Use of Amino Acids and Organic Waste Extracts to Improve the Quality of Liquid Nitrogen–Calcium–Magnesium Fertilizers

Abstract

Agriculture is one of the most important sectors of the global economy, but it increasingly faces sustainability challenges in meeting rising food demands. The intensive use of mineral fertilizers not only improves yields, but also causes negative environmental impacts such as increasing greenhouse gas emissions, water eutrophication, and soil degradation. To develop more sustainable solutions, the focus is on organic fertilizers, which are produced using waste and biostimulants such as amino acids. The aim of this study was to develop and characterize liquid nitrogen–calcium–magnesium fertilizers produced by decomposing dolomite with nitric acid followed by further processing and to enrich them with a powdered amino acid concentrate Naturamin-WSP and liquid extracts from digestate, a by-product of biogas production. Nutrient-rich extracts were obtained using water and potassium hydroxide solutions, with the latter proving more effective by yielding a higher organic carbon content (4495 ± 0.52 mg/L) and humic substances, which can improve soil structure. The produced fertilizers demonstrated favourable physical properties, including appropriate viscosity and density, as well as low crystallization temperatures (eutectic points from –3 to –34 °C), which are essential for storage and application in cold climates. These properties were achieved by adjusting the content of nitrogenous compounds and bioactive extracts. The results of the study show that liquid fertilizers enriched with organic matter can be an effective and more environmentally friendly alternative to mineral fertilizers, contributing to the development of the circular economy and sustainable agriculture.

1. Introduction

One of the most important and consistent sectors of the world economy is agriculture [1,2]. The extremely high nutritional needs of the world’s population have increased global cereal production up to 70%, which is necessary to avoid poverty and to ensure adequate nutrition for a rapidly growing population [3]. Due to intensive agriculture and rapid soil degradation, fertilization is a necessary step in creating the right conditions for plant growth and development and for producing quality crops [4]. Nitrogen fertilizers and their impact on agriculture are a very important issue, both for food production—since the world population is projected to reach 9.7 billion people by 2050—and for environmental concerns. Nitrogen, one of the three main elements required by plants, is an essential nutrient, as it is crucial for protein production and plant growth. However, the use of nitrogen fertilizers has both positive and negative consequences: while it improves crop yields and quality, its overuse leads to water and soil pollution and poses risks to human health [5,6,7,8,9,10]. In this context, it is necessary to strike a balance between the use of nitrogen fertilizers and environmental protection to ensure the long-term sustainability of agriculture. A balanced use of environmentally friendly nitrogen fertilizers can contribute to improved environmental quality and a reduction in greenhouse gas emissions [11,12,13,14]. The uncontrolled use of mineral fertilizers degrades the quality of land, with leaching losses of up to 75% due to their extremely high solubility, which means that mineral fertilizers are minimally helpful for plant growth [15]. Calcium and magnesium oxides are vital secondary nutrients that contribute significantly to plant growth and development. They support various physiological processes, including cell wall formation, enzyme activation, and nutrient uptake efficiency [16,17].

Amino acids (biostimulants)—which are microorganisms of inorganic or organic origin used in small quantities—can be a solution to reduce the negative impact of mineral fertilizers and improve soil health. Amino acids used in mineral fertilizers act as building blocks of proteins that provide the plant with organic nitrogen and promote cell growth. They are particularly important for plant growth and development, especially when used as foliar fertilizers [18,19,20].

Currently, great attention is paid not only to organic fertilizers, but also to the reuse of waste from other industries (especially food, bioenergy, and agriculture). There is a lot of information in the literature about their production, and compost, municipal, or agricultural waste can be an excellent raw material for the production of organic fertilizers [12,13,14]. There is an increasing need to partly replace mineral fertilizers with organic ones, the advantage of which is that their use improves the organic carbon content of the soil, as well as leads to a slower release of nutrients necessary for the plant, which does not harm plant growth [21]. However, it is very important to note that such raw materials may contain pathogens and toxic elements, so all risks must be taken into account [14,22]. For this reason, waste cannot be directly applied to the soil.

Quite a large amount of organic carbon and nutrients can be extracted from waste by solid–liquid extraction [23,24]. Humic substances, also known as organic carbon, are extracted by a variety of methods, the choice of which depends on the nature of the raw material, the quality of the product to be obtained, and its cost-effectiveness. During the extraction, water or alkaline substances can be used as a solvent, and the extracts obtained in this way are considered biostimulants. This improves soil quality and compensates for nutrient deficiencies [25]. Extraction using lye produces extracts that contain significantly higher levels of organic carbon and nutrients, as the solubility of the waste material used is increased in the alkaline medium. Potassium or sodium alkalis (KOH or NaOH) can be used as alkaline solvents at different concentrations (0.05–2 M), but potassium alkaline is a better agronomic choice due to the addition of potassium [26,27,28]. One important aspect that is highlighted when comparing alkali with water is that the use of alkali as a solvent produces humic substances that contribute to soil improvement and water retention, as well as reduce the risk of plant diseases [14,29]. In general, these humic substances’ alkaline extraction method is a simple, widely used and relatively efficient technology. More sophisticated methods such as resin columns or biosynthesis with micro-organisms are sometimes used to obtain higher-quality extracts. Alternative technologies such as hydrothermal treatment or the use of chelating agents are also developing rapidly. While more advanced methods may provide better-quality extracts or a more sustainable production process, they often require higher investments in equipment, energy, or raw materials. Therefore, from an economic point of view, alkaline extraction remains one of the most attractive alternatives, especially if there is the possibility of the efficient management of spent reagents and wastes. However, in order to achieve more sustainable agriculture and environmental goals, there is an increasing focus on innovative, bio-based production methods that may become not only environmentally but also economically attractive in the future [30,31].

