Substrates (media) for the cultivation of berry plants are an important component of a sustainable food production chain. The use of suitable growing substrates in modern industrial horticulture meets the needs of plants and ensures their productivity. Peat currently represents 77–80% of the growing substrates used annually in the horticultural industry in Europe [1
]. Peat is an extremely important component in substrates; however, its extraction threatens sensitive ecosystems, causes carbon sinks, and increases greenhouse-gas emissions [2
]. Different studies on bogs have confirmed that these ecosystems can substantially contribute to reducing atmospheric greenhouse gases [5
Therefore, substrates in which peat can be replaced by alternative components of organic or mineral origin are relevant to preserving unique wetland ecosystems. The suitability of various growing substrates in horticulture has been studied, i.e., certain quality parameters have been evaluated, including the degree of decomposition, the content of extractable nutrients, pH, bulk density, electrical conductivity and porosity. Various scientific sources indicated the possibility of using tree or coconut fibers, compost, tree bark, perlite, and other components that can be mixed to create appropriate growing substrates [7
]. When studying substrate compositions, the vegetative growth of plants should be assessed because the substrate should provide plants with an appropriate amount of water and nutrients [9
Recently, there has been a rapid increase in interest in blueberries and cranberries in Europe and around the world. Consequently, plantations of species from the Ericaceae Juss. family have increased the demand for large quantities of planting material. Wild species of the genus Vaccinium
L. grow in areas such as high moors and bogs, where the soil may be peaty and the pH ranges from 2.6 to 6.0 [10
]. As Hoover et al. [11
] reported, blueberries tolerate a wide range of soils. Notwithstanding, it was found that standard substrates containing higher amounts of fertilizers, especially nitrogen fertilizers, are not suitable for these plants [12
]. When searching for substrates for blueberry, it is important to consider the characteristics of its roots. Blueberry roots do not have root hairs, and the thin roots that are responsible for water and nutrient absorption are inhabited by mycorrhiza [13
In the production of substrates, renewable resources, such as wood chips and tree bark, can be used. Such substrates contribute to the utilization of logging waste [14
]. Lignin, which is found in plant cell walls, degrades more slowly compared to cellulose or hemicellulose and the degradation process of the substrate also slows down [16
]. Meanwhile, the bark of trees is rich in organic compounds (lignin, terpenes, fats, resins, sterols, glycosides, tannins, saccharides, acids, and others), which can change the quality of the substrates and affect the germination of plant seeds or the growth of saplings. It was determined that the quantitative and qualitative chemical composition of the bark of different tree species varies, and it is important to determine these variations. The bark’s compounds can affect the chemical characteristics of the substrates differently, for example, approximately 8% mannose has been found in spruce bark and approximately 9% arabinose in pine bark [7
]. Other studies showed that the phloem and the outer bark are richer in chemical compounds than the wood and also differ significantly among wood species [18
]. Kemppainen et al. [19
] investigated Norway spruce bark and detected a significant amount of tannins, 10.0%. Other researchers reported that fibrous materials are strong contenders in the replacement of peat in growing media, with a focus on the physical properties [16
]. As Vandecasteele et al. [20
] indicated, plant fibers have the potential for peat replacement and can provide protection against plant diseases.
Perlite is a non-renewable resource and is used throughout the world in horticultural applications. The physical and chemical properties of perlite as an component of substrates and the effect of this material on human health have been analyzed, and different studies confirmed that perlite can improve porosity and oxygenation to plant roots [21
Based on these previous studies, we hypothesized that the mix of spruce and pine fibers or perlite additions with peat could ensure the growth and quality of blueberry saplings. In this experiment, the effects of mixes of peat with different rates of spruce or pine fibers and perlite on vegetative growth, the content of extractable macronutrients and chlorophyll fluorescence in the leaves of blueberry saplings were studied.
2. Materials and Methods
2.1. Plant Material and Substrate Composition
Five hundred saplings of the highbush blueberry cultivar ’Duke‘ were purchased from a commercial nursery PLANTIN (Poland). Plants were propagated in in-vitro cultures in the laboratory of this nursery and were replanted to multi-pots after acclimatization. For this experiment, saplings with 1–2 lateral branches reaching a height of 8–12 cm were used. The saplings were transplanted into 2.0 L plastic containers filled with the appropriate substrates. In each substrate variant, thirty saplings were planted.
The substrates were composed of Scots pine Pinus sylvestris
L. and Norway spruce Picea abies
(L.) H.Karst. wood or bark fibers and high moor peat, which were used in various proportions. The experiment included 15 treatments: five mixes (peat + fiber of pine wood, peat + fiber of spruce wood, peat + fiber of pine bark, peat + fiber of spruce bark, and peat + perlite), each with three rates (Figure 1
, Table 1
The blueberry saplings were grown under natural light conditions in the greenhouse. The greenhouse temperature and relative humidity were maintained at 25 °C (day) and 15 °C (night) and 60%, respectively.
2.2. Content of Extractable Macronutrients and Organic Carbon and Peat Decomposition in Substrates
Before the planting of the rooted saplings, the content of extractable macronutrients was evaluated in the substrates. Additionally, the content of organic carbon and the degree of peat decomposition were assessed.
