Waste-Derived Fertilizers: Conversion Technologies, Circular Bioeconomy Perspectives and Agronomic Value

Academic Editor: Claudio Ciavatta Received: 27 July 2025 Revised: 8 September 2025 Accepted: 10 September 2025 Published: 11 September 2025 Citation: Hidalgo, D.; MartínMarroquín, J.M.; Corona, F.; Verdugo, F. Waste-Derived Fertilizers: Conversion Technologies, Circular Bioeconomy Perspectives and Agronomic Value. Agronomy 2025,15, 2167. https://doi.org/10.3390/ agronomy15092167 Copyright: © 2025 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/). Review Waste-Derived Fertilizers: Conversion Technologies, Circular Bioeconomy Perspectives and Agronomic Value Dolores Hidalgo * , Jesús M. Martín-Marroquín , Francisco Corona and Francisco Verdugo CARTIF Technology Centre, Circular Economy Area, 47151 Valladolid, Spain; [email protected] (F.V.) *Correspondence: [email protected] Abstract The urgent need to reduce the environmental footprint of agricultural inputs, alongside the rising cost and limited availability of mineral fertilizers, has encouraged the exploration of organic waste materials as alternative nutrient sources. This review examines how organic and nutrient-rich waste streams—from livestock excreta and food industry residues to sewage sludge and combustion by-products—can be converted into effective fertilizers through biological, thermal, and chemical processes, reducing dependence on mineral fertilizers and supporting more circular and resilient cropping systems. It also analyzes the agronomic performance of these waste-derived materials in terms of nutrient release dynamics, impact on soil functions, and crop response under different management conditions. Examples of commercial waste-derived fertilizers are included, together with their effects on soil quality and crop productivity, and regulatory frameworks and commercialization challenges are considered, with emphasis on current European policies and real-world product examples. The broader contribution of these practices to climate mitigation, resource recovery, and rural development is also discussed. While promising, the implementation of waste-based fertilization strategies requires further efforts in standardization, safety assurance, and farmer engagement. This article provides an integrated overview of the topic and identifies key areas for future research, innovation, and policy development in support of sustainable nutrient management. Keywords: biochar; biowaste; composting; environmental regulation; nutrient recovery; organic waste; soil health; sustainable fertilization 1. Introduction The mineral fertilizer industry is currently experiencing an unprecedented crisis due to the escalating cost of energy and raw materials. In 2024, global nitrogen fertilizer prices remained significantly elevated compared to pre-pandemic levels, with average nitrogen–phosphorus–potassium fertilizer costs reaching approximately USD 327 per tonne in May—more than three times higher than the 2019 average of around USD 100 per tonne [ 1 ]. This persistence reflects the strong dependency of nitrogen fertilizer manufacturing on natural gas, which accounted for about 90% of input costs during the 2022 peak [ 2 ]. Although natural gas prices declined from their mid-2022 highs, they remained elevated compared to historical norms. In Europe, benchmark gas prices averaged around €50/MWh in early 2025—about twice the level observed a year earlier—after peaking at €59/MWh in February 2025 [ 3 ]. Consequently, fertilizer production costs continued to be buoyed by energy prices. This price volatility, coupled with growing concerns about food Agronomy 2025,15, 2167 https://doi.org/10.3390/agronomy15092167 Agronomy 2025,15, 2167 2 of 28 security, is challenging the economic viability of conventional fertilization and accelerating the search for more sustainable alternatives. Simultaneously, environmental concerns related to the use of mineral fertilizers are intensifying. Excessive or poorly managed application of highly soluble nitrogen fertilizers contributes significantly to nitrogen losses through leaching and volatilization, resulting in nitrate accumulation in groundwater and emissions of nitrous oxide, a potent greenhouse gas with a global warming potential 298 times greater than carbon dioxide [ 4 ]. In the European Union (EU), agriculture accounts for over 90% of total ammonia emissions, largely due to nitrogen fertilizer use and livestock waste management [5]. Eutrophication of freshwater bodies, biodiversity loss, and degradation of drinking water sources are direct consequences of nutrient overloading from synthetic fertilizers. At the same time, the production of mineral fertilizers, particularly nitrogen-based products, is highly energyintensive and carbon-intensive. The Haber–Bosch process used to fix atmospheric nitrogen requires large amounts of natural gas, making fertilizer production one of the largest industrial consumers of fossil fuels. Globally, the International Fertilizer Association estimates that mineral fertilizer production contributes about 1.3% of total CO 2 emissions [ 6 ]. This double environmental burden—during both production and use phases—raises the urgency of identifying alternative fertilization strategies that not only reduce dependency on fossil energy but also mitigate nutrient losses. These challenges underscore the need to transition toward fertilizer systems that are both economically viable and environmentally sustainable. Addressing these challenges requires the identification of nutrient sources that can both reduce reliance on finite, fossil-based inputs and minimize environmental impacts. In this regard, valorizing residual organic streams offers a dual benefit: mitigating waste management issues while providing renewable nutrient inputs for agriculture. Organic waste materials generated from agricultural, agro-industrial, municipal, and biomass-based processes have emerged as promising sources of nutrients. These include livestock manure, compost, sewage sludge, digestates, vinasse, ashes, and biochar, all of which contain varying levels of nitrogen, phosphorus, potassium, and organic carbon [ 7 ]. Depending on origin and processing, these materials can partially or fully replace synthetic fertilizers, reducing reliance on non-renewable resources while contributing to waste management objectives. However, the direct land application of untreated organic residues poses risks such as odor emissions, pathogen transmission, heavy metal accumulation, and the introduction of microplastics or organic contaminants [ 8 ]. These risks necessitate the use of treatment technologies to stabilize and sanitize the material and improve its agronomic performance. Current strategies include biological treatments such as composting and anaerobic digestion, thermochemical methods like pyrolysis and hydrothermal carbonization (HTC), and chemical processes including struvite precipitation and ammonia stripping [9]. Each of these technologies transforms organic residues into more uniform and manageable products. For example, composting converts biodegradable organic matter into stable humus-like material rich in nutrients and beneficial microbes [ 10 ]. Anaerobic digestion yields digestate, a nutrient-rich slurry that can be used as a fertilizer or further processed into concentrated nutrient fractions [ 11 ]. Thermochemical routes, on the other hand, generate biochar or ash products that can serve as soil amendments, immobilize contaminants, and enhance nutrient retention [ 12 ]. Table 1provides a selection of commercial fertilizers currently produced from some biowaste streams, highlighting their processing methods, primary nutrient contents, and additional components. These examples demonstrate the technological versatility and growing market interest in alternative fertilizers derived from waste valorisation approaches. Most of the listed products are pelletized or granulated to Agronomy 2025,15, 2167 3 of 28 facilitate handling and field application, and several integrate mineral additives or undergo