The Effect of Newly Developed Microbial Biopreparations on the Chemical Composition of Strawberry (Fragaria × ananassa Duch.) Fruit Grown in an Organic Farming System

Abstract

Non-chemical methods of fertilisation and protection have been gaining importance in recent years. This trend is closely linked to current European Union (EU) agricultural policy and the growing consumer awareness of the impact of nutrition on health. Newly developed biopreparations have to be tested for their agricultural efficiency alongside a quality assessment of the resulting food. The aim of this study was to determine whether the use of newly developed microbially enriched fertilisers in organic strawberry cultivation had an effect on fruit chemical composition and heavy metal accumulation. In the research, five biopreparations (K2–K6 combinations) containing selected Bacillus strains and plant extracts were tested in 2021 and 2022 on three strawberry cultivars: ‘Honeoye’, ‘Rumba’, and ‘Vibrant’. After the vegetation period, the collected fruit samples were frozen, freeze-dried, and subjected to chemical analyses to determine the total carbon and nitrogen content, as well as the concentration of microelements (Mn, Fe), macroelements (Na, Mg, K, Ca, P) and heavy metals (Cd, Pb, Cu, and Zn). The application of the tested biopreparations did not significantly impact the total carbon content of strawberry fruit. For most of the tested traits, cultivars reacted differently to the tested preparations. A higher total nitrogen content was found for treatments treated with biopreparations, especially for the ‘Vibrant’ cultivar—ranging from 15.2 g·kg⁻¹ K2 (BacilRoots) to 16.3 g·kg⁻¹ K3 (BacilRoots + BacilExtra) and K5 (BacilRoots + BacilExtra + BacilHumus)—being about 10–18% higher than on the control object (K1). The content of sodium, phosphorus, calcium, and magnesium did not change significantly under the influence of biopreparations. The use of the K3 and K5 treatment resulted in significantly lower iron contents when compared to those of the control (strawberries sprayed with water with no biopreparations added)—respectively, by 16.1% and 17.9%. ‘Vibrant’ treated with water (control treatment) showed the highest contents of iron, copper, and zinc when compared to those treated with biopreparations. No exceedances of the permissible heavy metal content were found in the samples tested.

1. Introduction

A review of the international literature reveals an increasing awareness among farmers of the necessity to reduce the use of chemical inputs in plant cultivation and to introduce organic products instead [1]. This fact facilitates the development of biological alternatives based on, among others, beneficial microorganisms that can protect plants from diseases, stimulate their growth and development, provide the necessary mineral nutrients, and improve soil quality [2].

The negative effects of excessive chemical input have led to a necessity for the replacement of these inputs with organic alternatives that do not pose a risk to human health and the environment [1]. One method of reducing the utilisation of fertilisers is to enhance their efficacy through the incorporation of beneficial microorganisms, which may encompass filamentous and arbuscular fungi (AMF), in addition to bacteria that facilitate the growth and physiological condition of plants. These microorganisms can also contribute to soil quality improvement and to the protection of plants from diseases and pests [1]. The limited availability of manure, as the majority of both conventional and organic farms do not keep livestock, is often a significant challenge, especially for organic farming. This is why organic farmers are searching for alternatives that can effectively fulfil the role of mineral fertilisers [2]. Şener and Cantemur [3] found that the consumer interest in premium foodstuffs being produced in an environmentally conscious manner can be attributed to a heightened environmental awareness and also a decline in consumer confidence in the quality and safety of mass-produced foodstuffs, which are often contaminated with dioxins, bacterial pathogens, and heavy metals.

A diet based on food from conventional, intensive agriculture can be contaminated with plant protection product (PPP) residues. This can translate into an increased incidence of cancer and chronic diseases among consumers. Therefore, it is important to provide farmers with solutions, including safe formulations for fertilisation and plant health improvement, that will promote yield quantity and food quality and are safe for consumers.

Strawberry fruits, derived from Fragaria chiloensis and Fragaria virginiana, date back to the 18th century. Currently, these are of great interest to consumers, mainly for their tastiness, but especially for their numerous health-promoting properties and high content of bioactive compounds [4,5,6].

The high content of phenolic compounds in strawberries, including anthocyanins, flavonols, and derivatives of hydroxycinnamic acid and ellagic acid, contributes to their antioxidant activity [7]. Additionally, strawberries are rich in amino acids, which provide health benefits such as antimutagenicity, a reduction in blood sugar, and a decrease in coronary heart diseases [8,9]. They are a source of minerals (iron, potassium, calcium, phosphorus) and vitamins A and C [10,11,12,13,14]. They also contain pectin, fibre, carotenoids [10,12,15,16,17], and glutathione [10].

Camargo et al. [18] found that organic strawberries had a higher anthocyanin content than that of conventional ones. Organic products are often considered safer as they contain lower levels or are completely free of pesticide residues, making them a preferred choice for consumers concerned about food safety and health [19,20,21]. On the other hand, some authors showed that organic crops are more likely to be contaminated by mycotoxins [22,23], while Brodal et al. [24], on the basis of a literature study, concluded that a farming system itself does not increase the risk of mycotoxin contamination. Organic products were also chosen by consumers as they had better sensory attributes, such as taste, texture, and aroma [25,26]. Consumer perception and willingness to pay also play a significant role in the preference for organic fruits. Some consumers are willing to pay a premium for organic fruits due to their perceived environmental benefits, such as reduced chemical input and support for sustainable agricultural practices [27,28].

According to Jamiołkowska and Hetman [29], increasing consumer demands for the quality of food consumed as well as a growing interest in environmental issues are the main causes of the increasing share of alternative, sustainable production methods in agriculture. Currently, the safety of plant protection products and promotion of biological and other non-chemical methods to improve plant health are of great interest both to agricultural scientists and farmers [29,30].

As a safer alternative, it is recommended to use PGPR—plant-growth-promoting rhizobacteria—which positively affect plant health and growth by suppressing pathogenic organisms and accelerating nutrient availability and assimilation [31]. In contrast, Mynett et al. [32] emphasise the importance of PGPMs (plant-growth-promoting microorganisms), which stimulate the growth of young plants, enhance root system development, and improve their resistance to abiotic and biotic stresses. PGPMs include, among others, Trichoderma harzianum, Bacillus amyloliquefaciens, Serenolipita indica, and Gliocladium virens, which are used in bioprotection against numerous fungal diseases.

According to Schönbichler et al. [33], the intensive use of chemical plant protection products has led to substantial environmental pollution, which poses a significant threat to human and animal health. This is mostly due to the translocation of pesticides’ active substances through food chains, which can ultimately risk the health of people. As an alternative, synthetic crop protection products could be replaced by rhizobacteria, which naturally stimulate the growth and health of certain groups of plants [33]. Grzegorczyk et al. [34] highlighted two main advantages of biological plant protection: the increased nutritional value of crops and possibility to utilise pathogen antagonists to degrade mycotoxins. Such activities are in line with the Sustainable Development Goals of soil and food conservation, as well as the implementation of the Farm-to-Table Strategy and thus the European Green Deal.

