Diagnosis and management of nutritional constraints in berries

C H A P T E R

40 Diagnosis and management of nutritional constraints in berries Rolf Nestbya, Jorge B. Retamalesb,*

a

˚ s, Norway Division Food and Society (Horticulture), Norwegian Institute of Bioeconomy (NIBIO), A b Head ISHS Division Vine and Berry Fruits, Vin˜a del Mar, Chile *Corresponding author. E-mail: [email protected]

O U T L I N E 1 Availability of nutrients

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2 The roots and the rhizosphere

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3 Strawberry (Fragaria × ananassa Duch.) 3.1 Accumulation of minerals in the plant 3.2 Causes of nutrient constraints 3.3 Diagnosis of nutrient constraints 3.4 Management of nutrient constraints

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4 Highbush blueberry (Vaccinium sp.) 4.1 Accumulation of minerals in the plant 4.2 Causes of nutrient constraints 4.3 Diagnosis of nutrient constraints 4.4 Management of nutrient constraints

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References

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1 Availability of nutrients Today, growers have the possibility to highly modernize the irrigation and thereby the fertilization, by adding nutrients through the irrigation system (fertigation). Through modern fertigation technique using valves, sensors, timers, and IT support, growers avoid a lot of time-consuming manual work. However, data achieved from sensors have to be in a context to make the fertigation optimally adjusted at all developmental stages, also taking into account the variations in climate, mainly sunlight and temperature. Several companies deliver such equipment today. The competition is high, which is a driver for continuous improvements of the systems. This chapter starts with a general section that describes the conditions for nutrient uptake by the roots, which will highlight soil conditions, interactions with soil organisms (the biome), and the importance of the rhizosphere (growth medium in the immediate proximity of the root hairs). In the following three main sections, we will focus on the diagnosis of nutrient constraints (stresses) in berry crops. We will show how the knowledge of these constraints is helpful to manage optimal growing conditions using modern fertilization technology and help the growers to achieve this. Since there is a wide range of berry crops, the focus will be on strawberry and highbush blueberry. There are similarities and differences between these species on nutrient demand and especially of soil preferences, discussed in two separate sections. Throughout the world, strawberries tend to grow in light soils, but the result can be very good in heavy and nutrient-rich soils, properly drained. Blueberries prefer acid soils, typically with pH of 4.5 to 5.5. We will discuss the general need of macronutrients and boron (B), at different developmental stages. This will vary between the two species and between cultivars of each species. We will concentrate on general demands of the species and a few cultivar-related differences.

A.K. Srivastava, Chengxiao Hu (eds.) Fruit Crops: Diagnosis and Management of Nutrient Constraints https://doi.org/10.1016/B978-0-12-818732-6.00040-X

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© 2020 Elsevier Inc. All rights reserved.

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2 The roots and the rhizosphere Mineral nutrients are transported from the soil to the different plant organs (roots, crowns, leaves, runners (strawberry), and fruits), generally starting with the roots and transported from there to the other plant organs. Leaves can also absorb mineral nutrients, but that is of minor importance; however, this property is often used to alleviate acute deficiencies. When the mineral nutrients have entered the plant, they move between plant organs depending on plant signaling. A special plant organ will be of interest to the grower, which for the berries is a high fruit yield of good eating and postharvest quality. To achieve this, it is important that transport of minerals from the soil through the roots is sufficient and that all plant organs have optimal levels of macro- and micronutrients, at all developmental stages. Unless unfavorable aboveground conditions are dramatically limiting for plant health, the plant health depends on interactions occurring at the root-soil-microorganism interface (Benbrook, 2017). Successful berry cultivation has to start with a healthy plant, and this chapter will deal with strawberry and blueberry as examples of berries. A healthy plant in strawberry could be a short-day plant where flowers are not yet initiated, or more commonly, a fully initiated short-day plant ready to sustain a proper yield shortly after planting (60-day plants). Another alternative of increasing interest is remontant (long-day) strawberry plants. In blueberry 1- to 2-year-old in vitro or plants from cuttings are used. In strawberries, the plant is established in a bed normally mulched with one of the different types of polyethylene, and the planting system varies from one to several rows on the bed. Plant density (number of plants per square meter) will depend on the cultivar. However, the plants will compete for nutrients (from both soil and fertilizers), and the closer they are planted, the greater the competition. In the end, fruit yield, size, and quality will decide the density. There is also a limit to the sum of cations that can be simultaneously absorbed by the roots (Greenwood and Stone, 1998). The growth of strawberry roots, typically, looks like in Fig. 40.1, which shows a cross section of a bed mulched with black polyethylene film, supported with two drip lines for fertigation. The majority of root growth occurs just below the two drip lines where most of the water and nutrients end up. In this case, it was intentionally fertigated down to 30 cm, to avoid leaching of nutrients (Nestby and Guery, 2018). The roots seek for water and mineral nutrients and grow between the soil particles. The roots active in uptake of nutrients—the root hairs, which are 5–20-μm long–are surrounded by the rhizosphere (Waisel et al., 1996). The rhizosphere microbiome (population of microbes) extends the functional repertoire of the plant (Bakker et al., 2013). In the rhizosphere, abundant organisms are found that help the plant to absorb nutrients, and organisms also enter into the root hairs; that is the case of the mycorrhiza that surround the roots and provide them with minerals and get assimilates in return. Additionally, beneficial microbes in the microbiome improve the conditions for root uptake of mineral nutrients. The rhizosphere is well described in several chapters of the classic book Plant Roots: The Hidden Half (Waisel et al., 1996). However, given the unraveling processes that drive selection and activities of the rhizosphere microbiome, it has open up new avenues to manipulate crop health and yield (Bakker et al., 2013). Later, several authors have shown that using biofumigation (green manure) and adding biofertilizers are beneficial to improve uptake of mineral nutrients from the soil (Koron et al., 2014; Chakraborty et al., 2017; Kapur et al., 2018; FIG. 40.1 Root development of “Sonata” strawberry in a silt loam bed with two drip lines. Soil depth between tubes and plow sole is 30–35 cm.