chemical stabilization to meet agronomic and safety requirements. Table 1. Examples of commercial fertilizers derived from biowaste sources. Biowaste Origin Processing Method or Additive Agents Main Nutrients (%) Secondary Components Physical Form Reference Sewage sludge Treated with oxidants (e.g., ClO2, ferrates), ammonia or phosphoric acid N: 10, P2O5: 23, K2O: 12 Fe: 1, S: 20 Granular [13,14] Sewage sludge Alkali hydrolysis using NaOH or KOH N: 4.5, P2O5: 7, K2O: 2.5 S, Ca, Fe, Mg Liquid [15] Sewage sludge Acid-ammonia treatment (H2SO4, H3PO4, NH3)N: 13–20, P2O5: 1–13 S: 14–24, Fe: 0.5–3 Granular/pellets [16] Sludge–ash blend Stabilized with fly ash and lime kiln dust N: 0.5, P2O5: 0.3, K2O: 0.1 Ca: 10, Mg: 4 Powdered/soil-like [17] Sludge-derived solid Treated with Fe salts, acids, and ammonia N: 16, P2O5: 4.6 S: 16, Fe: 1 Granular [18] Biosolids Mixed with conventional mineral fertilizers N: 4–15, P2O5: 1–10, K2O: 4–15 Ca: 2, Fe: 1 Granular [19] Poultry manure ash Blended with KCl, TSP, and chalk P2O5: 5–14, K2O: 12–20 Ca: 15 Granular [20] Poultry litter Composted or dried P2O5: 6, K2O: 3 Ca: 12 Pellets [21] Agro-industrial residues Pressed cakes and vegetable meals P2O5: 2.8–9, K2O: 2.5–15 S: 2–15 Granular [22] Livestock manure and digestate Composting and drying P2O5: 4, K2O: 3 S: 3 Granular [23] Meat and bone meal Combined with dolomite, lime or acids P2O5: 21–23, K2O: 3–4 SO3: 4.5 Granular [24] Municipal wastewaters Struvite precipitation using magnesium compounds N: 5, P2O5: 28 Mg: 10 Granular/pellets [25–27] Sewage sludge ash Phosphate recovery via acid–alkali extraction and precipitation P2O5: 21–22 Ca: 15–18 Powdered/granular [28,29] Note: All listed products provide a combination of macronutrients (N, P, K) and secondary/micronutrients (e.g., Ca, Mg, S, Fe), as shown in the “Main nutrients” and “Secondary components” columns. The relative agronomic value of each product is determined by this combined nutrient profile rather than a sole focus on one category. Agronomically, the performance of these waste-derived fertilizers varies depending on the form and availability of nutrients, organic matter content, and interactions with soil and crops. Some materials, such as struvite, offer controlled nutrient release and low leaching potential, while others, like untreated digestate, may have high nitrogen content but also high salinity or low phosphorus availability [ 30 ]. Field trials reported in the literature demonstrate that bio-based fertilizers can perform comparably or even surpass conventional mineral fertilizers in terms of crop yield and nutrient use efficiency, particularly when applied under site-specific agronomic conditions and integrated with optimized management strategies [31]. In addition to agronomic considerations, regulatory frameworks play a critical role in determining the market viability of waste-derived fertilizers. In the EU, regulation establishes common criteria for placing fertilizing products on the market, including those containing organic and secondary raw materials [ 32 ]. This regulation sets specific requirements for safety (e.g., limits for heavy metals and pathogens), nutrient content, and labeling. However, other legislative instruments continue to impose strict requirements for materials of urban or animal origin [ 33 ]. Under EU law, processed Category 2 and 3 animal by-products—i.e., materials not intended for human consumption, such as manure, former foodstuffs, or slaughterhouse by-products—must meet defined processing temperatures, Agronomy 2025,15, 2167 4 of 28 pathogen-inactivation standards, traceability and end-point criteria before they can be incorporated into CE-marked fertilising products. Despite the existence of a regulatory framework, practical implementation remains challenging. Many waste-based products fall into legal grey zones, leading to uncertainty for producers and users alike. Moreover, certification procedures for new products can be lengthy and costly, particularly for small or decentralized processing facilities. Market acceptance is also influenced by farmers’ perceptions, which are often shaped by concerns about product reliability, safety, and compatibility with existing fertilization regimes [34]. From a broader perspective, the valorization of organic residues into fertilizers embodies the core principles of the circular bioeconomy. First, it promotes nutrient loop closure by returning essential elements such as nitrogen, phosphorus, and potassium to agricultural soils, reducing dependence on finite mineral resources. Second, it mitigates environmental burdens by diverting biodegradable waste from landfills or uncontrolled disposal, thus lowering greenhouse gas emissions, nutrient leaching, and other pollution pathways. Third, it enables the creation of new value chains in which agricultural, agro-industrial, and municipal residues become feedstocks for bio-based fertilizer production, fostering rural development and job creation. This integrated approach directly contributes to global policy objectives, particularly the Sustainable Development Goals (SDGs) on zero hunger (SDG 2), clean water and sanitation (SDG 6), responsible consumption and production (SDG 12), and climate action (SDG 13), by advancing resource-efficient, low-emission, and resilient agricultural systems [8]. Furthermore, when integrated within local or regional contexts, waste-derived fertilizer production can foster rural development, support decentralized bioeconomy models, and enhance resilience to input volatility and supply disruptions. Nonetheless, a number of critical barriers still hinder the widespread adoption of these alternative fertilizers. Chief among them are regulatory uncertainties, particularly when materials originate from urban or animal waste streams, as well as burdensome certification processes and limited social acceptance. Farmers’ perceptions regarding the safety, consistency, and agronomic reliability of biobased products play a decisive role in their uptake, and remain shaped by past experiences, information asymmetry, and lack of standardized quality metrics. In addition, technical gaps persist around the optimization of formulations, delivery systems, and field performance under diverse agro-climatic conditions. As noted by Hidalgo et al. [ 9 ], modular and cost-effective processing units, robust environmental assessment methods, and harmonized agronomic testing protocols are urgently needed to advance this transition. This review aims to address these challenges by offering a narrative and critical overview of current knowledge, technologies, and regulatory aspects of fertilizer production from biowaste streams, integrating the identification of suitable residues, analysis of conversion processes, evaluation of agronomic performance, and discussion of sustainability and policy implications. The review draws from scientific literature, technical reports, and commercial examples to support science-based decision-making in policy, research, and practice. In addition, it provides an updated synthesis of EU regulatory developments and a comparative assessment of major waste streams, highlighting both mature and emerging processing technologies with proven scalability. Ultimately, this work seeks to provide actionable insights for farmers, industry stakeholders, and policymakers to facilitate the adoption of safe, efficient, and sustainable nutrient management solutions, thereby contributing to improved soil health, crop productivity, and circular economy objectives. 