According to these principles, accelerating the EU’s transformation to a sustainable food system involves the production of healthy and quality food products. Thus, the heavy metal content of strawberries can be a serious concern due to the potential health risks associated with their consumption. Heavy metals such as lead (Pb), cadmium (Cd), copper (Cu), and zinc (Zn) can accumulate in strawberry fruit, posing a risk to human health. Moreover, heavy metals accumulated in food can be toxic even in low concentrations [3,35] due to their direct absorption through the gastrointestinal tract. The chosen farm management strategy as well as the choice of the production method can affect the chemical composition of fruit. An organic farming system might have some advantages here, as it does not rely strongly on industrial inputs, which conventional farming does. A study by Şener [3] highlighted that strawberries grown in a conventional system had higher concentrations of heavy metals compared to those grown organically, suggesting that farming practices have a significant impact on strawberry fruit safety and quality.

In view of the increasing number of newly discovered microorganisms and their potential different co-interaction, it is necessary to continue research that will enable the development of effective biopreparations. Their performance will be thoroughly and succinctly tested by scientific bodies, and then, after passing this stage, they will be put on the market [36].

A review of the existing literature reveals a deficit in the research findings exploring the impact of biopreparations on the quality of plant products, including their macro- and micronutrient composition, which is related to their direct nutritional value. Therefore, the aim of this study was to investigate the potential influence of newly developed biopreparations enriched with beneficial microbes and plant extracts on strawberry chemical composition, with the aim of enhancing fruit quality.

2. Materials and Methods

2.1. Experimental Study

The field experiment was set up at the Agricultural Experimental Station of the Institute of Soil Science and Plant Cultivation, State Research Institute in Puławy, located in Grabów nad Wisłą (Mazowieckie region, Poland: 51°21′18.7” N 21°39′22.2” E) in May 2019 on a certified organic field on Luvisolstype soil on a grey-brown podzolic soil created from strong loamy sands on light loam. The frigo strawberry seedlings were delivered by the Agronom Plants (Zienki 14, 21-230 Sosnowica, Poland) company. The experimental site had an area of 11.5a and was divided into 72 plots, each with an area of 16 m² and with 48 plants cultivated. The pre-crop for the strawberry was red clover. In the first year (2019), the flowers from the strawberry plants were removed in order to strengthen the aboveground parts and roots of the strawberry for the coming seasons. The experimental site was divided into six strips: control (K1) and five strips where the preparations were applied (K2–K6). The formulations tested were developed from locally sourced microbial strains and tested as part of wider study. Furthermore, the microbial products tested in this research are currently undergoing the patenting process and are slated for market introduction as microbial fertilising products. The detailed composition and trade names of the newly developed preparations patented by Bacto-tech Sp. z o.o. are detailed in

Table 1. Details of tested biopreparations.

Abbreviation	Trade Name	Details
K1	-	Control object
K2	BacilRoots	Preparation containing Bacillus sp. AF75BC and Bacillus subtilis AF75AB2 on a carrier that consists of dry humic acids, mustard, rapeseed oil, and clove oil in micronised dolomite (109 CFU/plant)
K3	BacilRoots + BacilExtra	Preparation containing Bacillus sp. AF75BC and Bacillus subtilis AF75AB2 on a carrier that consists of dry humic acids, mustard, rapeseed oil, and clove oil in micronised dolomite (109 CFU/plant), and Bacillus subtilis AF75AB2 and Bacillus sp. Sp115AD on a carrier that consists of plant extracts (nettle, horsetail, calendula) and humic acids in micronised dolomite (105 CFU/cm2)
K4	BacilRoots + BacilHumus	Preparation containing Bacillus sp. AF75BC and Bacillus subtilis AF75AB2 on a carrier that consists of dry humic acids, mustard, rapeseed oil, and clove oil in micronised dolomite (109 CFU/plant), as well as Bacillus sp. Sp116AC*, Bacillus sp. Sp115AD, humic acids, and yeast culture effluent in micronised dolomite (105 CFU/cm2)
K5	BacilRoots + BacilExtra + BacilHumus	Preparation containing Bacillus sp. AF75BC and Bacillus subtilis AF75AB2 on a carrier that consists of dry humic acids, mustard, rapeseed oil, and clove oil in micronised dolomite (109 CFU/plant), and Bacillus subtilis AF75AB2 and Bacillus sp. Sp115AD on a carrier that consists of plant extracts (nettle, horsetail, calendula) and humic acids in micronised dolomite (105 CFU/cm2), as well as Bacillus sp. Sp116AC*, Bacillus sp. Sp115AD, humic acids, and yeast culture effluent in micronised dolomite (105 CFU/cm2)
K6	BacilExtra + BacilHumus	Preparation containing Bacillus subtilis AF75AB2 and Bacillus sp. Sp115AD on a carrier that consists of plant extracts (nettle, horsetail, calendula) and humic acids in micronised dolomite (105 CFU/cm2), as well as Bacillus sp. Sp116AC*, Bacillus sp. Sp115AD, humic acids, and yeast culture effluent in micronised dolomite (105 CFU/cm2)

Source: Nakielska et al. [37], modified.

.

According to current Polish marketing criteria, the preparations that were used were classified as microbial fertilising products. This implies they have not been registered as biostimulators; however, their composition suggests effects beyond plant health promotion. Additionally, other fertilisers and products approved for organic farming were used to keep the nutrient content of soil at an optimal level (

Table 2. Applied bioformulations and fertilisers during 2021 and 2022 seasons.

Treatment Type	Product and/or Formulation	Manufacturer	Dose per Hectare	Number of Treatments
Fertilisation	Redarom Activstart	Biodevas Laboratoires Savigné-1’ Évêque, France	1.5 L	2
	Olibio	Biodevas Laboratoires Savigné-1’ Évêque, France	2 L	2
	Aminosol (N)	AZELIS POLAND Sp. z o. o., Poznań, Poland	3 L	2
	Potassium sulphate Patentkali®	K+S Polska Sp. z o.o., Poznań, Poland	250 kg	1
Fertilisation	Potassium salt	K+S Minerals and Agriculture GmbH, Kassel, Germany	60 kg	1
	Carbonate lime Polcalc	Polcalc Nawozy Wapniowe Sp. z o.o., Warsaw Poland	500 kg	1
Biopreparations (microbial fertilising products)	K2 (BacilRoots)	Bacto-Tech Sp. z o.o., Toruń, Poland	50 kg	3
	K3 (BacilRoots + BacilExtra)	Bacto-Tech Sp. z o.o., Toruń, Poland	50 kg	3
	K4 (BacilRoots + BacilHumus)	Bacto-Tech Sp. z o.o., Toruń, Poland	50 kg	3
	K5 (BacilRoots + BacilExtra + BacilHumus)	Bacto-Tech Sp. z o.o., Toruń, Poland	50 kg	3
	K6 (BacilExtra + BacilHumus)	Bacto-Tech Sp. z o.o., Toruń, Poland	50 kg	3

Source: Nakielska et al. [37].

