1 Introduction

Developing nations like Turkey discharge 75% of wastewater generated from industries directly into the environment, which may cause adverse environmental impacts (Bhat and Qayoom 2021). Biogas production and the stabilization of wastes through microbial decomposition in an oxygen-free environment are becoming increasingly widespread worldwide. Anaerobic digestion takes place in biogas plants, which are recognized as versatile options for recovering sustainable energy from agricultural waste. In Europe, the number of biogas plants in Germany, France, the Czech Republic, and Denmark was 11,084, 837, 574, and 141 in 2019, respectively (Gustafsson and Anderberg 2022), while those in Turkey were 84 in 2019 (Şenol et al. 2021).

The primary goal of biogas production is to generate electricity and heat in the energy sector and to utilize waste in terms of waste management (Selvaraj et al. 2022). The slurry formed at the end of anaerobic digestion in fermenters is separated into solid and liquid phases. The solid matter of the digestate (6–12% wet base) obtained from the fermenters can be increased to 20–30% using separators. Furthermore, the whole digestate is separated into liquid (80–90% of the mass) and solid (10–20% of the mass) fractions (Fuchs and Drosg 2013). The liquid part is sent to the digestate storage tank. The stabilized whole digestate that has been produced as a result of the process now poses a new issue that needs to be addressed. This means the solid and liquid digestate produced during the biogas process should be accurately utilized. Although spreading the digestate for agriculture is the simplest method, it presents numerous drawbacks, including biological contamination, heavy metal pollution, ammonia volatilization, and leaching of nutrients (Selvaraj et al. 2022). In addition, digestate storage results in emission of greenhouse gases like CH4, N2O, and others (Lukehurst et al. 2010). Traditionally, digestates are utilized in agriculture as crop nutrients (Czubaszek and Wysocka-Czubaszek 2018) and for the growth of microalgae (Monfet et al. 2018). Struvite precipitation is a promising eco-technology to recover primary plant nutrients (nitrogen and phosphorus) from liquid fraction from digestate, which lessens the environmental effect of wastes (Macura et al. 2019). Struvite is a recovered crystalline chemical obtained from different types of sources. The chemical formula of struvite is NH4MgPO4·6H2O, and it contains 12.5% P, 5.7% N, and 9.9% Mg (Latifian et al. 2012). The theoretical fertilizer value of struvite varies depending on the source and the recovery process. It has been stated since 1850 that struvite can be used as a fertilizer source due to the nutrients in its chemical composition and could support plant growth. Struvite has a low water solubility (Massey et al. 2009; Cabeza et al. 2011). However, the solubility of struvite is increased by organic acids exudated by the roots of the plants (Ahmed et al. 2015; Talboys et al. 2016). Thus, struvite can be considered as a slow-release P source that better meets crop P requirement during the growing period (Ackerman et al. 2013). Several studies have shown that struvite is an effective water-soluble phosphorus fertilizer in neutral and slightly acidic soils (Plaza et al. 2007; Massey et al. 2009). Struvite has a significant amount of citrate-soluble P, which varies by the struvite source and/or methods. This means struvite can be dissolved by organic acids exuded by plant roots (Talboys et al. 2016). Studies have shown that struvite is superior or comparable to most of the conventional P fertilizers for various plants such as ryegrass, maize, lettuce, triticale-oat-corn-alfalfa, cabbage and maize (Plaza et al. 2007; Gell et al. 2011; Ryu et al. 2012; Hilt et al. 2016; Wen et al. 2019; Valle et al. 2022).

Although there are various studies concerning the struvite production and the usage of struvite on plant growth, much less is known about how the struvites obtained at different molar ratios from biogas liquid digestate affect plant nutrition and growth. Therefore, this study aimed (1) to investigate the effect of two different struvites produced from biogas liquid fermented product with different additives on P nutrition and growth of lettuce, and (2) to examine the contribution of struvites to the mineral nutrition of plants with various nutrients in their composition. Furthermore, the efficacy of the struvites was tested in acidic and calcareous soils on lettuce plants grown under greenhouse conditions in response to four commonly used chemical fertilizers.

