A Preliminary Evaluation of the Use of Solid Residues from the Distillation of Medicinal and Aromatic Plants as Fertilizers in Mediterranean Soils

Abstract

The current study focuses on a preliminary evaluation of the use of solid residues produced from the distillation of selected medicinal and aromatic plants (MAP) as fertilizers for alkaline soils. Specifically, the residues of hemp (Cannabis sativa L.), helichrysum (Helichrysum Italicum (Roth) G. Don), lavender (Lavandula angustifolia Mill.), oregano (Origanum vulgare L.), rosemary (Rosmarinus officinalis L.) and sage (Salvia officinalis L.) were added in an alkaline and calcareous soil at the rates of 0 (control), 1, 2, 4 and 8%, in three replications (treatments), and the treated soils were analyzed. The results showed that upon application of the residues, soil electrical conductivity (EC), organic C, total N and the C/N ratio significantly increased, especially at the 4 and 8% rates. The same was found for soil available P, K, B, Cu and Mn. The effects of the residues on soil pH, cation exchange capacity (CEC) and available Zn and Fe were rather inconclusive, whereas soil available N significantly decreased, which was somewhat unexpected. From the different application rates tested, it seems that all residues could improve soil fertility (except N?) when they were applied to soil at rates of 2% and above, without exceeding the 8% rate. The reasons for the latter statement are soil EC and available Mn: the doubling of EC upon application of the residues and the excessive increase in soil available Mn in treatments with 8% residues raise concerns of soil salinization and Mn phytotoxicity risks, respectively. This work provides the first step towards the potential agronomic use of solid residues from MAP distillation in alkaline soils. However, for the establishment of such a perspective, further research is needed in respect to the effect of residues on plant growth and soil properties, by means of at least pot experiments. Based on the results of the current study, the undesirable effect of residues on soil available N should be investigated in depth, since N is the most important essential element for plant growth, and possible risks of micronutrient phytotoxicities should also be studied. In addition, application rates between 2 and 4% should be studied extensively in order to recommend optimum application rates of residues to producers.

1. Introduction

Globally, the industrial use of medicinal and aromatic plants (MAP) has increased due to changes in people’s lifestyle and dietary habits [1,2]. These plants are rich in bioactive compounds, such as essential oils and phenolic compounds [3,4,5]. In addition, the well-established knowledge of the antimicrobial and anti-inflammatory properties of essential oils [3] has led to their use in a range of products in the food industry, cosmetology, medicine and pharmacology [3].

During the distillation of any aromatic and/or medicinal plant, essential oils are separated and collected [1,6]. Essential oils are present in plants in exceptionally low amounts, i.e., 0.5–8% of dry yield, depending on the species and variety of the plant [3,7]. Hence, because of the low yield of essential oils, considerable amounts of solid residues are produced during the distillation process [4,8]. In most of the cases, the solid residues from distillation are disposed of in landfills or fields as piles near the distilleries or are incinerated, contributing to environmental degradation and/or pollution [3,9].

Human population growth has led to intensive cultivation of soils and overuse of inorganic fertilizers because of the increased need for food [10]. The high prices of inorganic fertilizers and their non-renewable nature, as well as the need to protect the environment, have led to the production and use of more environmentally friendly fertilizers, made by recycling and valorizing industrial wastes. Moreover, the recycling and reuse of by-products from industrial activities are consistent with the cycle economy theory [11]. Although some industrial wastes (e.g., olive pomace, winery distilleries, and cheese whey) have been valorized as fertilizers [12,13], in the case of solid residues from MAP distillation, there is a lack of information concerning their use as soil amendments and fertilizers.

In recent years, there have been studies regarding different uses of solid residues from MAP distillation, in the perspective of reducing waste production and enhancing the added value of wastes. These studies include the production of compost, vermicompost, biochar, biofuel, biogas and biopesticides [3,4,7,14,15,16]. The use of solid residues from MAP distillation for co-composting with other types of wastes provides a promising method for eco-friendly solid waste management. Solid residues from MAP distillation were used in compost production as bulking and anti-pathogenic agents with positive results [3,16,17]. In addition, solid residues from MAP distillation and java citronella (Cymbopogon winterianus Jowitt) were mixed with cow dung, and vermicompost with high nutrient composition was produced [9,18]. Furthermore, there are certain studies focusing on the replacement of a part of peat with solid residues from MAP distillation in culture substrates [8,19]. Moreover, the application of a secondary extraction to solid residues from MAP distillation, for collecting remaining bioactive compounds and using them as antimicrobial agents in the food industry, has been investigated [20]. However, also in the latter case, the issue of rational management of the final solid residues remains.

Although there are studies referring to the production of compost and vermicompost by mixing solid residues from MAP distillation with other materials, there is a lack of information regarding the effects of these products on soils and plants [5,21]. Furthermore, there are no studies on the effects of using only solid residues from MAP distillation on soil properties and plant growth, without previously mixing them with other materials. Consequently, it is crucial to extend our limited knowledge regarding effects of particular residues on the inhibition–activation of soil microorganisms [3,22] and soil nutrient availability–immobilization, as well as on plant growth [23], in the perspective of their use in agriculture.

In the Mediterranean region, there are 25,000 naturally found species of which more than 1200 are considered MAP [1]. It is reported that 6620 species and subspecies are grown in Greece, including numerous MAP [24]. Lavender (Lavandula angustifolia Mill.), oregano (Origanum vulgare L.), rosemary (Rosmarinus officinalis L.) and sage (Salvia officinalis L.) are the most cultivated MAP in Greece [25]. In addition, the cultivation and exploitation of helichrysum (Helichrysum Italicum (Roth) G. Don) and hemp (Cannabis sativa L) has increased lately.

