Efficiency of Vivianite from Water Purification Depending on Its Mixing with Superphosphate and Application Method

Citation: Ayeyemi, T.; Recena, R.; García-López, A.M.; Quintero, J.M.; del Campillo, M.C.; Delgado, A. Efficiency of Vivianite from Water Purification Depending on Its Mixing with Superphosphate and Application Method. Agronomy 2024,14, 2639. https://doi.org/10.3390/ agronomy14112639 Academic Editor: Koffi Djaman Received: 10 October 2024 Revised: 3 November 2024 Accepted: 5 November 2024 Published: 8 November 2024 Copyright: © 2024 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). Article Efficiency of Vivianite from Water Purification Depending on Its Mixing with Superphosphate and Application Method Tolulope Ayeyemi 1, Ramiro Recena 1, Ana María García-López 1, JoséManuel Quintero 1, María Carmen del Campillo 2and Antonio Delgado 1,* 1Department of Agronomy, University of Sevilla, ETSIA, Ctra. Utrera km 1, 41013 Sevilla, Spain 2Department of Agronomy, University of Córdoba, Edificio C4, Campus de Rabanales, 14071 Córdoba, Spain *Correspondence: [email protected] Abstract: Vivianite precipitation is gaining attention in phosphorus (P) removal from water purification. It is an iron (Fe)- and P-rich compound that can be used as a slow-release P fertilizer. However, this slow release can constrain P supply to crops in the initial growing stages. This limitation can be overcome by mixing with soluble P fertilizers and with banding application. Thus, the objective of this study was to assess the fertilizer effect of vivianite and superphosphate mixtures and determine the most effective application method for vivianite and its mixture with superphosphate as a soluble fertilizer. A pot experiment was conducted by growing sunflowers in calcareous soil under controlled conditions involving two factors. The first factor was the combinations of vivianite and superphosphate: 100% Vivianite + 0% Superphosphate –T2–, 70% Vivianite + 30% Superphosphate –T3–, 30% Vivianite + 70% Superphosphate –T4–, 0% Vivianite + 100% Superphosphate –T5—at a single P rate of 50 mg P kg −1 and a non-fertilized control –T1–. The second factor was the application method: (i) mixing vivianite powder with the bulk soil and (ii) applying it in bands at three points around the plants. The dry matter (DM) yield in the roots and shoots of the sunflower when all P was applied as superphosphate was higher than when it was applied as vivianite. However, the combination of superphosphate and vivianite in different proportions (T3 and T4) led to a considerably higher DM yield compared to sole vivianite application (T2). The highest plant P uptake was observed in T5, while the lowest was in T1 and T2. The replacement values on a dry matter (PFRV DM ) and P uptake (PFRV P Uptake ) basis and the nutrient use efficiency of T3 and T4 were higher than that of T2. However, the PFRV DM and the PFRV P Uptake were in the same range as the proportion of the superphosphate added to the fertilizer mix. Thus, increased P use efficiency could be achieved with mixtures of vivianite and superphosphate. However, the contribution of vivianite to the fertilizer mix is difficult to access in a short growing cycle. Hence, further research is recommended on the residual effect of vivianite in such fertilizer mix on subsequent growing cycles. Keywords: vivianite; superphosphate; fertilizer; sunflower 1. Introduction Increased agricultural production has been made possible with a considerable use of fertilizers containing nitrogen (N), phosphorus (P), and potassium (K) for decades [ 1 ]. Phosphorus fertilizers have been produced from phosphate rock since the end of the 19th century, and this raw material now faces challenges of rapid depletion [ 2 , 3 ]. Of all three primary macronutrients, the continued supply of P fertilizers for sustainable and global food security has been described as a looming crisis by