The use of amino acids as biostimulants is an important step in the development and use of organic fertilizers in agriculture, but even more important is the reuse of the inherent waste when incorporated in the production of liquid fertilizers in the circular economy. The aim of these liquid organic fertilizers is to compete with the mineral fertilizers available on the market, as the overuse of these fertilizers is increasingly causing serious problems for the environment, for plant growth and yield, and even for humankind. The scientific literature provides data that the –OH group of the humic phenol of natural and synthetic humic acids was involved in the formation of a stable Cd²⁺–humic complex. The hydrophobic part of the humic acid can form a cage-like conformation around the Cd²⁺ ion, providing the desired stability of the Cd²⁺–humic complex [32]. This explains the positive effect of humic substances on soil contaminated with heavy metals, but the influence of humic acids on the properties and stability of liquid fertilizers has not been studied or described. Therefore, this article focuses on the production of liquid nitrogen–calcium–magnesium fertilizers by decomposing dolomite with nitric acid followed by further processing and on improving their properties by enriching them with organic liquid bioactive substances, including a powdered amino acid concentrate and liquid organic extracts derived from digestate.

2. Materials and Methods

2.1. The Production of Liquid Nitrogen–Calcium–Magnesium Fertilizers

For the production of liquid nitrogen–calcium–magnesium fertilizers, uncalcined dolomite (crushed to <0.25 mm) was used as the source of calcium and magnesium. The dolomite was obtained from the Petrašiūnai, Lithuania, quarry and supplied by AB “Dolomitas”. This carbonate-class mineral consists of calcium and magnesium carbonates. Nitric acid (HNO₃, chemically pure, 65% concentration, “Achema”, Jonava, Lithuania) was used for dolomite decomposition, and ammonia water (NH₄OH, chemically pure, 25% concentration, “Achema”, Jonava, Lithuania) was used to neutralize the resulting solution. Additional nitrogen sources included urea (CO(NH₂)₂, chemically pure, “Achema”, Jonava, Lithuania) and ammonium nitrate (NH₄NO₃, chemically pure, “Achema”, Jonava, Lithuania).

For the decomposition of dolomite with nitric acid, 160 g of dolomite and 490 cm³ of 45% nitric acid (which is 140% of the stoichiometric amount) were used. The decomposition process was carried out in a fume hood and lasted approximately 50 min. The maximum reaction temperature did not exceed 50 °C. The resulting dolomite decomposition solution was filtered and neutralized to different pH values (6.1 and 7.3). These pH values were chosen to be close to neutral so that the produced fertilizer would not change the pH of the soil medium. Neutralization was performed using 25% ammonia water. The amount of ammonia water added depended on the target pH: 147.2 cm³ when the pH was 6.1 and 152.9 cm³ when the pH was 7.3. The neutralization process lasted about 30 min. To minimize ammonia loss during neutralization, ammonia water was introduced through a tube to the bottom of the reaction mixture, added very slowly, and continuously stirred. Subsequently, different amounts (from 0 to 40%) of nitrogen-containing components (urea and ammonium nitrate) were added to the neutralized dolomite decomposition solutions, and the crystallization temperatures of these solutions were investigated.

During the production of liquid nitrogen–calcium–magnesium fertilizer, the reagents were used in the following sequence: dolomite → 45% nitric acid → 25% ammonia water → urea → ammonium nitrate.

2.2. Bioactive Substances Used in the Liquid Fertilizers

Two types of bioactive substances were additionally added to the nitrogen–calcium–magnesium fertilizers: powdered amino acid concentrate (AAC) Naturamin-WSP (total organic carbon concentration—80%, importer—“Daymsa”, Zaragoza, Spain) and a liquid organic matter concentrate produced in our laboratory from digestate.

The organic matter extracts were produced in order to reuse waste generated during biogas production. During the production of the extracts, UAB “Kurana” (KP), dried at 60 °C, whole digestate (WD) from 3 to 5 mm fraction, and crushed digestate (CD) to <2.0 mm fraction were used. The concentrations of water/acid-soluble plant nutrients in the digestate were as follows: 10.78/2.69% N, 1.60/4.44% P₂O₅, 0.76/0.88% K₂O, 0.09/3.12% CaO, 0.25/6.01% MgO, and 59.49% C. A universal dry material crusher (Blender 80111S, Waring, Torrington, USA) was used to grind the raw materials. The resulting raw material fractions were separated according to particle size using RETSCH (Retsch, GmbH, Haan, Germany) woven wire sieves with mesh sizes of <0.25, 0.5, 1.0, 2.0, 3.15, 5.0, 7.0, and >10.0 mm [33]. Based on previous research [34], two optimized extraction methods were selected for preparing digestate extracts. In the first method, CD was extracted in an ultrasonic bath at 50 °C for 9 h using distilled water as the solvent. In the second method, WD was treated using a magnetic stirrer at 90 °C for 6 h with 0.5 M potassium hydroxide as the solvent.

2.3. Analytical Methods for Determining Plant Nutrient Concentrations

The nitrogen concentrations (N) were determined using the Kjeldahl method. Samples were mineralized with concentrated H₂SO₄ in a Turbosog TUR/TVK mineralizer (TUR/TVK, Gerhardt, Germany), and the resulting solutions were analysed using an automatic Gerhardt VAPODEST 45s system (VAP 45s, Gerhardt, Germany) with an accuracy of 0.5%. The VAPODEST 45s is a programmable distillation system specifically designed for nitrogen and Kjeldahl analysis, capable of storing up to 20 programs [35].

The concentrations of calcium and magnesium oxides were determined by complexometric titration (Trilone B) using different indicators (calconcarboxylic acid and dark blue chromogen) [36].

The organic matter content in the digestate was determined in accordance with LST EN 13039:2012 “Soil improvers and growing media—Determination of organic matter content and ash” [37]. Total organic carbon (TOC) concentration was measured using two methods: the chemical bichromate method for raw materials and a Shimadzu TOC-L analyser (Japan) for extracts. Organic carbon content in the dried raw materials was analysed with a T70/T80 UV-VIS spectrophotometer (PG instruments LTD, Leicestershire, England) at 590 nm. The concentration of organic carbon in the extracts was determined using a TOC analyser (TOC-L, Shimadzu Corporation, Kyoto, Japan) following the LST EN 1484:2002 (EN 1484:1997) standard [38].