).

The experimental factors of the experiments were as follows: (1) the recently developed K2–K6 preparations and (2) three strawberry varieties (‘Honeoye’ (Figure 1), ‘Rumba’ (Figure 2), and ‘Vibrant’ (Figure 3)). These are early varieties recommended for organic cultivation. ‘Rumba’ shows fairly good resistance to leathery rot and verticilliosis, as well as the powdery mildew of strawberries. It is considered one of the most popular early varieties due to its high resistance to most diseases, frost resistance, as well as good fruit quality and flavour. [38]. ‘Vibrant’ is a dessert variety with large fruits. It is valued for its flavour, high yielding potential, and long post-harvest shelf life. The variety is moderately resistant to powdery mildew, leathery fruit rot, and strawberry crown rot. An additional advantage is easy and fast harvesting due to the plant height and large fruits [39]. ‘Honeoye’ is a variety showing good resistance to grey mould and leaf diseases, but it is quite sensitive to verticilliosis and has low frost resistance. It is prized for its juicy and very sweet fruits, which are recommended for processing [40]. The plantation under investigation was not irrigated.

The formulations were applied three times throughout the growing season via a Fragaria boom sprayer (Figure 4). The dose per treatment was 50 kg·ha⁻¹ of formulation, dissolved in 700 L of water. The formulation used was a prototype; the target formulations for the market are 10 times more concentrated. The results presented herein are for the 2021 and 2022 harvest seasons, which represent the second and third years of plantation fruiting, respectively. The strawberry fruits were harvested at intervals of 2–3 days. Chemical analyses were conducted on the fruit harvested on 14 June 2021 and 13 June 2022. The fruits were harvested from the same eight plants on each harvest date. Following harvesting, the fruits were frozen and then freeze-dried (lyophilised). The chemical analysis with an ICP-MS spectrometer (for metal content analysis) and a TOC/TN analyser (vario Macro cube CN elementary analyser (Elementar Analysensysteme GmbH; Langenselbold, Germany)) (for carbon and nitrogen content analysis) was used. The analysis of metal concentration was conducted using inductively coupled plasma mass spectrometry (Agilent quadrupole 7500CE ICP-MS; Santa Clara, CA, USA). The extracts were prepared in concentrated nitric acid by microwave digestion. A blank sample and certified reference material (NIST1400 and CRM028-050) were included in the analyses for quality control of the entire analytical process. The basic validation of the parameters included the recognition of recovery, ranging from 90 to 97% for the analysed metals, and precision, defined as a relative standard deviation < 3%. The limit of detection (LOD) ranged from 0.007 mg/kg to 0.099 mg/kg.

2.2. Meteorological Conditions

Both the years of the study, with the exception of April, were quite dry and warm compared to the multi-year average (

Table 3. Weather conditions during the 2021 and 2022 growing seasons.

Month	Precipitation [mm]			Temperature [°C]
	2021	2022	Multi-Annual Average	2021	2022	Multi-Annual Average
April	51.2	42.4	42.0	6.4	6.5	7.5
May	49.9	23.3	53.0	12.5	14.4	12.4
June	70.1 *	31.5	110.0	19.5	19.5	16.7
July	61.7 *	80.6	105.0	21.8	19.1	17.8

* The rainfall values may deviate from the actual ones due to hail damage to the rain gauge.

). In particular, May and June 2022 had significantly lower rainfall than the multi-year average for these months.

2.3. Statistical Analysis

Basic statistical analyses were performed to reject random and coarse errors and to create a database of analysed variables. The data were analysed using two-way analysis of variance (ANOVA) followed by a post hoc Fisher’s test. The significant differences between the groups of variables were determined at p < 0.05. As there were no significant differences between years or interactions of years × biopreparation combinations, the results for the individual traits are presented as an average of 2021–2022. In order to classify the varieties and treatments based on the element content in the strawberry fruits, Principal Component Analysis (PCA) was used to perform ordination. The data were analysed using STATISTICA software (Statistica v. 10, Statsoft Inc., Tulsa, OK, USA).

3. Results and Discussion

The tested biopreparations developed under the EcoFruits project were created in response to the needs of organic strawberry and raspberry farmers. A positive effect of the selected preparation combinations on the strawberry fruit yield was observed on the same experimental site as in the present study (paper by Nakielska et al. [37]). The tested products contained bacteria of the genus Bacillus, which when used as biocontrol agents have a positive effect on plants by improving nitrogen fixation; solubilising soil phosphorus; and producing numerous secondary metabolites, such as antioxidants and cell wall-degrading enzymes, which directly support plant resistance against pathogens [33]. Previous research indicates that tested microbial biostimulants affect the development of pathogenic microorganisms and the quality of fresh strawberries that have beneficial influences on fruits. Although the presence of bacteria and fungi in strawberries fruit was observed, the phytopathogens were not present, which may have been a consequence of the application of the tested carriers and microbial preparations [41].

The tested microbial preparations also contained humic acids, which, according to Mynett et al. [32], can change plant metabolism and increase the permeability of cell membranes, which results in the faster transport of mineral compounds in the plant, which in turn increases the intensity of cell respiration and enhances cell division processes.

Sas-Paszt et al. [1] and Mynett et al. [32] found that intensive mineral fertilisation does not always have a positive effect on plant growth. On the other hand, the use of microbial preparations containing the strains Bacillus sp. and Bacillus amyloliquefaciens, Paenibacillus can significantly reduce the use of mineral fertilisers [1,32]. The introduction of natural plant breeding technologies and the use of biofertilisers is an ideal alternative to intensive mineral fertilisation, which increases production costs and negatively impacts the environment [1].

Ayaz et al. [42] emphasise that an integrated approach combining breeding with biological control agents is crucial to maintain biodiversity and ecosystem health from further depletion and to maintain sustainable agricultural practices. The authors note the need for further research on, inter alia, the development of potential biological conservation agents and the use of biotechnology and ‘omics’ techniques to improve biological control measures [42]. In contrast, Grzegorczyk et al. [34] emphasise that an important direction of research is the search for other positive mechanisms involved in biological plant protection.

Macro- and micronutrients are essential for human health. Ca, P, Mg, S, and F are the building materials of the human body. Na, K, Ca, and Mg are important for the maintenance of osmotic pressure and acid–base balance; Fe and Zn are part of compounds that are crucial for metabolic processes, while Mg and Ca are important for maintaining normal muscle contractility. Strawberry plants are able to easily uptake minerals from the soil, resulting in relatively a high content of minerals in fruits [43].

According to Bojarska et al. [43], strawberry plants readily take up minerals from the soil, so their fruit contains a fairly high amount of these elements.

One of the basic parameters for assessing the quality of an agricultural product, along with an assessment of the impact of the fertiliser product, is the dry weight. The dry matter content of the strawberry fruit differed significantly between cultivars. On average, the highest dry matter content was found in ‘Honeoye’ (11.18%) and the lowest in ‘Vibrant’ (8.75%) (

Table 4. Dry matter content of strawberry fruit [%].