The solid residues from the distillation of MAP seem to maintain their initial inorganic composition [7,26], as well as certain amounts of bioactive ingredients [6]. Since there is a lack of information concerning application of the aforementioned solid residues to cultivated soils, such kind of research could be considered valuable for the recycling and valorization of distilleries’ wastes. The objectives of the study were the preliminary evaluation of the use of solid residues from the distillation of specific MAP, which are cultivated in the Mediterranean region, as amendments and fertilizers for alkaline and calcareous soils. These soils are the majority of soils in the Mediterranean countries, because the climate (long periods of high temperatures and low rainfall) favors their formation. The pH of alkaline soils is above 7.0 and, in most cases CaCO₃ is precipitated making the alkaline soils also calcareous. The high-pH, Ca-rich conditions of alkaline soils can lead to deficiencies of certain essential for plants micronutrients (especially Fe, Zn and B) and macronutrients (especially P). In addition, formation of a calcic horizon in these soils can impede the drainage of water in soil and the root growth [27].

In detail, the objectives of this study were the assessment of the effects of solid residues from the distillation of hemp, helichrysum, lavender, oregano, sage and rosemary on the chemical and microbial properties and fertility of an alkaline and calcareous soil. We added the aforementioned residues in an alkaline and calcareous soil at various rates. Then, we determined certain soil chemical and microbiological properties, as well as the concentrations of available macro- and micronutrients. This was done to see if there will be an improvement in the soil properties in order to initially establish an optimum rate or range of rates for the application of the residues to alkaline soils.

2. Materials and Methods

2.1. Sampling of the Soil and the Solid Residues fromMAP Distillation

An arable soil, classified as Endisol according to Soil Taxonomy, loamy in texture [27], alkaline in reaction and calcareous, was selected from Thessaloniki plane in northern Greece and adequate quantity of it was collected. Then, the soil sample was air-dried, ground to pass a 10 mm sieve and used for the treatments.

Adequate quantity of solid residues was collected from different distilleries of essential oils of northern Greece, right after MAP steam distillation, during July to September of 2022. The MAP, which had produced the solid residues, were hemp, helichrysum, lavender, oregano, rosemary and sage. After collection, the residues were transferred in a protected place and left for air-drying, until constant weight was achieved. Then, flowers together with leaves of the residues of helichrysum, lavender and rosemary were separated from the shoots and all residues were cut into small species and used for the preparation of the treatments. Furthermore, sub-samples of the residues, in five replications, were used for the determination of their composition [28].

2.2. Treatments of the Soil with the Solid Residues from MAP Distillation

In the beginning of October 2023, the solid residues from MAP distillation were added to the air-dried soil at the rates of 0 (control), 1, 2, 4 and 8%, (treatments), in three replications, and the treatments were left for equilibration with periodic wetting and air-drying for almost 30 days. The experimental design was the completely randomized (CR), with two factors (residues x rates) and three replications. The treated soils were ground to pass through a 2 mm sieve and analyzed for the following properties: the pH and electrical conductivity (EC) were measured in 1:2 w/v and 1:5 w/v suspension with water, respectively [29], and the cation exchange capacity (CEC) was determined by the method of hexaammine cobalt (III) chloride [Co(NH₄)₆Cl₃] [30]. The organic C was determined by the wet oxidation method [31] and the total N by the Kjeldahl method [32]. The soil-available NO₃-N and NH₄-N were extracted with 1 M potassium chloride (KCl) and were determined by ultraviolet spectrometry and the sodium salicylate–sodium nitroprusside method, respectively [33]. The soil available P was extracted by the Olsen method [34], K by the ammonium acetate method (CH₃COONH₄, pH 7) [35], B with hot water [36] and Cu, Zn, Fe and Mn were extracted with DTPA [37]. In the extracts, P was determined by the molybdenum blue–ascorbic acid method [34], K by flame photometry, Cu, Zn, Fe and Mn by atomic absorption spectrometry and B by the azomethin–H method [36].

Furthermore, the microbial respiration rate (MR) was estimated by trapping carbon dioxide (CO₂) in 1 M sodium hydroxide (NaOH) for 24 h and titrating excess of NaOH with 1 M hydrochloric acid (HCl) [38]. The microbial biomass N (Nmic) was determined by a colorimetric method using ninhydrin, after fumigation or not with chloroform (CHCl₃) [39]. Moreover, from the MR and Nmic values the metabolic quotient (qCO₂) was calculated.

2.3. Statistical Analysis

For each property of the soil treatments, within each kind of residues (flowers–leaves or shoots), two-way analysis of variance (ANOVA) (species × rate) was conducted. In all cases, means were compared using the protected least square difference (LSD) test, at p ≤ 0.05. Statistical analysis was carried out with IBM SPSS Statistics, version 29.