2. Biowaste Streams for Nutrient Recovery The transition towards circular and bio-based fertilization strategies relies fundamentally on the availability, characterization, and appropriate transformation of organic waste Agronomy 2025,15, 2167 5 of 28 streams. These streams, originating from agricultural, agro-industrial, municipal, and biomass-processing sectors, are not only abundant but also rich in essential nutrients such as nitrogen, phosphorus, potassium and carbon, often in organic or mineralizable forms. Their valorization into fertilizing products serves the dual objective of diverting waste from disposal and replacing fossil-derived mineral fertilizers [ 35 ]. The European regulatory framework—especially Regulation (EU) 2019/1009 on fertilizing products [ 36 ]—has opened the market to waste-derived materials that comply with specific safety, quality, and nutrient content criteria. However, their use is still constrained by complementary regulations. Fertilizers, under the EU’s Fertilising Products Regulation (FPR), are categorized into Product Function Categories (PFCs) and Component Material Categories (CMCs). PFCs define the product’s function (e.g., fertilizer, soil improver, etc.) while CMCs specify the types of materials that can be used to manufacture them. On the other hand, the Animal By-products Regulation EC 1069/2009 [ 37 ] governs the treatment and use of animal-origin wastes, requiring specific processing and end-point conditions for their legal transformation into fertilizers. Additionally, national interpretations of waste legislation and classification (e.g., whether a treated sludge is considered a product or a waste) can pose significant barriers to market access [38]. To fully unlock the bioeconomic potential of these materials, it is essential to distinguish between biowaste types, understand their nutrient profiles, and identify suitable treatment technologies for safe and efficient nutrient recovery. The following subsections provide a structured overview of major waste streams relevant to fertilizer production, summarizing their composition, transformation methods, agronomic potential, and regulatory context. 2.1. Livestock Manure Livestock manure is among the most abundant and widely generated organic residues in agriculture. It typically contains significant levels of organic matter and macronutrients, with concentrations ranging from 2 to 7% nitrogen, 1 to 3% phosphorus (as P 2 O 5 ), and 2 to 7% potassium (as K 2 O) on a dry matter basis, though these values vary depending on the animal species, feeding regime, and manure handling practices [ 9 ]. In its raw form, manure poses various environmental and sanitary risks, including ammonia volatilization, nitrate leaching, greenhouse gas emissions (particularly methane and nitrous oxide), and transmission of zoonotic pathogens. Additionally, its high water content and odor hinder long-distance transport and long-term storage. To address these issues, manure is increasingly subjected to stabilization and treatment processes aimed at improving its agronomic value and minimizing health and environmental hazards. Treatments such as composting, anaerobic digestion, drying, and pelleting improve stability, reduce volume and odor, and enhance storage and application properties [ 39 ]. The regulatory framework governing the use of manure-derived fertilizers in the EU is primarily defined by Regulation EC 1069/2009 [ 37 ], which categorizes animal by-products and prescribes mandatory hygienization treatments, particularly for Category 2 and 3 materials [ 37 ]. For manure, heat treatment to specific time–temperature combinations is required prior to its use as a fertilizing product, unless national rules establish less stringent conditions for certain uses. Additionally, products seeking CE marking under Regulation (EU) 2019/1009 must comply with pathogen thresholds, nutrient content limits, and labeling standards [36]. Despite these constraints, numerous commercial products based on livestock manure are available in EU markets. Companies such as Italpollina [ 23 ] and Fertikal [ 24 ] produce granular fertilizers by composting manure blended with vegetal materials or by mechanical dehydration and pelletizing. These products are often enriched with natural additives (e.g., leonardite, plant extracts, humic substances) or supplemented with mineral nutrients Agronomy 2025,15, 2167 6 of 28 to enhance agronomic performance [ 40 ]. Research and field trials confirm that manurebased fertilizers can improve soil structure, stimulate microbial activity, and deliver crop yields comparable to conventional mineral fertilizers, particularly when applied in integrated nutrient management schemes [ 39 ]. However, uptake by farmers often depends on factors such as odor control, ease of use, and product consistency. In this regard, advances in drying, pelletizing, and odor neutralization technologies have contributed to greater market acceptance and expanded international distribution [41]. 2.2. Sewage Sludge and Derivatives Beyond livestock manure, sewage sludge from urban wastewater treatment plants also represents a major nutrient-rich stream for fertilizer production (Figure 1). Sewage sludge, including its ash, represents a substantial byproduct of municipal wastewater treatment systems. It typically contains elevated levels of phosphorus—up to 6–7% P 2 O 5 on a dry weight basis—along with nitrogen, organic matter, and micronutrients. Owing to its high nutrient content, sewage sludge is widely considered a potential raw material for fertilizer production. However, its direct use is highly restricted by environmental regulations due to the frequent presence of heavy metals, pathogenic microorganisms, microplastics, and residues of pharmaceuticals. European legislation [ 36 ] sets stringent thresholds for contaminants, and national regulations often impose additional barriers, especially concerning sludge incineration residues. To mitigate these risks, sewage sludge is commonly subjected to stabilization processes such as alkaline treatment, composting, anaerobic digestion, or thermal technologies [ 42 ]. Among the latter, incineration is often the preferred strategy, especially in highly regulated contexts, as it reduces volume and inactivates pathogens. The resulting sewage sludge ash, while depleted in organic matter, retains a high mineral phosphorus content and can be valorized through chemical recovery techniques. These typically involve acidic or alkaline extraction followed by precipitation of phosphate salts, such as calcium phosphate or struvite [43]. Figure 1. Flowchart of the industrial process for converting sewage sludge into fertilizers. Industrial processes like TetraPhos, EuPhore, Metawater and Nippon PA [ 29 , 44 ] exemplify the commercial application of such wet-chemical recovery routes. The phosphate salts produced may be formulated into fertilizers, but their marketing as EU fertilizing products is contingent on compliance with CE marking rules, including contaminant limits and nutrient content specifications under Regulation (EU) 2019/1009 [ 36 ]. Still, regulatory gaps remain: for instance, ash derived from sewage sludge is not currently listed in Annex II of this EU regulation, which defines permissible component materials. Several European Agronomy 2025,15, 2167 7 of 28 countries have introduced additional national restrictions on the agricultural use of sewage sludge and its ashes, despite the existence of EU-wide regulations. For example, Poland explicitly prohibits the use of sewage sludge ash in commercial fertilizers. Germany is phasing out direct land application by 2032, mandating phosphorus recovery instead. In Sweden and Switzerland, policy trends point toward mandatory incineration or complete bans on land application. In Austria, certain regions have already banned the use of sludge and related products. These divergences reflect a fragmented regulatory landscape across the EU, driven by environmental, health, and societal concerns [ 45 ]. Such materials may only be used in specific contexts—such as soil reclamation—under quality-controlled composting or fermentation conditions. Even then, they must meet strict thresholds on contaminants and pathogen content, and their use remains controversial. Efforts are ongoing at the EU level to develop harmonized frameworks for including certain ashes as eligible component materials for fertilizers, but until that happens, Member States retain discretion over the authorization and classification of sludge-derived products. 