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Mihàlka et al., 2017; Tomic et al., 2018). Some amendments have reduced injury of Fusarium wilt in strawberry, which has been an important problem since the ban of methyl bromide (Borrero et al., 2017). Highbush blueberries have two types of root: thick storage roots (<11 mm in diameter) and fine, threadlike roots (as small as 1 mm in diameter). The former anchor plants and perform a storage function, while the fine roots absorb water and nutrients. Blueberries do not have root hairs, and a specific type of endotropic mycorrhiza, ericoid mycorrhizae, inhabits the roots (Coville, 1910; Jacobs et al., 1982). These fungi form symbiotic associations with blueberry roots and help them prosper in soils with low pH and high organic matter (Vega et al., 2009). Mycorrhizal inoculation increases plant, root, and shoot dry weight without influencing shoot/root ratios (Yang et al., 2002). Mycorrhizae increase the uptake of soil nutrients and the efficiency of fertilizer application, improve water use, and protect the blueberry plant from toxic elements, such as Al (Scagel and Yang, 2005). Mycorrhizal colonization of blueberries varies significantly with cultivar, rate of fertilizer application, and the amount and type of soil organic matter present in the soil. In highbush blueberry fields in Oregon, large variations were found in mycorrhizal infection levels (0.5%–44% of total root length). Most colonization occurred in the upper 15 cm of the soil profile (Scagel and Yang, 2005). In general, about 50% of blueberry roots are located within 30 cm of the crown, and 80%–85% are within 60 cm (Paltineanu et al., 2017). Mulching concentrates roots near the surface. Abbott and Gough (1987) found that high rates of irrigation tended to increase root depth and that mulched highbush plants had 83% of their roots in the top 15 cm of soil compared with 40% in nonmulched plants. Vaccinium spp. absorptive roots can have diameters of less than 50 mm compared with a typical diameter of more than 200 mm in most other woody species. Valenzuela-Estrada et al. (2008) studied the root system of mature northern highbush “Bluecrop” plants with minirhizotrons and established that the ephemeral portion of the root system was mainly in the first three root orders. First- and second-order roots, despite being extremely fine, had median life spans of 115–120 days. The more permanent portion of the root system occurred in fourth- and higher-order roots. Roots in these orders had the lowest specific root length, nitrogen/carbon (N/C) ratios and levels of mycorrhizal colonization. Young containerized highbush plants growing in sawdust had two peaks of root growth during the season (Abbott and Gough, 1987). The first (weaker) peak occurs near fruit set and extends to the immature green fruit stage. The second peak occurs once fruit harvest started and ends before plant dormancy.

3 Strawberry (Fragaria × ananassa Duch.) 3.1 Accumulation of minerals in the plant A root takes up mineral nutrients by absorption of nutrient ions from the soil solution: These ions are readily available, but their concentrations in the soil solution are usually very low. The most abundant is NO3 (nitrate) in concentrations as high as 5–10 mM and followed by SO4 2 , Mg2+, and Ca2+ in concentrations up to 2–5 mM, K+ up to 1–2 mM, and PO4 up to 4 μM, respectively. By releasing H+ and HCO3 as dissociation products of respiratory CO2, roots promote ion exchange at the surface of the clay minerals and humic particles, obtaining in return the nutrient ions (Larcher, 2003). From the roots, the minerals enter into the aboveground plant organs. The amount of mineral nutrients accumulated in the vegetative and generative part of the strawberry plant at the end of harvest season was published as early as 1978 for soil-growing plants (Albregts and Howard, 1978, 1980), and a little later in soilless growing systems (Lieten and Misotten, 1993). These relative early examinations indicated that cultivated strawberries of different genotypes did not differ much in the amount of macroelements accumulated and in the balance between them, despite that soil type and large differences in mineral content and balance, pH, and cultivation system could create some deviations. In strawberries, 31 kg N/ha was stored in the vegetative plant organs, and there was a similar amount harvested from the field as fruit yield at the end of harvest, if the fruit yield was approximately 30 t/ha. A higher or lower fruit yield would accordingly contain more or less N, respectively (Nestby et al., 2005). Later, the Haifa Group (Haifa, 2018a,b) published values that confirm the earlier findings of Albregts and Howard (1978). In addition, Haifa published a table showing removal of all macronutrients at different yields. Remontant strawberries produce higher yields than short-day strawberries and thereby remove more N from the field. However, it is important to establish that the amount of mineral nutrients needed and the balance between them change at different developmental stages. Additionally, it is decisive to secure a proper pH, which, in soil, has to be optimal before planting, and, in soilless culture, must be balanced continuously to secure an optimal pH of the fertigation water. With this knowledge in mind, it should be relatively easy to set up a plan to fertilize a given cultivar in soil

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or in soilless culture. Lower correlations are reported in the literature for the microelements, possibly because of the differences in availability of nutrients in various growing systems (Nestby et al., 2005). Accordingly, we know the approximate amount of nutrients needed to build a well-functioning plant with potential to develop high-quality fruit. In the next section, we will look further into diagnosis and management of nutrient constraints with focus mainly on macronutrients.

3.2 Causes of nutrient constraints Setbacks may arise in the strawberry culture, because of problems with the growing medium being soil or soilless, lack of regularly monitoring of the field situation, climate, pest and insects, etc. If plants show symptoms of nutrient deficiencies, the problems have started long before these are visible. Sometimes, there are no clear deficiency symptoms, but the yield or the fruit size or quality does not reach their potentials. This could be because of a climatic situation, such as below optimal temperature for flower development in a biannual growing system, too high temperatures during fruit development that stresses the plants resulting in small fruits, and sometimes discoloring. However, it could also be that the rhizosphere is not well developed or that plants in spite of enough mineral nutrients in the soil have reduced availability of the nutrients. Too high or too low pH will also influence availability of mineral nutrients. Therefore, it is important to keep pH within an optimal range. Soilless production has increased dramatically in the last 25 years. However, in England (as in the rest of Europe), the essentials of the systems implemented have not changed much since a booklet on protected cultivation by Dennis Wilson was published in 1998 (Wilson, 1998). In the beginning, crops were mostly soil grown with moving polyethylene tunnels between fields, but because of constraints caused by soilborne diseases, more than 50% of the English strawberry production was soilless in 2015 (Moore, 2015). The situation is much the same in the rest of Europe and in varying degree in the rest of the world. There have been reports of direct or indirect effects on fruit yield and quality for all mineral nutrients. However, the optimal values change between cultivars, and new cultivars are released every year, so there should be continuous adjustments placing new cultivars at least into a known nutritional group of cultivars.