2 Materials and Methods

2.1 Biogas Liquid Digestate and Struvite Production Procedure

In plant growth experiments, two different types of powdered struvite samples with different elemental compositions, struvite 1 (STR1) and struvite 2 (STR2), were used to test their effectiveness in terms of plant growth and mineral nutrition. The tested struvite samples were produced from the liquid digestate (LD, liquid fraction of a centrifuge decanter) through the struvite crystallization process during the treatment experiments, where the LD was subjected to a series of physico-chemical treatment procedures. The LD samples were collected from an anaerobic fermentation pool of a biogas plant (in Sandıklı, Afyonkarahisar, Turkey) where poultry manure is the substrate for biogas generation. Table 1 presents the typical characteristics of the LD. Dry and organic matter of the LD were determined according to AOAC (1990). The pH and EC of the LD were measured with a pH and EC meter (Milwaukee Mi150 and WTW Multi 340i, respectively). The COD concentration was determined according to the closed reflux colorimetric method as described in the APHA 5220D method. The concentrations of Mg2+, NH4+, and PO43− were measured using Merck Spektroquant test kits (cat. no. 100815 for Mg2+, 100683 for NH4+, 114842 for PO43−).

Table 1 Typical characteristics of the LD used for struvite crystallization experiments
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The treatment of the LD was performed by applying different molar ratios of the Mg2+ and PO43− sources (powder forms of KH2PO4 and MgSO4·7H2O) as overdose (> 1.0 for Mg2+ and/or PO43−) and underdose (< 1.0 for Mg2+ and PO43−) conditions, which were externally added into a glass vessel with a working volume of 5 L. In each experimental run, 1 L of LD sample was added to the external Mg2+ and PO43− source in a known amount to adjust the desired molar ratio of NH4+, Mg2+, and PO43− in the untreated LD. The reaction mixture was mixed for 60 min at a constant speed of 150 rpm from the bottom side of the reactor through a magnetic stirrer. An aqueous solution of NaOH (10 M) was used to adjust the pH to 8.5. The pH adjustment was performed after adding the external PO43− and Mg2+ sources under mixing (the pH adjustment lasted for 30 min until an equilibrium state was reached as indicated by a stable pH). Crystallization reaction was allowed for the remaining 30 min of the reaction time. Then, the magnetic stirrer was switched off, and the reaction mixture was immediately transferred into graduated cylinders to separate the produced struvite precipitates from the liquid phase. The precipitates were allowed to settle for 30 min. After separating and collecting the precipitates, they were dried in an oven at 50 °C for three days. The dried precipitates were then powdered in a grinding mill at 28,000 rpm for 30 s and stored in a vacuum desiccator until used in plant-growth experiments.

Among the various struvite samples produced from the LD at different molar ratios (NH4+/Mg2+/PO43−), STR1 (1.0/1.3/1.0) and STR2 (1.0/1.3/1.3) samples had the highest efficiencies of NH4+, Mg2+, and PO43− removal and the highest amount of struvite production. Therefore, the present study aimed to test whether there are differences in plant nutrition and growth.

2.2 Chemical Fertilizers and Struvites

Four different widely used commercial fertilizers were used to compare the effectiveness of struvites in terms of phosphorus supplying capacity and usability as fertilizer. These fertilizers are di-ammonium phosphate (DAP), monoammonium phosphate (MAP), triple superphosphate (TSP), and 20–20-20. Nutrient compositions of the fertilizers and struvites are given in Table 2. The amount of nutrients and their forms in the chemical fertilizers and struvites were determined with the same procedures described by Kacar and Kütük (2010).

Table 2 Nutrient amounts and forms of the chemical fertilizers
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2.3 Soils used for Plant Growth

This study used soils with two different pH values, one acidic (pH: 3.2) and the other calcareous (pH: 7.8). The acidic soil was collected from Rize, which is located in the north-east of Turkey and has a coast to the Black Sea. The calcareous soil was collected from the agricultural experimental site of Isparta University of Applied Sciences, the Faculty of Agriculture, Isparta. Table 3 presents some descriptive characteristics of the soils and the sufficiency levels of some parameters (Kacar 2009). The soils were air-dried and passed through a 4-mm sieve. The pH and EC of the soils were determined in suspension (the ratio of soil:water is 1:2.5) using a combined pH-EC meter (Hanna HI-5522, USA). The textures, CaCO3, and organic matter (OM) were measured using the methods of Bouyoucos (1951), Allison and Moodie (1965), and Walkley and Black (1934), respectively. The available P concentration was determined as described by Olsen (1954) in calcareous soil and by Bray and Kurtz (1945) in acid soil. Exchangeable K, Ca, and Mg, and DPTA-extractable Fe, Zn, Mn, and Cu were determined as described by Jackson (1967) and Lindsay and Norvell (1978).