Varieties	Biopreparations
	K1	K2	K3	K4	K5	K6	Average
‘Honeoye’	10.64 a * (±0.50)	11.58 b (±0.56)	11.06 ab (±0.92)	11.44 ab (±1.38)	10.95 ab (±0.77)	11.24 ab (±1.04)	11.18 C (±0.36)
‘Rumba’	8.78 a (±0.42)	9.04 a (±0.61)	9.81 a (±0.62)	10.02 a (±0.78)	9.78 a (±1.03)	9.83 a (±1.05)	9.59 B (±0.32)
‘Vibrant’	8.07 a (±0.46)	8.93 a (±0.45)	9.28 a (±0.47)	8.99 a (±0.81)	8.70 a (±0.75)	8.31 a (±0.51)	8.75 A (±0.24)
Average	9.17 a (±0.41)	9.85 a (±0.42)	10.05 a (±0.42)	10.15 a (±0.61)	9.81 a (±0.52)	9.79 a (±0.57)	9.84 (±0.20)

* The different lowercase letters in the rows indicate significant differences between biopreparations, and the different capital letters in the last column indicate significant differences between cultivars tested. The biopreparations × cultivar interactions were also not statistically significant at p < 0.05.

). The effect of biopreparations on the dry matter content was found only for the cultivar ‘Honeoye’ (higher dry matter content in fruit treated with K2 (11.58%) compared to the control with the lowest dry matter content of 10.64%). For the other two cultivars, a trend towards a positive effect of the biopreparations on the increase in the dry matter content of strawberry fruit was also found, to the greatest extent with the K4 combination, but this could not be confirmed statistically. Kilic [14], on the basis of her research on strawberry cultivars ‘Monterey’ and ‘Albion’, found that the application of fertilisers containing beneficial bacteria (including Bacillus subtilis, Bacillus amyloliquefaciens, Bacillus licheniformis, Bacillus mucilaginosus) and mycorrhiza can result in increased yields, as well as a positive effect on fruit quality due to the improved uptake rate of nutrients from the soil [14]. The positive effect of the tested microbial biopreparation on the strawberry yield was also confirmed by our research conducted on the same experiment [37].

Çakmakçi and Çakmakçi [44], based on review studies, pointed out that organic food has a higher content of nutrients and flavour compounds especially, including compounds such as sugars whose main component is carbon. Nevertheless, no statistically significant differences were found in the total carbon content of strawberries in the tested combinations (

Table 5. Total carbon content of strawberry fruit [g·kg⁻¹].

Varieties	Biopreparations
	K1	K2	K3	K4	K5	K6	Average
‘Honeoye’	446.6 a * (±20.0)	455.1 a (±14.0)	452.6 a (±13.8)	458.0 a (±15.6)	459.3 a (±13.4)	436.9 a (±7.7)	451.7 A (±5.4)
‘Rumba’	452.4 a (±19.5)	447.6 a (±16.3)	462.9 a (±15.7)	448.7 a (±12.2)	455.6 a (±18.0)	461.9 a (±18.5)	455.0 A (±6.4)
‘Vibrant’	439.9 a (±9.2)	459.2 a (±17.5)	449.4 a (±15.5)	456.1 a (±14.9)	454.7 a (±13.7)	452.7 a (±15.6)	452.7 A (±5.8)
Average	446.3 a (±9.0)	454.0 a (±8.7)	454.9 a (±8.3)	454.3 a (±7.8)	456.5 a (±8.2)	450.5 a (±8.3)	453.1 (±3.4)

* The different lowercase letters in the rows indicate significant differences between the biopreparations, while the different capital letters in the last column indicate significant differences between cultivars tested. The biopreparations × cultivar interactions were not statistically significant at p < 0.05.

). Despite the lack of significant differences, fruit that was not treated with biopreparations had an average 2.3% lower carbon content (446.3 g·kg⁻¹) compared to that of fruit treated with biopreparation K5 (456.5 g·kg⁻¹), which may indicate the occurrence of a certain trend of difference. According to Dinç et al. [45], the variety of the crop could be the most important factor that determines its nutritional quality; however, in the conducted studies, this effect was not visible.

The growing season and the region of origin of the plants have been found to have minimal influence on the average metal content of strawberries [43].

The results for nitrogen content were slightly different where no statistically significant differences were found for the total nitrogen content in ‘Honeoye’ and ‘Rumba’ treated with biopreparations, while ‘Vibrant’ exhibited lower total nitrogen content compared to that of other varieties. Significant differences in the total nitrogen content were also found between cultivars. ‘Rumba’ fruits had the highest (on average 18.8 g·kg⁻¹) and ‘Vibrant’ fruits the lowest (15.7 g·kg⁻¹) total nitrogen content (

Table 6. Total nitrogen content in strawberry fruit [g·kg⁻¹].

Varieties	Biopreparations
	K1	K2	K3	K4	K5	K6	Average
‘Honeoye’	14.8 a * (±1.3)	16.7 a (±2.2)	18.3 a (±2.3)	18.2 a (±1.7)	17.1 a (±1.7)	16.9 a (±1.6)	17.1 B (±0.7)
‘Rumba’	18.6 a (±2.4)	17.6 a (±2.1)	19.3 a (±1.2)	19.2 a (±2.1)	18.4 a (±1.5)	19.8 a (±2.4)	18.8 C (±0.7)
‘Vibrant’	13.8 a (±0.7)	15.2 ab (±1.5)	16.3 b (±2.3)	16.0 b (±1.6)	16.3 b (±1.6)	16.2 b (±1.5)	15.7 A (±0.7)
Average	15.7 a (±1.1)	16.5 ab (±1.1)	18.0 b (±1.1)	17.8 ab (±1.0)	17.3 ab (±0.9)	17.7 ab (±1.1)	17.2 (±0.4)

* The different lowercase letters in the rows indicate significant differences between the biopreparations, while the different capital letters in the last column indicate significant differences between cultivars tested. The biopreparations × cultivar interactions were not statistically significant at p < 0.05.

). The presence of nitrogen in the form of nitrates in the fruit of crops is a consequence of feeding plants with nitrogen, as well as the result of the natural circulation of nitrogen in the soil–plant trace. Nitrates are taken up by plants for protein synthesis. Their levels in plants are influenced by both the geochemical composition of the environment and ecological characteristics [46,47].

Kopeć et al. [48] listed fertilisation, soil pH, and meteorological conditions as well as water deficit during the growing season among the factors modifying the chemical composition of strawberries, in addition to the cultivar. This was supported by Shrestha et al. [49] who concluded that there are clear differences in the chemical properties of different strawberry varieties. Additionally, the build-up of trace elements in the soil can result in their accumulation in edible crops. Consumption of these crops by animals and humans can subsequently lead to the development of diseases such as cancer [50]. According to Şener and Cantemur [3], a significant amount of heavy metals that penetrate into living organisms with food through the skin or respiratory system and cannot be excreted through the lungs, skin, kidneys, liver, or intestines can accumulate in the bodies of humans and animals. Accumulation of these metals in organisms can lead to thyroid disease, infertility, neurological diseases, autism, and even death [3]. In our research, the heavy metal content in none of the samples exceeded the permissible content.