2.3. Digestates In addition to sludge-derived products, digestates from anaerobic digestion processes constitute another significant source of nutrients suitable for agricultural reuse. Digestates are residues from the anaerobic digestion of biodegradable organic materials such as livestock manure, sewage sludge, agro-industrial residues, and food waste. They are increasingly recognized as a valuable nutrient source within circular bioeconomy strategies. Typically, digestates are separated into a solid and a liquid fraction: the solid phase is enriched in organic matter, phosphorus, and residual fiber, while the liquid phase retains most of the ammonium nitrogen and soluble potassium [ 46 ]. However, raw digestates often present challenges such as low dry matter content, high electrical conductivity, and poor handling properties. To overcome these limitations and enhance their agronomic performance, digestates are commonly treated using technologies such as drying, pelletization, ammonia stripping, membrane filtration (e.g., ultrafiltration), nitrification, and nutrient recovery processes (e.g., struvite precipitation) [ 47 ]. These treatments allow the concentration and stabilization of nutrients, reduce odor and microbial risks, and result in tailor-made formulations suitable for field application. From a regulatory perspective, digestates derived from biodegradable waste and certain animal by-products can be used in the manufacture of CE-marked fertilizing products under PFCs 1 (fertilizers) or 2 (soil improvers), as defined in Regulation (EU) 2019/1009 [ 36 ], provided they meet the criteria for pathogen inactivation, hygiene, and end-of-waste status. In particular, digestates that include manure or sewage sludge must comply with processing conditions and pathogen reduction standards laid out in Regulation EC 1069/2009 [ 37 ], and additional restrictions may apply depending on the Member State. Several commercial fertilizer products are now produced from processed digestates, particularly in granular or liquid form. They must comply with stringent EU and national regulations on nutrient content and contaminant levels, which can affect their market access. These may be marketed as organic fertilizers or as organo-mineral formulations when combined with supplementary mineral inputs to balance the NPK ratios. Producers often add natural additives such as biochar, humic acids, or plant-based bio-stimulants to enhance nutrient uptake, reduce volatilization losses, and improve soil structure [ 40 ]. Despite these advances, barriers to full-scale deployment include seasonal variability in feedstock composition, logistical issues in transport and storage, and uneven regulatory interpretation among EU countries. Digestate-based fertilizers represent a promising avenue for nutrient recycling, especially when integrated into regionally adapted nutrient management plans that valorize local waste streams and reduce reliance on imported synthetic fertilizers. Fur- Agronomy 2025,15, 2167 8 of 28 thermore, in the context of the evolving European energy landscape—marked by increased biogas deployment as a renewable energy vector—it is expected that the availability of digestate will grow exponentially over the coming years [ 48 ]. This anticipated expansion underscores the urgency of developing standardized treatment pathways, robust quality criteria, and efficient logistics for digestate valorization, ensuring both environmental protection and agronomic efficacy. 2.4. Food and Agro-Industrial By-Products Apart from animal and urban waste, industrial waste has also proven to be a relevant source of nutrients for fertilizer production. Press cakes, vinasse, brewers’ spent grains, and other vegetable by-products from agro-industrial operations are nutrient-rich residues with significant potential for nutrient and carbon recovery. Studies show, for example, that sugarcane vinasse carries high levels of potassium and organic carbon, and its use in fertigation has demonstrated reduced reliance on mineral K fertilizers while enhancing soil structure and water retention [ 49 ]. Filter cake (sugar mill by-product) is similarly effective as a phosphorus-rich soil conditioner, improving microbial activity and soil fertility [ 50 ]. On the other hand, brewers’ spent grain contains abundant organic carbon and protein, but high moisture (70–80%) and perishability limit its direct use. Ongoing innovation in this field includes the development of precision mixing strategies that leverage synergies between agro-industrial residues and other organic inputs to produce consistent, cropspecific fertilizer blends [51]. Recent developments in the valorization of agro-industrial by-products have emphasized not only their nutrient content but also their potential to act as carriers for microbial inoculants and biostimulants, enhancing both nutrient cycling and plant resilience under stress. For instance, fermented spent grains and oilseed press cakes have been successfully inoculated with Trichoderma spp. and Bacillus strains, showing improvements in nitrogen mineralization rates and suppression of soil-borne pathogens [ 52 ]. Innovative processing methods—such as bokashi fermentation, hydrothermal carbonization, or ensiling—are increasingly applied to stabilize high-moisture residues like vinasse or vegetable pomace, converting them into agronomically valuable, pathogen-free amendments with prolonged shelf-life [ 53 ]. Additionally, the integration of these residues into modular decentralized production systems is gaining traction, especially in regions with high agri-food density but limited access to commercial fertilizers. Such systems enable the on-site transformation of organic waste into tailored organo-mineral blends, reducing transport needs and fostering circular bioeconomy models at the territorial scale [54]. 2.5. Biochar and Ashes Alongside these organic residues, combustion by-products and ashes offer alternative mineral sources of nutrients, particularly phosphorus. Biochar, derived from the pyrolysis of biomass or organic waste, is gaining attention as a soil improver and carrier for nutrients. While low in intrinsic NPK content, its porous structure enhances nutrient retention and microbial activity in soils. When enriched with digestate or mineral salts, biochar can function as a slow-release fertilizer [ 55 ]. Similarly, poultry manure ash and combustion residues are increasingly used in phosphorus-rich fertilizer blends [ 56 ]. Their agronomic value depends on solubility, mineral form, and impurity levels. Incineration produces ash that can be valorized as a fertilizer. The characteristics and fertilizing potential of ash vary considerably depending on feedstock origin. Biomass-derived ashes are typically rich in potassium [ 57 ], while ashes from the combustion of slaughterhouse and meatprocessing waste may contain high concentrations of phosphorus [ 58 ]. Ashes also exhibit valuable microelement content and liming potential. However, a major constraint is their Agronomy 2025,15, 2167 9 of 28 frequent contamination with heavy metals, which may disqualify them from fertilizer use due to environmental safety regulations. Although ashes can be applied for soil reclamation, their inclusion in commercial fertilizers is not permitted under European Commission (EC) Regulation No 2003/2003. Within the scope of EU legislation and circular bioeconomy guidelines, ash materials should be recycled and reused when possible. In the absence of harmonized EU-level provisions, Member States may apply their own regulatory frameworks. Notably, work is ongoing to consider adding ash materials to Annex II of the EU list of permissible inputs for organic fertilizers, specifically under the category “Heavy Metals and Organic Compounds from Wastes Used as Organic Fertilizers” [59]. 