3. Results

3.1. Properties of the Original Soil and the Solid Residues from the Distillation of MAP

The original soil was loamy in texture (30% sand, 43% silt and 20% clay), calcareous (CaCO₃ 5.0 ± 0.0%) and alkaline to strongly alkaline in reaction (pH 8.3 ± 0.0) and had low salinity (EC 0.27 ± 0.01 dS m⁻¹) [27]. As far as the soil available macronutrient content is concerned, NO₃-N (58.1 ± 1.9 mg kg⁻¹) was high [40], P (9.5 ± 1.9 mg kg⁻¹) was marginally sufficient [41] and K (112 ± 12 mg kg⁻¹) was low to marginally sufficient [42]. In addition, soil available micronutrients Β (0.55 ± 0.06), Cu (2.06 ± 0.07) and Zn (0.75 ± 0.03) ranged at sufficiency levels, whereas Fe (13.7 ± 0.6) and Mn (15.8 ± 1.1) ranged at high levels (sufficiency levels B 0.1–2, Cu 0.1–2.5, Zn 0.2–2, Fe 2.5–5 and Mn 1–5 in mg kg⁻¹) [43]. All the aforementioned mean and standard deviation values are reported for the properties and composition of control in the respective figures and tables of the current study.

It is well known that the plant nutrient concentrations differ among soils and plants, because differences in soil composition and location, and in plant species, tissues and growth stage [7,44,45]. However, it seems that they are not influenced by the distillation process and method [7,26,45]. As a consequence of the above, nutrient concentrations of the solid residues from MAP distillation, used in the current study, differed among MAP, as well as, between their tissues (flowers–leaves and shoots).

The specific residues were analyzed in a previous study of ours, where they were tested as amendments for acid soils [28]. The results of that study have showed that the solid residues from the distillation of hemp, helichrysum, lavender, oregano, rosemary and sage contained considerable amounts of macro- and micronutrients essential for plant growth. Among all residues tested, those of hemp contained the significantly highest concentrations of plant nutrients. Moreover, the residues from flower–leaf distillation had the highest concentrations of most macro- and micronutrients compared with the respective residues of shoots. As far as the C/N ratio of all residues is concerned, it ranged from 15.8 to 28.0, in the residues from flowers–leaves distillation and from 48.2 to 74.5 in the residues from shoots distillation. Consequently, attention is needed in respect to the use of the specific residues, since soil application of organic amendments with C/N ratio above 25, at high rates, could induce soil N immobilization [46,47].

3.2. The Soil Chemical Properties After the Application of the Solid Residues from the Distillation of MAP

The effect of solid residues from the distillation of MAP on soil pH was rather inconclusive. Specifically, pH of almost all treatments with the 4 and 8% rates of residues significantly decreased compared with control, whereas pH of the treatments with the 1 and 2% rates of residues significantly increased or was similar to that of control (Figure 1). The maximum decrease was observed in the treatments with the 8% rate of residues from flower–leaf distillation of helichrysum, oregano and lavender and additionally in those with the 4% rate of residues from flower–leaf distillation of lavender (Figure 1a).

The EC of all treatments with the 4 and 8% addition rates of all solid residues from MAP distillation significantly increased compared with control, whereas the effect of the lower addition rates of the residues on soil salinity was rather inconclusive (Figure 2). Within each addition rate, the treatments with residues (flowers–leaves or shoots) from the distillation of lavender showed the significantly highest value, in almost all cases (Figure 2a). The obvious explanation for this is that lavender contains higher amounts of water-soluble constituents than the other plant species used in the current study. As far as the CEC is concerned, although it significantly increased in the treatments with the 8% rate of residues from flower–leaf distillation of helichrysum, rosemary and sage, and in all treatments with the residues (flowers–leaves or shoots) from the distillation of lavender, it ranged at levels similar to the initial value of soil CEC (Figure 3).

It is well known that the addition of organic fertilizers to soils enhances their organic C and total N content [48,49], as a result of the high initial concentrations of organic substances in the specific fertilizers. In this study, the organic C content of all solid residues ranged from 41.8 to 49.7% and the total N content ranged from 1.26 to 2.65% for the residues from flower–leaf distillation and from 0.64 to 0.97 % for the residues from shoots distillation [28]. Consequently, the organic C significantly increased in all treatments with all kinds of the residues compared with control, with the increase being proportional to the increase in the application rate (Figure 4). The total N significantly increased upon addition of the 4 and 8% residues from flower–leaf distillation of all plants compared with control and the same was evidenced for the residues from shoots distillation of lavender and rosemary (Figure 5). It is worth noting that within each addition rate, apart from the 1%, the treatments with the residues from flower–leaf distillation of hemp had the significantly highest total N content (Figure 5a). Similar increases are reported by other researchers, after application of manure mixed with wastes from distillation of mint (Mentha arvensis L.) [26].

Except for the hemp, the C/N ratio significantly increased in almost all treatments with the 4 and 8% rates of residues from flower–leaf distillation compared with control (Figure 6). In addition, the C/N ratio significantly increased in almost all treatments with the residues from shoots distillation relative to control (Figure 6). Within each addition rate, the treatments with residues from helichrysum distillation had the highest values in most cases (Figure 6). However, despite the observed increase, in almost all treatments the C/N ratio ranged at levels similar or a little higher than the common levels for typical inorganic arable soils (range 8–15, with mean value 12) [27]. Furthermore, it is reported that after 60 days of incorporation of plant residues of sage and spearmint, the C/N ratio decreased at levels similar to the initial soil, probably because of the total decomposition of plant residues [50].

3.3. Soil Fertility and Microbial Properties After the Application of the Solid Residues from the Distillation of MAP

Soil available NO₃-N concentration was initially high [40] and upon the addition of solid residues from MAP distillation, control showed the highest NO₃-N concentration among treatments, except for those with the 8% rate of residues from flower–leaf distillation of hemp or from shoots distillation of lavender (

Table A1. Concentrations of available macronutrients (mean ± SD) in the alkaline soil treatments with the solid residues (flowers–leaves) from the distillation of MAP.