Cadmium toxicity causes molecular, and physiological changes in plants, affecting photosynthesis, gas exchange, plant growth, water balance, and also mineral uptake. High cadmium levels lead to plant DNA damage and rapid changes in gene expression in the nucleus and chloroplasts [51]. Cadmium antagonises the effects of Ca, K, Mn, Mg, and Fe as a result of disturbances in carbohydrate metabolism and mineral nutrition, as well as slowing plant growth [51]. Due to its good water solubility, cadmium easily penetrates plant tissues, leading to reduced photosynthesis and transpiration and reduced plant root growth [51].

The highest manganese content of 41.8 mg·kg⁻¹ was found for ‘Vibrant’, 24.4% higher than ‘Rumba’ (33.6 mg·kg⁻¹). Significantly, the lowest manganese content was found for the ‘Honeoye’ cultivar (29.2 mg·kg⁻¹) (

Table 7. Manganese content of strawberry fruit [mg·kg⁻¹].

Varieties	Biopreparations
	K1	K2	K3	K4	K5	K6	Average
‘Honeoye’	28.7 a * (±3.2)	33.3 a (±4.3)	30.1 a (±2.9)	30.0 a (±3.4)	26.3 a (±2.9)	26.7 a (±2.3)	29.2 A (±1.3)
‘Rumba’	34.6 b (±2.9)	34.0 b (±4.4)	34.9 b (±3.5)	37.3 b (±2.7)	24.6 a (±1.7)	36.5 b (±2.7)	33.6 B (±1.4)
‘Vibrant’	40.8 a (±4.3)	45.2 a (±3.9)	38.9 a (±6.1)	44.0 a (±4.4)	36.2 a (±2.2)	45.3 a (±2.1)	41.8 C (±1.7)
Average	34.7 ab (±2.4)	37.5 b (±2.6)	34.7 ab (±2.5)	37.1 b (±2.4)	29.0 a (±1.8)	36.2 b (±2.3)	34.9 (±1.0)

* The different lowercase letters in the rows indicate significant differences between the biopreparations, while the different capital letters in the last column indicate significant differences between cultivars tested. The biopreparations × cultivar interactions were not statistically significant at p < 0.05.

). The manganese content of the fruit ranged from 24.6 mg·kg⁻¹ (preparation K5 cultivar ‘Rumba’) to 45.3 mg·kg⁻¹ (preparation K6 cultivar ‘Vibrant’). On average, lower fruit manganese contents (29.0 mg·kg⁻¹) were recorded for all cultivars after application of the K5 combination compared to those for strawberries from sites where K2 (37.5 mg·kg⁻¹), K4 (37.1 mg·kg⁻¹), and K6 (36.2 mg·kg⁻¹) were applied. Hattab et al. [50] found the manganese content at a level of 0.387 μg·g⁻¹, while in the present study the manganese content ranged from 26.3 mg·kg⁻¹ for ‘Honeoye’ fruits treated with K5 to 45.3 mg·kg⁻¹ for ‘Vibrant’ fruits treated with the K6 biopreparation. Moreover, in a study conducted by Hattab et al. [50], the content of selected microelements and heavy metals (Fe, Mg, Mn, K, Ca, Na, Zn, Cu, Ni, Cd) in ‘Sabrina’ strawberry fruits cultivated under organic and conventional farming systems were analysed, and the results demonstrated significantly higher concentrations of micronutrients in organically cultivated fruits in comparison to in their conventionally grown counterparts. In the case of heavy metals Cd, Cu, and Zn, no significant differences were identified in the content of these elements between organic and conventional fruits. A significantly higher concentration of copper was observed in conventional fruits (0.62 μg·g⁻¹ dry weight) compared to in organic fruits (0.48 μg·g⁻¹ dry weight) [50]. In the present study, the mean copper content in organic strawberries was 2.57 mg·kg⁻¹ for the K2 treated object and 3.52 mg·kg⁻¹ for the control (K1).

The iron content in strawberry fruit on average for the tested cultivars remained at similar levels, from 33.0 mg·kg⁻¹ for ‘Honeoye’ to 35.8 mg·kg⁻¹ for ‘Vibrant’, and it did not differ significantly (

Table 8. Iron content of strawberry fruit [mg·kg⁻¹].

Varieties	Biopreparations
	K1	K2	K3	K4	K5	K6	Average
‘Honeoye’	32.7 a * (±3.4)	34.1 a (±4.7)	31.3 a (±0.4)	34.6 a (±4.5)	30.4 a (±1.3)	34.7 a (±1.6)	33.0 A (±1.2)
‘Rumba’	38.4 b (±5.6)	32.4 ab (±2.2)	36.6 ab (±4.6)	35.3 ab (±3.5)	30.6 a (±1.5)	37.2 ab (±1.5)	34.9 A (±1.3)
‘Vibrant’	46.4 b (±11.1)	35.4 ab (±4.8)	30.5 a (±1.6)	35.7 ab (±2.4)	35.3 ab (±1.2)	34.9 ab (±2.4)	35.8 A (±1.7)
Average	39.1 b (±4.2)	33.9 ab (±2.2)	32.8 a (±1.7)	35.2 ab (±1.9)	32.1 a (±0.9)	35.6 ab (±1.1)	34.5 (±0.8)

* The different lowercase letters in the rows indicate significant differences between the biopreparations, while the different capital letters in the last column indicate significant differences between cultivars tested. The biopreparations × cultivar interactions were not statistically significant at p < 0.05.

). Fruit sprayed with combinations of the K3 and K5 preparations had a significantly lower iron content than in the control. The iron content of strawberries in present study ranged from 30.4 mg·kg⁻¹ for ‘Honeoye’ strawberries from treatment K5 to 46.4 mg·kg⁻¹ for ‘Vibrant’ fruits from the control treatment (K1), while in the study of Hattab et al. [50] it was 43.26 μg·g⁻¹.

On average, for all cultivars, the significantly highest copper content was found in fruit from the K1 control (3.52 mg·kg⁻¹) and the significantly lowest in fruit sprayed with K2 (2.57 mg·kg⁻¹) (Figure 1). On average, there was significantly less copper in ‘Rumba’ fruits than in the ‘Honeoye’ and ’Vibrant’ fruits. For the ‘Vibrant’ cultivar, as with iron, the copper content of the fruit from the control (K1) was compared to that of strawberries from sites where K3 and K2 were applied.