Table 3 Some descriptive properties of the soils used for the experiment
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2.4 Pot Experiment

The study was performed with 2 kg of soil under greenhouse conditions. All the fertilizers and the struvites were applied in powder form to 200 mg kg−1 P soil. The amounts of the other nutrients added with chemical fertilizers and struvites besides 200 mg kg−1 P were given in Table 4.

Table 4 The amounts of the other nutrients added with chemical fertilizers and struvites (mg kg−1)
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All the materials were ground in a porcelain mortar and passed through a 0.5-mm sieve before soil application. Curly lettuce (Lactuca sativa L. var. crispa) was used as the plant material. One lettuce seedling was planted in each pot and left for growth for two months. During the growing period, the plants were watered with tap water. Irrigation intervals were adjusted according to the plant's demand and when a light peeling on the leaf’s irrigation was performed. The experiment was conducted on 72 pots consisting of six fertilizer materials (DAP, MAP, TSP, 20–20-20, STR1, and STR2), two P dosages (0 and 200 mg kg−1), two soils (acid and calcareous), and three replications. The experiment was carried out under a completely randomized design.

The experiment ended after a two-month-growing period. In order to determine the growth parameters, each plant was harvested together with the roots. At first, each pot was water-saturated to pull the plants from the soil easily. Then, soil was removed from the roots by hand, and the rest was washed with tap water. After the free water on the plants was removed, the roots were separated from the shoot. Both plant parts (shoots and roots) were weighted separately to obtain each fresh weight. Since lettuce is a freshly marketed plant, it was thought that it would be appropriate to give fresh weight values along with dry weights to determine the effect of treatments on marketable yield. Then, the leaves of the shoot were separated, washed with tap and distilled water, and left for drying at 65 °C until a stable weight was reached. Similarly, the roots were left to dry after washing with tap and distilled water. After drying, the dry weights of both shoots and roots were recorded. In order to determine the nutrient concentrations in leaves, the dried plant parts were crushed with a steel mill. In order to determine the P, K, Ca, Mg, Fe, Mn, Zn, and Cu concentrations, 0.5 g of plant samples were wet digested in HNO3 + HClO4 acid mixture using a hot plate, then filled up with distilled water at 50 ml. Leaf P concentration was determined calorimetrically (Shimadzu UV-1208, JPN), and the other nutrients were determined using an atomic absorption spectrophotometer (Varian FS240, USA). The Kjeldahl distillation method was used to determine shoot N concentration (Mills and Jones 1996). Nutrient uptakes were calculated by multiplying the shoot dry weights and shoot nutrient concentrations.

The apparent phosphorus recovery (APR) was used to evaluate the plant P uptake (PUfertilized) in fertilized soils in comparison to the no P applied control (PUcontrol) considering P application rate (Prate) as follows (Benjannet et al. 2019; Jamal et al. 2023).

$${\text{APR}}(\mathrm{\%})=({{\text{PU}}}_{{\text{fertilized}}}-{{\text{PU}}}_{{\text{control}}}/{{\text{P}}}_{{\text{applied}}})\times 100$$

The applied P was 200 mg kg−1 (400 mg for 2 kg of soil). The P uptakes from DAP-applied plants in acid soil were 12.4 mg, and the P uptake by the control plants was 5.7 mg. With the given values, the APR was calculated as 1.68%

2.5 Statistical Evaluation

The results were evaluated statistically using the Minitab (Version 20, USA) package program. The TUKEY test was conducted at a 5% significance level to separate the means for all treatments. No statistical comparison was performed between the soils to determine the effect of treatments on each soil individually, and each soil was evaluated within itself.

3 Results

3.1 Characteristics of Struvites

The nutrient amounts of the struvites are given in Table 5. The total P concentrations of STR1 and STR2 were 7.25% and 9.85%, respectively. The amount of citric acid-soluble (2%) struvites were 6.76% and 7.96% for STR1 and STR2, respectively. The amount of water-soluble P in STR1 was 0.35% and in STR2 was 1.95%. Based on the solubility of struvites in water and citric acid, it was found that 4.8% and 19.8% of total P in STR1 and STR2 were water-soluble and 93% and 81% of total P were citric acid-soluble in STR1 and STR2, respectively. The macronutrient concentrations of STR1 and STR2 were 2.61% and 2.77% for N, 4.16% and 4.39% for K, 0.88% and 0.82% for Ca, and 2.92% and 2.64% for Mg, respectively. The concentrations of Zn, Fe, Mn, and Cu were 186, 536, 72, and 7 mg kg−1 for STR1 and 117, 755, 67, and 11 mg kg−1 for STR2.

