Conversion of waste into organo-mineral fertilizers: current technological trends and prospects

One of agriculture’s most important goals is finding the proper equilibrium between sustainability and intensification of production. The exponential growth of the world population, climate variability, and soil degradation are essential factors that require the development of novel agricultural practices to achieve food security. In this context, organo-mineral fertilization has been proposed as a promising approach. Such a concept is based on novel fertilizers formulations combining organic and mineral resources features, which could simultaneously address soil fertility and health issues. The manufacturing processes of organo-mineral fertilizers (OMF) are highly versatile and revolve around the co-processing of organic and mineral matrices. OMF’s peculiarity resides in using systemic methodologies for waste valorization to generate cost-effective and eco-friendly products in alignment with the bio-circular economy. Despite their advantages, OMF adoption by farmers is still not satisfactory, which could be attributed to the difficulty of accessibility and a stagnant market. This work critically reviews recent advances in the organo-mineral fertilization concept. Our review provides an in-depth understanding of the chemical, biological, and thermal methodologies used for OMF generation through mineral and organic matrices co-processing. We also discuss the positive effect of such products on the plant-soil system by focusing on the mechanism of action. Furthermore, this review scrutinizes the innovation profile of OMF based on trends of patent submission during the last 20 years. It also provides future research and development pathways based on current drawbacks and limitations of the industry.

eco-friendly products in alignment with the bio-circular economy. Despite their advantages, OMF adoption by farmers is still not satisfactory, which could be attributed to the difficulty of accessibility and a stagnant market. This work critically reviews recent advances in the organo-mineral fertilization concept. Our review provides an in-depth understanding of the chemical, biological, and thermal methodologies used for OMF generation through mineral and organic matrices co-processing. We also discuss the positive effect of such products on the plant-soil system by focusing on the mechanism of action. Furthermore, this review scrutinizes the innovation profile of OMF based on trends of patent submission during the last 20 years. It also provides future research and development pathways based on current drawbacks and limitations of the industry.
Abstract One of agriculture's most important goals is finding the proper equilibrium between sustainability and intensification of production. The exponential growth of the world population, climate variability, and soil degradation are essential factors that require the development of novel agricultural practices to achieve food security. In this context, organo-mineral fertilization has been proposed as a promising approach. Such a concept is based on novel fertilizers formulations combining organic and mineral resources features, which could simultaneously address soil fertility and health issues. The manufacturing processes of organo-mineral fertilizers (OMF) are highly versatile and revolve around the coprocessing of organic and mineral matrices. OMF's peculiarity resides in using systemic methodologies for waste valorization to generate cost-effective and

Introduction
Limited raw material and land resources with the rising global population are currently critical factors hampering agriculture in achieving food security goals (Alves et al. 2014;Reynolds et al. 2016). The last green revolution has 10-20% fewer yields than conventional farms. Still, in terms of profits, the trend is inversed, which is attributed to lower operating costs, better plant stress resistance, the premium price of the end products, and a less complex supply chain. Conversely, Kirchmann (2019) stated that the current meta-analysis had overestimated the agricultural production of organic farming as the assessment is more based on a nature-related philosophical thinking; rather than relevant biological science, and statements corroborating the complete substitution of mineral fertilizers are scientifically irrelevant. Moreover, according to Swedish statistical data, conversion to organic farming without losing the yield will plausibly require increasing arable land by 50% (Kirchmann 2019).
Both conventional and organic farming have their respective advantages and drawbacks. One promising approach that could be applied is combining organic and mineral inputs. Both resources are theoretically compatible, and according to several research investigations, this is possibly the best solution to maximize the agronomic efficiency and crop productivity without neglecting sustainable soil health and fertility (Laird et al. 2010;Ayalew and Dejene 2012;Ichami et al. 2019;Liu et al. 2020). Such a concept has recently gained more interest, leading to the development of a novel category of fertilizer products, namely the organo-mineral fertilizers (OMF). These latter are fertilizers resulting from combining through a chemical reaction, inorganic fertilizers with a high content of nutrients with an organic matrix or soil improver (Antille et al. 2013a;Kominko et al. 2019). A key advantage of OMF products is that their manufacturing process is fundamentally based on organic waste valorization, which aligns with the circular economy concept (Barje et al. 2012;Bouhia et al. 2020Bouhia et al. , 2022aCosta et al. 2022). Furthermore, manufacturing methodologies are not limited to chemical processes. Depending on the nature of the feedstock, a diversity of techniques can be used, such as thermal conversion, anaerobic digestion, and solid-state fermentation. Implementing procedures for the co-processing of mineral and organic resources instead of single applications is justified by the gained additional value of the final products due to specific chemical interactions. This includes improved bioavailability of nutrients, higher chemical reactivity, and slow-release properties (Kominko et al. 2019). OMF are often reported for their multiple beneficial effects on agrosystems, including their ability to improve soil physico-chemical properties and biological functionalities and enhance plant physiological traits (Obalum et al. 2012;Carvalho et al. 2014;Pawlett et al. 2015;Boiffard et al. 2016;Zainab et al. 2016;Silva et al. 2017;Farrar et al. 2018).