Similarly, as in the case of copper, the zinc content in ‘Rumba’ fruit was significantly lower (7.37 mg·kg⁻¹) than in ‘Vibrant’ (9.66 mg·kg⁻¹) (Figure 5). The control treatment of ’Vibrant’ had a significantly higher zinc content than that of the K3 treatment (by 115.5%). ’Rumba’ fruits from the K4 treatment were characterised by a significantly higher (by 55.8%) Zn content than for the K5 treatment. Hattab et al. [50] reported a zinc content of 0.98 mg·kg⁻¹ for organic strawberries. However, the present study found a higher content of zinc, with levels ranging from 6.08 mg·kg⁻¹ for the ‘Rumba’ K5 treatment to as high as 15.30 mg·kg⁻¹ for the ‘Vibrant’ fruits from the control object (K1). In Hattab et al.’s [50] study, the sodium content in fruits ranged from 125.8 μg·g⁻¹, while in present study it varied a lot, with values ranging from 13.2 mg·kg⁻¹ for the ‘Honeoye’ control object (K1) to 34.0 mg·kg⁻¹ for the ‘Rumba’ K3 treatment.

Fresh strawberries contain about 90% water [52]. The limit values for cadmium (Commission Regulation (EU) 2023/915 of 25 April 2023) [53], which are 0.030 mg·kg⁻¹, are given for the content in fresh fruit mass. Given that in the tested study freeze-dried (lyophilised) fruits were used, the limit values were not exceeded in any sample (Figure 6). Fruits of the cultivars ‘Honeoye’ and ‘Rumba’ had a significantly lower cadmium content compared to that of ‘Vibrant’. On average, for all cultivars, a significantly lower cadmium content (0.03 mg·kg⁻¹) was recorded at site K5 compared to at site K2 (0.06 mg·kg⁻¹).

As in the case of cadmium, there were no exceedances of the permissible amounts of lead (0.10 mg·kg⁻¹ fruit) in strawberry fruit for any of the combinations tested (Commission Regulation (EU) 2023/915 of 25 April 2023 [53] (Figure 6). As in the case of cadmium, fruit of the Rumba cultivar had a lower lead content (0.013 mg·kg⁻¹) than that of fruit of the ‘Vibrant’ cultivar (0.029 mg·kg⁻¹). In the control treatment K1 of the ‘Vibrant’ cultivar, fruits were characterised by a significantly higher lead content (0.42 mg·kg⁻¹) compared to that of treatments sprayed with the biopreparation K6 (0.018 mg·kg⁻¹) and K5 (0.023 mg·kg⁻¹).

The biopreparations used did not significantly differentiate the sodium content of the strawberry fruit (

Table 9. Sodium content of strawberry fruit [mg·kg⁻¹].

Varieties	Biopreparations
	K1	K2	K3	K4	K5	K6	Average
‘Honeoye’	13.2 a * (±2.1)	14.2 a (±2.4)	13.2 a (±2.1)	18.5 a (±3.4)	16.0 a (±1.5)	14.9 a (±1.8)	15.1 A (±0.9)
‘Rumba’	33.9 a (±4.1)	24.3 a (±2.1)	34.0 a (±7.4)	24.7 a (±6.2)	24.3 a (±2.8)	31.4 a (±5.9)	28.5 C (±2.1)
‘Vibrant’	31.2 a (±10.0)	26.9 a (±4.7)	21.3 a (±2.8)	21.9 a (±1.8)	21.2 a (±2.5)	24.8 a (±4.2)	24.2 B (±1.7)
Average	26.1 a (±4.3)	21.8 a (±2.2)	22.9 a (±3.3)	21.7 a (±2.3)	20.5 a (±1.5)	23.7 a (±2.9)	22.6 (±1.1)

* The different lowercase letters in the rows indicate significant differences between the biopreparations, while the different capital letters in the last column indicate significant differences between cultivars tested. The biopreparations × cultivar interactions were not statistically significant at p < 0.05.

). Significant differences were only found between cultivars. ‘Rumba’ fruits had the highest sodium content (on average 28.5 mg·kg⁻¹), ‘Vibrant’ 15.1% less (24.2 mg·kg⁻¹), while ‘Honeoye’ showed 47% lower sodium content than that of ‘Rumba’ (15.1 mg·kg⁻¹). In a study by Bojarska et al. [43] conducted on 11 strawberry cultivars, the Na content of the fruit of the cultivar ‘Honeoye’ was 4.08 ± 0.11 mg·100 g⁻¹ of the dry weight and was more than double that recorded in their own study for the same cultivar for each combination.

The use of biopreparations significantly differentiated the potassium content of ‘Vibrant’ fruit, with treatments with biopreparation K2 having a higher content (21,033 mg·kg⁻¹) compared to with K3 (17,493 mg·kg⁻¹) (

Table 10. Potassium content of strawberry fruit [mg·kg⁻¹].

Varieties	Biopreparations
	K1	K2	K3	K4	K5	K6	Average
‘Honeoye’	18,381 a * (±1718)	18,828 a (±1568)	17,686 a (±1257)	18,322 a (±1464)	16,107 a (±631)	16,763 a (±795)	17,640 A (±503)
‘Rumba’	19,609 a (±590)	20,326 a (±1252)	21,953 a (±2024)	19,974 a (±1770)	19,509 a (±917)	19,392 a (±689)	20,158 B (±546)
‘Vibrant’	20,099 ab (±816)	21,033 b (±1531)	17,493 a (±1420)	17,757 ab (±923)	18,286 ab (±964)	20,469 ab (±1390)	19,136 B (±532)
Average	19,363 a (±639)	20,062 a (±821)	19,044 a (±1002)	18,684 a (±808)	17,968 a (±574)	18,874 a (±664)	18,978 (±319)

* The different lowercase letters in the rows indicate significant differences between the biopreparations, while the different capital letters in the last column indicate significant differences between cultivars tested. The biopreparations × cultivar interactions were not statistically significant at p < 0.05.

). Similarly to the sodium and magnesium contents, ‘Honeoye’ fruit had significantly less potassium (17,640 mg·kg⁻¹) than that of the other two cultivars. ‘Vibrant’ fruit had 8.5% and ‘Rumba’ fruit had 14.3% higher potassium content than that of ‘Honeoye’ fruit. In the study conducted by Hattab et al. [50], the potassium content of the ‘Sabrina’ strawberries was found to be at a level of 15,415 μg·g⁻¹. In contrast, Bojarska et al. [43] found a potassium content of 1656 ± 37 mg·100 g⁻¹ dry weight in the fruit of ‘Honeoye’. In the present study, the potassium content was higher for most treatments (with the exception of K5 treatment). The results of our study indicated a higher concentration of potassium, with values ranging from 16,107 mg·kg⁻¹ for K5 ‘Honeoye’ to 21,953 mg·kg⁻¹ for ‘Rumba’ fruits with K3 treatment.

The biopreparations used did not differentiate the calcium content of strawberry fruits for any of the varieties tested (

Table 11. Calcium content of strawberry fruit [mg·kg⁻¹].