Table 5 Elemental composition of struvites
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3.2 Crop Yield

The comparison of the effects of the struvites and chemical fertilizers on the growth of lettuce is presented in Supplementary Material, Fig. S1. In addition, the effects of treatments on the fresh and dry weights of lettuce grown in acid and calcareous soils are given in Figs. 1 and 2, respectively. In acidic soil, 136.0 g of STR2 and 132.3 g of STR1 had significantly higher effects (P < 0.05) on the shoot fresh weight. In addition, the 20–20-20 fertilizers showed a similar effect to the struvites on the shoot fresh weights (124.7 g) by taking in the same statistical group. The lowest root fresh weight was recorded in the control group plants. STR1 was the most effective application on root fresh weight (13.5 g) in acid soil. While 20–20-20 fertilizer was the least effective treatment on root dry weight (0.35 g) in acid soil, STR1 had the highest effect (1.27 g). The lowest shoot and total dry weights were obtained from the control and TSP treatments, whereas STR1 was the most effective treatment.

Fig. 1
figure 1

Comparison of the effects of the struvites and chemical fertilizers on the fresh (A) and dry (B) weights of lettuce grown in acid soils. Values in the same column followed by the same letter are not significantly different (P < 0.05). DAP di-ammonium phosphate, MAP monoammonium phosphate, TSP triple superphosphate, 20–20-20 NPK fertilizer, STR1 struvite1, STR2 struvite2. Bars above columns indicate standard errors. Letters above the bars represent the statistical significance between the treatments. Treatments sharing the same letter are not significantly different (p > 0.05)

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Fig. 2
figure 2

Comparison of the effects of the struvites and chemical fertilizers on the fresh (above) and dry (below) weights of lettuce grown in calcareous soils. Values in the same column followed by the same letter are not significantly different (P < 0.05). DAP di-ammonium phosphate, MAP monoammonium phosphate, TSP triple superphosphate, 20–20-20 NPK fertilizer, STR1 struvite1, STR2 struvite2. Bars above columns indicate standard errors. Letters above the bars represent the statistical significance between the treatments. Treatments sharing the same letter are not significantly different (p > 0.05)

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The control and TSP treatments in calcareous soil led to significantly lower effects on the shoot fresh weight. Both the struvites were the most effective applications on the shoot fresh weight. The control treatment resulted in the lowest root fresh weight. The fertilizers and struvites produced a similar effect on the root fresh weight. As for the total fresh weights, the control and TSP-applied plants showed the lowest growth. While the total fresh weight was 71.9 g under control conditions, it was 82.8 g under the TSP treatment. The other fertilizers and both struvites showed similar effects on the fresh weight. However, the highest values were obtained from STR1 (150.0 g) and STR2 (155.6 g). As in the fresh weights, the lowest shoot dry weights were determined from the control (3.09 g) and TSP treatment (3.66 g). The other treatments showed a similar effect on the shoot dry weight, with the values changing from 5.94 g (MAP) to 7.35 g (STR1). While MAP and TSP were the most effective fertilizers on the root dry weight, the control (0.31 g) was the least effective treatment. The control and TSP treatments yielded the lowest total dry weights, 3.40 g and 4.37 g, respectively. The highest dry weight was obtained from SRT1 (7.85 g), followed by STR2 (7.44 g), although having the same statistical effects as the other fertilizers.

When a general assessment of the fresh and dry weights of the plants was performed for both soil types, it was discovered that the control group plants had the lowest yield values, followed by the plants to which TSP was applied. Compared to the control, STR1 and STR2 treatments significantly increased the total fresh weight of plants grown in acidic soil by approximately 2.7 times and in calcareous soil by approximately 2.1 times. Among the other chemical fertilizers, the 20–20-20 fertilizer significantly increased the fresh weight by 145% and 99% on acidic and calcareous soils; the DAP fertilizers increased by 125% and 90%, and the MAP by 77% and 70%, respectively. The TSP fertilizer was the least effective chemical fertilizer on the total fresh weight of the plant, and it increased the yields by 5% in acidic soil and 14% in calcareous soil. There were no significant differences among STR1, STR2, and 20–20-20 treatments in either type of soil. In this case, it is possible to say that the applications are ranked as STR1 = STR2 = 20–20-20 > DAP > MAP > TSP = control in terms of their effectiveness on the plant fresh weight.