Review reports on OMF are scarce. The only relevant example is the work of Kominko et al. (2019) which mainly focused on the chemical coprocessing of sewage sludge and mineral fertilizers. In this context, this critical review aims to complement previous investigations by providing a more systemic analysis of the concept of organo-mineral fertilization. Conventional and novel technologies (not limited to chemical processes) used in products manufacturing are thoroughly described. The valorization of the mineral byproducts of the phosphate industry into OMF is also addressed for the first time. Furthermore, the effect of such products on soil physico-chemical and biological properties, plant agrophysiological traits, and agronomic productivity is depicted, focusing on the mechanism of action. Ultimately, we describe the current status of the market, highlight significant limitations and provide future perspectives on moving forward this vital segment of the fertilizer industry.

Manufacturing processes of organo-mineral fertilizers
The manufacturing processes used to convert organic and mineral wastes into organo-mineral fertilizers are highly diverse (Fig. 1). Several research investigations demonstrated that chemical, thermochemical and biological methodologies are equally efficient for obtaining products with satisfactory fertilizing properties. The choice of an adequate process depends mainly on the nature of the used feedstock and the targeted market. This section systematically reviews the most notable chemical and biological methodologies employed in OMF development through valorizing organic/mineral waste and low-grade phosphate.
2.1 Conventional manufacturing processes of organo-mineral fertilizers based on organic waste and chemical fertilizers Implementing a process to produce OMF is mainly dependent on the raw material properties, which often include an organic waste and a mineral fraction. Diverse formulations have been assayed (Table 1). However, the granular and pelletized formulations are the most dominant. Considering the great diversity of the generated organic waste, the possibilities are almost limitless. The conversion of organic waste into OMF formulations requires achieving a double objective. First, generating a product with a high fertilization quality, and second, creating agronomic value from a given organic waste. For example, when processing sewage sludge, the most important aspect would be the elimination of microbial pathogens, which is usually carried out through the addition of alkali compounds such as lime, potassium hydroxide, and sodium hydroxide or acidic compounds such as phosphoric acid. Interestingly, these compounds have an additional benefit, increasing the final product's nutritional value. Adding inorganic fertilizers or their precursors is necessary to balance bio-waste derived fertilizer products (Kominko et al. 2019). Technically speaking, the most straightforward procedure consists of mixing the moistened organic raw material with an alkaline co-product such as gypsum, cement, lime, and fly ash. Then the NPK ratio/content is optimized by adding potash, phosphates, and liquid ammonia (Kominko et al. 2019;Gómez-de la Cruz et al. 2020). Several technologies have been proposed to produce OMF, and most of them are based on simple two or three-step chemical processes, including pretreatment. Pathogen elimination could also be carried out   Bakhashwain et al. (2013) Biosolid based poultry manure, peat, and straw Mixing the three organic fractions with diammonium phosphate (DAP) and KCl with a moisture less than 10% Granulated organo-mineral fertilizer with 37.35% of carbon, an NPK ratio of (9:4:4), 1.78% Mg 2+ and 0.59% Ca 2+ Mazeika et al. (2016) Unstabilized sewage sludge and poultry litter Sewage sludge is milled, and the ash is mixed with poultry litter and mineral fertilizer. The obtained product was dried at 70 °C for 3 h Granulated organo-mineral fertilizer with an NPK ratio of (11.5:5.12:16.2) and a Zn content of 566 mg/kg Kominko et al. (2019) by heat treatment, given that the issue of toxic vapor emission is resolved. For example (Jourdain 1996) suggested a methodology for manufacturing solid OMF through processing organic wastes such as sewage sludge and liquid manure. Briefly, OMF granules (2, 3 mm) are produced by combining a mineral mixture obtained from mineral fertilizer and organic mixture (manure and sewage sludge). The generated paste is then granulated, and oven treated at more than 200 °C for less than 80 to eliminate possible pathogens. The ammoniacal vapor is recovered, cooled down, and returned to the mixture. Depending on the used raw organic material, heat treatment may not require elevated temperature, as demonstrated by patent (Antonius and Terlouw 2007). Those authors proposed technology for producing OMF pellets using four different materials, namely an inorganic fertilizer (DAP, MAP, PS, Roch phosphate…), urea, manure (chicken, poultry, cattle) and a lignin compound (ammonium lignosulfonate). Prior to blending, the organic materials are heat-treated at only 75 °C; then the mixture is palletized at a pressure between 90 and 120 bars, leading to an NPK 14-5-8 + 2MgO + 0.5 Fe formulation. The added value of such a product is its slow-release property, which is attributed to the formation of urea-lignin bonds. Alkaline Ammonia Pulse (AAP) is another interesting technology proposed by (Burnham 2015), which enabled the production of what he named an "inorganically-enhanced bio-organic fertilizer" with the following composition NPK SFe 10-10-10 + 1:20. Such technology is mostly based on varying the pH along the process to obtain a specific desired effect. Briefly, sewage sludge is supplemented with ammonia and lime to rise the pH mixture, which stresses pathogens and increases the N content. The mixture is then transferred to a second reactor where pH is decreased, and P content is enhanced by adding phosphoric acid waste. Ultimately, the granulation process is achieved by adding molasses and a binding agent. (Cabello-Fuentes 2010) developed a technology based on reverse AAP for treating sewage sludge. Compared to the AAP, the process includes a heat-based pretreatment for decontamination purposes. Afterward, sludge pH is reduced up to 2 by adding phosphoric acid, eliminating the remaining pathogens and enhancing P content. The mixture is then transferred to a second reactor where pH neutralization is performed by adding magnesium hydroxide containing lime, which induces tricalcium phosphate and dicalcium phosphate. Finally, the mixture is treated with sulfuric acid, granulated, and neutralized with phosphoric acid, ammonia, and potassium (Kominko et al. 2019).