Varieties	Biopreparations
	K1	K2	K3	K4	K5	K6	Average
‘Honeoye’	2899 a * (±142)	2536 a (±264)	2575 a (±263)	3176 a (±106)	2935 a (±222)	2792 a (±381)	2814 A (±105)
‘Rumba’	3092 a (±434)	2542 a (±182)	2604 a (±351)	2470 a (±261)	2796 a (±369)	2824 a (±109)	2700 A (±115)
‘Vibrant’	3792 a (±406)	3507 a (±269)	3014 a (±166)	3487 a (±352)	3861 a (±381)	3905 a (±329)	3583 B (±132)
Average	3261 a (±217)	2862 a (±172)	2731 a (±155)	3045 a (±175)	3198 a (±213)	3174 a (±204)	3032 (±78)

* The different lowercase letters in the rows indicate significant differences between the biopreparations, while the different capital letters in the last column indicate significant differences between cultivars tested. The biopreparations × cultivar interactions were not statistically significant at p < 0.05.

). Statistically significant differences were only found between cultivars; ‘Vibrant’ fruit had on average a significantly higher calcium content (3582 mg·kg⁻¹), being 24.6% higher compared to that of ‘Rumba’ (2700 mg·kg⁻¹) and 21.4% higher compared to that of ‘Honeoye’ (2814 mg·kg⁻¹). Despite the lack of significant differences, a trend towards higher calcium content was found in the control treatment K1 (3261 mg·kg⁻¹) compared to in the treatments where biopreparations were applied (K3—2731 mg·kg⁻¹; K5—3198 mg·kg⁻¹) on average for all cultivars. The calcium content in the present study ranged from 2470 mg·kg⁻¹ for ‘Rumba’ fruit from site K4 to 3905 mg·kg⁻¹ for ‘Vibrant’ strawberries from site K6. In contrast, Hattab et al. [50] reported a significantly lower calcium content in the fruit, at a 1762 mg·kg⁻¹ level. Similarly, in the study by Bojarska et al. [43], a lower Ca content of 207.3 ± 15.4 mg·100 g⁻¹ of dry weight was recorded for the ‘Honeoye’ fruit than in the present study (2536–3176 mg·kg⁻¹).

As in the case of sodium and calcium, the biopreparations used did not significantly differentiate the magnesium content of strawberry fruit (

Table 12. Magnesium content of strawberry fruit [mg·kg⁻¹].

Varieties	Biopreparations
	K1	K2	K3	K4	K5	K6	Average
‘Honeoye’	1488 a * (±109)	1437 a (±97)	1432 a (±100)	1467 a (±81)	1422 a (±102)	1388 a (±104)	1436 A (±38)
‘Rumba’	1793 a (±130)	1613 a (±125)	1720 a (±179)	1695 a (±135)	1573 a (±47)	1632 a (±64)	1664 B (±47)
‘Vibrant’	1912 a (±113)	1798 a (±148)	1582 a (±104)	1794 a (±158)	1856 a (±105)	1951 a (±111)	1810 C (±52)
Average	1731 a (±82)	1616 a (±77)	1578 a (±77)	1652 a (±77)	1617 a (±65)	1657 a (±76)	1637 (±31)

* The different lowercase letters in the rows indicate significant differences between the biopreparations, while the different capital letters in the last column indicate significant differences between cultivars tested. The biopreparations × cultivar interactions were not statistically significant at p < 0.05.

). The magnesium content of strawberry fruits differed significantly among the cultivars tested. As in the case of the sodium content, ‘Honeoye’ fruit contained the least magnesium on average for all combinations (1436.0 mg·kg⁻¹). ‘Vibrant’ fruit had significantly higher magnesium content (1810.1 mg·kg⁻¹) than that of the other two cultivars. Despite the lack of significant differences, on average the control treatment had the highest magnesium content—1731.2 mg·kg⁻¹—while in the treatments where biopreparations were used, the magnesium content ranged from 1577.7 mg·kg⁻¹ (preparation K3) to 1657.1 mg·kg⁻¹ (preparation K6). Hattab et al. [50] recorded a magnesium content of 1531.52 μg·g⁻¹ in organic strawberries, while in the present study the content of this element ranged from 1387.6 mg·kg⁻¹ for ‘Honeoye’ fruits treated with K6 to 1912.4 mg·kg⁻¹ for the ‘Vibrant’ control (K1) treatment. In contrast, Bojarska et al. [43] found a Mg content of 122.7 ± 3.1 mg·100 g⁻¹ of dry weight in the fruit of the cultivar ‘Honeoye’, which was lower than the Mg content found for the same cultivar in the present study.

The application of biopreparations significantly differentiated the phosphorus content only in ‘Vibrant’ fruits (

Table 13. Phosphorus content of strawberry fruit [mg·kg⁻¹].

Varieties	Biopreparations
	K1	K2	K3	K4	K5	K6	Average
‘Honeoye’	2074 a * (±111)	2339 a (±340)	2112 a (±119)	2234 a (±221)	2122 a (±227)	2003 a (±123)	2152 A (±84)
‘Rumba’	2310 a (±331)	2498 a (±244)	2579 a (±277)	2647 a (±331)	2741 a (±349)	2457 a (±281)	2552 B (±117)
‘Vibrant’	2284 ab (±336)	2285 ab (±176)	1905 a (±164)	2150 ab (±193)	2252 ab (±264)	2518 b (±278)	2229 A (±94)
Average	2223 a (±150)	2374 a (±144)	2199 a (±127)	2343 a (±148)	2372 a (±167)	2326 a (±141)	2311 (±59)

* The different lowercase letters in the rows indicate significant differences between the biopreparations, while the different capital letters in the last column indicate significant differences between cultivars tested. The biopreparations × cultivar interactions were not statistically significant at p < 0.05.

); the phosphorus content of fruit from treatment K6 (2518 mg·kg⁻¹) was significantly higher than that of K3 (1905 mg·kg⁻¹). Fruits of the ‘Rumba’ cultivar contained significantly more phosphorus (2552 mg·kg⁻¹) compared to that of the ‘Honeoye’ (2152 mg·kg⁻¹) and ‘Vibrant’ (2229 mg·kg⁻¹) cultivars. ‘Honeoye’ fruit had a 15.7% and ‘Vibrant’ a 12.7% lower phosphorus content compared to that of ‘Rumba’ fruit. In a study conducted by Bojarska et al. [43], the phosphorus content of the fruit of the cultivar ‘Honeoye’ was found to be 232.9 ± 11.0 mg·100 g⁻¹ of the dry weight, a result that is comparable to that obtained in the present study for the K2 treatment. As reported by Kumar et al. [51], phosphorus is crucial in reducing the uptake and translocation of cadmium through processes related to the binding of cadmium to the plant cell wall and the formation of cadmium phosphate complexes. Zinc, boron, and iron play an important rol in growth, flowering, and fruiting, which affects the quality of strawberries grown in the field [48,54].