several studies [ 1 , 4 , 5 ]. Therefore, there is an urgent and increasing need to make efficient use of the limited P reserves and search for alternative and sustainable sources of P to support crop production to feed the world’s rapidly growing population. Mineral P fertilizers such as superphosphate have been used by farmers to meet the P demand of agricultural crops for food production [ 6 , 7 ]. These fertilizers are highly Agronomy 2024,14, 2639. https://doi.org/10.3390/agronomy14112639 https://www.mdpi.com/journal/agronomy Agronomy 2024,14, 2639 2 of 15 water soluble, and when added to soils, they result in immediately high soil solution P concentrations for early crop growth [ 8 ]. However, a large proportion of the P from mineral P fertilizer is readily adsorbed by soil particles, thus resulting in a pool of residual P [ 9 ]. A proportion of this P is unavailable to plants [ 10 ]. On the other hand, in soils where sorbing capacity is saturated, there is a possibility that P can be lost through leaching, or when bound to soil particles, it can be lost through runoff or erosion, thereby causing eutrophication in waterbodies [ 11 , 12 ]. As a result of its reactions in soil, when P demand is high at a later stage of crop growth, there is less P available for plant uptake and growth [ 13 ]. This may partly explain why a high supply of P in soluble form does not automatically translate to high crop biomass production in P-deficient soils [ 14 ]. However, P limitation at a later stage of crop growth is believed to have a reduced impact on production compared to P limitation at the early stage of crop growth [15]. With the imminent phosphate rock scarcity crisis, attention has been drawn to the recovery and recycling of P from different sources [ 1 , 16 , 17 ]. Vivianite is one of the P- removal products that has been identified as a potential P fertilizer product from the water purification process [ 18 ]. Removal of P from wastewater as vivianite is gaining attention since it has a high P concentration, and the recovery of this product is easier than other products, such as struvite, due to its magnetic properties [ 14 ]. Some promising results have been obtained using vivianite as a P fertilizer [ 14 , 19 ]. However, its efficiency is not fully comparable with that of soluble mineral fertilizer. Unlike superphosphate, vivianite has a low solubility [ 20 , 21 ] and probably releases P slowly and in a controlled manner. With this attribute, it is expected to be less susceptible to losses through leaching or reactions promoting a decrease in its availability to plants, such as the precipitation of insoluble metal phosphates. Although the release of P from vivianite at the early stages of crop growth may be too slow to meet crop demands at the beginning of the growing cycle, it may be able to serve as a source of P for crops at a later growing stage and a residual source of P for subsequent crops when compared to readily soluble P fertilizers. With the combined application of soluble phosphate and vivianite, the demand for P at the early stage of crop growth is supplied by the soluble form, while demand at the later stages of growth can be supplied by vivianite. This will allow crops to access P throughout all stages of growth. However, there is no evidence of the potential benefits of mixtures of soluble P fertilizers and vivianite on crops. According to the 4R nutrient stewardship, the method of fertilizer application plays a crucial role in ensuring positive nutrient use efficiency [ 22 ]. The banding of P fertilizers in soils with limited P availability has improved crop yields [ 23 ]. This is ascribed to the increased P availability in a volume of soil with a high root density. This enhances the absorption of the nutrient, which essentially moves to roots through diffusion [ 24 ]. In addition, the banding promotes a high concentration of P in a restricted