When it comes to formulation, advanced methodologies such as a polymer based-coating technique may be efficiently applied to OMF. For example, Antille et al. (2013b) succeeded in elaborating an OMF product using spray coating. Granules of NPK OMF were produced through granulating 80 °C heated digested sewage sludge and then coating the generated granules via spraying with a liquid mixture containing urea (46% N) and potassium (60% K 2 O). More recently, Gonçalves et al. (2020) assayed an OMF formulation based on sugarcane wastes. Briefly, the sugarcane filter cake was composted and supplemented with mineral nutrients, namely, urea, potassium chloride, monoammonium phosphate, boron, and oxysulfate. Then OMF pellets (3.9 mm × 9.1 mm) were produced after adding an organic polymer. Overall, the methodologies used in the chemical manufacturing of OMF based on organic waste showcase several similarities with the routinely used processes in the chemical fertilizer industry. Mandatory steps such as grinding, granulation, acid treatments, heating, and coating are often reported, which pose the question of economic viability. In other terms, at an equal scale are OMF economically justified compared to chemical fertilizer? This furthers the need for in-depth technical-economical evaluation taking into consideration machinery, labor, consumables, waste collection/ transportation, and added value to farmers.

Production of organo-mineral fertilizers based on co-processing of mineral and organic byproducts
From a theoretical standpoint, OMF technologies are highly suitable for the valorization of mineral waste, especially those generated by the phosphate industry. Such matrices are widely abundant and rich in several fertilizing compounds. Tow mineral wastes are produced along the phosphate value chain (from ore beneficiation to fertilizer production), phosphate washing sludge (PWS), and phosphogypsum (PG) which is the result of the "wet process" used to produce phosphoric acid through the processing of RP by sulfuric acid. PG is the most significant by-product of the phosphate industry. Worldwide production of PG is estimated at between 200 and 250 million tons per year, among which only 15% is recycled. Moreover, by 2025 total discarded PG is estimated to reach 8 billion tons (Tayibi et al. 2009). PG is a very low water-soluble acidic powder constituted of oxides of sulfur, calcium, silicon, aluminum, iron, P, fluorosilicate, fluorine aluminate, and fluoride phosphate. Although scarce and mainly limited to compost and biochar production, research investigations dealing with PG processing into OMF have shown promising results. For example, Karim et al. (2019) evaluated the possibility of producing a nutrient richer and less toxic biochar (compared to PG) through the coplasma processing of banana peduncles and PG. The operations were carried out in an extended arc thermal plasma reactor using three different plasmagen gases (argon, ammonia, and oxygen). Using argon allowed to obtain biochar with higher potassium (12.7%) and sulfur (13.3-17.8%) contents compared to PG, while the use of ammonia resulted in a lower leachable fraction of fluoride and heavy metals in biochar. In another study, (Yang et al. 2015) attempted to address the issue of the by-products of kitchen waste composting (greenhouse gas emission) using PG (10%) as an additive. Results showed that PG significantly reduced methane and ammonia emissions by 85.8% and 23.5%, respectively, without affecting compost maturity. The reduction of gas emission was attributed to a lower pH, higher NH 4 + concentration, and a higher content of sulfate in the PG treatment, which may simultaneously suppress methanogens and improve the compost agronomic value. Likewise, works of (Elfadil et al. 2020) demonstrated that cocomposting cattle manure with PG and the waste of the phosphate flotation enhanced the composting process and improved the nutritional value of the final product. Agronomic assays showed that compostbased on PG and phosphate flotation wastes resulted in higher biomass production of chickpea compared to the sole application of compost-based manure. All those studies demonstrated that eco-friendly and simple technologies such as compost or biochar are efficient for developing OMF through processing intricate mineral matrices. Even more straightforward, recently (Matveeva et al. 2021) proposed a methodology for the industrial process of PG into OMF via simple mixing of lignin waste sludge (75-80%), PG (20-25%) and mineral NPK. Agronomic trials under controlled conditions revealed that mixing the soil with the OMF in equivalent quantity induced optimal plant growth. It also reduced PG toxicity due to limited strontium assimilation by plant tissue, which was attributed to calcium/strontium competition.