2.6. Municipal Wastewaters Phosphorus recovery from municipal effluents via controlled precipitation (mainly as struvite: MgNH4PO4·6H2O) is a mature and expanding approach. Struvite is particularly valued for its slow-release characteristics and low leaching potential. Companies like Ostara, Phospaq, and AirPrex have operational installations in EU wastewater treatment plants [ 60 ]. The use of recovered struvite in agriculture is authorized under Regulation 2019/1009 when it meets the criteria for PFCs 1 (fertilizers) and specific CMCs. However, permitting varies by member state depending on whether the recovered material is classified as a waste or product at the national level [ 7 ]. Recent developments in nutrient recovery focus on integrated processes that combine crystallization with stripping technologies to simultaneously recover phosphorus and nitrogen from liquid effluents. These systems can achieve near-complete nutrient recovery (>90% P and N) by coupling magnesium addition for struvite precipitation with ammonia stripping columns that concentrate nitrogen into ammonium salts [ 61 ]. Pilot-scale implementations—such as the demonstration unit in the municipal wastewater treatment plant of Valladolid, Spain (Figure 2)—have shown the feasibility of producing marketable struvite while reducing nutrient loads in discharge streams. Figure 2. Pilot-scale nutrient recovery system installed at the Valladolid municipal wastewater treatment plant, combining ammonia stripping and struvite crystallization. Source: CARTIF. Agronomy 2025,15, 2167 16 of 28 three years with compost application [ 100 ]). This contributes to long-term soil fertility and can gradually enhance yields over time. Compost also tends to be safe in terms of pollutant uptake—even though it may introduce small amounts of heavy metals, field data show minimal transfer of these potentially toxic elements to crops in the short term [ 98 ]. In summary, compost alone can maintain moderate yields (often comparable to manure-based fertility) but typically cannot fully replace mineral N for high-demand crops without yield tradeoff. Farmers often use compost in integrated regimes, adding some mineral fertilizer to meet crop needs while relying on compost for soil health improvement and slow-release nutrition [101]. Anaerobic digestate contains more readily available nitrogen than compost, mostly as ammonium, and thus behaves more similarly to a mineral N fertilizer. Field trials in the last five years indicate that liquid digestate can effectively replace a substantial portion of mineral N fertilizer for crops like cereals and maize. For instance, a Belgian study on silage maize found that the liquid fraction of digestate, when incorporated into soil at equivalent N rates, produced biomass yields statistically on par with calcium ammonium nitrate fertilizer. In that trial, plots fertilized with liquid digestate achieved yields not significantly different from those receiving synthetic N, whereas unamended digestate or more concentrated ammonia products underperformed (yielding no better than the unfertilized control in that case) [ 99 ]. Similarly, a two-year field experiment in Croatia comparing digestate vs. mineral NPK on maize reported that full-dose digestate-based fertilization sustained 95–97% of the grain yield of conventional fertilizer (10.5–11.8 t/ha vs. 11.2–12.1 t/ha). However, halving the digestate application (to test partial substitution) led to significant yield reductions, underscoring that sufficient nutrient application rate is critical even with organic sources. These results suggest that when digestate is applied at agronomically equivalent N rates and properly incorporated to minimize ammonia losses, it can produce yields nearly equivalent to mineral N fertilizer [102]. Co-application trials also highlight the benefits of blending digestate with mineral fertilizer: combining digestate with some mineral N or P often yields equal or higher output than mineral fertilizer alone. In one study, a 50:50 mix of digestate (liquid or solid fraction) with mineral NPK produced maize yields as high as or higher than full mineral fertilization, taking advantage of both immediate and slow-release nutrient fractions [99]. On the other hand, limitations of raw digestate include its high water content (in liquid form) and ammonia volatility if not promptly incorporated. Field studies emphasize that injection or immediate incorporation of liquid digestate is needed to prevent N losses and odors, especially on warm days. Solid digestate (fiber fraction) releases N more slowly and often contains more phosphorus; its field performance is closer to that of compost—improving soil organic matter but providing less immediately available N. Overall, digestates are effective fertilizers, especially for nitrogen, but may require complementary P/K inputs (since many digestates have N-rich but P-lower nutrient ratios) and careful handling to match the timing of crop uptake [103]. 4.2. Biochar Amendments: Yield Effects and Soil Health Biochar itself is not a nutrient-rich fertilizer (unless enriched with nutrients), but its porous structure can retain nutrients and improve soil properties. Field results with biochar in the last five years have been mixed and context-dependent. In temperate Europe, adding pure biochar alone often shows little immediate yield increase and can even transiently depress crop growth if high rates are applied, due to nitrogen immobilization or pH effects. For example, a 4-year trial in Germany found that co-composting biochar with manure led to a slight yield decrease for the first-year cereal crop (spelt) at higher biochar rates, and marginal yield declines in winter wheat as well. No significant effects on a leguminous Agronomy 2025,15, 2167 17 of 28 fodder crop were observed, and only by the fourth year did maize yields show a slight increase in the biochar-amended plots [ 100 ]. These modest short-term impacts align with other field studies indicating that biochar’s benefits to yields often materialize under specific conditions (such as degraded, acidic, or drought-prone soils) or after an initial period as soil structure and microbial communities adjust [104]. The primary agronomic value of biochar lies in soil health improvements: increased soil organic carbon (with long-term carbon sequestration), better moisture retention, and reduced nutrient leaching. In the German trial, both compost and biochar applications raised soil C stocks significantly, with the combination yielding up to 10 t/ha additional soil organic carbon in three years [ 100 ]. Enhanced soil water holding capacity and cation exchange from biochar can support crop growth under stress conditions, even if routine yields under optimal conditions are unchanged. Researchers have also explored biochar-based fertilizers—soaking or co-composting biochar with nutrient sources to create slow-release formulations [ 105 ]. These “biochar-enriched” fertilizers often show more promise: a scoping review reported that biochar composites enriched with nutrients can improve yields more effectively than raw biochar alone [ 106 ]. In summary, biochar as a standalone amendment yields inconsistent short-term results in temperate field trials, but as a component of organic fertilizer blends (e.g., compost + biochar), it contributes to long-term soil quality and nutrient retention. Its advantages (carbon sequestration, soil improvement) must be balanced against its cost and the need to pair it with nutrient sources for significant yield impact. 