volume of soil, leading to a saturation of the sorbent capacity of soils and, consequently, to an increased availability to plants [ 25 ]. In the case of vivianite, an increased solubilization and P release in the rhizosphere due to the release of low-molecular-weight acids has been shown in a study conducted by Yang et al. [ 26 ]. All these suggest that placing vivianite close to the root system with a banding application may increase its efficiency as a P fertilizer. Soluble fertilizers are usually applied in granule form to soils. However, mixing P fertilizers, especially less soluble products like struvite, in powder form with soil was effective in supplying P to crops in a study by Degryse et al. [ 27 ]. Thus, particle size and application method may have an effect on the efficiency of less soluble P fertilizers, such as vivianite. Hence, it is relevant to assess the best application method to achieve maximum efficiency in the use of vivianite as a P fertilizer. Therefore, the objective of this study was to evaluate the fertilizer effect of mixtures of a soluble P form, superphosphate, and vivianite and to determine the most effective application method, i.e., banding or mixed as powder with soil, when vivianite is applied as a P fertilizer to crops. Agronomy 2024,14, 2639 3 of 15 2. Materials and Methods 2.1. Experimental Design 2.1.1. Soil The soil used for this experiment was obtained from an olive grove in the Aljarafe area of Seville, Spain. It is located in the municipality of Bollulos de la Mitación (37 ◦ 19 ′ 10.6 ′′ N, 6 ◦ 07 ′ 58.7 ′′ W). The soil sample was taken at a depth of 0–15 cm, after which it was air-dried, homogenized, and sieved to 4 mm. This soil was selected because of its low Olsen P content (3.5 mg P kg −1 ). The soil was classified as a Calcic Haploxeralf, according to the Soil Taxonomy of the USDA [ 28 ]. The properties of the soil used in this experiment are shown in Table 1. Table 1. Soil properties. Clay Silt Sand CCE SOC pH EC CEC Ca Mg K Na Olsen P Fe Mn Zn Cu %dS m–1 cmolckg−1mg kg−1 15 19 66 3.30 0.45 8.34 0.13 10.45 9.43 0.53 0.42 0.07 3.5 10.7 13.4 0.43 19.6 CCE, Ca carbonate equivalent; SOC, soil organic C; EC, electrical conductivity; CEC, cation exchange capacity; Ca, Mg, K, and Na, exchangeable cations; Fe, Mn, Zn, and Cu extracted with DTPA, Diethylenetriaminepentaacetic acid. 2.1.2. Experimental Factors: Fertilizer Treatments and Application Methods In this experiment, two treatment factors were considered. 1. Fertilizer Treatment: Two sources of P (vivianite and superphosphate) were used in this experiment. We used four different combinations of vivianite and superphosphate, and a non-fertilized control making a total of five fertilizer treatments. The summary of the five fertilizer treatments is given below: 0% Vivianite + 0% Superphosphate −Control −T1 100% Vivianite + 0% Superphosphate −T2 70% Vivianite + 30% Superphosphate −T3 30% Vivianite + 70% Superphosphate −T4 0% Vivianite + 100% Superphosphate −T5 The vivianite used in this experiment was recovered from Wastewater Treatment Plants (WWTPs) during the process of water purification, and it was provided by Wetsus (European Centre of Excellence for Sustainable Water Technology) from Leeuwarden, the Netherlands. The elemental composition was determined with the use of ICP-OES after acid digestion. The Fe 2+ to Fe 3+ ratio was determined by Mössbauer and X-ray photoelectron spectroscopy (XPS). The scanning electron microscope image of the recovered vivianite was obtained with the use of a microscope (JEOL JSM 6300, Freising, Germany) at the SCAI— University of Cordoba (Spain) facilities. Monocalcium phosphate (Ca(H 2 PO 4 ) 2 .H 2 O) was used as soluble P fertilizer (superphosphate). 