3.3 Phosphorus Concentrations, Uptakes and APR by the Shoot of Lettuce

The effects of treatments on the P concentration and uptake and the APR of the lettuce shoot are given in Table 6. Treatments significantly (P < 0.05) affected the shoot P concentrations in all the conditions. The highest P concentrations in the shoot of lettuce growing in acidic soil were measured from the plants growing in the pots treated with STR1 (0.41%) and STR2 (0.48%). The control, DAP, and TSP-treated plants yielded lower P concentrations while the P concentration determined from the MAP and 20–20-20-applied plants followed the struvites. The struvites increased the P concentration of the lettuce shoot three times compared to the control treatment in calcareous soil. Three distinct treatments groups existed in terms of P concentration of the plant grown in calcareous soil. The struvites were the highest group. The second group had the lower plant P concentration and consisted of the control, DAP, MAP, and 20–20-20 fertilizers. The third group consisted of TSP, which was more effective than the second group.

Table 6 Comparison of the effects of the struvites and chemical fertilizers on the shoot P concentrations, uptakes, and recovery
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Treatments significantly (P < 0.05) affected the shoot P uptakes in all conditions. Struvites-applied plants took about five and seven times more P when compared to the control in both acidic and calcareous soils. It is apparent that STR1 and STR2 increased the P uptakes of plants 2–5 times compared to chemical fertilizers in acidic soil and three times in calcareous soil. The treatments significantly (P < 0.05) affected the APR in all the conditions. The highest APR was obtained from STR1-applied plants with 6.33%, followed by STR2 (6.28%) in acidic soil and STR1 (10.13%) followed by STR2 (9.55%) in calcareous soil. It should be noted that STR1 and STR2 increased the plant APR approximately 3–35 times compared to chemical fertilizers in acidic soil and 4–5 times in calcareous soil.

3.4 Shoot N, K, Ca and Mg Concentrations and Uptakes

Table 7 gives the means of the shoot N, K, and Ca concentrations and uptakes of lettuce grown in acidic and calcareous soils. All the parameters examined were affected significantly (P < 0.05) by the treatments in acidic and calcareous soils. The lowest (1.39%) N concentration was obtained from the control plants, and the highest N concentration (3.39%) was obtained from the plants treated with the 20–20-20 fertilizer in acidic soil. The struvites-applied plants took the second place after the 20–20-20 fertilizer, with the values of 2.34% (STR2) and 2.08% (STR1). As for calcareous soil, the highest plant N concentration was obtained from the 20–20-20 fertilizer with 3.5%, followed by STR1 fertilizer (2.7%). N removed by the lettuce shoot depending on the treatments showed a similar tendency as in N concentration in acidic and calcareous soil. The highest N removal (194 mg) was achieved by for the 20–20-20-applied plants, followed by struvite applications in acidic and calcareous soil. The highest plant K concentration was obtained from STR2 with 3.8%, followed by the 20–20-20 fertilizer (3.2%) in acidic soil, and TSP with 6.3%, followed by the 20–20-20 fertilizer (6.2%) in calcareous soil. On K uptakes, STR2 was the most efficient material, followed by STR1 and the 20–20-20 fertilizers in acidic soil and STR2 followed by the 20–20-20 fertilizers in calcareous soil. The control and TSP were the most ineffective applications on the K uptakes by lettuce shoot in all the conditions. In acidic soil, the amount of Ca determined in the plants treated with the 20–20-20 and TSP was statistically higher. Plants grown with the application of the 20–20-20 fertilizer in pots had the highest Mg (0.54%) concentrations in acidic soil. The control group plants took up the least Ca (27 mg pot−1); on the contrary, the plants grown under 20–20-20 and DAP-applied conditions took up the highest amount of Ca (76 and 73 mg pot−1, respectively) from the soil in acidic soil. The other fertilizers had a similar effect on the plant Ca uptake. As for calcareous soil, the struvite applications yielded relatively lower plant Ca concentrations than both the control and chemical fertilizers. The highest Ca uptake was obtained from the 20–20-20 fertilizer application. The 20–20-20 fertilizer on the shoot Mg concentration showed the strongest effect. The control group plant had the lowest Mg concentration in acidic soil. The plants grown under other fertilizers and struvites took up higher Mg from the soil in acidic soil. In calcareous soil, the struvite applications yielded more plant Mg concentrations than the control and chemical fertilizers. The results of the Mg uptake are in parallel with the Mg uptake.