3.3 Diagnosis of nutrient constraints 3.3.1 Nitrogen (N) Nitrogen is absorbed in roots as NO3 (nitrate) and NH4+ (ammonium) and is an essential component of protoplasm and enzymes and is found as NO3 in vacuoles. During the transition from the vegetative to the reproductive phases and during the mobilization of storage proteins for new shoots of perennial plants, organic N compounds are transferred in large quantities. In leaves, growing shoots, and ripening fruits, the transport of N as organic compounds provides amino groups for the synthesis of amino acids and for transamination and serves as building blocks in protein synthesis and cell growth. Ammonium is metabolized in the roots, where it reacts with sugars. On the other hand, nitrate transported to the leaves is reduced to ammonium and then reacts with sugars. At high temperature, the plant’s respiration increases, making the sugars less available for ammonium metabolism in the roots. The practical conclusion is that at higher temperatures applying a lower ammonium/nitrate ratio is advisable (Larcher, 2003; SFM, 2018). To avoid N deficiency, there are standards for appropriate level of N and other mineral nutrients in strawberry plants. For macroelements, these levels are expressed in the percentage of dry matter (DM) (and in parts per million for micronutrients) of young mature leaves as indication of the situation of the whole plant. Recommendations throughout the world vary; for example, Haifa (2018a), situated in Israel, recommends an optimal N range of 1.9%–2.8%, a little lower than the 2.5%–3.5% recommended in Australia (Lawrence, 2010), while a general recommendation in Norway is 1.8%– 2.2% (Yara, 2018). The differences are probably a result of the nature of leaf sampling. Haifa (2018a) and Lawrence (2010) recommend leaf sampling preferably when the plants are actively growing at early harvest, while Yara based their recommendation on sampling after harvest when the growth is slowing down (Yara, 2018). The drop in N content of the leaves from start to end of harvest was confirmed by Bottoms et al. (2013). In addition, the optimal level may be different between cultivars, and cultivars that grow in the warm climate of Israel and Australia are different from those grown in the colder climate of Norway. Fertilization affects root growth such that low availability of N improves root growth but restrains shoot growth (Vamerali et al., 2003; Arevalo et al., 2005), while high N supply enhances shoot growth more than root growth. A similar effect was achieved in strawberry seedlings using pure organic fertilizers, but the effect was stronger by adding a combined organic/inorganic fertilizer. Simultaneously, the concentrations of indole-3-acetic acid (IAA) and abscisic acid (ABA) in the roots decreased when seedlings were fertilized from the initial to the late growth phase, while

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isopentenyl adenosine (iPA) levels increased at all growth stages. This suggests that the concentrations of endogenous phytohormones in strawberry plants could be responsible for the morphological changes of roots due to fertilization (Wang et al., 2009). Adding N in strawberry at 58.8 kg N/ha in the spring of the fruiting year, in addition to N supply from the soil, resulted in slightly less firm fruits than in unfertilized strawberries (Shoemaker and Greve, 1930). This effect was confirmed by some researchers (Overholser and Claypool, 1932; Miner et al., 1997), but not by Darrow (1931) who showed that fruits receiving extra N were firmer than those in plants receiving no extra N and that plants with low or moderate growth tended to have firmer fruits. Others found no influence of N on fruit firmness (Cochran and Webster, 1931; Haut et al., 1935; Bell and Downes, 1961). The divergence in these early results concerning fruit firmness is probably due to the variable amount of readily available N in the soil and that N leaching was not considered. It is therefore difficult from these experiments to establish a maximum limit of added N in fertilizers for plants growing in soil for avoiding soft fruits. However, as mentioned earlier, short-day strawberry (plant + fruit), depending on cultivar, contains 61 kg N/ha, important to keep in mind when managing fertilization of strawberries. Nitrogen also has effects on the number of flowers initiated, fruit size and malformation. Adding 100 kg N/ha increased the number of flowers in the first inflorescence; however, fruit malformation and greater number of small fruits (<2.0 g) occurred, resulting in reduced yield (Yoshida et al., 1991; Kopanski and Kaweci, 1994). Others confirmed the effect of N on fruit malformation (Kirsch, 1959; Yoshida et al., 1991; Yoshida, 1992). However, fruit malformation was reduced by applying N after sepal differentiation (Yoshida, 1992). This amount of fertilizer, in short-day strawberry growing in a well-balanced soil, is usually more than necessary to give a good strawberry development and will rather reduce than improve yield and fruit quality. However, the results often vary by soil type and field age. Nestby (1998) showed that there were influences on yield parameters in a nutrient-rich silt loam supplied with a basal fertilizer at weekly intervals in the summer season of the planting year and from flowering to end of harvest in harvesting years. The yield increased only in the second fruiting year giving 124 kg N/ha extra in both fruiting years, but fruit size decreased. On the other hand, in a nutrient-poor sandy loam, fruit yield strongly increased in both years giving extra 62 kg N/ha, with a small significant increase above this by doubling the amount of fertilizer, but fruit size decreased. The percentage of malformed fruits increased in the first fruiting year using extra fertilization, but in the second fruiting year, malformation decreased. A level of fertilizer estimated according to Albregts and Howard (1978) was used by Gariglio et al. (2000) applying 53 kg N/ha, which increased strawberry yield, a response due to increased fruit number, but not fruit weight, while fruit weight increased in everbearing strawberries by adding 40 kg N/ha (Burgess, 1997). 3.3.2 Potassium (K) Plants adsorb K as K+, which has good transportability in plants. It regulates hydration and is synergistic to NH4 + and Na+ and antagonistic to Ca2+. Besides, it has electrochemical effect (membrane potential and osmoregulation) and activates enzymes in photosynthesis (Larcher, 2003; Raviv and Lieth, 2008). The recommendations of contents of K in leaf samples vary; however, for example, in Israel, Haifa (2018a) recommend an optional K range of 1.6%–2.5% similar to Lawrence (2010) in Australia, while Yara (2018) recommend 1.2%–1.8% for Norwegian growers. The same comments as for N are valid here. According to Haifa (2018a), the plant is deficient at 1.2% K in leaf DM, which is equal to the minimum of the optimum range recommended in Norway. Besides deficiency symptoms in leaves shown as discoloration, fruits from deficient plants failed to develop full color; their texture was pulpy and insipid in taste (Ulrich et al., 1980; Raviv and Lieth, 2008). In early stages of K deficiency, dead calyces appeared, and in advanced stages of deficiency, wilting and drying up of pedicles and peduncles resulted in shriveling of fruits (Lineberry and Burkhart, 1943). Too much K in the soil can restrict the uptake of Ca and Mg ions and may create physiological disorders such as tip burn of emerging leaves and flowers. Main causes of tip burn disorders is high K, low Ca, high salinity, and cation imbalance; K deficiency symptoms could be seen on susceptible cultivars and under unfavorable environmental conditions (Kaya et al., 2002; Trejo-T
ellez and Gómez-Merinoand, 2012; Nestby et al., 2005; Raviv and Lieth, 2008). Because of the limited substrate volume in soilless cultivation, an unbalanced nutrient solution has larger influence on the crop compared with soil cultivation (Adak, 2009). For optimum crop performance, the electrical conductivity (EC) of the nutrient solution was more important than the K + : Ca2 + : Mg2 + ratio (Neocleous and Savvas, 2013). In regard to interactions between cations, the ratio K:Ca:Mg (Neocleous and Savvas, 2013), the levels of K (Seyedi et al., 2014), and the pH (Kim et al., 2005) are particularly important in terms of yield and quality in various horticultural crops. Likewise, high levels of K+ and high EC in the nutrient solution increased fruit dry matter, total soluble solid content (SSC), and lycopene concentration of tomato (Fanasca et al., 2006). A short application of high EC during ripening may be of practical interest for enhancing lycopene in fruit without affecting overall growth or yield (Wu and Kubota, 2008). Trejo-T
ellez and Gómez-Merinoand (2012) reported that strawberry plants with a good supply of K can