2. Fertilizer Application Method: Two fertilizer application methods were used in the experiment. (a) Banding (fertilizer product placed at certain points in the soil in the pot around the plants). (b) Powder mix (mixing of fertilizer product with soil). Agronomy 2024,14, 2639 4 of 15 2.1.3. Plant Material The crop selected for this experiment was sunflower (Helianthus annuus L.) variety LG CLP clearfield 5 2 1 High oleic. Sunflower was selected because it can provide a substantial amount of biomass and high nutrient uptake in short-term experiments. The seeds were pre-germinated in darkness for four days, after which they were transplanted. 2.1.4. Pot Experimental Design The pot experiment was conducted in a randomized block design with five replicates for each treatment. Each pot corresponded to a replication with one sunflower plant. As mentioned above, two factors were involved in this experiment (i) fertilizer treatments—vivianite and superphosphate combined in four different proportions at a single application rate of 50 mg P kg −1 and control without the application of P fertilizer (five fertilizer treatments), and (ii) two fertilizer application methods (banding and powder mix). 2.1.5. Experimental Setup A 300 g portion of soil was weighed (Ohaus AR3130, Shanghai, China) and placed in transparent cylindrical polyethylene pots with a capacity of 350 mL (height 150 mm, diameter 55 mm). For the powder mix fertilizer application method, the corresponding dose of the fertilizer product was carefully mixed with the soil in powder form two days before transplanting the sunflower seedlings. For the banded (or localized) application, the corresponding dose of the fertilizer product was applied at three points in the pot at a depth of approximately 10 cm. Special care was taken to ensure that the small root of the sunflower did not come into direct contact with the applied products or the walls of the pot. Pots were then placed in the growing chamber with temperatures of 25 ◦ C/18 ◦ C day/night. During the first week of the experiment, the sunflower plants were only irrigated with water, after which a P and Fe nutrient-free Hoagland solution (pH 6–6.5) was applied between 30–40 mL per irrigation periodically. The composition of the solution was (all concentrations in mmol L –1 ): MgSO 4 (2), Ca(NO 3 ) 2 (5), KNO 3 (5), KCl (0.05), H 3 BO 3 (0.024), MnCl 2 (0.0023), CuSO 4 (0.0005), ZnSO 4 (0.002) and H 2 MoO 4 (0.0005). The volume of irrigation solution at the end of the cropping period was 1.64 L pot−1. 2.2. Collection and Analysis of Plant and Soil Samples 2.2.1. Collection of Plant and Soil Samples At the end of the experiment (94 days), the root, shoot, and flower heads of sunflower plants were collected separately. The root was carefully separated from the soil, washed with deionized water, and dried. All parts were left to dry in a forced air oven dryer (J.P Selecta, Incubig, Barcelona, Spain) at a constant temperature of 65 ◦ C for 48 h, after which the dry matter (DM) in each organ was weighed (Ohaus AR3130, Shanghai, China). Soil samples were also collected for Olsen P and DTPA Fe analyses. These samples were airdried and milled to pass through a 2 mm screen. 2.2.2. Plant and Soil Analysis at the End of the Experiment Plant organs were ground separately. 0.25 g of ground plant material was mineralized in porcelain crucibles in a muffle furnace (Carbolite, ELF 11/14B, Hope Valley, UK) at 550 ◦ C for eight hours. Subsequently, ashes were dissolved with 1 M HCl and heated at 100 ◦ C on a hotplate (J.P. Selecta, Barcelona, Spain) for 15 min. Phosphorus concentrations were determined by the colorimetric method according to Murphy & Riley [ 29 ], with subsequent measurement in a spectrophotometer (UNICAM UV2-200, Cambridge, UK) at 882 nm. Iron was determined by atomic absorption spectrophotometry (AAS) (UNICAM Solaar M5, Cambridge, UK). The total P uptake was determined as the sum of the product of the dry weight of each organ and its P concentration. Available P in soil after the sunflower crops were harvested was determined by the