PWS is another important environmental concern of the phosphate industry. It is estimated that 28 tons of PWS are produced in Morocco alone each year (Mobaligh et al. 2021). PWS is usually dumped in decantation basins without prior treatment. Although the importance of such mineral waste, little has been done concerning its valorization into Agri-inputs. The only example is the work of Mobaligh et al. (2021), who investigated the co-composting of PWS, olive mill waste sludge, green waste, and sugar lime sludge. Using PSW at a rate of 20% resulted in better compost quality in terms of hygienic properties, humification process, and nutrient content compared to the single-use of sugar lime sludge. Those results are valuable, but further studies need to be performed to ascertain the technical feasibility of producing OMF from PSW. This latter could plausibly showcase a significant physico-chemical inconsistency due to variation in RP properties and the environmental effect of the decantation basin.

Development of high added value
organo-mineral fertilizers via direct valorization of valuable ore: the case of natural rock phosphate Low-grade RP is currently gaining significant momentum in the organic farming market as a source of plant nutrients. While highly attractive, RP mainly displays an unavailable P form, and efficacy depends on the rock reactivity and soil pH. Using biological methods in OMF development could enhance RP agronomic efficiency and widen the targeted market as the generated products are highly suitable for producing organically labeled crops. The process used in the development of RP-based OMF is exclusively based on co-composting or organic wastes and RP under reactor systems or composting windrow and may require the addition of microbial inoculum to simultaneously enhance the degradation of recalcitrant organic compounds and rock solubilization. An example of this is the work of Pandey et al. (2009), who showed that the organic matrix based poultry manure with the addition of RP and P solubilizing microorganisms resulted in concentrations up to 2.32 mg g −1 of available P in the OMF product. Using various organic waste (urban waste and plant residues), Naher et al. (2018) reported that co-composting with RP addition increased the P solubilization dynamic. Similarly, the addition of phosphate solubilizing microbes, such as Aspergillus niger, Aspergillus flavus, and Trichoderma harzianum during waste treatment production increased the P availability to almost 8.19% without affecting the decomposition rate (Gaind 2016). Vermicomposting is also a promising technology that can improve P solubilization.
Research investigation by Mupondi et al. (2018) demonstrated that using Eisenia fetida earthworms during co-compositing of RP and a manure-paper mixture resulted in significantly higher resin extractable P, better humification and lower heavy metals contamination. In mechanistic terms, increased P solubilization is attributed to the direct intervention of microbial enzymes such as phosphatase and dehydrogenase (Kutu et al. 2019). In addition, the ligands of stable organic matter compete with P during cation adsorption, which makes P more available in the soil (Pawlett et al. 2015;Tang et al. 2019). Biswas & Narayanasamy (2006) investigated the enrichment of OMF product using rice straw (C/N = 86.2), low-grade RP (8.62% total P), waste mica (10% total K), and a fungal inoculum as the phosphate solubilizer (Aspergillus awamori), showing optimization of P availability in the OMF final products. Various mixtures were prepared with and without P/K and phosphate solubilizer, notably, 2 concentrations of P and K (2 and 4%) were added to 15 kg of straw rice, which was supplemented with urea to adjust the C/N ratio and cow dung (5 kg per 100 kg straw rice) as a natural inoculum. The obtained results showed that the addition of RP and the fungal inoculum significantly increased the Olsen P content (higher with 4% RP). (Naher et al. 2018) studied the biochemical features of an OMF produced from urban waste, plant residue, and RP. Sugarcane waste was added to enrich the compost with NPK and RP was used at 5 and 8%. Moreover, adding other organic matrices such as (rice straw, mustard waste, and sugarcane waste) into OMF significantly reduced the decomposition rate. More importantly, this product enhanced the overall microbial activity but was more selective toward phosphate solubilizing microbes when RP was added. Bustamante et al. (2016) investigated the efficiency of developing an OMF by the co-composting process with RP (27% P 2 O 5 ) and green waste with the addition of elemental sulfur (0.5%). In this study, two concentrations of RP (2.3 and 4.6%) were used, and several green wastes, including palm, prunings, grass (Lolium perenne L.) clippings, and a mixture (1:1) of olive tree (Olea europaea L.) and conifer. Results showed that the extractable water P increased in all the mixtures. However, such an increase was surprisingly higher in the RP-free treatments. These observations are in line with the works of Biswas and Narayanasamy (2006) and Lu et al. (2014), who reported a reduction of water-extractable P after RP addition due to the precipitation of P with calcium. At the end of the experiment (maturation phase), P release was more significant in the 2.3% RP supplemented treatment, which was attributed to the nitrification process and sulfate addition.