4.3. Recovered Nutrient Fertilizers: Struvite, Ammonium Salts, and Ash-Based Products Struvite is a waste-derived phosphorus fertilizer (often recovered from wastewater) that has gained attention as a slow-release P source. Field studies show that struvite can perform comparably to conventional mineral P fertilizers (like superphosphate) in supplying crops with phosphorus, especially over a full growing season [ 97 ]. Because struvite granules dissolve slowly, immediate P availability can be lower than that of fully water-soluble fertilizers; however, plant-available P is released gradually in step with crop demand. A recent meta-analysis confirmed that crop responses (biomass yield, P uptake) to struvite increase in acidic soils (where struvite dissolves faster) and tend to match or even exceed responses to triple superphosphate in low-pH conditions. For instance, struvitefertilized plants in acidic soils (pH < 6) yielded higher biomass and P uptake than those fertilized with ammonium phosphate or superphosphate in one field experiment. By contrast, in neutral to alkaline soils, struvite’s slower dissolution can lead to slightly lower early growth or yield if no other P source is available [107]. That said, many European field trials and demonstrations (including on cereals, maize, and grassland) have found no significant yield penalty when using struvite as the P source, provided it is applied at agronomically equivalent P rates and appropriately placed (e.g., banding in the root zone). In some cases, struvite has even improved P use efficiency by reducing P losses—its slow-release nature means less leaching and runoff risk compared to soluble P fertilizers [ 107 , 108 ]. Advantages of struvite include its high P content ( ~5–12% P by weight) and low impurity levels, as well as its contribution of some nitrogen (around 5% N), which is released concurrently. It effectively “recycles” P from waste streams into a plant-accessible form, helping close the phosphorus loop. Limitations are that it is less effective in calcareous or high-pH soils (where dissolution is hindered) and its upfront cost can be higher given current recovery processes. Farmers may mitigate struvite’s slow initial release by granule size reduction or co-application—e.g., using a blend of struvite plus a small amount of soluble P fertilizer to ensure sufficient early-season P for seedlings. Agronomy 2025,15, 2167 18 of 28 Overall, real-world use has shown struvite to be a viable substitute for mined P fertilizers, with field trials in Europe reporting comparable crop yields and no adverse soil impacts when struvite replaces conventional P sources [109]. Ammonium-rich liquids and crystals obtained from waste (such as ammonium sulfate or ammonium nitrate derived from scrubbing biogas digestate or manure storage emissions) function very similarly to synthetic mineral N fertilizers. Chemically, these products contain nitrogen entirely in mineral forms (NH 4+ or NH 4 NO 3 ), so their plant availability is immediate and high. For example, ammonium sulfate solutions recovered via ammonia stripping of digestate have 100% of nitrogen in ammoniacal form, just like commercial mineral N fertilizers [99]. Agronomic tests indicate that such recovered N can achieve yields on par with conventional fertilizer N when applied at equivalent N rates and with proper handling. A large European field study (18 bio-based fertilizers across 4 sites) found that nitrogen-rich wastederived fertilizers generally produced similar crop yields to a mineral N reference at the same total N application [ 97 ]. The first-year replacement value of these fertilizers averaged ~70% of mineral N, meaning crops took up roughly 70% as much N from the bio-based products as from synthetic N in year one. This reflects some inevitable N losses or slower mineralization for certain organic-N materials in that mix, but many refined N products (like liquid ammonium sulfate) approach 80–100% equivalence. In practice, trials with ammonium sulfate from digestate report that it can substitute directly for synthetic N with no yield loss; any shortfall is usually due to handling or timing (e.g., volatilization if surface-applied). One noted benefit is that using these waste-derived N solutions can reduce overall farm emissions by capturing ammonia that would otherwise be lost to air, and then utilizing it on crops. Indeed, field measurements have shown that plots fertilized with digestate-derived N can have comparable soil nitrate levels and crop N uptake to those with calcium ammonium nitrate or urea, without increasing N 2 O emissions or leaching when managed properly [ 99 ]. The practical limitations mainly involve logistics: recovered N liquids tend to be dilute (e.g., 5–10% N), meaning larger volumes must be handled and sprayed, and storage can be an issue due to odor. There may also be trace contaminants (for instance, residual organics or chloride in certain processes) to monitor, though these are typically low if the product meets quality standards [ 110 ]. In summary, ammonium salt fertilizers from waste perform equivalently to conventional mineral N in agronomic terms, offering an effective way to recycle nitrogen if application is well-timed and calibrated. Various ashes from burned wastes—such as sewage sludge incineration ash, poultry litter ash, or wood ash—are used as sources of phosphorus, potassium, and lime. Their agronomic effectiveness depends on the nutrient content and solubility of the ash. Poultry litter ash is typically rich in P (often >10% P) and K, and is somewhat soluble; field trials have shown that it can produce significant yield responses. In one study, applying poultry manure ash at ~60 kg P/ha to a grassland resulted in higher herbage yields than an unfertilized control, demonstrating that crops could utilize nutrients from the ash [ 111 ]. Wood ash, commonly used in Nordic countries, provides K, Ca, and magnesium and acts as a liming agent; on acidic soils it has improved cereal yields and soil pH, though on neutral soils its effect is mainly to supply K. Sewage sludge ash contains substantial total P (often 5–8% P), but much of it is in insoluble mineral forms (e.g., aluminum or calcium phosphates). As a result, untreated sludge ash tends to release phosphorus slowly and often shows little short-term yield impact unless supplemented. For instance, in the same grassland trial, sewage sludge ash performed similarly to the no-P control in terms of crop yield over the 15-month period, indicating limited P availability to plants [ 111 ]. This aligns with other findings that sludge ash requires processing (such as acid extraction, thermal treatment, or granulation with additives) to become an effective P fertilizer [98]. Agronomy 2025,15, 2167 19 of 28 Advantages of ash products are that they recycle nutrients (especially P and K) from waste streams and can replace mined rock phosphate or K 2 O fertilizers. They also often have liming value (raising soil pH), particularly poultry and wood ash, which are alkaline. However, limitations and concerns include the potential for heavy metal accumulation (since metals in the original waste concentrate in the ash). Long-term field monitoring has found that repeated applications of sludge or compost-derived ash can introduce net inputs of metals like cadmium, copper, or zinc to the soil [ 112 ]. So far, plant uptake and soil toxicity effects have generally been negligible in trials, but regulatory limits may constrain how much ash fertilizer can be applied [ 113 ]. Another limitation is nutrient imbalances: ashes may supply excess P relative to N (since N is lost during incineration) and thus are best used in combination with a N source. In practice, ash-based fertilizers are often granulated or blended with other nutrients to improve their handling and efficacy (for example, some products mix sewage sludge ash with sulfuric acid to solubilize P, creating a fertilizer comparable to superphosphate). Field demonstrations in Europe (e.g., in the Ferticovery and Fertitec projects) are ongoing to optimize such formulations. In summary, ash fertilizers can effectively contribute P, K, and liming, but their field performance is variable—poultry litter ash is readily effective, while sewage sludge ash is a slow-release P source requiring further treatment or long-term perspective. Careful management is needed to avoid contaminant buildup and to integrate ashes into balanced fertilization plans (often by co-applying with N or using in soils that also need pH correction). 