method of Olsen et al. [ 30 ]. To this end, two grams of each soil sample were weighed (Ohaus AR3130, Shanghai, China) into Agronomy 2024,14, 2639 5 of 15 50 mL falcon tubes, and 40 mL of NaHCO 3 0.5 M extractant at pH 8.5 was added. This combination was shaken (Heidolph, Promax 2020, Schwabach, Germany) for 30 min at 180 rpm and then centrifuged (Eppendorf AG, 5810, Hamburg, Germany) at 900 g for 10 min. The P content of the extract was determined by colorimetry following the methodology of Murphy & Riley [ 29 ] in a spectrophotometer (UNICAM UV2-200, Cambridge, UK) at 882 nm. The DTPA (Diethylenetriaminepentaacetic acid) extractable Fe determination was carried out according to Lindsay & Norvell [ 31 ] with slight modifications. Five grams of soil were weighed (Ohaus AR3130, Shanghai, China) into 50 mL falcon tubes, and 20 mL of DTPA/CaCl 2 TEA (triethanolamine) was added and stirred (Heidolph, Promax 2020, Schwabach, Germany) for two hours at 160 rpm. The suspension was then placed in the centrifuge (Eppendorf AG, 5810, Hamburg, Germany) for 15 min at 900 g. The Fe content of the extract was determined with the use of an Atomic Absorption Spectrometer (AAS) (UNICAM Solaar M5, Cambridge, UK). 2.3. Fertilizer Efficiency Indexes The P fertilizer replacement value (PFRV) was adapted from [ 32 ] as the amount of commercial mineral P fertilizer that is saved or replaced when using an alternative fertilizer (vivianite in this case) to obtain the same P uptake or yield. This was estimated on a P uptake basis as: PFRVP Uptake = Puptakei−Puptakec Puptakes−Puptakec where, Puptake i is the P uptake by the crop in the treatment with alternative fertilizer i, Puptake c is the average P uptake in the non-fertilized control treatment, and Puptake s is the average P uptake in the superphosphate treatment at the same P rate as i. The PFRV was also estimated on a total dry matter (DM) basis as: PFRVDM = DMi−DMc DMs−DMc where, DM i is the DM of the crop in the treatment with alternative fertilizer i, DM c is the average DM of the control treatment, and DM s , is the average DM of the superphosphate treatment at the same P rate as i. PFRV DM can also be referred to as Agronomic Replacement Value (APV). The nutrient use efficiency (NUE) of the sunflower plant was estimated through the agronomic efficiency (AE), apparent P recovery (APR), and Olsen P recovery (OPR). Agronomic efficiency was adapted from [ 33 , 34 ], which is defined as the efficiency of applied fertilizer nutrients in increasing biomass yield. It is calculated as: AE = DMi−DMc Applied P rate where DM i is the DM of the crop in the treatment with alternative fertilizer i, DM c is the average DM of the control treatment and applied P rate is the rate of P fertilizer (50 mg P kg−1) used for the experiment. Apparent P recovery (APR) was adapted from [ 33 , 34 ] and is defined as the difference in P uptake between pots where P is applied and control pots where P is not applied relative to the amount of P fertilizer applied. It is calculated as: APR = Puptakei−Puptakec Applied P rate where, Puptake i is the P uptake by the crop in the treatment with alternative fertilizer i, Puptake c is the average P uptake in the non-fertilized control treatment, and applied P rate is the rate of P fertilizer used for the experiment. Agronomy 2024,14, 2639 6 of 15 Olsen P recovery (OPR) is defined as the difference in soil P availability index (measured as Olsen P) between pots where P is applied and control pots where P is not applied relative to the amount of P fertilizer applied. It is calculated as: OPR = Olsen Pi−Olsen Pc Applied P rate where Olsen P i is the Olsen pvalue of the treatment with alternative fertilizer i , Olsen P c is the Olsen pvalue of the control treatment and applied P rate is the rate of P fertilizer used for the experiment. 