Rate (%)	Species
	Hemp	Helichrysum	Lavender	Oregano	Rosemary	Sage
	ΝO3-Ν (mg kg−1)
0	58.1 ± 1.9 aB*	58.1 ± 1.9 aA	58.1 ± 1.9 aA	58.1 ± 1.9 aA	58.1 ± 1.9 aA	58.1 ± 1.9 aA
1	15.6 ± 0.4 aE	10.4 ± 0.9 bE	9.5 ± 0.9 bD	10.3 ± 0.6 bD	11.6 ± 0.7 bD	8.9 ± 0.8 bE
2	20.2 ± 1.0 aD	14.7 ± 0.8 bD	11.5 ± 2.5 bD	12.8 ± 1.3 bD	13.4 ± 0.8 bD	13.2 ± 0.5 bD
4	34.3 ± 0.7 aC	24.4 ± 1.6 cdC	27.1 ± 4.4 bcC	28.0 ± 0.5 bC	21.6 ± 1.8 dC	21.4 ± 1.4 dC
8	63.8 ± 3.5 aA	49.5 ± 6.2 bB	51.1 ± 4.4 bB	51.6 ± 2.6 bB	39.6 ± 1.4 cB	41.3 ± 1.2 cB
	p F-test
Rate	<0.001
Residue	<0.001
Interaction	<0.001
	ΝH4-Ν (mg kg−1)
0	11.0 ± 2.0 aC	11.1 ± 2.0 aA	11.0 ± 2.0 aA	11.0 ± 2.0 aA	11.0 ± 2.0 aA	11.0 ± 2.0 aA
1	9.7 ± 0.2 aC	9.3 ± 0.2 abB	8.4 ± 0.3 bB	8.2 ± 0.2 abB	8.1 ± 0.5 bB	8.2 ± 0.9 abB
2	10.4 ± 0.3 aC	8.9 ± 0.1 abB	8.1 ± 0.4 bB	8.2 ± 0.1 bB	7.5 ± 0.3 bBC	8.0 ± 0.2 bB
4	12.9 ± 0.7 aB	8.7 ± 0.3 bB	8.6 ± 0.7 bB	7.8 ± 0.5 bB	7.3 ± 0.2 bcBC	5.9 ± 0.4 cC
8	16.5 ± 0.5 aA	9.3 ± 0.5 bB	9.0 ± 0.1 bB	8.2 ± 0.2 bcB	6.5 ± 0.6 dC	6.9 ± 0.1 cdBC
	p F-test
Rate	<0.001
Residue	<0.001
Interaction	<0.001
	P (mg kg−1)
0	9.5 ± 1.9 aE	9.5 ± 1.9 aBC	9.5 ± 1.9 aC	9.5 ± 1.9 aD	9.5 ± 1.9 aC	9.5 ± 1.9 aC
1	23.6 ± 0.8 aD	9.1 ± 0.7 cC	7.5 ± 0.4 cC	12.6 ± 3.5 bC	8.4 ± 0.4 cC	8.5 ± 0.7 cC
2	39.8 ± 2.0 aC	11.1 ± 1.9 cBC	7.9 ± 0.3 dC	14.5 ± 1.0 bC	10.7 ± 1.0 cC	10.3 ± 0.3 cdC
4	71.7 ± 2.5 aB	11.9 ± 1.7 eB	12.5 ± 1.5 deB	26.6 ± 2.4 bB	16.5 ± 0.8 cB	14.8 ± 1.1 cdB
8	122 ± 0.6 aA	18.9 ± 0.2 dA	18.2 ± 2.0 dA	41.6 ± 2.6 bA	26.1 ± 0.4 cA	26.2 ± 1.8 cA
	p F-test
Rate	<0.001
Residue	<0.001
Interaction	<0.001
	K (mg kg−1)
0	112 ± 10 aE	112 ± 10 aE	112 ± 10 aE	112 ± 10 aE	112 ± 10 aE	112 ± 10 aE
1	163 ± 3 aD	136 ± 7 bD	163 ± 6 aD	175 ± 3 aD	175 ± 3 aD	155 ± 5 abD
2	267 ± 15 bcC	161 ± 3 eC	263 ± 6 cdC	313 ± 31 aC	287 ± 12 bC	243 ± 12 dC
4	397 ± 21 bB	243 ± 15 dB	413 ± 6 bB	470 ± 36 aB	453 ± 12 aB	337 ± 15 cB
8	720 ± 10 cA	373 ± 12 eA	857 ± 12 bA	887 ± 15 aA	840 ± 26 bA	557 ± 15 dA
	p F-test
Rate	<0.001
Residue	<0.001
Interaction	<0.001

* Different lower-case letters indicate significant differences among means within the same row, whereas different capital letters indicate significant differences among means within the same column, using the protected LSD test, at p ≤ 0.05.

and

Table A2. Concentrations of available macronutrients (mean ± SD) in the alkaline soil treatments with the solid residues (shoots) from the MAP distillation.