4.4. Influence of Formulation and Application Strategies on Performance How waste-derived fertilizers are formulated and applied strongly influences their agronomic outcomes. Researchers have experimented with pelletizing, blending, and coated formulations to improve the handling and nutrient release of biobased fertilizers. Many waste fertilizers (compost, digestate solids, struvite, etc.) have been processed into pellets or granules. Pelletizing increases bulk density and makes application with conventional spreaders feasible. It can also moderate the release rate—for example, dried digestate pellets release N slightly more slowly than liquid digestate, potentially reducing leaching. Recent field studies with pelletized digestate-based organo-mineral fertilizer showed excellent performance (nearly matching mineral NPK yields) and noted that the product’s slow-release nature helped provide sustained nutrition through the season [ 114 ]. Pelleted forms also ease transport and storage by reducing volume and odors. The drawback is some extra energy and cost for processing, and in some cases, pellets may need adequate moisture to break down in soil. Overall, converting wet, bulky wastes into granular fertilizer greatly improves farmer adoption and allows for more precise dosing. Combining waste-derived fertilizers with conventional fertilizers or with each other can capitalize on their complementary strengths. Many trials document that integrated nutrient management—using a portion of mineral fertilizer along with organic amendments—achieves the best outcomes. In two long-term experiments, substituting 25–50% of mineral N with organic wastes (compost, manures) maintained yields while improving soil fertility, whereas using either alone was suboptimal [ 115 ]. Co-application is especially useful for balancing nutrients, e.g., adding mineral N to compost, which is P-rich but N-poor, or adding a small amount of readily available P (like superphosphate) to struvite or sludge ash at planting. Such strategies ensure early crop needs are met by mineral nutrients while the waste fertilizer releases its nutrients more gradually. Field results support this approach—as noted, a 50:50 digestate plus mineral N regime produced maize yields equal to or above the mineral-only treatment in one trial [ 99 ]. Likewise, combining compost or biochar with mineral N often yields more than either alone, and Agronomy 2025,15, 2167 20 of 28 reduces the required synthetic fertilizer input by 20–50% without yield loss [ 115 ]. These findings underscore that partial substitution of mineral fertilizer with biowaste products can be performed successfully; a common recommendation is to replace a portion of N or P and monitor crop response, rather than 100% replacement in one go. The effectiveness of waste-derived fertilizers can be enhanced by precision in application. Techniques like banding or row placement have been beneficial for slow-release fertilizers (e.g., placing struvite in seed rows improved its P uptake efficiency, yielding higher grain P uptake than broadcasting it [ 99 ]. Timely application is also critical for N-rich wastes—applying digestate just before peak crop N demand, or splitting applications, can improve N recovery. Some trials in cooler climates show that delaying application of organic fertilizers until soils warm can avoid early-season nutrient immobilization [ 115 ]. In general, aligning nutrient release with crop growth stage (through timing or using nitrification inhibitors, etc.) is a key strategy to boost the agronomic performance of waste fertilizers. Inspired by enhanced-efficiency fertilizers in the mineral fertilizer sector, researchers have begun testing coatings on waste-derived fertilizers to further control nutrient release. For example, coating compost pellets with humic substances or biochar, or coating recovered ammonium sulfate with a polymer, could reduce initial nutrient losses [ 116 ]. While still experimental, these methods aim to combine the environmental benefits of recycled fertilizers with the targeted efficiency of slow-release products. Early greenhouse studies suggest that such modifications can reduce N 2 O emissions and nitrate leaching from digestate-based fertilizers without hurting yields, but field-scale evidence is limited so far [117]. 4.5. Performance Variability Across Soils and Climates Waste-derived fertilizers do not behave identically in all situations—soil type, climate, and crop choice can influence outcomes. Interestingly, some broad studies have found performance to be robust across different conditions. A recent multi-site European trial reported no significant interaction between fertilizer type and soil or climate variables, meaning the suite of biobased fertilizers tested performed consistently relative to mineral fertilizer across diverse sites (soils ranging from sandy loam to clay, and climates from cool temperate to Mediterranean). This suggests that many recycled fertilizers can be reliably used in various regions [97]. Nonetheless, specific products show known sensitivities: Soil pH: As discussed, struvite excels in acidic soils but may underperform in highpH calcareous soils [ 111 ]. Conversely, sewage sludge ash or basic wood ash releases P more readily under acidic conditions (and also helps raise pH). Biochar tends to have more positive effects in acidic or degraded soils (partly by liming effect and improving cation exchange), whereas in neutral fertile soils it might do little. Compost provides more noticeable benefits on sandy, low-humus soils (improving water and nutrient retention) than on heavy soils that already have high organic matter [118]. Soil texture and nutrient status: On light, sandy soils with low nutrient retention capacity, slow-release fertilizers—such as struvite, coated digestate, or biochar-amended compost—can significantly reduce leaching losses and help stabilize yields under droughtprone conditions. Conversely, on fertile soils with high baseline nutrient levels, particularly phosphorus, crop response to additional fertilization—whether mineral or organic—may be limited. This effect has been demonstrated in both field and lysimeter experiments. For example, Mancho et al. [ 119 ] observed that in sandy soils, struvite substantially reduced phosphorus leaching compared to soluble mineral fertilizers such as MAP or NPK. However, in P-rich soils, struvite still supplied adequate phosphorus without overapplication, whereas in low-P soils, its lower solubility risked limiting yield unless dosing and placement were optimized. Agronomy 2025,15, 2167 21 of 28 Climate and weather: Weather extremes can overshadow fertilizer effects. In field trials, year-to-year yield differences due to rainfall patterns have been observed regardless of fertilizer type [ 99 ]. However, waste-derived fertilizers sometimes show benefits under stress conditions, e.g., compost and biochar improving yield resilience in drought years by enhancing soil moisture, or slow-release N forms mitigating leaching during wet seasons. In cold climates or early spring applications, mineralization of organic N can be delayed; farmers in such regions may need to adjust timing (e.g., apply digestate a bit earlier than mineral N to account for any lag in availability). Overall, while the average performance of waste-derived fertilizers is comparable to that of mineral fertilizers in moderate conditions, farmers should consider site-specific factors. Fine-tuning application methods (as noted above) can help ensure these products meet crop demands under varying soil and weather scenarios. 