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synthetize more sugar and thereby develop sweeter fruits. In contradiction, Pivot and Gillioz (2001) indicated that excessive K in a soilless closed system reduced fruit quality due to lower sugar content. 3.3.3 Calcium (Ca) Ca enters the roots as Ca2 + and has very poor transportability in plants, where it is transported as ion and chelate, and binds in the tissue as pectates. It regulates hydration, activates amylase and ATPase, and regulates elongation growth. Further, it is a signaling substance via calmodulin and in the form of Ca pectate is responsible for stabilizing the cell walls of plants (Larcher, 2003; Raviv and Lieth, 2008). The recommendations of Ca content in leaf samples vary; for example, Haifa (2018a) recommend an optimal Ca range of 0.7%–1.7%; the range in Australia is much narrower, 1.0%–1.2% (Lawrence 2010), and the Norwegian recommendation is 1.0%–1.5% (Yara, 2018). The differences are smaller than for N and K. When Ca is deficient, new tissues such as root tips, young leaves, and shoot tips often exhibit distorted growth from improper cell wall formation. In addition, Ca2+ shows antagonism with K+ and Mg2+. Calcium moves from the roots to the rest of the plant via evapotranspiration by the water-conducting elements of the plant (i.e., xylem). If the evapotranspiration is high, such as on a hot dry day, calcium will move from the roots and up through the plant. Conversely, lengthy spells of cool, humid weather will reduce evapotranspiration, and subsequently, calcium movement can be restricted. Plant organs such as the fruit and developing leaves do not transpire as much as a fully mature, expanded leaf and therefore would tend to be the first to express Ca deficiency (Larcher, 2003; Bolda, 2010). 3.3.4 Magnesium (Mg) Mg enters the roots as Mg2+ and has partly good transportability. Mg regulates hydration and is antagonistic to Ca2+, is important in basal metabolism (photosynthesis and phosphate transfer), and is synergistic with Mn2+ and Zn2+ (Larcher, 2003). The recommendations of Mg content in leaf samples vary; for example, Haifa (2018a) recommend an optimal Mg range of 0.30%–0.49 % of DM, the range in Australia is one step up to 0.40%–0.60% of DM (Lawrence, 2010), and the Norwegian recommendation of 0.20%–0.30% is on the lower end of the scale (Yara, 2018). There are few reports of deficiency symptoms on fruits caused by low Mg, but Mg application increased fruit size in 1 of 3 years for “Tribute” strawberry (Lamarre and Lareau, 1997). According to Ulrich et al. (1980), in Mg-deficient plants, fruit appears nearly normal, except for a lighter red color and a tendency to albinism, and the upper margins of mature blades develop scorching, moving inward. 3.3.5 Phosphorus (P) P enters the roots as HPO4 2 or H2 PO4 and has good transportability in organically bound form. It incorporates in the plant as free ions, in esteric compounds, nucleotides, phosphatides, and phytin and functions in plants in basal metabolism and syntheses (phosphorylation). The recommendations of P content in leaf samples vary; for example, Haifa (2018a) recommend an optimal P range of 0.25%–0.40% of DM, the range in Australia is not much different (0.30%–0.50% of DM) (Lawrence, 2010), and the Norwegian recommendation is 0.20%–0.30% of DM, a little lower than for the other two. The effect of P on fruit quality has received little attention. However, Haut et al. (1935) found that P had no effect on fruit firmness, but flowers and fruits of P-deficient plants tended to be smaller than normal. Fruits of susceptible cultivars occasionally developed albinism (Ulrich et al., 1980) and had a lower SSC (Valentinuzzi et al., 2015). There was positive correlation between P and SSC in 24 strawberry cvs, and the concentration of P and SSC both increased from the lower part toward the tip of the fruit. After irrigating with 6.0-mM phosphoric acid, the SSC increased, and a KH2PO4 solution was the most effective (Zhang et al., 2017). 3.3.6 Boron (B) B is a microelement that is noticed here since it is an essential element for vascular plants and influences fruit set and quality. It enters the roots as HBO3 2 and H2 BO3 and plays a role in carbohydrate transport and metabolism, in phenol metabolism, and in activation of growth regulators (Shkolnik, 1974; Larcher, 2003). The recommendations of B content in leaf samples vary; for example, Haifa (2018a) recommend optimal B range of 30–64 ppm, in Australia 30–50 ppm (Lawrence, 2010), and in Norway 20–40 ppm (Yara, 2018). Several authors have reported influence of B on pollen germination and fruit set (Visser, 1955; Vasil, 1964; Guttridge and Turnbull, 1975; Ulrich et al., 1980). Since many years, blasting of flowers and distorted fruits and a tendency to produce fascinated fruits ( Johanson, 1963; Willis, 1945) were reported. Vitamin C content of “Redcoat” strawberry

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grown on light soil deficient in B increased linearly with B application rates of 0–8 kg/ha and sugar for rates up to 4 kg/ha (Cheng, 1994). Later, Lieten (2000) showed that a nutrient solution without boron produced a strongly reduced number of fruits and a high percentage of malformed fruit.