Rate (%)	Species
	Helichrysum	Lavender	Rosemary	Helichrysum	Lavender	Rosemary
	ΝO3-Ν (mg kg−1)			ΝH4-Ν (mg kg−1)
0	58.1 ± 1.9 aA*	58.1 ± 1.9 aB	58.1 ± 1.9 aA	11.0 ± 2.0 aA	11.0 ± 2.0 aA	11.0 ± 2.0 aA
1	10.1 ± 0.9 aD	10.1 ± 2.6 aD	10.0 ± 2.1 aC	8.3 ± 0.2 aB	8.7 ± 0.3 aB	9.0 ± 0.3 aB
2	15.7 ± 0.0 aC	13.6 ± 0.7 aD	9.4 ± 0.6 bC	7.9 ± 0.5 aB	7.7 ± 0.1 aB	8.1 ± 0.7 aB
4	16.0 ± 0.6 bC	41.9 ± 5.8 aC	12.3 ± 0.4 cC	7.0 ± 0.1 aB	7.4 ± 0.2 aB	7.9 ± 0.3 aB
8	32.1 ± 1.7 bB	113 ± 4 aA	17.8 ± 1.2 cB	7.2 ± 0.7 aB	7.4 ± 0.1 aB	7.5 ± 0.4 aB
	p F-test			p F-test
Rate	<0.001			<0.001
Residue	<0.001			NS #
Interaction	<0.001			NS
	P (mg kg−1)			K (mg kg−1)
0	9.5 ± 1.9 aB	9.5 ± 1.9 aD	9.5 ± 1.9 aCD	112 ± 10 aC	112 ± 10 aE	112 ± 10 aE
1	8.9 ± 1.4 aB	10.0 ± 1.8 aD	8.7 ± 0.5 aD	129 ± 7 bBC	185 ± 4 aD	169 ± 2 aD
2	7.8 ± 0.1 bB	12.3 ± 0.7 aC	10.6 ± 0.9 aC	140 ± 8 bBC	303 ± 15 aC	277 ± 6 aC
4	8.7 ± 0.5 bB	15.3 ± 0.8 aB	15.1 ± 0.3 aB	163 ± 1 cB	523 ± 15 aB	410 ± 20 bB
8	14.1 ± 1.0 cA	32.3 ± 0.3 aA	25.6 ± 0.6 bA	280 ± 0 cA	1077 ± 55 aA	707 ± 64 bA
	p F-test			p F-test
Rate	<0.001			<0.001
Residue	<0.001			<0.001
Interaction	<0.001			<0.001

* Different lower-case letters indicate significant differences among means within the same row, whereas different capital letters indicate significant differences among means within the same column, using the protected LSD test, at p ≤ 0.05. ^# Non-significant.

). Surprisingly, in all other treatments NO₃-N concentration significantly decreased, upon application of all kinds of the residues compared with control (Table A1 and Table A2). This resulted in a decrease in the NO₃-N concentrations from the initial high levels (control) to levels that could be characterized moderate to sufficient (most of the treatments with residues) [40]. Moreover, similar results were obtained for NH₄-N (Table A1 and Table A2).

Soil available P significantly increased in almost all treatments with the 4 and 8% rates of residues (Table A1 and Table A2). In addition, upon the 8% addition rate of all residues to soil, P increased from marginally sufficient (control) to high or very high levels [41], apart from the treatments with residues from shoots distillation of helichrysum (Table A1 and Table A2). Soil available K concentration significantly increased in all soil treatments with the residues compared with control (Table A1 and Table A2). Especially from the addition rate of 2% and above, K increased from marginally sufficient (control) to high levels (treatments with residues) [42]. In addition, within the same addition rate, soil treatments with the residues from the distillation of helichrysum showed the lowest K content (Table A1 and Table A2), which is consistent with the low concentration of K in the specific distillation residues [28].

As far as MR is concerned, it increased in almost all treatments with the 4 and 8% rates of residues from flower-leaves distillation in comparison with control, except for treatments with the 4% rate of residues from the distillation of helichysum or sage (Figure 7a). However, no significant differences were observed in all treatments with the residues from shoots distillation, except for the treatment with the 8% rate of residues from helichrysum distillation (Figure 7b). The Nmic increased in all soil treatments with the residues from flower–leaf distillation of hemp and lavender (Figure 8). In addition, within each addition rate, the specific residues showed the significantly highest values (Figure 8), whereas the treatments with residues from the distillation of rosemary and sage had the lowest values (Figure 8). In contrast, within each residue, in the treatments with residues from the distillation of sage and rosemary the Nmic significantly decreased in the 8% and 4 and 8% rates, respectively (Figure 8) compared with control. Furthermore, the qCO₂ was not affected by treatment.

As far as the soil available micronutrients are concerned, B significantly increased in all treatments with the 4 and 8% addition rates of the distillation residues (

Table A3. Concentrations of available micronutrients (mean ± SD) in the alkaline soil treatments with the solid residues (flowers–leaves) from the distillation of MAP.