Trace elements (iron, manganese, copper, chromium, and zinc) and heavy metals (cobalt, nickel, lead, and cadmium) in the liquid fertilizers were analysed using atomic absorption spectroscopy (AAS). These measurements were conducted on a Perkin Elmer AANALYST 400 spectroscope (Perkin Elmer, Waltham, MA, USA) with an accuracy of ±0.01 mg·cm⁻³.

2.4. Physical Methods for Determining Properties of Liquid Fertilizers

The following methods were used to determine the physical properties of the intermediate solutions and the final products: viscosity was measured using a glass capillary viscometer (Ø 1.2 mm), density was determined using aerometers, and pH was measured using a HANNA 211 calibrated pH meter (Woonsocket, RI, USA) [35].

The solubility of solid nitrogen-containing substances (urea and ammonium nitrate) in a liquid multicomponent system obtained by dolomite decomposition with HNO₃ and subsequent neutralization with NH₄OH, as well as the crystallization temperature of these systems, was determined using the polythermal method. The same method was applied to study the influence of bioactive substances (“Naturamin-WSP” and organic matter extracts) on the crystallization temperature of liquid nitrogen–calcium–magnesium fertilizers. Using a mixture of dry ice and ethanol as a cooling agent, the temperature at which the first crystal appeared and the temperature at which the last crystal melted were measured and fixed in liquid multicomponent systems. The result of the crystallisation temperature was the arithmetic mean of five replicate measurements [39].

2.5. Instrumental Analysis Methods for Determining Properties of Liquid Fertilizers

X-ray diffraction analysis (XRD) was conducted with a BRUKER AXS D8 ADVANCE diffractometer (Bremen, Germany), employing CuKα radiation with a nickel (Ni) filter. The detector movement step size was 0.02°, with an intensity measurement time of 0.2 s per step. The system operated at an anodic voltage of 40 kV and a current of 40 mA. This method identifies compounds based on diffraction peaks, compared against reference radiographs in the SearMach database [40].

Scanning electron microscopy with energy-dispersive spectroscopy (SEM-EDS) was conducted using an FEI Quanta 200 FEG microscope with a Schottky electron gun. This system is equipped with a BRUKER (“Hitachi”, Tokyo, Japan) XFLASH 4030 X-ray energy-dispersive spectrometer, enabling simultaneous chemical microanalysis. The microscope’s working distance ranged from 2 to 10 mm, and the energy resolution (Kα) was up to 133 eV, provided by a silicon drift detector [41].

2.6. Statistical Analysis

To ensure the reliability of the results and to minimise random errors, the analysis of the dry crude digestate and its extracts was carried out several times. The results are presented as the arithmetic means of at least three independent measurements and the corresponding standard deviations (±SD). All statistical analyses were performed at the 95% confidence level and the significance threshold was set at p ≤ 0.05. Differences between group means were assessed using one-way analysis of variance (ANOVA).

3. Results and Discussion

3.1. Decomposition of Dolomite by Nitric Acid

For the production of liquid fertilisers, it is important to choose process conditions that achieve the highest possible concentration of plant nutrients in the solution. In order to achieve a higher degree of decomposition in the dolomite and the highest possible concentration of calcium and magnesium, it is necessary to use calcined dolomite. However, dolomite is usually calcined at a high temperature (1050 °C), which is very energy-intensive. For this reason, and after considering the results of previous studies [42], uncalcined dolomite was chosen and decomposed with 45% nitric acid. The decomposition time was 50 min. The amount of nitric acid required was calculated according to the stoichiometry, but dolomite contains not only calcium and magnesium carbonates, but also impurities, therefore a higher amount (140%) of nitric acid was used for the dolomite’s decomposition. The chemical composition of the Petrašiūnai dolomite decomposition solution obtained under these conditions was as follows: 32.7 ± 0.35% CaO, 17.4 ± 0.02% MgO, and 2.0 ± 0.01% N.

In this way, by decomposing dolomite with nitric acid, it is possible to produce liquid nitrogen–calcium–magnesium fertilizers with the formula 2–0–0+33CaO+17MgO. The most important physical properties characterizing liquid fertilizers were also determined: crystallization temperature—19.5 °C, kinematic viscosity—3.6 ± 0.15 mm²/s, and density—1.369 ± 0.18 g/cm³.

Considering that during decomposition with nitric acid, other minerals contained in dolomite are also decomposed along with calcium and magnesium carbonates, it is very important to assess the concentration of micronutrients and heavy metals in the solution. For this reason, the concentrations of lead, cadmium, iron, chromium, manganese, nickel, copper, zinc, and cobalt in the nitric acid digestion solution of Petrašiūnai dolomite were analysed by AAS, and the results obtained are presented in

Table 1. Concentrations of heavy metals and micronutrients in dolomite decomposition solution.

Chemical Element Concentration, %
Cu	Fe	Co	Mn	Cr	Ni	Pb	Cd	Zn
2.4 · 10−5	1.5 · 10−3	5.0 · 10−5	5.5 · 10−4	1.6 · 10−4	–	2.5 · 10−5	–	1.1 · 10−4

.

It can be seen that heavy metals such as nickel and cadmium were not detected in the solution, and chromium and lead are present in the digestion solution, but the amounts are very low and do not exceed the permissible limits [43]. The micronutrients detected during the analysis were copper, iron, manganese, cobalt, and zinc. Unfortunately, the concentrations of these chemical elements in the solution were very low (only trace amounts were detected) and cannot be declared as micronutrients according to the established standards [44].

XRD analysis data (Figure 1) confirmed that this mineral mainly contains CaCO₃ and MgCO₃, with only trace amounts of other compounds such as SiO₂ and Al₂O₃.

Using the SEM-EDS method, the surface of the dolomite powder was analysed for elements, and elemental mapping was conducted (Figure 2), which allows us to evaluate the structure of dolomite. As can be seen from the SEM images, although at lower magnification (Figure 2a), the particles appear rough, irregularly shaped with a heterogeneous size distribution and aggregated morphology. At a magnification of up to 250× (Figure 2b), the crystalline and angular structure characteristic of dolomite becomes apparent. This morphology suggests a relatively high surface area [45,46].

