccinium corymbosum and the pH-tolerant Vaccinium arboreum in response to near neutral pH stress using RNA-Seq BMC Genomics 18 1 580
Peng, C., Wang, X., Chen, J., Jiao, R., Wang, L., Li, Y., Zuo, Y., Liu, Y., Lei, L., Ma, K., Huang, Y. & Chen, Z. 2014 Biology of ageing and role of dietary antioxidants BioMed Res. Intl. 2014 1 13
Pinto, R.M., Mota, M., Oliveira, C.M. & Oliveira, P.B. 2017 Effect of substrate type and pot size on blueberry growth and yield: First year results Acta Hort. 1180:517 522
Pozo, J., Alvaro, J.E., Morales, I., Requena, J., Mazuela, P.C. & Urrestarazu, M. 2014 A new local sustainable inorganic material for soilless culture in Spain: Granulated volcanic rock HortScience 49 1537 1541
Pozo, J., Urrestarazu, M., Morales, I., Sánchez, J., Santos, M., Diánez, F. & . Álvaro, J.E. 2015 Effects of silicon in the nutrient solution for three horticultural plant families on the vegetative growth, cuticle, and protection against Botrytis cinerea HortScience 50 1447 1452
Prior, R.L., Cao, G., Martin, A., Sofic, E., McEwen, J., O’Brien, C., Lischner, N., Ehlenfeldt, M., Kalt, W., Krewer, G. & Mainland, C.M. 1998 Antioxidant capacity as influenced by total phenolic and anthocyanin content, maturity, and variety of vaccinium species J. Agr. Food Chem. 46 7 2686 2693
Retamales, J.B. & Hancock, J.F. 2012 Blueberries. Crop Production Science in Horticulture, 21. CABI, London, UK
Rodríguez, D., Reca, J., Martínez, J., Lao, M.T. & Urrestarazu, M. 2014 Effect of controlling the leaching fraction on the fertigation and production of a tomato crop under soilless culture Scientia Hort. 179 153 157
Rodríguez, D., Reca, J., Martínez, J., López-Luque, R. & Urrestarazu, M. 2015 Development of a new control algorithm for automatic irrigation scheduling in soilless culture Appl. Math. Inf. Sci. 9 1 47 56
Rosen, C.J., Allan, D.L. & Luby, J.J. 1990 Nitrogen form and solution pH influence growth and nutrition of two Vaccinium clones J. Amer. Soc. Hort. Sci. 115 83 89
Schmid, A., Suter, F., Weibel, F.P. & Daniel, C. 2009 New approaches to organic blueberry (Vaccinium corymbosum L.) production in alkaline field soils Eur. J. Hort. Sci. 74 3 103 111
Sonneveld, C. & Straver, N.B. 1994 Nutrient solution for vegetables and flowers grown in water or substrates Voedingspolossingen glastijnbouw 8 1 33
Sonneveld, C. & Voogt, W. 2009 Plant nutrition of greenhouse crops. Springer, Dordrecht, Netherlands
Statgraphics Centurion X.V.I.I 2018 Statgraphics Net for Windows 7. 10 Jan. 2018. <https://www.statgraphics.net/descargas-centurion-xvii/>.
Strik, B.C. & Yarborough, D. 2005 Blueberry production trends in North America, 1992 to 2003, and predictions for growth HortTechnology 15 391 398
Tasa, T., Starast, M., Vool, E., Moor, U. & Karp, K. 2012 Influence of soil type on half-highbush blueberry productivity Agr. Food Sci. 21 4 409 420
Townsend, L.R. 1971 Effect of acidity on growth and nutrient composition of the highbush blueberry Can. J. Plant Sci. 51 5 385 390
Urrestarazu, M. 2004 Tratado de cultivos sin suelo. 3rd ed. Mundi-Prensa, Madrid, Spain
Urrestarazu, M. 2015 Manual práctico del cultivo sin suelo e hidroponía. Mundi-Prensa, Madrid, Spain
Urrestarazu, M., Gallegos, V.M. & Álvaro, J.E. 2017 The use of thermography images in the description of the humidification bulb in soilless culture Commun. Soil Sci. Plan. 48 13 1595 1602
Urrestarazu, M., Nájera, C. & Gallegos, V.M. 2016 Efectos del silicio en cultivos hortícolas FASAGUA: Nuestro campo 46 19 23
Urrestarazu, M., Salas, M.C., Valera, D., Gómez, A. & Mazuela, P.C. 2008 Effects of heating nutrient solution on water and mineral uptake and early yield of two cucurbits under soilless culture J. Plant Nutr. 31 3 527 538
USDA Organic 2015 Citric acid and salts. Technical evaluation report. 23 Jan. 2018. <https://usdasearch.usda.gov/search?utf8=%E2%9C%93&affiliate=usda&query=citric+acid&commit=Search>.
Valera, D.L., Belmonte, L., Molina-Aiz, F.D. & López, A. 2016 Greenhouse agriculture in Almería: A comprehensive techno-economic analysis. Serie Economía, 27. Cajamar Caja, Rural Almería, Spain
Van Bockhaven, J., De Vleesschauwer, D. & Höfte, M. 2013 Towards establishing broad-spectrum disease resistance in plants: Silicon leads the way J. Expt. Bot. 64 5 1281 1293
Vargas, O.L. & Bryla, D.R. 2015 Growth and fruit production of highbush blueberry fertilized with ammonium sulfate and urea applied by fertigation or as granular fertilizer HortScience 50 479 485
Vargas, O.L., Bryla, D.R., Weiland, J.E., Strik, B.C. & Sun, L. 2015 Irrigation and fertigation with drip and alternative micro irrigation systems in northern highbush blueberry HortScience 50 897 903
Voogt, W., van Dijk, P., Douven, F. & van der Maas, R. 2014 Development of a soilless growing system for blueberries (Vaccinium corymbosum): Nutrient demand and nutrient solution Acta Hort. 1017 215 221
Xie, Z.S. & Wu, X.C. 2009 Studies on substrates for blueberry cultivation Acta Hort. 810 513 520
Xu, C., Ma, Y. & Chen, H. 2014 Technique of grafting with Wufanshu (Vaccinium bracteatum Thunb.) and the effects on blueberry plant growth and development, fruit yield and quality Scientia Hort. 176 290 296
Zhu, Y. & Gong, H. 2014 Beneficial effects of silicon on salt and drought tolerance in plants Agron. Sustain. Dev. 34 2 455 472