Choosing the correct dose of RP in the OMFbased compost is paramount. The current observations prove that RP concentration and the release of water-soluble P are negatively correlated. Additionally, to other factors such as the nature of the feedstock, the manufacturing process, and chemical features of RP. Theses aspects were well documented by Singh (1985), who studied the effect of RP addition (20.6% P 2 O 5 ) at 5, 10, and 25% during OMF production using the composting process of farm wastes consisting of grasses, wheat straw, bean, and tree leaves. This author showed that RP addition significantly enhanced the biodegradation of organic matter, which was optimal in the treatment supplemented with 10% RP. Most importantly, a further increase of RP content (25%) reduced the loss of organic matter, which was attributed to the toxic effect induced by salt excess in RP. Moreover, both organic P and citric soluble P were higher at 10% RP supplemented treatment. Such organic P is critical as it represents mostly P entrapped in microbes. This organic P could operate as a slow-release fertilizer due to the slow rate of decomposition. Thus, providing available P to the plant for a more extended period. Overall, the formulated product demonstrated an agronomic efficiency equivalent to single super phosphate.  (2015) and Wright et al. (2008) showed that amending a sandy, loamy soil with OMF based on municipal biosolids composted waste or wood biochar material corrected the soil salinity efficiently. Likewise a co-application of two OMF based on green waste/biosolid compost and biochar in saline-sodic soil decreased the EC value by 79% compared to unamended soil (Chaganti et al. 2015). Such organic matrices also positively affect bulk density parameters concomitantly with improving soil nutrient and SOM content (Qiao et al. 2020). The high soil CEC depends mainly on soil physico-chemical properties and cations storing properties. The high retention capacity of soil can be significantly improved following the application of stable organic matter such as humic substances and biochar (Indraratne et al. 2007;Karimi et al. 2019). When these fractions interact with soil oxygen and water after an extended period, a higher level of oxidation occurs, thus resulting in an increment of soil negative charges and CEC. Soil organic matter aggregation becomes highly associated with those negative charges (Ouyang et al. 2013;Calvo et al. 2014). Reactive surfaces characterize biochar and humic substances due to reactive functional groups such as (-C-H, -C-O, C=O, -COOH, -NH). Using such fractions during OMF formulation is highly appreciated. The temperature and moisture conditions directly influence the humic acid (HA) polymerization and composition, which could vary drastically. At low temperature and under adequate humidity (Tundra landscape), only 20-45% of soil extractable humic acids can resist acid hydrolysis. However, this value is much higher (64-75%) in standard temperature and moisture conditions (Steppe landscape). Additionally, such environmental conditions affect HA composition. The same authors found that a lesser C: N induced a higher biodegradation resistance and long persistence. Similar results were reported by Veen and Paul (1981), who suggested that soil humic fractions are more resistant to degradation and turnover (up to thousands of years) because of their association with soil mineral colloids. More stability suggests more aromatic cores and more reaction with mineral surfaces, directly increasing soil CEC.
Due to their richness in organic carbon, OMF can be considered an efficient carbon sequestration solution. Such application tends to form a black color with a strong resistance to degradation, unlike the naturally occurring SOM in the environment (Wattel-Koekkoek et al. 2003). Additionally to aromatic structures, SOM also contains aliphatic carbon compounds, easily degraded and oxidized (Dergacheva et al. 2012;Jindo et al. 2014). Adding more SOM may lead to an increase in the soil's ability to retain plantavailable nutrients. The use of OMF based compost and biochar showed the ability to retain cations, and even some anions such as P. These properties make these two organic matrices recoverable in agricultural lands, able to improve crop yield while reducing the impact of environmental pollution induced by nutrient leaching (Schmidt et al. 2014). Overall, the functionalities of the organic matrix depend on the initial organic compounds composition. For example, a very rich substrate in functions such as lignin, phenols, or lipids can theoretically improve the final product's chemical function, thereby increasing nutrient use efficiency upon soil application. Conversely, the opposite result could be obtained in a silty-clayey or clayey soil, as the association with this stable organic matter could prevent the plant from accessing these nutrients. Hence, the choice of the organic matrix must be well studied and should depend not only on the soil's nutritional needs but also on its nature.