Table 7 Comparison of the effects of struvites and chemical fertilizers on N, K, Ca, and Mg concentrations and uptakes of shoot of the lettuce
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3.5 Shoot Fe, Cu, Zn and Mn Concentrations and Uptakes

The shoot micronutrient concentrations and uptakes are given in Table 8. The plant Fe concentrations varied between 174 and 306 mg kg−1 in acidic soil. It can be said that the control and the 20–20-20 group plants had higher Fe concentrations; on the contrary, the STR1 group plants had the lowest concentrations. Similarly, the 20–20-20 fertilizer resulted in the highest plant Fe uptake. The control, MAP, and TSP had the weakest effect on the Fe uptake. In calcareous soil, the highest Fe concentration and Fe uptake were acquired from the DAP-fertilized plants. The plant Cu concentrations were found to be between 1.3 and 4.50 mg kg−1 in acidic soil and between 8 and 14.5 mg kg−1 in calcareous soil. The highest Cu concentration and Cu uptake were obtained from the STR2-fertilized plants in acidic soil and the DAP-fertilized plants in calcareous soil. The concentration of Zn in the control was the highest in the plants grown in acidic soil. However, the lowest Zn concentration was detected in the plants grown in the STR1-applied pots. The plant Zn uptake, contrary to the plant Zn concentration, was lower in the control and TSP-applied plants. The 20–20-20 was the only fertilizer distinguished from the other fertilizers with the highest effect on the plant Mn uptake.

Table 8 Comparison of the effects of struvite and chemical fertilizers on Fe, Cu, Zn, and Mn concentrations and uptakes of shoot of the lettuce
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4 Discussion

Struvite is a magnesium ammonium phosphate crystal with a chemical formula of MgNH4PO4·6H2O. According to the chemical formula, the theoretical fertilizer value of struvite is 12.5% P, 5.7% N, and 9.9% Mg (Latifian et al. 2012). While the results obtained in some studies were compatible (Johnston and Richards 2003; Uysal et al. 2014) with the theoretical values, they are incompatible in other studies (Li and Zhao 2003; Ryu et al. 2012; Achat et al. 2014; Ryu and Lee 2016; Uysal and Kuru 2015; Liu et al. 2016). Our results showed that the P content for both STR1 and STR2 was lower than the theoretical value. Given that the current legislation of the European Commission (EC 2019) prescribes a minimum P content within the precipitated phosphate salt of 7% on a dry basis, it can be concluded that the precipitates produced had a satisfactory P content. Regarding the results of N and Mg, the values we determined are below the theoretical values. It should be noteworthy to mention that the STR1 and STR2 were produced from a waste stream and it is possible that they might contain some impurities other than the constituent ions of struvite (NH4+, Mg2+, PO43−), implying the reason for why the determined content of P, N, and Mg for the STR1 and STR2 were lower than that of the theoretical values. Despite all this, our results are in agreement with the struvite properties previously obtained from different sources with different processes (Li and Zhao 2003; Achat et al. 2014). Wadchasit et al. (2023) studied struvite precipitation from liquid anaerobic digestion effluents at a pH range of 8–10 without external sources of Mg2+ and PO43−. According to the results, the precipitates formed under the optimum pH condition of 9.0 was mainly composed of the elements of the constituent ions of struvite (NH4+, Mg2+, PO43−) with a mass% of 32.6% oxygen (O), 5.9% P, 6.9% Mg, and 2.4% N as well as 36.7% carbon, 5.8% K, 4.5% Na, and other elements such as Ca, Si, Cl, and S with lower mass% values. Based on the determined elemental composition, it was concluded that the presence of N, Mg, P, and O indicated that the crystalline component mainly consisted of struvite. In contrast, the presence of the other elements suggested that the precipitation process not only led to the formation of struvite crystals. In conclusion, it was reported that it was difficult to obtain pure struvites because liquid anaerobic digestion effluents had other organic components that gathered on the struvite crystals, which might result in observing an elemental composition for Mg, N, and P lower than that of the theoretical values. Differences in nutrient contents of struvites may be due to the variations of the source and recovery process as reported by different studies mentioned by Ahmed et al. (2018). The struvites have a low solubility and are slowly dissolved into water. However, they have a relatively high solubility in weak acid. The solubility of STR1 and STR2 showed that 4.8% and 19.8% of the total P were water-soluble, while 93% and 81% of the total P were soluble in citric acid. The results indicating the solubility of both struvites are in accordance with previous studies (Kern et al. 2008; Cabeza et al. 2011). Compared to the solubility of struvites with chemical fertilizers, it can be said that their water solubility is low (Massey et al. 2009).