3.4 Management of nutrient constraints Growing conditions of strawberries in open field and in protected culture vary because of differences in growth medium and climate. In addition, pests and diseases may influence the crop. Strawberry plants have a shallow root system and are sensitive to water stress and salinity (Sauvageau et al., 2017). Earlier, we have dealt with the influence of macronutrients and boron on crop yield and fruit quality and focused on what the plant needs to grow optionally. The focus here will be to discuss management to achieve optimal plant growth and fruit quality when growing in soil and to some extent in soilless growing. However, this is an extensive topic, and only general recommendations are given. Growers should search for local information to manage strawberry fields (e.g., Haifa, 2018a,b; Queensland, 2008; Strik, 2013; NCDA, 2015. Bolda et al., 2010, local extension reports). The level of organic matter in soil is important, and to bring the content to an optimal level should start early and be continuous. Soils with higher levels of fine silt and clay usually have higher levels of organic matter than those with a sandier texture. There are some general guidelines. For example, 2% organic matter in a sandy soil is very good and difficult to reach, while a soil with 50% clay would need 6% organic matter to reach an aggregation level similar to a soil with 16% clay and 2% organic matter (SARE, 2012). To maintain a good level of organic matter, green manure crops (cereals, millet, legumes, clover, etc.) in between strawberry crops are recommended (Queensland, 2008), and higher soil activity was shown in intercropping with beans and clover (Dane et al., 2016). Other amendments such as vermicompost, biochar, N2-fixing bacterium (Gluconacetobacter diazotrophicus), seaweed extract (Ascophyllum nodosum), other microinoculants, and fermented food waste had positive influence on growth conditions of the soil (Chalanska et al., 2016; Yadav et al., 2016; Delaporte-Quintana et al., 2017; Hou et al., 2017; de Tender et al., 2016; Patil et al., 2016; Holden and Ross, 2017). In addition, pH has to be within the optimal range (6.5). For strawberry, the general recommendation is to use lime or dolomite (if soil Mg is low) at least 6 months before planting (Lawrence, 2010). Soil analyses will give a good indication of the demand of preplanting fertilizer to add, at least 4–5 months before planting or earlier if there is a long period of cold winter and frozen soil in between. The soil should be prepared before planting to obtain a good structure. Adding fertilizer and lime/dolomite should be compatible with modern methods, such as broadcasting using a tractor with Global Positioning System (GPS) before laying the plastic mulch. Together with information of variation in mineral nutrients and pH, the broadcasting equipment can deliver fertilizer or lime/dolomite in varying amount depending on soil analysis. The use of slow release fertilizers is an option, and they are best applied during bed formation in a band 10–15 cm below the surface. The benefit of slow release fertilizers is that they supply nutrients to the plants at a regular rate over 3–9 months, giving more even plant growth and reducing the risk of fertilizer burn of the roots (Lawrence, 2010). When the field is established, drones carrying modern photo equipment can be used to map the nutritional situation in the field, and again, GPS on a tractor (or a robot) can be used to add a balanced supplementary fertilization that may be necessary on a low fertile soil (Bossin, 2016). In open field, there is one more problem, and that is water availability. This will vary a lot between regions and states and time. In some places, the average rainfall can cover the need of the plants. However, the rain does not necessarily fall when it is needed, and when it falls, it may be too little or too much. Too high rainfall will increase nutrient leaching, and a fertilization plan has to be adapted, preferably after analyses of the runoff. To escape the effect of too much rain, especially during fruit growth and ripening, causing leaching and the infection of fruit rot (Botrytis cinerea), the field could be protected with polyethylene tunnels or rain roofs. This is frequently used, for example, in Huelva (Andalucia, Spain) where high tunnels are dominating (Gu
ery et al., 2018). This creates a dessert-like condition under the shelter, and watering becomes necessary, but now, it can be completely controlled preferably using drip irrigation. Again, modern techniques using sensors for analyzing volume water content (VWC) and EC are valuable tools. These sensors can connect to an automated sensor-based control that starts and stops watering depending on sensor readings (Gu
ery et al., 2018). The drip water is an excellent carrier of fertilizers. Combined with EC measurements and drainage lysimeters, the use of water and fertilizer can be programed to fit the actual needs of the plants (Nestby and Guery, 2018; Gu
ery et al., 2018; Garcia-Tejero et al., 2018). Moving strawberry into soilless culture has been more common. However, comparing soil-grown strawberries with soilless production showed that SSC, glucose, fructose, ascorbic acid, tocopherol, and total polyphenolic compounds were significantly higher in soil-grown strawberries. This should be a topic in future research (Treftz and Omaye, 2015).

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4 Highbush blueberry (Vaccinium sp.) 4.1 Accumulation of minerals in the plant The information provided in this review refers mainly to high chill or northern highbush (NHB) and low chill or southern highbush (SHB) blueberries. The nutrient demand of blueberries is low, compared with fruit trees (Table 40.1). However, in most situations, regular fertilizer applications are usually necessary for commercial fields (Hanson and Hancock, 1996; Krewer and NeSmith, 1999). There are various conditions, in both the plant and the soil, that explain the low nutritional requirements of blueberries compared with other fruit crops. Blueberries are said to be calcifuge plants, which means they are adapted to acidic soil conditions. Best growth and productivity are obtained when blueberries grow in soils with a pH in the range of 4.2–5.5. At this pH, the availability of most soil nutrients is limited, and this reduces the amount of mineral elements that are absorbed by the plant (Korcak, 1989; Hanson and Hancock, 1996). Blueberry roots are shallow and devoid of hairs (which limits the surface area in contact with the soil or substrate), and in natural habitats, they are colonized by a specialized type of fungus called ericoid mycorrhizae (ErM). Studies on NHB blueberries showed that increasing fertilization rates decreased ErM colonization of “Duke” but had little influence on colonization of “Reka” (Golldack et al., 2001). This type of cultivar-specific response in sensitivity to ErM colonization by nutrient availability may be responsible for some of the differences in the frequency and intensity of colonization detected among different highbush blueberry cultivars (Scagel, 2005). In addition, the fine root system of blueberries demands a loose soil, which makes sandy loams high in organic matter preferable for their cultivation.