2.4. Statistical Analysis Statistical analysis was carried out with Statgraphics Centurion XVI (Statgraphics Technologies Inc., The Planes, VA, USA). Analysis of variance (ANOVA) was used to assess the effect of the factors (source of P—fertilizer treatment and fertilizer application method) and the interaction between factors. The homogeneity of the variance (Levene’s test) and normality (Smirnof-Kolmogorov test) were verified in all variables, transforming the data if necessary (power transformation). Means were compared using the Bonferroni test at p< 0.05. 3. Results 3.1. Fertilizer Properties The properties of the recovered vivianite used as a fertilizer in this experiment are shown in Table 2. Vivianite had 108 g kg –1 of P and 280 g kg –1 of Fe; it also contained significant amounts of Ca and Mg. There was a variation between the Fe 2+ to Fe 3+ ratio obtained by Mössbauer spectroscopy (75 to 25%) and that of XPS (X-ray photoelectron spectroscopy) (41 to 59%). A high heterogeneity in crystallinity and uneven crystal sizes is observed in the vivianite, as shown in the electron microscope image (Figure 1). Table 2. Elemental composition and proportion of Fe2+ and Fe3+ in the recovered vivianite. C nd N nd P108 g kg−1 K0.25 g kg−1 Ca 8.9 g kg−1 Mg 9.2 g kg−1 Fe 280 g kg−1 Zn 0.16 g kg−1 Mn 1.14 g kg−1 Cu 0.04 g kg−1 Mössbauer Fe speciation Fe2+ 75% Fe3+ 25% XPS Fe Speciation Fe2+ 41% Fe3+ 59% nd: not detectable, XPS, X-ray photoelectron spectroscopy. 3.2. Effect of Fertilizer on Plant Development and Nutrition The application methods did not have a significant effect on most of the parameters measured. There was also no interaction between the two factors considered in this experiment, i.e., fertilizer treatment and application methods, except for Fe uptake by crops. As a result, the effect of the fertilizer treatment as a main factor can be analyzed across both application methods. The dry matter yield obtained with the use of the tested fertilizer products is displayed in Figure 2. The two lowest shoot dry matter (DM) yields were observed in the control (T1) and 100% vivianite (T2) treatments; there were no significant Agronomy 2024,14, 2639 7 of 15 differences between these two treatments. The highest shoot dry matter (DM) yield was observed in T5 (100% Superphosphate). T3 (70% Vivianite + 30% superphosphate) and T4 (30% Vivianite + 70% Superphosphate) promoted significantly higher shoot DM yields than T1 and T2. The root DM yield followed a similar trend as the shoot DM yield, with the lowest root DM yield in T1 and T2 and the highest in T5, i.e., with all the P applied as superphosphate. This treatment had the highest P uptake, while the lowest was observed in the non-fertilized control treatment (T1; Figure 3). Although P uptake in the T2 treatment was slightly higher than in T1, there was no significant difference between the two. P uptake in T3 and T4 was significantly higher than in T1 and T2. In terms of Fe uptake, T4 and T5 had the highest Fe update, with no significant differences between them (Table 3). The lowest Fe uptake was obtained with the non-fertilized control (T1 treatment), which was not significantly different from the T2 and T3 treatments. Iron uptake was the only variable where a significant interaction between both factors was observed (Table 3). This was ascribed to the increased Fe uptake with banding relative to mixing with soil in the T4 treatment (Figure S1). The Fe concentration was observed to be significantly higher in the roots than in the aerial parts in all treatments (Table S1). However, the highest Fe concentration in the root was observed in the T2 treatment (all P applied as vivianite). Agronomy 2024, 14, 2639 6 of 15 Olsen P recovery (OPR) is defined as the difference in soil P availability index (measured as Olsen P) between pots where P is applied and control pots where P is not applied relative to the amount of P fertilizer applied. It is calculated as: OPR = Olsen P−Olsen P  Applied P rate where Olsen P i is the Olsen p value of the treatment with alternative fertilizer i , Olsen P c is the Olsen p value of the control treatment and applied P rate is the rate of P fertilizer used for the experiment. 