According to the element mapping (Figure 2c), it can be seen that the chemical composition of the particles is uniform (elements such as Ca, Mg, C, Si, Al, Fe, S, and K are evenly distributed) and does not depend on the particle size. Heavy metals were not detected by this method due to their very low concentration in dolomite.

3.2. Neutralization and Nitrogen Enrichment of Dolomite Decomposition Solutions

During the decomposition of dolomite with nitric acid, a solution was formed containing dissolved calcium, magnesium, and nitrate ions, with a strongly acidic pH of 2. To make this solution suitable for agricultural applications, such as foliar or soil fertilization, it is necessary to neutralize the pH to near-neutral values. For this purpose, a 25% ammonia solution was selected. Additionally, the addition of an ammonia solution enriched the solution with nitrogen, thereby increasing its fertilizing value by supplying a higher concentration of nitrogen essential for plant growth.

The use of different volumes of ammonia water enabled the preparation of solutions with different pH values (6.1 and 7.3) and very similar densities (1.351 ± 0.29 and 1.355 ± 0.14 g/cm³), which were subsequently used for the production of a liquid nitrogen–calcium–magnesium fertilizer. These two solutions with different pH values were then enriched by the addition of nitrogenous components (urea and ammonium nitrate).

It is known that the composition of liquid fertilizers is characterized by the concentration of saturated solutions that crystallize at 0 °C, a parameter commonly referred to as the salt-out (crystallization) temperature (ISO 23381:2020) [39,47]. For this reason, studies have been conducted on the solubility of urea in a neutralised dolomite decomposition solution. Up to 40% urea was added to two solutions with different pH values. The resulting polytherms representing the solubility of urea are presented in Figure 3.

Figure 3a presents the polytherm of crystallization temperature as a function of the urea concentration of the solution containing calcium, magnesium, ammonium, and nitrate ions at a pH of 6.1. The curve exhibits typical eutectic behaviour until the concentration of the urea increases from 0 to 14%, and the crystallization temperature decreases, reaching a minimum (the first point has a urea concentration of 5% and temperature of −35.3 °C). This indicates that the system achieves its lowest freezing point at a specific composition, corresponding to the eutectic mixture.

In the 14–30% concentration region, the curve exhibits multiple breaking points (points 3, 4, and 5 from −34 to −27.5 °C), suggesting complex crystallization processes involving several different ions. These breaks may reflect the sequential precipitation of different double-salt crystallization processes involving calcium, magnesium, ammonium, and nitrate ions (e.g., Ca(NO₃)₂·NH₄NO₃, Mg(NO₃)₂ NH₄NO₃, etc.) [48,49,50].

After the sixth point, the further addition of urea causes a steady increase in the crystallization temperature. The steep increase in crystallization temperature at concentrations above 30% reflects the limited solubility of urea, the approach to saturation, and the dominant crystallization of urea itself.

The polytherm in Figure 3b also shows eight more or less accentuated breakpoints, which can also be attributed to the formation of a crystalline phase of different compositions. The curve exhibits a typical eutectic behaviour until the concentration of urea increases from 0 to 8%, but the lowest crystallisation temperatures were found at the first and third breaking points, which are −36 °C. The other breaking points (from the fourth to the seventh points) are not very clearly expressed, and from the eighth point, a sharp rise in temperature begins. The polytherm reaches a temperature of 0 °C at a 33% urea concentration. The error bars indicate experimental variability and show relatively good reproducibility, especially in the higher-temperature region.

In summary, it can be seen from Figure 3 that by selecting the composition of liquid fertilizers according to the crystallization temperature of 0 °C, ~36 or ~33% urea can be added to the neutralized dolomite decomposition solution, respectively, when the solution has a pH of 6.1 or a pH value of 7.4. It means that, at lower pH values, more urea can be added to the neutralised decomposition solution because the solubility of urea increases in more acidic media [51]. By estimating the concentrations of calcium and magnesium from dolomite and of nitrogen from nitric acid, ammonia water, and urea, the composition for the liquid fertilizer produced would be 17.6 ± 0.38% N, 3.8 ± 0.53% CaO, and 2.2 ± 0.09% MgO or the formula 18–0–0+4CaO+2MgO (Figure 3a) and 17.8 ± 0.42% N, 3.3 ± 0.41% CaO, and 2.1 ± 0.13% MgO or the formula 18–0–0+3CaO+2MgO (Figure 3b). The kinematic viscosity values of these liquid nitrogen–calcium–magnesium fertilizers were 4.523 ± 0.61 and 4.759 ± 0.48 mm²/s, with densities of 1.325 ± 0.15 g/cm³ and 1.331 ± 0.33 g/cm³, respectively. When urea was added to the neutralized solutions, the pH values increased slightly and were 6.4 and 7.6.

In order to fully investigate the variation in the chemical composition of liquid nitrogen–calcium–magnesium fertilisers and to achieve the highest possible nitrogen concentration in liquid fertilisers, solubility studies on nitrogen sources (ammonium nitrate) were further carried out. The polytherms of the solubility of ammonium nitrate in liquid fertilizers of 18–0–0+4CaO+2MgO and 18–0–0+3CaO+2MgO are shown in Figure 4. As can be seen in Figure 4, the nature of the curves is partly similar to that when urea was added to the neutralised dolomite decomposition solution (Figure 3). Up to 40% ammonium nitrate was added to the liquid fertilizers with pH values of 6.4 and 7.6, respectively. The dependence of the crystallisation temperature on the concentration of ammonium nitrate in the liquid fertilizers shows that the eutectic point’s crystallisation temperature fluctuates from −34.0 °C to −35.0 °C. It was found that at 0 °C, 35% ammonium nitrate could be added to the lower-pH (Figure 4a) solution and 30% ammonium nitrate to the higher-pH solution (Figure 4b).

