applied sciences Article Effects of Organic and Mineral Fertilization on Yield and Selected Quality Parameters for Dried Herbs of Two Varieties of Oregano (Origanum vulgare L.) Natalia Matłok 1,* , Agnieszka Ewa St˛epie´n 2,3 ,Józef Gorzelany 1, Renata Wojnarowska-Nowak 4 and Maciej Balawejder 5 1 Department of Food and Agriculture Production Engineering, University of Rzeszow, St. Zelwerowicza 4, 35601 Rzeszów, Poland; [email protected] 2 Department of Dietetics, Institute of Health Sciences, College of Medical Sciences, University of Rzeszow, al/Mjr. W.Kopisto 2a, 35310 Rzeszów, Poland; [email protected] 3 Centre for Innovative Research in Medical and Natural Sciences, College of Medical Sciences, University of Rzeszow, Warzywna 1A, 35310 Rzeszów, Poland 4 Institute of Physics, College of Natural Sciences, University of Rzeszow, Pigonia 1 Str, 35959 Rzeszow, Poland; [email protected] 5 Department of Chemistry and Food Toxicology, University of Rzeszow, St. Cwikli´nskiej1a,´ 35601 Rzeszów, Poland; [email protected] * Correspondence: [email protected] Received: 8 July 2020; Accepted: 5 August 2020; Published: 9 August 2020 Abstract: The purpose of the conducted study was to determine the effect of the applied organic and mineral fertilizer on the yield and the selected quality parameters of the obtained dried mass of Origanum vulgare L. A pot experiment was carried out involving two varieties of oregano (Aureum and Hot & Spicy), which were fertilized using organic and mineral fertilizer. The produced raw material underwent a drying process using the CPD-VMFD combination method, which involved 1 convective pre-drying (40 ◦C and 0.8 m s− ) and vacuum-microwave finish drying at 240 W magnetron power. The resulting dry products were assessed in terms of color in CIE L * a * b * (Colour Ques spectrophotometer) and spectrophotometric analyses in order to determine the total content of polyphenols and antioxidant properties (ABTS and DPPH). Headspace solid phase micro-extraction (HS-SPME) from the obtained products was carried out, and their composition was determined by gas chromatography (GC-MS). The organic fertilizers used have resulted in the production of dry oregano herbs with a much higher bioactive potential than that of the herbs treated with material fertilizer. This phenomenon could be recognized as an elicitation effect associated with the use of common nettle extract. The organically fertilized oregano also contained an increased carvacrol content in the headspace, which resulted in a more intense odor of the dry herbs. The use of organic fertilizers in the cultivation of Origanum vulgare L. contributes to the production of raw material for direct consumption and drying, which has a higher bioactive potential and better organoleptic properties. Keywords: Origanum vulgare L.; variety; fertilization; organic fertilization; yield; antioxidant activity; total polyphenolic content; carvacrol; thymol 1. Introduction Oregano (Origanum vulgare L.) belongs to the Lamiaceae family, whose natural habitat is the Mediterranean region. The species Origanum vulgare L. is derived from the Euro-Siberian and Irano-Siberian regions and has a wide range of morphological and chemical diversity [1,2]. Currently, this species is widespread and grown in Europe, Asia, North Africa and the Americas [3]. Appl. Sci. 2020, 10, 5503; doi:10.3390/app10165503 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 5503 2 of 10 Because of its composition and its characteristics, Origanum vulgare L. is a principal culinary herb used worldwide, both fresh and dried [1,4]. The oregano herb is also used in traditional medicine in many countries as a plant medicine with therapeutic (sudorific, carminative, antispasmodic, antiseptic, toning) properties [5,6]. The medicinal properties of oregano are attributed to the biological properties of this herb and to the composition of the essential oils contained in it [7,8]. Essential oils contained in oregano ranging from 0.5% to 7% are rich in two isomeric phenols, carvacrol and thymol, as well as their precursor monoterpenes: p-cymene and γ-terpinene [9,10]. The active ingredients of oregano also include polyphenols, including flavonoids, tannins, glycosides, saponins, alkaloids and sterols [11]. These ingredients not only determine the taste and smell of the herb but also its properties, such as antioxidant and antimicrobial ones [12]. The bioactive content and the efficiency and composition of the essential oils in oregano depend on a number of factors, such as variety and crop conditions, including climate, habitat and water stress as well as the fertilization method and harvesting time [13–15]. The use of organic fertilizer with plant extracts in cultivating herbs results in a raw material with higher levels of bioactive compounds due to the occurrence of the elicitation phenomenon [16]. Elicitation is an intervention intended to induce a plant’s defense reaction. This reaction causes a number of biochemical processes leading to the creation of multiple groups of secondary metabolites. Plant extracts or chemical compounds such as abscisic acid, methyl jasmonate and trans-anethole [17] can be used as elicitors. The aim of the present paper was to determine the impact of the variety as well as of the organic fertilizers applied on the yield of Origanum vulgare L., the content of the selected bioactive compounds and the composition of the volatile organic compounds responsible for the scent of the dried raw material. 