4.2 Causes of nutrient constraints To obtain optimum yields, plants must have sufficient nutrient levels during active growth. Nutrient imbalances will affect fruit yield and quality. The degree of the effect will depend on the magnitude, opportunity, and duration of the deviation of nutrient levels from the optimum (Marschner, 1986). The high yield and quality required in commercial plantings demand constant field monitoring to satisfy the nutrient requirements to avoid nutrient deficiencies or excesses (Hart et al., 2006). Weather (temperatures, wind, and rainfall), fruit load, shoot growth, soil moisture, pruning intensity and timing, yield, and insect and disease load can affect plant functioning and the nutrient status of the plant (Stiles and Reid, 1991). In a 2-year study on the seasonal evolution of nutrients in NHB blueberries (“Aurora,” “Bluecrop,” “Draper,” “Duke,” “Legacy,” and “Liberty”), Strik and Vance (2015) found that the pattern of nutrient changes was similar between organic and conventional sites, but they had fewer differences in nutrient concentrations among cultivars at the organic site. In addition, the cultivar had a significant effect on all fruit nutrients except for P at the conventional site. Nitrogen is usually the mineral nutrient most frequently applied to blueberries (Hanson and Hancock, 1996). Fertilizer rates can be lower in soils high in organic matter since they have a higher N supply (Eck et al., 1990). Ca is another important nutrient because of its impacts on fruit quality. The goal of fertilization is to remove limitations to yield and quality by supplying the blueberry crop with ample nutrition in advance of demand. Fertilizer applications should be based on soil and plant analysis, information on environmental conditions, plant performance and management, and grower’s experience. A fertilizer application should produce a measurable change in plant growth, plant performance, and/or nutrient status. Results from nutrient applications can vary from year to year and from field to field (Retamales and Hancock, 2018). Fertilizers are only one part of a complete management package. If some parts of the blueberry-growing system are not working properly, extra fertilization is no solution (Hart et al., 2006). Plants interact with the environment (nutrients, light, water, and biotic factors) to generate growth. The amount of growth and the balance between reproductive and vegetative growth determine yield. Adequate nutrition is based on the soil/plant interaction (Marschner, 1986). Plant growth requires satisfaction of certain biological, physical, and chemical conditions by the soil (Retamales and Hancock, 2018). To establish the nutritional status and pH soil analysis is important before planting. Once fields are planted, nutrient management should be based on leaf analysis and soil pH monitoring. To be useful, leaf sampling must follow strict procedures. Estimation of fertilizer needs is based on nutrient demand/supply and fertilizer use efficiency. Fertigation has expanded due to higher efficiency and ease of use. Organic nutrient management is also expanding for blueberries. Whatever the method, efficiency increases with greater number of applications and as the rate of each is reduced. Except for some micronutrients and on specific occasions, foliar feeding is usually inefficient and costly (Retamales and Hancock, 2018).

575

4 Highbush blueberry (Vaccinium sp.)

TABLE 40.1

Sufficient or normal foliar concentrations of macronutrients (%) and micronutrients (ppm) for northern highbush (NHB) blueberries (Hanson and Hancock, 1996), and apple (Hanson, 1998; Stiles and Reid, 1991) in Michigan, USA.

Nutrient

NHB blueberry

Apple

Macronutrients (%) N

1.70–2.10

2.00–2.16

P

0.08–0.40

0.16–0.30

K

0.40–0.65

1.30–1.50

Ca

0.30–0.80

1.10–1.60

Mg

0.15–0.30

0.30–0.50

S

0.12–0.20

–

Micronutrients (ppm) B

25–70

25–50

Cu

5–20

10–20

Fe

150–250

Mn

50–80

Zn

20–40

Data on dry-weight basis.

4.3 Diagnosis of nutrient constraints 4.3.1 Nitrogen (N) Soil N is in constant flux, moving in the soil profile and having different chemical forms (Subbarao et al., 2006). The N cycle is mediated by microorganisms, whose activity depends on chemical and physical soil conditions. Some processes of the N cycle increase N availability for the plants (nitrification and mineralization), while others have the opposite effect (immobilization, denitrification, volatilization, and leaching). Plants deficient in N are usually stunted, initiate fewer canes, and have low vigor and pale green to chlorotic (yellow) leaves. The chlorosis is uniform across the leaf. Symptoms appear first on older leaves and will eventually include the entire plant if no N is applied. Leaves drop early, and yields are usually reduced. Excessive N results in plants with numerous, vigorous shoots and large, dark green leaves. Growth occurring at the end of the season may not harden properly before winter. The tips of these shoots often suffer freezing injury by low winter temperatures. Plants with excessive N have lower yields and smaller berries that ripen later (Hanson and Hancock, 1996; Hart et al., 2006). Several reports confirm the preference of blueberries for NO3 or NH4 + . They concluded that differences in response to form of N may be due to variability in rhizosphere pH. Absorption of NO3 is accompanied by net release of excess OH , which will raise the rhizosphere pH, while NH4 + uptake requires net release of H+ with a concomitant drop in rhizosphere pH (Merhaut and Darnell, 1995). The recommended N rates vary greatly among various producing regions. While 73 kg N/ha is suggested for fields older than 7 years in Michigan (Hanson and Hancock, 1996), 185 kg N/ha is advised in Oregon (Hart et al., 2006). For NHB blueberries grown under mulch, rates of 158–170 kg N/ha in year 1 and 238–257 kg N/ha for later years have been estimated in Oregon. However, in recent trials in Oregon comparing N sources (urea vs (NH4)2SO4) and mode of application (granular vs fertigation), the highest yields were obtained in plants fertigated with 63–93 kg N/ha/year (Vargas and Bryla, 2015). 4.3.2 Potassium (K) Leaf K is rarely low in blueberries, except on sandy soils. As with other fruit crops, the first deficiency symptom is chlorosis of older leaves margins. Greater K deficit can cause scorching of leaf margins, cupping, curling, and necrotic spots and dieback of shoot tips (Hart et al., 2006). Low leaf K can be due to lower root function, flooding, poor drainage, high N, drought, and very acid soils (Stiles and Reid, 1991). Root growth is important for K nutrition. In clayey or compacted soils, root growth will be reduced; then, even though soil nutrient analysis can show high K, leaf K could be low (Shaw, 2008). Excessive K (leaf K > 0.9%) can generate Mg and Ca deficiencies (Stiles and Reid, 1991).