Bebek Markovinović et al. [55] assessed the microelement content of strawberries of the ‘Albion’ cultivar and the juice produced from them, as well as the by-products from strawberry juice production, and they ranged as follows: Cu 0.077–0.415 mg·kg⁻¹, Zn 0.988–3.12 mg·kg⁻¹, and Pb < 0.03–0.076 mg·kg⁻¹. In our study, the values for these elements ranged as follows: Cu from 1.95 mg·kg ⁻¹ (K4, ‘Rumba’) to 4.66 mg·kg⁻¹ (K1, ‘Vibrant’), Zn from 6.08 mg·kg⁻¹ (K5, ‘Rumba’) to 15.30 mg·kg⁻¹ (K1, ‘Vibrant’), and Pb from 0.008 mg·kg⁻¹ (K1, K2, K3, ‘Rumba’) to 0.042 mg·kg⁻¹ (K1, ‘Vibrant’).

In order to confirm the relationship between the macroelement and microelement content and the tested varieties and treatments, the ordination method of PCA (Principal Component Analysis) was used (Figure 7 and Figure 8). The PCA analysis showed that the points corresponding to the tested varieties generally grouped together, indicating that the variety had a greater effect than that of the biopreparations on the chemical composition of the strawberry fruits. The points corresponding to the variety Honeoye are grouped together in the graph in Figure 8 at the position corresponding to the dry weight of the fruit in Figure 7. The points corresponding to the variety Rumba in Figure 8 are correlated with the contents of C, N, P, K, and Na in Figure 7. The highest content of micronutrients and heavy metals in the fruit is correlated with the position of the points corresponding to the variety Vibrant (Figure 8): Mg, Fe, Mn, Zn, Ca, Cd, Cu, and Pb (Figure 7).

Research conducted by Bojarska et al. [43] on 11 strawberry cultivars confirms that the fruit of this species differs in terms of the macronutrient content depending on the cultivar. Of the 11 varieties tested, the fruit of the ‘Honeoye’ variety was the most abundant in magnesium, sodium, calcium, phosphorus, and potassium. In contrast, in the present study, the same cultivar had a lower content of Mn, Na, Mg, K, and P than that of the cultivars ‘Vibrant’ and ‘Rumba’. The effectiveness of microorganisms in biopreparations and their effect on the growth, development, and yield of the cultivated plant are often determined by the method of inoculation. Foliar application increases the growth of the aboveground parts of the plant, while soil application stimulates growth mainly in the roots [1,56]. The efficacy of biopreparations is contingent upon the utilisation of organic matter by the constituent microorganisms as a source of energy, carbon, and electrolytes. This enables the microorganisms to proliferate and reproduce, while the organic matter is gradually biodegraded into innocuous end products. Biopreparations typically comprise bacteria, nutrients, a carrier, and, optionally, enzymes [36]. Babalola [31] mentions, among the advantages of the use of PGPR, the compensation of the decline in plant growth caused by heavy metals, weeds, drought, and salinity [31].

Excessive use of synthetic fertilisers contributes to the toxic accumulation of heavy metals, soil acidification, and the formation of soil crust, which lead to a decrease in soil organic matter and humus substance. In turn, soil acidification leads to an increase in the concentration of harmful ions in the soil, limits plant growth, and also reduces the uptake of phosphate by plants [57,58]. As disadvantages of the use of synthetic fertilisers, Babalola [31] lists the pollution of water bodies and the destruction of microorganisms and beneficial insects, which make crops more susceptible to diseases. In the long run, synthetic fertilisers can also reduce natural soil fertility, which leads to irreversible damage to the entire ecosystem.

The use of microbiological preparations may become a necessity to face current agronomic challenges. A crucial strategy for mitigating the negative effects of external factors, like climate change’s negative impact, is plant biostimulation, which enhances plant resilience disturbances, accelerates physiological processes, and promotes the growth and development of plants. This approach is particularly effective when bacterial or bacterial–fungal consortia are incorporated into biopreparations [59].

Among such products are microbiological fertiliser preparations, exemplified by the K2–K6 formulations tested in the experiment described in this study. These preparations contained bacteria from the Bacillus genus. According to Furtak [60], products containing Bacillus species can positively influence seed germination, enhance root system development, improve plant resilience to prolonged stress conditions, and ultimately contribute to higher crop yields [60].

4. Summary and Conclusions

The present study investigated the effects of newly developed microbial biopreparations on the quality and chemical composition of strawberries grown under organic farming conditions. While there was a trend towards the increased total carbon content in strawberries from plots treated with biopreparations, this effect was not statistically significant. The biopreparations also did not result in significant differences in sodium, magnesium, and calcium levels. However, notable cultivar-specific responses to biopreparations were observed, especially in terms of the dry matter; total nitrogen; and several elements, including heavy metals.

Among the strawberry cultivars tested, significant differences were found for dry matter and the content of elements, such as nitrogen, manganese, copper, zinc, cadmium, lead, sodium, magnesium, potassium, calcium, and phosphorus. The ‘Honeoye’ cultivar showed the lowest levels of manganese, sodium, magnesium, potassium, and phosphorus, while ‘Vibrant’ had the highest manganese, zinc, lead, and magnesium content. ‘Rumba’ exhibited the lowest levels of heavy metals (copper, zinc, cadmium, and lead) and calcium, alongside the highest levels of total nitrogen, sodium, potassium, and phosphorus. The biopreparations notably influenced the dry matter content in ‘Honeoye’, total nitrogen in ‘Vibrant’, and several other elements across different cultivars.

Specific combinations of biopreparations had significant effects. For instance, application of the K2 preparation increased the dry matter in ‘Honeoye’, while K3–K6 enhanced the total nitrogen content in ‘Vibrant’. The K5 preparation increased the iron content in Rumba, and K2–K3 reduced copper accumulation in ‘Vibrant’. Furthermore, K6 and K5 reduced lead accumulation in ‘Vibrant’, indicating potential for selective nutrient management through biopreparation application.

These findings suggest that while microbial biopreparations can influence specific nutrient levels and quality parameters in strawberries, their effects are largely cultivar-dependent. This study showed that the effects of microbial biopreparations on the chemical composition of strawberry fruits varied widely among cultivars and treatments, with no clear or uniform pattern emerging across all the tested combinations. While certain trends were observed, such as the increased nitrogen content in ‘Vibrant’ fruits treated with K4, K5, and K6, or the reduced heavy metal content in specific treatments, these effects were not consistent across all cultivars or treatments. This shows the complex interactions between the biopreparations, plant genotype, and environmental conditions in organic farming systems. Further optimisation of biopreparation applications to specific cultivars and farming contexts can optimise their benefits. No clear pattern suggests that further research is needed to understand the mechanisms underlying these variable responses and to refine the use of microbial biopreparations in organic agriculture. This also includes further studies needed to test the effect of the preparations in other locations with soil conditions and on other varieties as well as to confirm the effectiveness of the tested biopreparations for different cultivation conditions. Future studies should expand to include a wider range of strawberry varieties commonly cultivated in Poland to assess the universality of these biopreparations. Additionally, testing across varied geographic locations and soil types would be valuable to determine how these factors influence the uptake and accumulation of nutrients, efficacy of biopreparations, and microbial survival in soil over time, thereby providing more targeted recommendations for organic strawberry production.