In the liquid urea fertilizers, with the addition of ammonium nitrate, the low temperature is maintained over a very wide concentration range, followed by a sharp increase to a positive value. The curves show pronounced second or third breakpoints and a number of minor fifth and sixth points. In Figure 4a, at a low temperature of −34.0 degrees, two breakpoints are visible, at 5 and 8% NH₄NO₃ concentrations, the first of which can be attributed to typical eutectics. However, as the ammonium nitrate concentration increases, the temperature rises, and many very small breakpoints are visible. The polytherm at 0 °C crosses the concentration axis at the 34% value. Meanwhile, in Figure 4b, the breaking point at the lowest temperature can be attributed to an atypical eutectic, because before the second eutectic point (at a temperature of −35 °C and 8% NH₄NO₃), there is a first breaking point (at a temperature of −31.7 °C and 6% NH₄NO₃). From the eutectic point, a similar curve is observed, as in the case of Figure 4a. In that branch, there are many unclearly expressed breaking points, which may be related to the formation of various calcium and magnesium nitrate crystal hydrates. It should be noted that solutions of this composition become very viscous and difficult to crystallise at low temperatures.

According to the results presented, it is possible to produce liquid concentrated nitrogen–calcium–magnesium fertilizers of 23.3 ± 0.13% N, 2.4 ± 0.30% CaO, and 1.5 ± 0.17% MgO or 23–0–0+2CaO+2MgO (Figure 4a) and 22.1 ± 0.09% N, 2.9 ± 0.06% CaO, and 1.8 ± 0.04% MgO or 22–0–0+3CaO+2MgO (Figure 4b). The physical properties of the resulting liquid nitrogen–calcium–magnesium fertilizers were investigated: their kinematic viscosity values were 4.307 ± 0.11 and 4.354 ± 0.05 mm²/s, their density values were 1.368 ± 0.02 and 1.363 ± 0.25 g/cm³, and their pH values, which did not change with the addition of ammonium nitrate, were 6.4 and 7.6, respectively.

3.3. Influence of Amino Acid Concentrate Naturamin-WSP on the Crystallization Temperature of Liquid Fertilizers

Considering that the unbalanced use of concentrated fertilizers reduces the number of microorganisms in the soil that affect the soil’s fertility, the current goal is to produce effective and environmentally friendly fertilizers. It is known that substances such as humic acids, amino acids, or algae extracts stimulate root development and the activity of microorganisms in the rhizosphere. Their presence can also reduce the negative effects of mineral fertilizers and generally reduce the need for such fertilizers, as they naturally strengthen plant immunity [52,53,54]. Even small amounts of these substances cause significant positive changes in plant growth and development. Often such bioactive substances are simply called biostimulants, which play a vital role in sustainable agriculture by improving nutrient use efficiency and plant resistance [55].

The bioactive material chosen for the study was the dry amino acid concentrate Naturamin-WSP. An AAC up to 20% was added to neutralised dolomite digestion solutions (of different pH values), which were standardised using only 35% urea, i.e., liquid fertilizer formulas of 18–0–0+4CaO+2MgO and 18–0–0+3CaO+2MgO. The resulting AAC solubility polytherms at different pH values of the liquid fertilizers are shown in Figure 5.

During the study, when the pH value of the liquid fertilizer was 6.4, a curve (Figure 5a) was obtained showing five breakpoints, of which the first three are very pronounced. These points are characterized by the crystallization temperature and AAC concentration, which were as follows: the first point was at −33.0 °C and 3% AAC, the second point was at −28.8 °C and 5% AAC, and the third point was at −33.2 °C and 8% AAC. Thus, it can be stated that, in this system, there are two eutectic points at which different solid phases crystallize. From the third breaking point, the temperature increases with the increase in AAC concentration. There are no major breaks in this part of the polytherm, but the fourth point (−28.3 °C and 9% AAC) and fifth point (−19.0 °C and 17% AAC) show changes in the phases formed in the system at equilibrium. A substantially similar curve, but with three clearly expressed eutectic breakpoints, was obtained when up to 20% of AAC was added to liquid fertilizers with a neutral pH value, i.e., 7.4 (Figure 5b). In this polytherm, the crystallization temperature of eutectic point 1 was the highest and the AAC concentration was the lowest (−28.0 °C and 3% AAC). Meanwhile eutectic point 3 has the same crystallization temperature and the same AAC concentration as point 3 in Figure 5a, −33.2 °C and 8% AAC, which indicates the stable eutectic of this system, independent of pH value. After adding 17% ACC and raising the temperature to −20.3 °C, another breaking point is visible, but it is not as pronounced as the previous ones and does not form another eutectic. It should be noted that the polytherms in Figure 5 exist only at a negative temperature, at a concentration of 20% AAC, and when temperatures of −17.7 °C (pH of 6.4) and −19.3 °C (pH of 7.4) are reached.

From the obtained results, it can be assumed that, regardless of the pH value, the amino acid concentrate reduces the crystallization temperature of the liquid fertilizers of 18–0–0+4CaO+2MgO and 18–0–0+3CaO+2MgO. It can be seen that the crystallization temperature starts to increase at 9% (Figure 5a) and 13% (Figure 5b) amino acid concentrate contents, but the temperature increase is very slow—the liquid nitrogen fertilizers with the AAC Naturamin-WSP remain highly stable. Based on the literature data [56], which states that the content of bioactive substances in fertilizers should not exceed 10–20%, and after evaluating the influence of the amino acid concentrate Naturamin-WSP on solubility polytherms (Figure 5 and Figure 6), a 5% AAC additive was selected for the liquid nitrogen–calcium–magnesium fertilizers. Therefore, it can be concluded that by adding 35% urea and 5% AAC to a neutralized solution of dolomite decomposed with nitric acid, it is possible to obtain two types of liquid concentrated bioactive nitrogen–calcium–magnesium fertilizers with very similar compositions: 15.3 ± 0.11% N, 3.4 ± 0.34% CaO, and 2.4 ± 0.38% MgO (formula: 15–0–0+3CaO+2MgO) (Figure 5a) or 15.6 ± 0.09% N, 3.3 ± 0.30% CaO, and 2.4 ± 0.26% MgO (formula: 16–0–0+3CaO+2MgO) (Figure 5b). The physical properties of these fertilizers were investigated. The pH values were 4.9 and 6.4, the kinematic viscosity values were 5.203 ± 0.61 and 4.897 ± 0.47 mm²/s, and the densities were 1.343 ± 0.16 and 1.329 ± 0.039 g/cm³, respectively.