576

40. Diagnosis and management of nutritional constraints in berries

K availability for crops depends on soil exchange dynamics. There is an active K fraction for immediate uptake and another long-term passive fraction. The passive fraction does not supply K during a growth cycle. The active fraction is composed of K in the soil solution (0.1%–0.2% of total soil K), exchangeable K and nonexchangeable K (occluded within phyllosilicate clays). If Na+ and NH4 + levels are high, K absorption is reduced. In contrast, optimum soil K levels improve uptake of Cu, Mn, and Zn. Release of exchangeable K is often slower than plant uptake, and thus, the K supply in some soils does not always satisfy crop K demand. Soil K application efficiency is 40%–60% and depends on the fertilizer form and dose, as well as the crop’s absorption capacity (Guerrero-Polanco et al., 2017). As fruit is an important sink for K in the plant, fruit load greatly influences leaf K levels. Fruit K increases strongly as fruits mature, approaching 60 mg per berry for ripe fruit (Hart et al., 2006). At harvest, 10 t of fruit removed 6.5, 7, and 8 kg K/ha in “Brigitta,” “O’Neal,” and “Duke” NHB blueberries, respectively (Hirzel, 2014). When leaf K is deficient, yields have been increased with K fertilization on various soil types. In Oregon, K fertilizers are not recommended if soil levels are >150 ppm and leaf K is >0.40%. If soil test readings are 101–150 ppm and tissue K is 0.21%–0.40%, up to 84 kg K2O/ha is suggested for fertilization. Application of 84–112 kg K2O/ha is advised when soil levels are 0–100 ppm and leaf K is <0.2% (Hart et al., 2006). Similar rates are recommended in Michigan, but it is suggested that if crop load is high, 0.35–0.40 leaf K would be adequate (Hanson and Hancock, 1996). 4.3.3 Calcium (Ca) Blueberries are calcifuges that thrive in low pH, are efficient in Ca2+ uptake, and have low Ca requirements relative to other temperate fruit crops. Healthy bushes typically have 0.3%–0.8% Ca in leaves (Eck, 1988) compared with 1%– 3% in temperate tree crops (Shear and Faust, 1980). Blueberries are seldom deficient in leaf Ca (Hanson and Hancock, 1996; Hart et al., 2006); however, even when leaf levels indicate adequate Ca supply to plants (Hanson et al., 1993), Ca can affect several fruit quality characteristics (fruit texture, firmness, and ripening rate). Fruit deficient in Ca can occur due to insufficient mobilization of Ca from internal stores, dilution due to tissue growth, or reduced Ca supply through the xylem (often due to low transpiration rates; Hocking et al., 2016). A high K and N supply, as well as wide fluctuations in moisture during the season, can reduce even further Ca supply to the fruit (Hirschi, 2004). Ca deficiency symptoms are most common in plants growing in lower pH soils, which also tend to have low Ca levels. Low leaf Ca levels can also occur in heavily fertilized, vigorously growing plants (Stiles and Reid, 1991). Ca serves various roles within plant cells, including structural, defense, and communication among tissues. The Ca levels needed for each role are quite different; 10 4 M Ca is required for the structural role and 10 7 M for communication functions (a 1000-fold gradient). When Ca uptake exceeds plant needs, some Ca is sequestered to form Ca oxalates, which also helps in defense and detoxification caused by heavy metals (Franceschi and Nakata, 2005). This can in part explain the weak relationships usually found between Ca applications and changes in Ca-related processes such as firmness and decay prevention (Retamales and Hancock, 2018). Ca is preferably absorbed by young roots. Soil NH4+, K, and Mg interfere with Ca absorption by roots; hence, high levels of these nutrients will reduce fruit Ca levels. The fruit tends to accumulate most of its Ca in their early stages of development and when shoot growth is limited (White and Broadley, 2003). As Ca is a phloem-immobile nutrient, the fruit relies mainly on transpirational water flow for its accumulation. Fruit transpiration rate is highest at fruit set (e.g., in kiwifruit, it can be as high as 2.3 mmol/m2/s, but this quickly declines to as little as 10% of this value later in development), whereas leaf transpiration remains at more than 10 mmol/m2/s (Montanaro et al., 2015). As blueberry fruit grows near harvest, Ca levels decrease by dilution (Retamales and Hancock, 2018). There are contradictory results reported from trials with Ca sprays on blueberries during the season; thus, while, some researchers reported positive effects on fruit quality, (St€ uckrath et al., 2008) others (Hanson, 1995; Vance et al., 2017) found no effect. 4.3.4 Magnesium (Mg) Mg deficiencies have been reported in fields of many blueberry-growing areas (Eck et al., 1990). They occur occasionally in Georgia (Krewer and NeSmith, 1999) and periodically in Michigan (Hanson and Hancock, 1996). The high variation in Mg content among source materials reflects on the total soil Mg contents (0.05%–0.5%). Differences in soil silicate content also explain the higher Mg contents typically found in clay and silty soils than in sandy soils. Mg availability to plants depends on the distribution and chemical properties of the source rock material and its degree of weathering, site-specific climatic factors, and, to a high degree, the management practices of a specific field, including the cultivar used and the organic/mineral fertilization practices (Gransee and F€ uhrs, 2013). Lower Mg levels are common in low-pH fields. Mg deficiencies are most frequent in rapid growing plants or in those with heavier fruit loads. Although the leaf deficiency level would occur <0.1%, Mg deficiency has been found in bushes with 0.2% (Hanson and Hancock, 1996). High Ca and/or K reduce Mg absorption and may indicate a need