2. Materials and Methods 2.1. Pot Experiment In order to determine the impact of the applied fertilization on the bioactive compounds’ content of dried herbs of two varieties of Origanum vulgare L., a pot experiment was carried out. The pot experiment was conducted in a greenhouse from July to August 2019. The experimental layout was factorial in a complete randomized design (CRD), with three replications. Each replication included 32 production pots with a volume of 0.78 dm3 of soil, with 10 seeds placed at a depth of approximately 0.5 cm. The analyzed factor in the experiment was the type of fertilization used. Two varieties of oregano were tested, Aureum and Hot & Spicy, by organic and mineral fertilizing. The organic cultivation method relied on an innovative organic substrate consisting of sterile peat with a pH of 5.5 to 6.5 (70% in terms of weight) fortified with an extract from the common nettle (Urtica dioica L.) (10% by weight) and horse manure (20% in terms of weight). This method involved pre-sowing soil fertilization using organic fertilizer with controlled release Bioilsa N 12.5. The other (conventional) oregano cultivation method used a sterile peat substrate with pH 5.5–6.5 (100%) and involved a pre-sowing soil-applied mineral fertilizer. The composition of the substrate and the applied fertilization in the tested methods of oregano cultivation are shown in Table1. Table 1. Characteristics of the methods used in cultivation of Origanum vulgare L. Substratum Abundance 1 Nitrogen Fertilization Fertilization Substratum pH in (mg 100 g Substratum− ) N Total Method Composition KCl (%) Dose Type of P2O5 K2O Mg 1 (g N Plant− ) Fertilizer Ammonium mineral 6.4 40.4 244.2 62.0 0.74 1.2 nitrate neutral peat (70%), extract of common Bioilsa N organic 5.3 237.0 1282.0 269.0 1.88 0.6 nettle (10%), horse 12.5 manure (20%) Appl. Sci. 2020, 10, 5503 3 of 10 During the course of the experiment, a constant soil humidity of 60% of field water capacity was maintained. To this end, the pots were weighed daily and watered once the percentage of soil moisture went below 60%. The temperature during the experiment was approximately 25 ◦C during the day and about 15 ◦C at night. No chemical plant protection was carried out during the experiment. The crop harvest was carried out eight weeks after sowing, and the obtained biomass yield was then assessed. To this end, 96 plants were randomly taken from each variant of the experiment and then cut directly above the ground and weighed. 2.2. Plant Material Preparation and Analysis Method of drying (S1), drying kinetics determination (S2), HS-SPME GC-MS herbs analysis (S3), total polyphenolic content (S4), antioxidant activities (S5), color analysis (S6) and statistical analysis (S7) methodology are attached as Supplementary Materials [16,18,19]. 3. Results and Discussion 3.1. Yield When analyzing the obtained results (Table2) regarding the height and weight of the individual plants of Origanum vulgare L. produced in the pot experiment, different values of the measured parameters were found depending on the variety and the fertilization used. A significant impact of the applied organic fertilization on the increase in height and the mass of plants of both varieties of oregano was noted. This was probably due to the increased content of the main macro-components (N, P, K and Mg) in the peat substrate resulting from its fortification with horse manure, which contained these fertilizing compounds. The increased nutrient content for plants, including mainly nitrogen, increased the yield of oregano and other species of spices [20–23]. In addition to nitrogen, the yield and quality of the crop were influenced by potassium and phosphorus. Table 2. The height and weight of individual plants of Origanum vulgare L. depending on the variety and the fertilization used. Plant Height Plant Mass Veriety Fertilization Method (cm) SD (g) SD ± ± organic 18.2 2.4 b 1.96 0.28 b Aureum ± ± mineral 14.6 3.0 a 1.32 0.21 a ± ± organic 16.1 2.3 d 1.51 0.31 d Hot & Spicy ± ± mineral 11.9 3.0 c 1.19 0.33 c ± ± In the table: SD and n = 96, differences between the growing methods, assessed by Statistica 13.1 Student’s test, are indicated± by different letters (a, b, c, etc.), difference at p < 0.05. 3.2. The Kinetics of Drying The kinetics of drying (CPD 40 ◦C-VMFD 240 W) of the leaves of two varieties of Origanum vulgare L.