The degree of peat decomposition was determined according to LST 1957:2022 [22
] and the pH was determined according to LST EN 13037 with the potentiometric method [23
]. The pH values of the prepared substrate mixes ranged from 4.5 to 5.4. As Trehane [10
] reported, blueberries require a soil pH between 4.0 and 5.2. The content of organic carbon ranged from 27.54% to 41.66%, and the decomposition of peat was 29.1–38.8% (Table 2
The detection of elements available for plants (K, P, Ca, and Mg) was accomplished according to LST EN 13652 [24
]. In the water extracts, the phosphorus (P) concentration was detected using the spectrometric method with ammonium molybdenum complexes in Shimadzu UV 1800; the concentration of potassium (K) was measured using the flame photometric method with a Flame Photometer Sherwood M410; and the calcium (Ca) and magnesium (Mg) concentrations were determined using the atomic absorption spectrometric method using Atomic Absorption Spectrometer (Perkin Elmer AAnalyst 200, Waltham, MA, USA) (Table 2
Mineral nitrogen (N) was extracted in a 1:5 (wt./vol) substrate suspension of 1 M KCl solution. The suspension was shaken for 60 min at 20 ± 2 °C. After shaking, the suspension was filtrated and analyzed using a flow injection analysis (FIA) system with FIASTAR 5000 analyzer. After the addition of an acidic sulfanilamide solution, the nitrates in the substrate extract were converted to nitrites in the cadmium column. They, then, reacted with N-(1-naphthyl) ethylenediamine dihydrochloride to form a purple azo dye whose absorbance can be measured at 540 nm and 720 nm. The substrate extract was injected into a flowing carrier solution, where ammonium was mixed with sodium hydroxide to form gaseous ammonia, which passed through a gas-permeable membrane into the indicator stream. Acidic indicators changed color in this stream when they reacted with ammonium gas. Photometric measurements were performed at 540 nm and 720 nm. The calculation of mineral nitrogen involved adding the combined amounts of nitrate and nitrite nitrogen to the ammonia nitrogen.
The organic carbon content was determined using dry combustion, where the sample was heated to 900 °C in a stream of air, and the carbon dioxide formed was measured using infrared spectroscopy. The amount of organic carbon in the substrate was determined according to the standard ISO 10694:1995 with an organic carbon analyzer multi-EA 4000 Analytik Jena [25
2.3. Growth of Blueberry Saplings and Content of Extractable Macronutrients in the Leaves
Saplings’ height and fresh leaf weight per plant were determined for all variants of substrates by evaluating all thirty saplings. Leaves were collected from each plant separately and the average weight was determined at the end of the first growth flush of vegetative shoots, i.e., at 90–95 days after transplanting, in the last decade of July. Plant height was measured with a measuring tape. To determine the nutrient concentration, samples of fully expanded leaves from the current season shoots were prepared. From five to ten leaves were collected per plant and mixed before being sent to the laboratory. A total of 200 g of raw material per substrate was prepared. The leaves of the blueberry saplings were air dried, then ground and analyzed using the appropriate methods: nitrogen (N) with the Kjeldahl method, potassium (K) with the flame photometric method, phosphorus (P) with the photometric method with molybdovanadate, calcium (Ca) with the oxalic acid method, and magnesium (Mg) with atomic absorption spectrophotometry at 285·2 nm [26
]. The amounts of organic carbon in the blueberry leaves were determined according to the standard ISO 10694:1995 with the organic carbon analyzer multi-EA 4000 Analytik Jena [25
]. All analyses were performed in triplicate.
2.4. Determination of Chlorophyll Fluorescence
For the investigation of the maximum photochemical efficiency of Photosystem II (Fv/Fm), the leaves were fully dark, adapted, prior to the measurement. Dark-adapted leaf areas were achieved using lightweight leaf clips for 20 min. The chlorophyll fluorescence was measured with a chlorophyll fluorimeter (Pocket PEA Chlorophyll Fluorimeters, Hansatech Instruments Ltd., Norfolk, UK) with a Fv/Fm test duration of 1 s. A total of 5 measurements per plant were taken from leaves located in different directions, at an average height of 0.3–0.4 m, using leaf clips. Ten replications for each substrate variant were performed. The maximum photochemical efficiency of photosystem II was quantified (Fv/Fm) using the following relationship proposed by Maxwell and Johnson [30
According to Murchie and Lawson [31
], the Fv/Fm of non-stressed plant material should be in the range of 0.81–0.83. A much smaller relative interval (0.79 ≤ Fv/Fm ≤ 0.84) was indicated by Maxwell and Johnson [30
2.5. Statistical Analysis
In the evaluation of the average height and leaf weight of saplings, and chlorophyll fluorescence, a non-parametric Kruskal–Wallis test comparing the ranks of the samples was used. For all hypotheses, statistically significant differences were evaluated at a significance level of p = 0.05. The same level of significance was used in testing for differences between means by employing a one-way ANOVA with a multiple (pairwise) comparison procedure using Duncan’s test. The statistical calculations were carried out using the IBM SPSS Statistics 27 software application.