Although the two different soils were not statistically compared, it was observed that there was 33% difference in favor of calcareous soil compared to acidic soil in terms of the marketable fresh total weight of the plants grown in the control groups. It should be noted that the experiment was carried out with three replicates. It was thought that the yield difference observed between the two soils was due to their complex physical, chemical, and biological differences. In addition, the difference in soil–plant interaction in both soils and its different reflections on plant growth may be another reason for the difference in yields (Pinto et al. 2014; Schreiter et al. 2014). The results of this study showed that the chemical fertilizer and struvite applications used in the experiment had a positive effect on plant growth in both soils, and accordingly, an increase in plant fresh and dry weights was obtained. Based on the treatments on plant fresh and dry weights, it was seen that the effects of both struvites were superior or equal to other chemical fertilizers. Similar results were reported in previous studies conducted on maize -tomato, triticale-oat-corn-alfalfa, ryegrass, and some medicine plants (Uysal et al. 2014; Hilt et al. 2016; Watson et al. 2019; Yetilmezsoy et al. 2020). Struvites are available in powder form, which may play a significant role in their ability to promote plant growth and nutrient uptake as effectively as chemical fertilizers. Granular struvite was found to be ineffective in a study comparing the agronomic activities of struvite and MAP fertilizer in acidic and calcareous soils; however, ground struvite was found to be at least as effective as MAP (Degryse et al. 2017). According to Nelson (2000), N uptake increased as struvite crystal size decreased. On the other hand, inconsistent results were observed between struvite and various P sources on plant growth across various soils in various studies (Nongqwenga et al. 2017). These findings demonstrated that the effectiveness of the struvite and other P sources is closely related to various soil elements, including soil P sorption capacity, initial soil P, soil pH, ion balance, and so on (Uwumarongie-llori et al. 2012; Palma et al. 2015; Boukhalfa-Deraoui et al. 2015a; Boukhalfa-Deraoui et al. 2015b; Huang et al. 2015; Khan et al. 2022; Mihoub et al. 2022). A study examining the effectiveness of struvite in three different soils revealed that the results obtained vary according to the P adsorption capacity of the soils. While struvite and single super phosphate were less effective in soils with a high sorption capacity, they were more effective in soils with a low sorption capacity (Nongqwenga et al. 2017).