In order to study the influence of the amino acid concentrate (AAC) Naturamin-WSP on liquid nitrogen–calcium–magnesium fertilizers with compositions of urea and ammonium nitrate, further investigations were conducted on the solubility of this bioactive additive in liquid fertilizers with different pH values. The solubility of the AAC in the liquid fertilizers of 23–0–0+2CaO+2MgO (pH of 6.4) and 22–0–0+3CaO+2MgO (pH of 7.4) is presented in Figure 4.

Up to 20% AAC was added to ammonia water–neutralized nitric acid solutions resulting from dolomite decomposition, containing 35% urea and 35% ammonium nitrate at a pH of 6.1 (Figure 6a) and 35% urea and 30% ammonium nitrate at a pH of 7.3 (Figure 6b). It was observed that in the AAC concentration range of 3–8%, the polythermal curve intersects the 0 °C temperature axis multiple times, indicating uneven changes in crystallization temperature up to 7.6% AAC (Figure 6a). The eutectic points of the system correspond to the first and third points, with crystallization temperatures of −3 °C and −0.7 °C and AAC concentrations of 1% and 3%, respectively. From the fourth breakpoint (7% AAC), the crystallization temperature of the system begins to decrease consistently to −13.3 °C and remains below 0 °C throughout the investigated concentration range, i.e., up to 20%.

When the pH value of the system under study is 7.3, a similar polytherm is visible (Figure 6b). It also has two eutectic points, but in this case, the curve is only in the negative temperature range and does not cross the 0 °C axis. In it, the eutectic points are the first one (1% AAC at −7.2 °C) and the third one (6% AAC at −9.0 °C), and from point 4, when the ACC concentration reaches 9%, a consistent decrease in the crystallization temperature to −16.5 °C begins. It is important to emphasise that the determination of the crystallisation temperature is complicated by the fact that the solutions become extremely viscous and resistant to crystallisation. Additionally, the colouring of the amino acid concentrate further hinders accurate temperature determination.

Comparing the polytherms with ammonium nitrate and urea additives (Figure 6) to those with only urea additives (Figure 5), it can be observed that although the eutectic points of the polytherms in Figure 6 occur at higher temperatures, higher concentrations of the biologically active additive “Naturamin-WSP” gradually reduce the crystallization temperature. Based on the presented results, it can be seen that by adding 35% urea, 30–35% ammonium nitrate, and 5% AAC to a neutralized solution of dolomite decomposed with nitric acid, a liquid concentrated nitrogen–calcium–magnesium biofertilizer was obtained, the composition of which was as follows: 13.9 ± 0.15% N, 3.1 ± 0.11% CaO, and 2.2 ± 0.14% MgO or 14.4 ± 0.36% N, 3.1 ± 0.01% CaO, and 2.2 ± 0.06% MgO. The formulas were the same in both cases: 14–0–0+3CaO+2MgO. The physical properties of these fertilizers were analysed as follows: pH values of 4.8 and 5.7, kinetic viscosities of 6.680 ± 0.68 and 5.734 ± 0.55 mm²/s, and densities of 1.346 ± 0.26 and 1.337 ± 0.81 g/cm³, respectively.

3.4. Influence of Organic Digestate Extracts on the Crystallization Temperature of Liquid Fertilizers

Since humic and fulvic acids have a positive effect on plants, it is appropriate to use them in combination with liquid fertilizers, thereby ensuring a greater positive effect on plants. There is a known method for producing humic substance concentrates from leonhardite by extraction using NaOH and KOH and adding surfactants. These products are used as a fertilizer, plant activator, and/or nutrient and as part of a process for obtaining this composition [57]. However, currently, much attention is paid to industrial and organic waste and its utilization. One such waste is digestate, a biogas production waste that can be widely used, since it contains nutrients necessary for plants. The X-ray diffraction analysis curves (Figure 7) show peaks characteristic of crystalline materials. The peaks were analysed using the RSDA data analysis software Search Match, automatically and in a targeted manner, searching for the elements H, C, N, O, Mg, P, K, and Ca, which were identified by chemical analysis. The peaks visible in the curve were assigned to the following substances: (NH₄) (PO₃), MgSO₄·7H₂O, K(NH₄)₂H(SO₄)₂, CH₇N₅O₃, and C₁₆H₃₄O. The different structures of the samples and the chemical compounds (salts) were identified: (NH₄) (PO₃) and K (NH₄)₂ H (SO₄)₂).

Scanning electron microscopic analysis reveal irregularly shaped, layered particle aggregates, typical of dried and compacted materials (Figure 8). At lower magnifications (50×), plate-like particles with rough, fractured edges and possible fibrous structures are observed. As the magnification increases (100×), the surface texture becomes more defined, with visible linear or layered features, suggesting potential crystalline structures. These morphological characteristics indicate that the material may be of mixed origin: both organic and mineral.

Previous studies [51] were conducted with digestates from three different companies, and their extracts were produced under different conditions. In this work, water and potassium hydroxide (alkali)-based KP digestate extracts (DEs) were produced in an ultrasonic bath, stirred with a magnetic stirrer, and exposed to different temperatures. The total organic carbon (TOC) concentration in the extracts was investigated, and the results are presented in

Table 2. TOC concentrations in water and alkaline extracts prepared using different extraction conditions.