4.6. Regulatory Considerations Throughout the manuscript, regulatory aspects influencing the use and market access of waste-derived fertilizers in the EU have been discussed in relation to specific product categories, including composts, digestates, biochar, struvite, ammonium salts, and ash-based products. These considerations encompass requirements under the Fertilising Products Regulation (EU) 2019/1009 [ 36 ], the Animal By-products Regulation EC 1069/2009 [ 37 ], and complementary national rules that can either enable or restrict commercialization. However, because these challenges are addressed in different parts of the text, a consolidated view can help clarify their scope and highlight common patterns. Table 4compiles this information into a single comparative overview, summarizing the main regulatory requirements and barriers for each fertilizer type. Table 4. Summary of key regulatory challenges for waste-derived fertilizers in the EU context, by product type. Fertilizer Type/Context Key Regulatory Challenge Practical Implication General/Cross-cutting Fertilising Products Regulation (EU) 2019/1009 (FPR) sets CE-marking criteria for safety, contaminants, nutrient content, and labeling; complementary regulations like EC 1069/2009 impose hygiene and processing rules for animal-origin materials. Producers must navigate multi-layered compliance; certification can be costly and time-consuming, especially for SMEs; national implementation differences add complexity. Compost Accepted under CMC of FPR when from source-separated biowaste; if containing animal by-products, must comply with EC 1069/2009 hygiene treatments. Additional processing required for ABP-containing compost; differences in national rules affect cross-border marketability. Digestate Recognized under FPR CMCs if from source-separated biodegradable waste; inclusion of manure/sewage sludge triggers EC 1069/2009 pathogen and hygiene standards. Dual compliance requirements (FPR + ABP); some Member States impose extra restrictions or permit limitations. Manure-derived fertilizers Regulated under EC 1069/2009 as Category 2/3 ABPs; must undergo specific hygienization (time–temperature) before use; FPR imposes additional safety and nutrient standards. Hygienization increases costs; national derogations may create uneven market access. Biochar Eligible under FPR only if produced from listed biomass materials; must comply with contaminant thresholds and not contain excluded waste types. Feedstock restrictions limit eligible sources; quality control required to meet contaminant and safety standards. Struvite Listed as a permissible CMC in FPR; must meet contaminant limits, nutrient content specifications, and labeling requirements. Slow inclusion in markets due to farmer awareness and placement strategies; stricter requirements in certain Member States. Recovered ammonium salts Permitted under FPR if purity, contaminant, and safety criteria are met; handling and storage must comply with chemical safety legislation. Transport and storage logistics affected by dilute nature; odor and handling concerns may influence acceptance. Sewage sludge ash Not currently listed in Annex II of FPR; national bans or phase-outs in some Member States; must comply with contaminant thresholds for specific uses. Market access restricted to non-fertilizer uses (e.g., soil reclamation); regulatory inclusion under discussion. Other ashes (e.g., poultry litter, wood ash) Some ashes permitted under FPR if meeting safety and contaminant criteria; others excluded unless included in future Annex II updates. Inconsistent acceptance across EU; potential heavy metal contamination can limit fertilizer use. Emerging materials Excluded from current CMC list; Draft CMC 11 proposed to include residues from agro-industrial, fermentation, and smelting sectors. Until adopted, such materials remain subject to national rules, causing fragmented market access. Beyond the regulatory dimension, environmental performance assessments also influence policy decisions and market acceptance. Several life cycle assessment (LCA) studies Agronomy 2025,15, 2167 22 of 28 have quantified the potential of waste-derived fertilizers to lower greenhouse gas emissions, reduce fossil energy demand, and improve nutrient cycling compared with conventional mineral fertilizers [ 85 , 120 – 122 ]. Such evidence supports ongoing efforts to align regulatory frameworks with sustainability objectives. 5. Conclusions and Future Perspectives in Circular Fertilization The valorization of organic waste into fertilizers stands as a key strategy to advance toward more sustainable, resilient, and circular agricultural systems. Unlike previous reviews, this work integrates a cross-cutting comparative assessment of major waste streams with the most recent EU regulatory developments, and synthesizes current evidence on mature and emerging processing technologies with proven agronomic performance and scalability. This review has shown that a broad range of residual streams—including livestock manure, sewage sludge, digestates, agro-industrial by-products, wastewater, and ashes—can be transformed via biological, thermochemical, or chemical processes into agronomically valuable fertilizing products. These approaches contribute to reducing dependence on mineral fertilizers, mitigating the environmental burdens associated with intensive nutrient use, and closing nitrogen, phosphorus, and carbon cycles in farming systems. Well-established technologies such as composting, anaerobic digestion, and struvite precipitation have demonstrated technical feasibility and agronomic effectiveness under field conditions. Among the different options reviewed, biochar and digestates stand out for combining high agronomic potential with technical feasibility for large-scale production, making them key candidates for broader deployment in circular fertilization strategies. However, their broader adoption is still constrained by regulatory barriers, processing costs, logistics, and end-user acceptance. Further efforts are needed to improve product standardization, ensure safety and quality, and tailor nutrient formulations to specific crop demands. Looking ahead, several emerging directions offer transformative potential. Integrated treatment systems combining anaerobic digestion with microalgae cultivation or insect rearing are particularly promising, enabling the production of novel biofertilizers such as frass or algal biomass while maximizing nutrient recovery. These modular and flexible platforms can be adapted to local contexts and diversified product portfolios. In parallel, advances in hydrothermal carbonization, functionalized biochar, controlled-release formulations, and the incorporation of biostimulants or microbial consortia are opening new opportunities to develop multifunctional fertilizers that not only supply nutrients but also enhance soil health and crop resilience. Future research should focus on: - Optimization of treatment pathways to enhance agronomic performance, minimize environmental impacts, and ensure economic viability—especially for decentralized systems. - Development of multifunctional fertilizers incorporating biostimulants, microbial inoculants, or controlled-release mechanisms. - Harmonization of regulatory frameworks and certification procedures to facilitate market access and recognition of waste-derived products. - Long-term field trials under diverse soil–climate conditions to assess nutrient release, crop yield, and soil health outcomes. - Stakeholder engagement through training, field demonstrations, and transparent communication to build trust and encourage adoption. Author Contributions: Conceptualization and writing—original draft preparation, D.H. and J.M.M.-M. ; writing—review and editing, D.H., J.M.M.-M., F.C. and F.V. All authors have read and agreed to the published version of the manuscript. Agronomy 2025,15, 2167 23 of 28 Funding: The authors gratefully acknowledge the financial support provided by the Institute for Business Competitiveness of Castilla y León (ICE-JCYL) through the ECOVITA project (Project No. 04/18/VA/0201), and by the European Commission through the Ferticovery project (Project No. 101181936). Conflicts of Interest: The authors declare no conflicts of interest. Abbreviations The following abbreviations are used in this manuscript: CMCs Component Material Categories EC European Commission EU European Union FPR Fertilising Products Regulation HTC Hydrothermal Carbonization PFCs Product Function Categories References 1. FAO. Developments in International Fertilizer Markets. In Proceedings of the Committee on Commodity Problems, Seventy-Sixth Session, Rome, Italy, 11–13 September 2024. Available online: https://openknowledge.fao.org/server/api/core/bitstreams/e379 efe6-a955-4ffc-bb53-0c9a89f11f1a/content (accessed on 21 July 2025). 2. European Commission. 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