Rate (%)	Species
	Hemp	Helichrysum	Lavender	Oregano	Rosemary	Sage
	B (mg kg−1)
0	0.55 ± 0.06 aE*	0.55 ± 0.06 aD	0.55 ± 0.06 aD	0.55 ± 0.06 aD	0.55 ± 0.06 aC	0.55 ± 0.06 aC
1	0.87 ± 0.04 aD	0.75 ± 0.13 abC	0.71 ± 0.02 bC	0.65 ± 0.08 bcD	0.55 ± 0.01 cC	0.65 ± 0.02 bcC
2	1.15 ± 0.10 aC	0.79 ± 0.05 bcBC	0.77 ± 0.04 cC	0.93 ± 0.04 bC	0.68 ± 0.05 cBC	0.69 ± 0.06 cBC
4	1.59 ± 0.10 aB	0.91 ± 0.04 cB	1.12 ± 0.14 bB	1.15 ± 0.10 bB	0.89 ± 0.10 cA	0.89 ± 0.08 cA
8	2.34 ± 0.07 aA	1.28 ± 0.14 cA	1.70 ± 0.26 bA	1.34 ± 0.22 cA	0.80 ± 0.07 dAB	0.84 ± 0.02 dAB
	p F-test
Rate	<0.001
Residue	<0.001
Interaction	<0.001
	Cu (mg kg−1)
0	2.06 ± 0.07 aC	2.06 ± 0.07 aA	2.06 ± 0.07 aD	2.06 ± 0.07 aB	2.06 ± 0.07 aA	2.06 ± 0.07 aAB
1	2.06 ± 0.04 bC	1.93 ± 0.03 cC	2.53 ± 0.10 aC	2.16 ± 0.11 bB	2.11 ± 0.06 bA	2.11 ± 0.05 bA
2	2.14 ± 0.04 cBC	2.04 ± 0.05 cAB	2.63 ± 0.06 aBC	2.32 ± 0.32 bA	2.06 ± 0.06 cA	2.09 ± 0.03 cA
4	2.21 ± 0.02 bAB	2.03 ± 0.06 cdAB	2.91 ± 0.01 aA	2.10 ± 0.03 bcB	1.93 ± 0.03 dB	2.06 ± 0.07 cAB
8	2.30 ± 0.06 bA	1.99 ± 0.05 cdAB	2.71 ± 0.06 aB	2.08 ± 0.08 cB	1.90 ± 0.05 dB	1.96 ± 0.04 cdB
	p F-test
Rate	<0.001
Residue	<0.001
Interaction	<0.001
	Zn (mg kg−1)
0	0.75 ± 0.03 aE	0.75 ± 0.03 aC	0.75 ± 0.03 aE	0.75 ± 0.03 aE	0.75 ± 0.03 aE	0.75 ± 0.03 aE
1	0.87 ± 0.02 bD	0.68 ± 0.03 eD	0.80 ± 0.04 dD	0.82 ± 0.04 cdD	0.86 ± 0.02 bcD	0.93 ± 0.02 aD
2	1.03 ± 0.03 bC	0.77 ± 0.00 eC	0.88 ± 0.02 dC	0.91 ± 0.02 cdC	0.93 ± 0.01 cC	1.09 ± 0.02 aC
4	1.30 ± 0.07 aB	0.90 ± 0.01 eB	0.97 ± 0.02 dB	1.11 ± 0.02 bB	1.02 ± 0.02 cB	1.31 ± 0.02 aB
8	1.72 ± 0.03 aA	1.12 ± 0.04 dA	1.07 ± 0.00 eA	1.41 ± 0.03 bA	1.22 ± 0.01 cA	1.75 ± 0.03 aA
	p F-test
Rate	<0.001
Residue	<0.001
Interaction	<0.001
	Fe (mg kg−1)
0	13.7 ± 0.6 aA	13.7 ± 0.6 aA	13.7 ± 0.6 aC	13.7 ± 0.6 aAB	13.7 ± 0.6 aB	13.7 ± 0.6 aC
1	10.7 ± 0.3 bB	12.1 ± 0.9 bB	15.2 ± 0.4 aB	14.3 ± 1.2 aAB	15.3 ± 1.4 aA	15.6 ± 0.6 aAB
2	10.7 ± 0.4 dB	13.7 ± 1.5 cA	15.8 ± 0.4 abB	15.1 ± 0.8 bcA	15.5 ± 0.5 abA	16.7 ± 0.7 aA
4	9.9 ± 0.8 dB	13.7 ± 1.6 cA	17.3 ± 0.8 aA	9.9 ± 0.0 dC	15.2 ± 0.9 bA	15.0 ± 0.7 bcB
8	10.0 ± 1.3 dB	12.0 ± 2.4 bcB	17.7 ± 1.0 aA	13.1 ± 0.1 bB	10.8 ± 1.3 cdC	10.2 ± 0.7 dD
	p F-test
Rate	<0.001
Residue	<0.001
Interaction	<0.001
	Mn (mg kg−1)
0	15.8 ± 1.1 aD	15.8 ± 1.1 aD	15.8 ± 1.1 aD	15.8 ± 1.1 aE	15.8 ± 1.1 aE	15.8 ± 1.1 aD
1	35.7 ± 2.6 cCD	38.2 ± 2.0 cCD	45.9 ± 3.8 cCD	300 ± 7.3 aD	96 ± 1.6 bD	47 ± 5.6 cD
2	51.4 ± 7.2 cC	67.0 ± 5.8 cC	60.9 ± 1.5 cC	407 ± 1.0 aC	207 ± 15 bC	183 ± 15.5 bC
4	85.0 ± 15.7 eB	193 ± 14 dB	209 ± 23 dB	463 ± 33 aB	370 ± 61 bB	331 ± 37.2 cB
8	154 ± 5.2 eA	376 ± 29 dA	413 ± 23 cA	546 ± 40 aA	426 ± 18 bcA	455 ± 31.5 bA
	p F-test
Rate	<0.001
Residue	<0.001
Interaction	<0.001

* Different lower-case letters indicate significant differences among means within the same row, whereas different capital letters indicate significant differences among means within the same column, using the protected LSD test, at p ≤ 0.05.

and

Table A4. Concentrations of micronutrients (mean ± SD) in the alkaline soil treatments with the solid residues (shoots) from the distillation of MAP.