Extraction Solvent
H2O				0.5 M KOH
Temperature, °C	Time, Hours	TOC Concentration, mg/L		Temperature, °C	Time, Hours	TOC Concentration, mg/L
		WD	CD			WD	CD
Magnetic Stirring
23	3	266 ± 0.01	417 ± 0.56	23	3	1170 ± 0.59	1517 ± 0.08
	6	290 ± 0.07	442 ± 0.25		6	2351 ± 0.41	2419 ± 0.21
	9	293 ± 0.17	417 ± 0.46		9	1930 ± 0.52	2080 ± 0.54
50	3	166 ± 0.02	235 ± 0.55	50	3	2672 ± 0.03	2926 ± 0.56
	6	271 ± 0.02	347 ± 0.41		6	2084 ± 0.21	3834 ± 0.22
	9	363 ± 0.01	161 ± 0.01		9	3315 ± 0.20	3407 ± 0.91
70	3	368 ± 0.01	508 ± 0.26	70	3	1884 ± 0.36	3050 ± 0.25
	6	483 ± 0.02	541 ± 0.09		6	3954 ± 0.68	3649 ± 0.41
	9	571 ± 0.15	503 ± 0.05		9	3466 ± 0.54	3615 ± 0.43
90	3	534 ± 0.26	574 ± 0.01	90	3	4008 ± 0.50	3552 ± 0.58
	6	534 ± 0.05	424 ± 0.01		6	4495 ± 0.52	4072 ± 0.33
	9	513 ± 0.02	444 ± 0.54		9	4032 ± 0.05	4232 ± 0.48
Ultrasonic Bath
30	3	247 ± 0.01	648 ± 0.02	30	3	1704 ± 0.25	1526 ± 0.02
	6	407 ± 0.01	523 ± 0.03		6	1781 ± 0.05	2193 ± 0.02
	9	366 ± 0.01	689 ± 0.51		9	2865 ± 0.54	2555 ± 0.39
50	3	322 ± 0.16	576 ± 0.02	50	3	1900 ± 0.95	1808 ± 0.42
	6	401 ± 0.06	568 ± 0.09		6	2484 ± 0.52	2508 ± 0.85
	9	438 ± 0.08	715 ± 0.08		9	3060 ± 0.15	2685 ± 0.45
60	3	310 ± 0.01	609 ± 0.45	60	3	2732 ± 0.02	2560 ± 0.66
	6	378 ± 0.02	599 ± 0.63		6	2682 ± 0.50	2483 ± 0.64
	9	465 ± 0.02	634 ± 0.22		9	3023 ± 0.51	2752 ± 0.44

.

According to the TOC concentration, one water-based digestate extract and one potassium hydroxide-based extract were selected. As shown in Table 2, the digestate from AB “Kurana,” after being exposed to water in an ultrasonic bath for 9 h at a temperature of 50 °C, yielded an extract with the highest organic carbon concentration (715 ± 0.08 mg/L). Meanwhile, the same digestate, after being crushed and stirred in potassium hydroxide using a magnetic stirrer for 6 h at a temperature of 90 °C, yielded an extract with the highest organic carbon concentration (4495 ± 0.52 mg/L) among the potassium hydroxide-based extracts. Thus, these extracts were selected for the preparation of liquid nitrogen–calcium–magnesium fertilizers, and solubility polytherms were studied.

As mentioned earlier, the composition of liquid fertilizers is characterized by a concentration of solutions that crystallize at a temperature of 0 °C. For this reason, the influence of liquid extracts obtained from digestate extracts on the crystallization temperature of liquid nitrogen–calcium–magnesium fertilizers was studied. Although two fertilizers with different pH values were produced, fertilizers with a lower pH value were chosen for the studies, because although some researchers claim that the pH of liquid organic fertilizers can be 3–5, it is usually indicated that the best quality liquid fertilizers have a pH of 3–7 [58]. Thus, up to 20% of DEs prepared on the basis of water (DEW) and alkaline (DEA) were added to liquid fertilizers of 18–0–0+4CaO+2MgO with a pH of 6.1. The obtained solubility polytherms of the digestate extracts are shown in Figure 9.

When DEW was added to the liquid fertilizer (Figure 9a), a polytherm was obtained with seven breakpoints characterized by the following respective DEW concentrations and crystallization temperatures: first—2% DEW at −30.0 °C, second—5% DEW at −28.0 °C, third—6% DEW at −31.0 °C, fourth—9% DEW at −25.5 °C, fifth—10% DEW at −26.0 °C, sixth—13% DEW at −24.0 °C, and seventh—14% DEW at −27.0 °C. From the third point onwards, the temperature of crystallisation increased with increasing DEW concentration, but a slight decrease in temperature was observed at the seventh point. The crystallization temperature remained low at −20.5 °C when the DEW concentration increased to 20%. A similar crystallization curve, also with seven breakpoints, was obtained using DEA (Figure 9b). In this system, the first eutectic point had an insignificantly higher DEA concentration (3%) and an insignificantly lower crystallization temperature (−34.0 °C) than the first eutectic point in Figure 9a. It can be stated that the eutectic in both systems was stable and was not significantly affected by the type of extracting solvent. It was observed that the crystallisation temperature starts to increase at a 14% DE concentration (Figure 9a) and at 3% (Figure 9b), but the increase is very slight. At the highest DE concentration of 20%, in both cases (DEW and DEA), the crystallization temperature remained low, at about −21.0 °C.

Based on these results, it was determined that concentrated liquid bioactive nitrogen, calcium, and magnesium fertilizers can be prepared by adding 35% urea and 5% DE to a neutralized dolomite solution digested with nitric acid. Two types of liquid biofertilizer were obtained: 16.6 ± 0.33% N, 3.3 ± 0.15% CaO, and 2.1 ± 0.51% MgO using DEW and 17.1 ± 0.41% N, 3.3 ± 0.35% CaO, and 2.2 ± 0.10% MgO using DEA. The formulas are the same: 17–0–0+3CaO+2MgO. However, with DEA, there was a low concentration of potassium, which cannot be declared according to the fertilizer regulation [44]. The physical properties of these fertilizers were also investigated and are as follows: pH values of 5.8 and 6.2, kinematic viscosity of 5.106 ± 0.03 and 4.953 ± 0.01 mm²/s, and density of 1.333 ± 0.09 and 1.341 ± 0.05 g/cm³, respectively.