4 Highbush blueberry (Vaccinium sp.)

577

for Mg application. Desirable ranges of the percentage of bases (as proportion of the CEC) in soil samples are 60%–80% Ca, 15%–30% Mg, and 10%–15% K. Mg should be applied if Mg is <4% of the bases or if K exceeds Mg as a percentage of the bases (Hanson and Hancock, 1996). For fruit crops in general, a ratio in leaf samples of the percentages of K:Mg greater than or equal to 4:1 usually indicates that the Mg supply is inadequate (Stiles and Reid, 1991). If foliar levels indicate a deficiency, soil pH should help decide which fertilizer to use. If soil pH is >4.5, magnesium sulfate (MgSO4, Epsom salts) or Sul-Po-Mag (21%–24% K2O, 21% S, and 10%–18% magnesium oxide (MgO)) should be chosen. Epsom salts can be fertigated. However, if the pH <4.5, dolomitic lime at 1 t/ha should be used. All soil applications should be in autumn (Hart et al., 2006). 4.3.5 Phosphorous (P) As with other fruit crops, P deficiencies are infrequent (Stiles and Reid, 1991). However, P supply to the roots of perennial crops is particularly constrained in acid, calcareous/alkaline, and old, highly weathered soils (Plassard and Dell, 2010). P is by far the least mobile and least available nutrient to plants in most soil conditions. Most soil P is bound tightly to the surface of soil particles or tied up as organic P compounds and thus mostly unavailable for plant uptake (Kochian, 2012). In addition, organic materials in the soil (e.g., from manure, organic mulch, or crop debris) can bind phosphate, in particular phytate (inositol compounds). It is estimated that plants use 10%–25% of applied inorganic P. The effects of excessive P are unusual in blueberry fields (Stiles and Reid, 1991; Krewer and NeSmith, 1999). Leaf P levels are highest early in the season and lowest at harvest. Tissue levels are little affected by crop load and moisture status. Threshold foliar P levels to establish deficiency vary. They are defined as 0.07% in Michigan and 0.09% in Wisconsin and Minnesota (Hart et al., 2006). In Oregon, P applications are only recommended if soil test (Bray) readings are <50 ppm and leaf P is <0.10%. At 26–50 ppm, soil P and 0.08%–0.10% leaf P up to 45 kg P2O5/ha are suggested. When soil P is <25 ppm and leaf P is <0.07%, 45–67 kg/ha of P2O5 is suggested (Hart et al., 2006). In Michigan, although recommendations are based on soil P levels, they suggest to apply P only when foliar P levels are <0.08% (Hanson and Hancock, 1996). 4.3.6 Boron (B) B deficiency is one of the most common plant micronutrient deficiencies worldwide (G€ urel and Başar, 2016). Low B is particularly prevalent in light-textured soils, where water-soluble B readily leaches and becomes unavailable to the plants. Deficiency is aggravated with dry weather and heavy crop load. Low plant B may accentuate deficiencies of other nutrients because of impaired root function (Ganie et al., 2013). In fruit crops in general, low B is often associated with Ca deficiency problems (Stiles and Reid, 1991). Its incidence varies across the blueberry-producing regions: while common in Oregon (Hart et al., 2006), it has not been found in Michigan (Hanson and Hancock, 1996). If B is deficient, 11–22 kg borax/ha (11% B) applied in the autumn or early spring prior to rain is beneficial. Alternatively, 0.9–2.7 kg Solubor (20% B) in 950 L water/ha can be sprayed before bloom or after harvest and before leaf senescence. An annual application of 560 g B/ha has been suggested (Hart et al., 2006). Foliar and soil B treatments (four applications of 0.2 kg B/ha between early bloom and 6-week post bloom) to “Bluecrop” NHB blueberries increased leaf and flower B levels, as well as fruit SSC, but had no effect on plant vigor, number of flowers per cane, fruit set, or yield (Wojcik, 2005). In contrast, a 4-year study in Missouri (United States) on the effect of autumn and spring foliar B sprays to “Blueray” and “Collins” NHB blueberries increased yield by an average of 10% for all seasons, mostly due to greater fruit number per plant. B sprays also reduced tip dieback symptoms (Blevins et al., 1998). Foliar B levels need frequent monitoring, as toxic B levels can rapidly appear (Hanson and Hancock, 1996).

4.4 Management of nutrient constraints The soil pH range recommended for highbush blueberries is 4.5–5.5. Soil pH influences nutrient availability for plants. Blueberries grown in high soil pH have yellow leaves with green veins. These leaves are small and often turn brown and fall from the plant before the season is over. Little growth occurs, and some plants may die. Plants stunted by high soil pH usually do not recover, even if soil pH is reduced (Hart et al., 2006). Plants established in high-pH soils often require replanting. Fe, Mn, or Cu deficiencies are common in high-pH soils; thus, instead of applying these elements to the soil, correcting the pH will usually be more effective. Soils are acidified either with elemental S incorporated before planting or with sulfuric acid (H2SO4) applied through the irrigation system. When soil S is applied, its conversion into acid is mediated by microorganisms, which require time, moisture, and warm temperatures. Thus, soil pH should be corrected at least 1 year before planting. Two variables affect the amount of elemental S needed to drop

578

40. Diagnosis and management of nutritional constraints in berries

soil pH: (1) the initial soil pH and (2) soil CEC. The higher the difference between the initial and the desired pH and the higher the CEC (or the soil buffer capacity), the more elemental S will be required to adjust soil pH. If more than 3.4 t/ha of S is necessary, the dosage should be split. Elemental S has to be thoroughly blended and incorporated in the top 20 cm of the soil (Retamales and Hancock, 2018). The increasing popularity of blueberries all across the world has forced growing the crop outside its natural habit (acid soils, high organic matter, and loose soil) in different soil conditions. Besides modifying soil acidity, the maintenance of moisture near the soil surface is critical because of the shallow rooting of blueberries. The most successful and widespread practice to enhance soil moisture and reduce weed infestation has been the use of mulch (Retamales and Hancock, 2018). A mulch is defined as any covering applied to the soil surface. This broad definition includes crop residues, weeds and other plant material, and artificial materials such as paper and plastic (Kumar et al., 2013). Mulches are common in blueberry production, and it is a prevalent approach to weed management in organic blueberry production (DeVetter et al., 2015). An Oregon survey showed that 58% of blueberry growers used some type of mulch (Scagel and Yang, 2005). Mulches can improve crop growth and productivity because they mitigate soil temperature fluctuations and reduce water loss from the soil surface (Clark and Moore, 1991; Burkhard et al., 2009; Cox, 2009). Within the soil profile, roots will grow primarily where organic matter is present. Mulching has improved root growth in the upper soil layer (Scagel and Yang, 2005), and a large part of the root system is developed at the soilmulch interface (Cox et al., 2014). Mulches from plant-derived materials supply organic matter and provide plant mineral nutrients (Clark and Moore, 1991; Himelrick et al., 1995). Sawdust or bark derived from Douglas fir is common for mulching throughout Washington, United States. Generally, compost is not recommendable for blueberry production, as they typically have high pH, EC, and K content, all of which are undesirable for blueberry growth and development (Sullivan et al., 2014; DeVetter et al., 2015). Plastic mulches are used in many blueberry-growing regions. Most benefits from plastic mulches occur in the first years after planting, as this is the period when competition for water, light, and nutrients is strongest. Depending on the type of mulch, they can last from 2 to 7 years, with weed mats (black landscape fabric made from woven polypropylene or polyethylene) having the longest active life (Cox, 2009). In blueberries, plastic mulches should be combined with fertigation, as the fertilizer placed under the plastic often depletes after 1–2 years (Williamson et al., 2006). Plastic mulching reduces fluctuations of soil moisture and temperature in the top 25 cm where most blueberry roots grow (Kader et al., 2017). On heavy, wet, clay soils, plastic mulches restrict soil microbial activity, leading to anaerobic soil conditions (Retamales and Hancock, 2018). Plastic mulching provides mechanical protection of the top soil, enhances root growth, stabilizes soil aggregates, increases mucilage production, and promotes soil fauna activity. The accelerated soil processes under plastic mulch can thus alter soil organic matter composition and quality (Steinmetz et al., 2016). Mulching has led to slight increases in total microbial diversity compared with nonmulched soil. The enhanced productivity under plastic mulches has often resulted in lower soil contents of Mg, K, P, and N than bare soils (Steinmetz et al., 2016). However, the effects of soil-mulch on microbial activity and soil nutrient contents would depend on both the type of mulch and the environmental conditions of the research site (Retamales and Hancock, 2018).

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