Effect of organo-mineral fertilizers on soil biological traits
Soils are complex mediums that permanently change according to several aspects, including edaphic, pedoclimatic, fertilization, and land use factors, directly affecting indigenous microorganisms' diversity, distribution, and functionalities (Fig. 2). Supplementing the soil with SOM significantly affects its biota and its microbial component (Zhang et al. 2010). Those variations result from the relative stability and the quality of available carbon in these formulations with the main products presented in (Table 2). The positive effect of OMF on soil microbial activity is attributed to a better soil porosity resulting in higher substrate availability and a more efficient enzymatic activity in the vicinity of organic particles (Głąb et al. 2016;Tang et al. 2019).
Many studies reported that OMF supplemented with phosphate solubilizing microorganisms (PSM) could significantly improve microbial P solubilization, which depends on soil and plant characteristics, feedstock, and the manufacturing process (Jones and Oburger 2011;Saxena et al. 2016;Qian et al. 2019). However, raw materials such as wood, industrial waste, and sewage sludge during OMF development could negatively affect microbial biomass and diversity (Bouhia et al. 2021). The SOM fraction of OMF formulations have a natural ability to absorb inorganic nutrients and some low-weight organic compounds. The low decomposition confers a progressive release of nutrients and carbon sources in the soil. Consequently, each microbial community (actinobacteria, bacteria, and fungi) behaves differently according to the physico-chemical properties of soils and SOM porosity (Quilliam et al. 2013;Jaafar et al. 2014).  revealed that OMF based on compost and mineral fertilizer (rice straw and pig manure) improved sugar content and yield attributes of sugarcane compared to the single-use of mineral fertilizers, which was mainly attributed to the higher  microbial diversity and functions of added SOM fractions. These authors showed that energy metabolism and the abundance of some species, such as acidobacteria, proteobacteria, chloroflexi, and gemmatimonadetes, were higher in OMF treatment, which was strongly correlated with the dynamic of soil nutrients. Similarly, using vermicompost extract bacteria species such as Azotobacter vinelandii, Bacillus megaterium, and Frateuria aurantia improved tomatoes yield and quality (Ruiz and Salas Sanjuan 2022). Other works based on predictive metagenomic showed that applying organic inputs (liquid and palletized digestate), OMF-based digestate, and slow-release mineral fertilizers, with equivalent N quantities resulted in significant differences in tomato root-associated microorganisms with respect to diversity and functionality (Caradonia et al. 2019). Such effect was peculiarly striking in the OMF treatments with palletized digested. The number of rhizosphere operational taxonomic units was significantly higher than other treatments, which was attributed to the level of plant-available N that might reduce specific microbial abundance and activity in case of significant availability (mineral fertilizers). Other works showed that co-application of OMF-based compost or biochar and mineral fertilizer improved microbial biomass and reduced N leaching compared to single-use of mineral fertilizer (Trupiano et al. 2017). The application of OMF improved soil organic matter, which increased carbon availability for microorganisms, resulting in a stimulation of microbial activity and its requirement for high nitrogen demand, especially in nitrate. OMF does not only increase plant nutrient concentration and total soil carbon content, but has also been reported to promote biological nitrogen fixation through better nodulation (Coyotl et al. 2018). Arbuscular mycorrhizal fungi (AMF) are another critical microbial component of agro-systems that may be influenced by OMF application, which may positively or negatively affect nutrient cycling. According to many authors (Warnock et al. 2007;Hale et al. 2013;Mukherjee and Zimmerman 2013;Conversa et al. 2015), root colonization by AMF is affected by SOM depending on the applied doses and properties. AMF is regulated by phytoavailable P in the soil. The higher the P content, the lesser the symbiosis efficiency. As stated previously, the SOM fraction of OMF bestows a slow-release property to the final product. Hence, we could theoretically predict  Makinde et al. (2011) that the use of OMF should result in better AMF functionality and diversity. In a comparative study, Van Geel et al. (2016) investigated the effect of the P sources on AMF features, OMF-based compost and mineral P, and slow-release P fertilizers. The application of organic-based slow-release fertilizer induced higher AMF diversity than its fast release inorganic counterpart in the rhizosphere (dominated by inferior AMF mutualists), which was mainly attributed to soil P content.