2.4. Statistical Analysis Statistical analysis was carried out with Statgraphics Centurion XVI (Statgraphics Technologies Inc., The Planes, VA, USA). Analysis of variance (ANOVA) was used to assess the effect of the factors (source of P—fertilizer treatment and fertilizer application method) and the interaction between factors. The homogeneity of the variance (Levene’s test) and normality (Smirnof-Kolmogorov test) were verified in all variables, transforming the data if necessary (power transformation). Means were compared using the Bonferroni test at p < 0.05. 3. Results 3.1. Fertilizer Properties The properties of the recovered vivianite used as a fertilizer in this experiment are shown in Table 2. Vivianite had 108 g kg –1 of P and 280 g kg –1 of Fe; it also contained significant amounts of Ca and Mg. There was a variation between the Fe 2+ to Fe 3+ ratio obtained by Mössbauer spectroscopy (75 to 25%) and that of XPS (X-ray photoelectron spectroscopy) (41 to 59%). A high heterogeneity in crystallinity and uneven crystal sizes is observed in the vivianite, as shown in the electron microscope image (Figure 1). Figure 1. Electron microscope image of recovered vivianite. Table 2. Elemental composition and proportion of Fe 2+ and Fe 3+ in the recovered vivianite. C nd N nd P 108 g kg −1 K 0.25 g kg −1 Ca 8.9 g kg −1 Mg 9.2 g kg −1 Fe 280 g kg −1 Zn 0.16 g kg −1 Figure 1. Electron microscope image of recovered vivianite. Table 3. Effect of fertilizer treatment on sunflower plants Fe uptake and soil DTPA Fe. Fertilizer Treatment Fe Uptake (mg Plant−1) DTPA Fe (mg kg−1) T1 (non-fertilized control) 0.70 ±0.05 b 4.82 ±0.26 ab T2 (100% vivianite) 0.99 ±0.18 b 4.97 ±0.23 a T3 (70% vivianite + 30% superphosphate) 1.50 ±0.27 b 4.25 ±0.15 bc T4 (30% vivianite + 70% superphosphate) 3.43 ±0.80 a 4.22 ±0.15 bc T5 (100% superphosphate 3.43 ±0.57 a 4.20 ±0.22 c Anova pValue Fertilizer Treatment (A) 0.0000 0.0005 Fertilizer Location (B) 0.0074 0.5627 A×B 0.0004 0.2770 Means ± standard error; means with the same letter were not significantly different according to the Bonferroni test (p< 0.05). DTPA, Diethylenetriaminepentaacetic acid. Agronomy 2024,14, 2639 8 of 15 Agronomy 2024, 14, 2639 8 of 15 Figure 2. Effect of fertilizer treatments on dry matter yield of sunflower plants. Means with the same letter were not significantly different according to the Bonferroni test (p < 0.05). Error bars represent the standard error for each fertilizer treatment. T1, non-fertilized control; T2, 100% vivianite; T3, 70% vivianite + 30% superphosphate; T4, 30% vivianite + 70% superphosphate; T5, 100% superphosphate. Figure 2. Effect of fertilizer treatments on dry matter yield of sunflower plants. Means with the same letter were not significantly different according to the Bonferroni test (p< 0.05). Error bars represent the standard error for each fertilizer treatment. T1, non-fertilized control; T2, 100% vivianite; T3, 70% vivianite + 30% superphosphate; T4, 30% vivianite + 70% superphosphate; T5, 100% superphosphate. Agronomy 2024, 14, 2639 8 of 15 Figure 2. Effect of fertilizer treatments on dry matter yield of sunflower plants. Means with the same letter were not significantly different according to the Bonferroni test (p < 0.05). Error bars represent the standard error for each fertilizer treatment. T1, non-fertilized control; T2, 100% vivianite; T3, 70% vivianite + 30% superphosphate; T4, 30% vivianite + 70% superphosphate; T5, 100% superphosphate. Figure 3. Effect of fertilizer treatments on P uptake of sunflower plants. Means with the same letter were not significantly different according to the Bonferroni test (p< 0.05). Error bars represent the standard error for each fertilizer