Rate (%)	Species
	Helichrysum		Lavender		Rosemary
	B (mg kg−1)
0	0.55 ± 0.06 aD*		0.55 ± 0.06 aD		0.55 ± 0.06 aC
1	0.67 ± 0.01 aC		0.59 ± 0.07 abCD		0.54 ± 0.05 bC
2	0.71 ± 0.03 aC		0.67 ± 0.04 aC		0.61 ± 0.11 aBC
4	0.90 ± 0.04 aB		0.94 ± 0.10 aB		0.68 ± 0.11 bB
8	1.22 ± 0.04 aA		1.09 ± 0.12 bA		0.85 ± 0.05 cA
	p F-test
Rate	<0.001
Residue	<0.001
Interaction	0.004
	Species
	Helichrysum	Lavender	Rosemary	Helichrysum	Lavender	Rosemary
	Cu (mg kg−1)			Zn (mg kg−1)
0	2.06 ± 0.07 aA	2.06 ± 0.07 aB	2.06 ± 0.07 aB	0.75 ± 0.03 aC	0.75 ± 0.03 aC	0.75 ± 0.03 aE
1	2.05 ± 0.00 bA	2.35 ± 0.14 aA	2.14 ± 0.06 bAB	0.75 ± 0.04 bC	0.80 ± 0.03 bBC	0.87 ± 0.06 aD
2	2.08 ± 0.09 bA	2.31 ± 0.25 aA	2.16 ± 0.04 abAB	0.77 ± 0.01 bC	0.80 ± 0.06 bBC	0.93 ± 0.01 aC
4	2.08 ± 0.03 aA	2.12 ± 0.10 aB	2.21 ± 0.07 aAB	0.84 ± 0.02 bB	0.84 ± 0.03 bB	1.06 ± 0.03 aB
8	2.06 ± 0.06 bA	2.13 ± 0.10 abB	2.22 ± 0.05 aA	0.99 ± 0.06 bA	0.94 ± 0.00 bA	1.29 ± 0.04 aA
	p F-test			p F-test
Rate	NS #			<0.001
Residue	0.004			<0.001
Interaction	NS			<0.001
	Fe (mg kg−1)			Mn (mg kg−1)
0	13.7 ± 0.6 aAB	13.7 ± 0.6 aB	13.7 ± 0.6 aC	15.8 ± 1.1 aD	15.8 ± 1.1 aD	15.8 ± 1.1 aD
1	12.7 ± 0.8 bB	14.0 ± 0.7 abB	14.3 ± 0.8 aBC	34.1 ± 0.9 aD	31.7 ± 1.7 aCD	25.9 ± 0.3 aD
2	14.1 ± 1.1 bA	16.0 ± 1.2 aA	15.3 ± 0.7 abAB	78.6 ± 15.3 aC	62.0 ± 6.2 aC	80.2 ± 1.6 aC
4	14.4 ± 0.7 bA	14.9 ± 0.5 abAB	15.9 ± 0.6 aA	175 ± 35 bB	259 ± 36 aB	123 ± 5 cB
8	14.9 ± 1.6 abA	14.1 ± 0.3 bB	16.2 ± 0.9 aA	340 ± 40 bA	389 ± 35 aA	189 ± 21 cA
	p F-test			p F-test
Rate	<0.001			<0.001
Residue	0.004			<0.001
Interaction	NS			<0.001

* Different lower-case letters indicate significant differences among means within the same row, whereas different capital letters indicate significant differences among means within the same column, using the protected LSD test, at p ≤ 0.05. ^# Non-significant.

). Results of Cu were rather inconclusive, whereas Zn significantly increased upon soil application of all residues at all rates (Table A3 and Table A4). Moreover, the aforementioned three micronutrients ranged at sufficiency levels in all treatments [43]. Furthermore, results of Fe were rather inconclusive, whereas Mn significantly increased upon the addition of all residues at the 2% application rate and afterwards compared with control (Table A3 and Table A4). In all treatments, Fe ranged at high levels, whereas Mn was very high in almost all treatments with residues, especially those with the 4 and 8% application rates of residues [43]. Regarding Mn, it is worth mentioning that despite the alkaline to strongly alkaline pH of the soil used, Mn increased from two to 35 times in all treatments with residues compared to control (Table A3 and Table A4).

4. Discussion

4.1. Effect of the Solid Residues from the Distillation of MAP on the Soil Chemical Properties

pH is considered as one of the most important chemical property of soils, as it adjusts the availability of macro- and micronutrients. It is reported [48] that organic materials were used for buffering soil pH and it is a common practice their use for pH increase of acid soils. As far as pH of alkaline soils is concerned, pH decrease of an alkaline soil is reported, after application of enriched compost with waste biomass from the distillation of palmarosa [Cymbopogon martini (Roxb.) Wats.] [49]. On the other hand, pH of an alkaline soil was not affected by the incorporation of plant tissues of spearmint (Mentha spicata L.), sage (Salvia fruticosa Mill.) and basil (Ocimum basilicum L.) [50,51]. In agreement with the findings of the current study, inconclusive findings about soil pH are also reported by other researchers in the literature.

Commonly, soil incorporation of composts and other organic residues is expected to increase soil salinity at risky levels, possibly, because the specific materials contain considerable amounts of salts [48,49]. In the current study, although soil EC increased with the addition of solid residues, it did not exceed the double value of the initial EC upon the 8% addition rate of the residues. Consequently, it seems that no apparent risk of soil salinization after application of the particular solid residues from MAP distillation is expected, probably because the soil used contained considerable amount of clay (20%) and had relatively high CEC (Figure 3) [27]. However, the almost doubling of EC after the residues application at the 8% rate is somewhat concerning in the perspective of repeated applications. Moreover, the risk of salinization could be apparent for alkaline soils with low clay content and low to moderate CEC. Based on the above, the 8% addition rate should be avoided.