Effect of organo-mineral fertilizers on crop yields and agrophysiological properties
OMF application is often associated with improved crop yield, which is mainly attributed to better nutrient availability in the soil (Fig. 2). According to several research investigations, compared to chemical fertilization, the OMF effect can be directly related to their organic constituents (Table 2). For example, an application of an OMF based on straw, rice composts (5 t ha −1 ), and mineral NPK significantly enhanced shoot/root nutrient content and the overall biomass of maize compared to the sole application of mineral fertilizer (Sia et al. 2019). Similarly, Abd El-Mageed and Semida (2015) reported a higher yield (53%) of cucumber treated with an OMF mixture based on sulfate, compost, and potassium humate. As demonstrated by Tian et al. (2018), OMF showcases a long-lasting effect on soil fertility. These authors showed that applying an OMF based on polymer-coated biochar/NPK granules improved cotton yield significantly over three successive harvesting periods (2013, 2014, and 2015), which was attributed to better nutrient retention in the 0-20 cm soil layer. Pawlett et al. (2015) revealed that the application of an OMF formulation based on bio-solids, urea, and potassium with low NPK ratios (10:4:4 and 15:4:4) increased soil fertility through enhancing nutrients availability and retention by 13 and 15% for N and P respectively, compared to a single application of mineral NPK.
The nature of the used organic matter in OMF formulation is essential with respect to the agronomic efficiency of the final product. For example, using non-stabilized organic matter during OMF formulation can result in fast biodegradation due to favorable microbial development conditions, which may alter the slow-release properties of the final product.
Additionally, some minerals resulting from biodegradation may contribute actively to the precipitation of these elements. Several works reported that the use of untreated organic matter such as poultry manure, organic waste, or other natural products during OMF formulation could induce a similar beneficial effect (compared to treated SOM) on plant nutrients. According to Ayeni and Ezeh (2017), an OMF based on untreated manure and mineral fertilizer increased tomato yield by 600% compared to unfertilized treatment. Moreover, OMF treatment positively affected soil macro/micronutrients and the nutrient content of tomato shoots. Moreover, Makinde et al. (2011) investigated the effect of a single application of pacesetter's Grade B and untreated kola pod husk (UKPH), as well as their combination with mineral NPK fertilizer on nutrient uptake of Amaranthus Cruentus (L); these authors assayed several OMF formations based Organic/mineral at different ratios and found that the best P, N, K, Ca 2+ and Mg 2+ plant uptake was obtained following the application of UKPH/NPK (75:25) formulation. These results can be related to the high nutrient content in waste, directly related to its N-NH 4 + richness. However, a high NH 4 + /NO 3 − ratio could be toxic to soil and plants ( Barje et al. 2012;El Fels et al. 2014). Inversely, it was reported that using untreated organic waste could negatively affect plants' agro-physiological parameters at the early stages of plant development (Kapellakis et al. 2015;Tajini et al. 2020;Bouhia et al. 2022b). Dania et al. (2014) showed that using a mineral NPK and rich P/N poultry manurebased OMF (5 g/pot) decreased stem girth up to 8% compared to the control despite enhancing the nutrient content moringa shoots. The use of unstable organic matter affects the plant growth parameters and can also induce asphyxiation of the soil due to its instability and oxygen consumption. Moreover, the use of such organic compounds could bring a variety of pathogens such as Actinobacillus, Bordetella, Clostridium, Corynebacterium, Escherichia coli, Listeria, Salmonella, Mycobacterium, Streptococcus and Staphylococcus which present a potential danger to consumer health (Lovett et al. 1969;Lu et al. 2003;Stern and Robach 2003;Ngodigha and Owen 2009;Bolan et al. 2010).

Market, innovation profile, and legislation
Despite a significant diversity of already commercialized products (Table 3), available data on the OMF market size is low. Bridge Market (2020) estimates that the OMF market will reach USD 616 million by 2027, which corresponds to a compound annual growth rate (CAGR) of 4.7% in the forecasting period from 2020 to 2027. Those numbers are trivial compared to the massive market of inorganic fertilizer, estimated at USD 95.27 billion in 2020 (The Brainy Insights 2020), and even pale in comparison to the USD 2.3 billion worth biofertilizer based microbes' market (Research Market 2017). According to Bridge Market (2020), several factors are hindering the growth of the OMF market, notably the limited analytical value of the organic sources, the absence of data with respect to OMF physico-chemical properties, and the higher production costs. This latter is highly debatable as the cost may vary mainly depending on the source and availability of organic raw materials (produced in-farm or imported), the required machinery, and soil fertility status (Loncaric et al. 2013).
Evaluating the innovation profile of a given technology is helpful to determine key technological trends over a period, as well as gaps and market opportunities. We have thoroughly analyzed patents related to OMF based on wastes from 2000 till 2020 using several patent databases, namely, google patents, World Intellectual Property Organization, Espacenet, Patenscope, Lens, and United States Patent and Trademark Office. The analysis involved the number of patents per year, top countries issuing Fig. 3 Innovation profile of organo-mineral fertilizer technologies based on a number of patent applications and issued patents from 2000 to 2020, and b Top 10 of the most cited patent patents, and the most cited patents (Fig. 3). For example, the grant rate is helpful to understand the date from which the technology protection is established and the rate of successful applications over a period. Figure 3 shows that in the last 20 years, the number of patent applications had remained constant (10-13/year), except for 2002 and 2011, when patent applications reached 24. Similarly, between half and two-thirds of applications are constantly granted. A ranking of the top countries in which the earliest application was filed reveals that China is the country from which most patents originated with 46 patents between 2000 and 2020, followed by Korea (30), Russia (27), United States (22) and Germany (19). Overall, these data demonstrate that the OMF market is stagnating, which is further corroborated by a low CAGR compared to other segments of the fertilizer market.