The performance of struvites on shoot nutrient concentrations and uptakes were comparable or superior to most other chemical fertilizers in both acid and calcareous soils. Different studies conducted in acid soils with Chinese cabbage, bird rapeseed, wheat varieties, and canola showed that struvite is either comparable or superior to the commercial fertilizers in terms of plant growth or mineral nutrition, mainly P and N (Ryu et al. 2015; Liu et al. 2016; Talboys et al. 2016; Katanda et al. 2016). In calcareous soil, both struvites were similarly superior or comparable to most of the fertilizer used for comparison in calcareous soil. The results may be unexpected because struvites are insufficiently soluble in calcareous soil. However, it was observed that struvite nutrients were available in this experiment. This may be because the pH of calcareous soil is below the level that will cause insolubility and/or precipitation of struvites. As reported before, struvite becomes insoluble and precipitates at a high pH ranging from 8 to 11 (Kabdasli et al. 2009). Hilt et al. (2016) reported that struvite applied in a calcareous soil with a pH less than 8 would also degrade, though at a slower rate compared to an acidic environment. Cerrillo et al. (2015) found struvite comparable to DAP for lettuce yield and P concentration. In our study, struvites were found to be superior to DAP and MAP for P concentrations and uptakes. On the other hand, Ackerman et al. (2013) reported that struvite was inferior to MAP in calcareous soil, with a pH of 7.7 for both dry weight and P uptake by the canola plants. The main factor contributing to the effectiveness of struvite applications on plant P concentration and uptake can be linked to the fact that the starter P levels of both soils are less than adequate (Hilt et al. 2016). The superior effects of struvites on plant growth and P nutrition may also be attributable to the Mg. As the fertilizers (DAP, MAP, TSP, and 20–20-20) used in the study do not contain Mg, no comparison was made with Mg-containing fertilizers such as Epsom salt or kieserite. While Mg has a direct effect on the development of the plant, it may have encouraged its uptake due to its synergetic effect with P (Plaza et al. 2007; González‐Ponce et al. 2009). Although the water solubility of struvite is relatively low compared to other chemical fertilizers, one of the reasons why plants benefit from them more than other chemical fertilizers is the higher production of root exudates that increase struvite solubility (Lyu et al. 2016; Talboys et al. 2016). On the other hand, it has been suggested that a gradual release of P from struvites reduces P fixation by supplying the P gradually to the rhizosphere, thereby limiting fixation and matching the plant's P demands in the growing season (Nyborg et al. 1998). The results obtained from the APR showed that the proportion of total applied P input to P uptake by the crop is generally low for all chemical fertilizers and both struvites. However, the calculated APR rates were similar to the results of previous studies. A study conducted on different soils with pH levels varying between 5.1 and 6.2 determined that the rate of APR from TSP and struvites by potato plant varied between 1.1% and 15.3% depending on the soils (Benjannet et al. 2019). In another study, the APR calculated from struvite by maize and grass was 7.3% and 4.8%, respectively, was 18.4% by maize, and 8.1% by grass from the ammonium phosphate (Szymanska et al. 2019). Although contradictory results have been obtained in various studies (Everaert et al. 2017; Benjannet et al. 2019), the APR value measured from struvites was higher than that of other chemical fertilizers.

The shoot N concentrations and uptakes after struvite applications were lower than after the application of the 20–20-20 fertilizer for both acidic and calcareous soils. This showed that struvites can be an essential alternative to N sources (Ryu and Lee 2016; Liu et al. 2016). In addition to plant growth, struvites were superior to or competitive with other chemical fertilizers in terms of essential nutrient content such as N, K, and Mg as struvites contain significant amounts of N, Mg, and K in addition to P. In this study, in addition to 200 mg kg−1 P, 71 and 56 mg kg−1 N, 115 and 89 mg kg−1 K, 81 and 53 mg kg−1 Mg and 24 and 17 mg kg−1 Ca were also applied with the applications of STR1 and STR2, respectively. Gaterell et al. (2000) reported that the presence of Mg in struvite makes it an attractive alternative to chemical fertilizers. There was no superior effect of struvites on microelement concentrations of plants compared to the other chemical fertilizers. This might be because there are enough microelements in the original soils or insufficient struvite microelements available to the plants. Compared to the other fertilizers, struvites were the most inferior source of plant Ca nutrition in both soils. The least efficient chemical fertilizer for plant growth was TSP. Although it had the highest P level, it was the least efficient one on most of the shoot nutrient concentrations including P. When the effectiveness of fertilizers on the Ca nutrition of plants grown in calcareous soil was examined, it was unexpected that TSP, which contains Ca, led to a significantly lower Ca supply than some fertilizers that do not contain Ca. This may be due to lower dissolution of TSP granules in this soil (Lombi et al. 2005).

5 Conclusion

In this study, we have demonstrated that both struvites produced in different molar ratios were found to be competitive and even superior to the other commercial fertilizers, DAP (di-ammonium phosphate), MAP (monoammonium phosphate), TSP (triple superphosphate), and 20–20-20 (NPK fertilizer), in terms of plant growth, P nutrition, P uptake, and APR (the apparent phosphorus recovery) under acidic and calcareous soil conditions. Our results revealed that struvites made significant contributions not only to the P nutrition of the plant but also to other nutrients, especially Mg, N, and K. In these respects, they were superior to chemical fertilizers. The findings from the experiment indicate that both struvites have relatively high fertilizer efficiencies, and therefore, they can be used as an alternative fertilization material in both acid and calcareous soils. However, further investigations are required to explore the potential use of struvites under field conditions with different soil properties and different P sorption capacities and at lower application rates.