treatment. T1, non-fertilized control; T2, 100% vivianite; T3, 70% vivianite + 30% superphosphate; T4, 30% vivianite + 70% superphosphate; T5, 100% superphosphate. Agronomy 2024,14, 2639 9 of 15 3.3. Phosphorus Fertilizer Replacement Value and Nutrient Use Efficiency Treatments T3 and T4 had a significantly higher PFRV DM (29% and 65%, respectively) compared with T2 (100% vivianite), which had 2.6%. However, PFRV P Uptake followed a similar trend, with slightly higher values than PFRV DM . The PFRV P Uptake of T3 and T4 was 34% and 74%, respectively, while that of T2 was 4.9% (Figure 4). The nutrient use efficiency estimated by the parameters, agronomic efficiency (AE), apparent P recovery (APR), and Olsen P recovery (OPR) followed a similar trend (Table 4). The highest AE, APR, and OPR were observed in T5 (100% superphosphate), while the lowest was in T2. T3 and T4 showed significantly higher efficiency parameters than T2 (100% vivianite). Agronomy 2024, 14, 2639 9 of 15 Figure 3. Effect of fertilizer treatments on P uptake of sunflower plants. Means with the same letter were not significantly different according to the Bonferroni test (p < 0.05). Error bars represent the standard error for each fertilizer treatment. T1, non-fertilized control; T2, 100% vivianite; T3, 70% vivianite + 30% superphosphate; T4, 30% vivianite + 70% superphosphate; T5, 100% superphosphate. 3.3. Phosphorus Fertilizer Replacement Value and Nutrient Use Efficiency Treatments T3 and T4 had a significantly higher PFRV DM (29% and 65%, respectively) compared with T2 (100% vivianite), which had 2.6%. However, PFRV P Uptake followed a similar trend, with slightly higher values than PFRV DM . The PFRV P Uptake of T3 and T4 was 34% and 74%, respectively, while that of T2 was 4.9% (Figure 4). The nutrient use efficiency estimated by the parameters, agronomic efficiency (AE), apparent P recovery (APR), and Olsen P recovery (OPR) followed a similar trend (Table 4). The highest AE, APR, and OPR were observed in T5 (100% superphosphate), while the lowest was in T2. T3 and T4 showed significantly higher efficiency parameters than T2 (100% vivianite). Figure 4. Effect of fertilizer treatments on PFRV on DM and P uptake basis for sunflower plants. Means with the same letter were not significantly different according to the Bonferroni test (p < 0.05), with PFRV DM and PFRV P Uptake analyzed independently. Error bars represent the standard error for each fertilizer treatment. T2, 100% vivianite; T3, 70% vivianite + 30% superphosphate; T4, 30% vivianite + 70% superphosphate. Table 4. Effect of fertilizer treatments on agronomic efficiency, apparent P recovery, and Olsen P recovery. Fertilizer Treatment Agronomic Efficiency (AE) Apparent P Recovery (APR) Olsen P Recovery (OPR) kg DM g –1 P kg kg –1 kg kg –1 T2 (100% vivianite) 0.018 ± 0.005 d 0.016 ± 0.011 d 0.009 ± 0.014 ab T3 (70% vivianite + 30% superphosphate 0.214 ± 0.010 c 0.142 ± 0.006 c 0.000 ± 0.010 b Figure 4. Effect of fertilizer treatments on PFRV on DM and P uptake basis for sunflower plants. Means with the same letter were not significantly different according to the Bonferroni test (p< 0.05), with PFRV DM and PFRV P Uptake analyzed independently. Error bars represent the standard error for each fertilizer treatment. T2, 100% vivianite; T3, 70% vivianite + 30% superphosphate; T4, 30% vivianite + 70% superphosphate. Table 4. Effect of fertilizer treatments on agronomic efficiency, apparent P recovery, and Olsen P recovery. Fertilizer Treatment Agronomic Efficiency (AE) Apparent P Recovery (APR) Olsen P Recovery (OPR) kg DM g–1 P kg kg–1 kg kg–1 T2 (100% vivianite) 0.018 ±0.005 d 0.016 ±0.011 d 0.009 ±0.014 ab T3 (70% vivianite + 30% superphosphate 0.214 ±0.010 c 0.142 ±0.006 c 0.000 ±0.010 b T4 (30% vivianite + 70% superphosphate) 0.489 ±0.028 b 0.314 ±0.014 b 0.030 ±0.007 ab T5 (100% superphosphate) 0.747 ±0.035 a 0.427 ±0.014 a 0.044 ±0.004 a