4.2. Effect of the Solid Residues from the Distillation of MAP on Soil Fertility and Microbial Properties

The observed increase in P and K, upon the addition of solid residues from MAP distillation to soil compared with control, was somewhat expected and it was attributed to the decomposition of the residues’ organic matter [13,14]. The unexpected was the observed decrease in both forms of soil available N, i.e., NO₃-N and NH₄-N, in comparison with control, because N mineralization and nitrification were expected instead of N immobilization. This means that a part of the initial soil available N, considerable in the case of NO₃-N, was used probably by the soil microorganisms. The increase in Nmic in the treatments with the residues from the distillation of hemp, helichrysum and lavender maybe is an indication that a certain part of N was immobilized into the microbial biomass. However, the C/N ratio values obtained upon soil addition of the residues were low to justify growth of soil microbial community adequate to cause N immobilization. On the other hand, the Nmic reduction and MR increase in the treatments with the residues from the distillation of sage and rosemary indicated a limited factor, even though not significant differences in qCO₂ among treatments were observed. Anyway, apart from N immobilization, also the suppression of N mineralization and nitrification in the treatments with residues cannot be excluded.

The aforementioned conclusion regarding N results of the present study is supported by other researchers’ findings. It is well known that the solid residues from MAP distillation contain bioactive compounds, like phenolics and flavonoids [6]. In addition, the phenolic compounds have been shown to be inhibitory to the digestion of cell walls, carbohydrates and proteins by soil microorganisms [3]. It is reported that the application of grape pomace in soil was associated with N mineralization increase and ammonium accumulation, but inhibited nitrification due to its phenolic compounds [52]. Furthermore, reduced microbial communities in an alkaline soil are reported after application of basil dry matter, which was attributed to the high content of surface phenolics in plant tissues [51]. However, decrease in available N after application of manure mixed with waste from distillation of mint was noticed and it was attributed to N immobilization than to suppression of soil microbial activity [26].

Regarding soil available micronutrients, it is worth mentioning that the excessive increase in Mn is somewhat concerning in respect to the risk of Mn phytotoxicity. Furthermore, Mn toxicity affected N mineralization by reducing soil nitrification and suppressing mineralization in an acid soil [53]. The high available concentrations of Mn, especially in soil treatments with residues of oregano, sage and rosemary, in combination with the presence of phenolic compounds [20] in the residues could also influence N mineralization and Nmic.

Several researchers reported similar findings to the aforementioned results of the current study for macro- and micronutrients. Specifically, the application of basil tissues to soil increased the availability of macronutrients (N, P and K) and micronutrients (Zn, Cu and Mn) [51]. In addition, soil application of Chinese medicinal herbal compost residues increased the available N and P [48]. Moreover, increased available nutrients are reported upon soil addition of solid residues from distillation of oregano (Origanum dubium Boiss.) and sideritis (Sideritis cypria Post) [21]. Furthermore, composted solid residues from distillation of sage, rosemary and basil mixed with commercial compost produced a suitable fertilizer supplement [5].

Although the solid residues from the distillation of MAP, alone or combined with other materials, can enrich biochar, vermicompost, compost and various substrates with macro- and micronutrients, the high application rates of these residues could cause adverse effects on plant growth and soil or substrate properties [8,21,49,54]. For example, a rate-dependent negative effect of extracts of compost derived from solid residues from distillation of basil and commercial composts on the seed germination and root elongation of cress (Lepidium Sativum L.) is reported [5]. In addition, a rate-dependent effect of Chinese medicinal herbal compost on the growth of tomato (Lycopersicon esculentum Mill.) and Chinese cabbage (Brassica rapa subsp. Chinensis L.) is reported [48], with the 5% being the optimum application rate. Limited phytotoxicity effects of solid residues from distillation of rosemary on lettuce (Lactuca sativa L.), tomato and perennial ryegrass (Lolium perenne L.) are also reported [55]. Furthermore, although the effects of residual essential oils content of the residues on soil microbial community were not investigated in the current study, it is worthy to be studied, since limited information is available.

5. Conclusions

According to the findings of the current study, the solid residues from the distillation of hemp, helichrysum, lavender, oregano, rosemary and sage contain considerable amounts of macro- and micronutrients essential for plant growth, which can be released in plant available forms (except N?) after mixing them with alkaline (and calcareous) soils. Regarding N, the decrease in the initial soil available concentrations of both N forms (NO₃-N and NH₄-N) upon residues application is very concerning, since N is the most important essential nutrient for plant growth and soil productivity, Thus, the N mineralization-nitrification-immobilization issue needs further clarification and thorough investigation. From the different application rates tested, it seems that all kind of residues, i.e., from distillation of flowers–leaves or shoots, can improve soil fertility when they are applied in soil at the 2% rate and above. However, caution is needed at high application rates of the residues, in respect to possible soil salinization and Mn phytotoxicity risks, since EC was doubled upon the application of 8% residues and soil available Mn excessively increased. Moreover, application rates above 4% are not practical regarding the proper handling of the particular residues in the field.

Nevertheless, this work provides the first step to the potential management of solid residues from the distillation of MAP as amendments for alkaline (and calcareous) soils, commonly found in Mediterranean countries, converting them into value-added materials for sustainable environment. However, for the establishment of such a perspective, further research is needed in respect to the effect of the residues on soil chemical properties, fertility and microbial community and on plant growth, by means of at least pot experiments. Moreover, the effects of the residual essential oils and phenolic compounds, contained in the solid residues from MAP distillation, on the aforementioned properties should be thoroughly investigated, since there is lack of information about these issues.