Figure 10 shows the dependence of the crystallization temperature on the DE concentration in liquid fertilizers that were produced with the addition of both urea and ammonium nitrate. In both cases, a typical eutectic dependence on the DE concentration is observed in the region of low DE concentrations (0–6%). In the initial range of DE concentrations (0–3%), the temperature decreases until the minimum eutectic point is reached. In liquid fertilizers of the formula 23–0–0+2CaO+2MgO, when DEW is used as an additive (Figure 10a), the eutectic point is fixed at a 2% DEW concentration and a temperature of –4.0 °C, and when DEA is used as an additive (Figure 10b), the eutectic point is fixed at a 2% DEA concentration and a crystallization temperature of −3.5 °C; therefore, it can be said that, in both cases, the parameters characterizing the eutectic point are the same.

The further increase in DE content to 5–6% increased the crystallization temperature to a positive value, but from 7–8% DE, the crystallization temperature in both cases began to decrease consistently again, as in the case when AAC was added to the fertilizer of this formula (Figure 6). This allows us to assume that in fertilizers of this composition, calcium and magnesium nitrates or ammonium calcium nitrates, which have low crystallization temperatures, crystallize in the solid phase, regardless of the origin of the organic additive. However, the composition of such complex multicomponent systems has not been analysed, and it is not possible to say exactly what specific salt is formed in the solid phase. Our assumptions about the formation of double salts in liquid fertilizers with the addition of organic matter are based on our previously conducted studies [59] and works of other authors analysing similar multicomponent systems [60]. When adding 20% of liquid DE, the lowest crystallization temperatures of liquid fertilizers of 23–0–0+2CaO+2MgO were determined: −9.25 °C (with DEW, as shown in Figure 10a) and −14.5 °C (with DEA, as shown in Figure 10b), but a greater increase in the concentration of organic matter extracts is inappropriate.

It was determined that, in this case, the system using a water-based extract (Figure 10a) has a slightly lower crystallization temperature at a slightly higher DEW concentration compared to the system using an alkali-based extract (Figure 10b). Both polytherms show that using a DE concentration higher than 10% decreases the freezing point of the solution, which is very important for the stability of fertilizers at low temperatures. Based on the results obtained, it can be stated that by adding 5% of DE to the 23–0–0+2CaO+2MgO fertilizer, it is possible to obtain a liquid concentrated nitrogen–calcium–magnesium fertilizer with a crystallization temperature of 0 °C, containing 21.9 ± 0.15% N, 3.1 ± 0.17% CaO, and 2.2 ± 0.01% MgO. The physical properties of these fertilizers were measured and are as follows: their pH values were 5.9 and 6.3; their kinematic viscosities were 6.570 ± 0.18 and 5.630 ± 0.65 mm²/s; and their densities were 1.258 ± 0.02 and 1.305 ± 0.41 g/cm³.

It is known that alkaline commercial products of bioactive substances are incompatible with acidic environments, as they quickly restore the original humic acid, which precipitates and forms flocculations that make it difficult or impossible to use the product in the field, as they clog the filters and nozzles of the equipment. So, in this case, the initial pH values (6.4 and 7.3) of the liquid nitrogen–calcium–magnesium fertilizer were very appropriately chosen [61]. Also, mixtures containing multiple amino acids are commonly used in agriculture, but the effects of individual components within these mixtures are less understood [62]. We have not found any published scientific studies describing the interaction of humic organic matter with other fertilization products or the influence of such additives on the properties and stability of liquid fertilizers; the data presented in this article are completely new and very valuable.

In comparison to commercial liquid Ca–Mg nitrate fertilizers [63], which typically contain soluble calcium nitrate and magnesium nitrate in fixed formulations and are intended for foliar or drench applications, our laboratory-developed fertilizers allow for adjustable pH values and are enriched with organic bioactive extracts that supply both nutrients and humic substances. In addition, the nitrogen concentration of our experimental fertilizers can be modified, making them adaptable to specific plant needs.

4. Conclusions

The experimental results showed that liquid concentrated nitrogen–calcium–magnesium fertilizers with pH values ranging from 6.1 to 7.3 can be produced by neutralizing the dolomite decomposition solution with ammonia water and adding nitrogen-containing substances such as urea or ammonium nitrate. The pH can be adjusted as needed by varying the amount of ammonia water used for neutralization.

Complex multicomponent systems consisting of Ca²⁺, Mg²⁺, NH⁴⁺, NO³⁻ ions, and H₂O with numerous breaking points were obtained. These ions interact in the liquid phase, and upon crystallization to the solid phase, they likely form double salts of calcium ammonium or magnesium ammonium nitrates, which are characterized by very low eutectic temperatures.

The liquid nitrogen–calcium–magnesium fertilizers (18–0–0+3CaO+2MgO and 22–0–0+3CaO+2MgO) prepared in the laboratory by the described method can be enriched with organic matter—either the commercial powdered amino acid concentrate Naturamin-WSP or aqueous and alkaline digestate extracts produced in the laboratory. The highest TOC concentration in the alkaline extract (4495 ± 0.52 mg/L) was obtained after 6 h of extraction at 90 °C with magnetic stirring. In contrast, the highest TOC concentration in the aqueous extract (715 ± 0.08 mg/L) was achieved after 9 h of ultrasonic extraction at 50 °C.

By supplementing these fertilizers with 5–10% of plant growth-promoting bioactive substances (powdered AAC or digestate-based liquid extracts), the resulting liquid fertilizers (17–0–0+3CaO+2MgO and 23–0–0+2CaO+2MgO) remain stable and become more environmentally friendly due to the presence of humic substances. The properties of these bioactive-enriched liquid fertilizers meet the requirements for liquid fertilizers: the crystallization temperature remains around 0 °C, the density ranges from 1.258 ± 0.02 to 1.341 ± 0.05 g/cm³, the kinematic viscosity ranges from 4.953 ± 0.01 to 6.570 ± 0.18 mm²/s, and the pH falls between 5.8 and 6.3.

Thus, it can be stated that liquid nitrogen–calcium–magnesium fertilizers enriched with bioactive substances may be a promising alternative to conventional liquid nitrogen fertilizers. However, further studies are needed to evaluate the agronomic efficiency of these fertilizers and the plant response to them under field conditions.