In terms of legislation and quality requirements, both the Brazilian commission and the European consortium of the organic-based fertilizer industry (ECOFI) have extensively addressed those aspects. Following their 2014 meeting, the ECOFI proposed a new definition for OMF, which state that "OMF means a complex fertilizer obtained by industrial co-formulation of one or more inorganic fertilizers with one or more organic fertilizers and organic soil improvers into solid forms (except for dry mixtures) or liquids. The organic C and the mineral nutrients must be present in each unit". From this definition, we can conclude that a clear distinction has been made between OMF and essential blends where all raw materials can still be distinguished. Interestingly, the fertilizer regulation (EC) No 765/2008 categorized the OMF products within the microbial plant biostimulant segment, which can be delivered in two forms: (1) solid form characterized by structural rigidity and resistance to change of shape or volume, and in which atoms are closely related to each other, either in a regular or irregular geometric form. Regarding the contaminant threshold, the OMF content of toxic trace elements should not exceed 1.5; 2; 1; 50; 120, and 40 mg kg −1 dry matter for cadmium (Cd), chromium VI (Cr VI), Mercury (Hg), Nickel (NI), lead (Pb) and Arsenic (As) respectively. (2) Liquid form refers to a suspension or a solution. A suspension is a two-phase dispersion in which solid particles are maintained in suspension in the liquid phase, and a solution is a liquid free of solid particles. Moreover, the Copper (Cu) and Zinc (Zn) concentration must be lesser than 300 and 800 mg kg −1 dry matter, respectively. Similarly, in the OMF product, the bacterial pathogen charge should be reduced to a minimum of 1000 CFU/g of Salmonella spp. Within the final product. In the case of non-stable OMF, the nitrification degrees must be controlled and should demonstrate a 20% reduction in ammoniacal nitrogen (NH 3 -N) oxidation 14 days after application at the 95% confidence level. In the case of the Brazilian legislation, quality criteria remain broad, and they mainly suggest that a quality product should have a minimum of 8% organic carbon; 80 mmol kg −1 ; 10% isolated primary macronutrients (N, P, K), or a mixture (NK, NP, PK, NPK); 5% of secondary macronutrients; 1% micronutrients and 30% maximum moisture (European Parlaient and Council 2019).
Regarding safety aspects, ECOFI addressed mainly the heavy metals issue. ECOFI proposition suggested that the contamination limit in OMF should be as follow: Cd concentration should not exceed 3 mg kg −1 dry weight for OMF with P 2 O 5 concentration lower than 5%, similarly to inorganic fertilizer when P 2 O 5 is higher than 5%. The maximum limit of Cr, Hg, Ni, Pb, Cu and Zn is 2, 2, 50, 120, 200, and 600 mg kg −1 dry weight, respectively.

Conclusion and future perspectives
The current literature review demonstrates that the concept of OMF is highly promising. Such a novel segment of the fertilizer industry will assuredly gain further attraction as it simultaneously addresses pivotal challenges of the agribusiness, namely food production, and waste management, which aligns with the highly desirable "circular economy." Nevertheless, further studies are required to address key technical and business challenges. For example, the influence of the composition of the used raw materials in OMF production may play a role as precursor when the humification/polymerization process is dominant. The primary function in this substrate needs to be studied to select the right combination with inorganic fertilizers to improve interactions and provide the possibility of a lengthy association.
During the OMF production process, several interactions occur. Those interactions may be highly complex and unpredictable, which can hinder the quality consistency of the final product. Therefore, simulation of interaction and modeling of complex valorization routes for the conversion of waste to OMF are required to achieve better optimization of the process.
OMF development through the co-processing of organic waste and phosphate mineral by-products is still scarcely addressed. It should be further investigated as the implication on the phosphate value chain could be substantial.
At the agronomic level, long-term trials are needed to investigate the persistence and evolution of OMF constituents after successive crop cycles and culture rotation and their effect on plants' agro-physiological traits and soil properties (aggregation, pH, enzymes, microbial activity). Studies focusing on the biostimulants properties of OMF are required, namely the effect of OMF on root architecture and development and their effect on plant physiological features. Moreover, the effect of such product on plant tolerance to biotic and abiotic stresses needs to be clarified through conducting targeted studies. OMF positively affects soil biology, biochemistry, and nutrient cycling; hence plant resistance to drought, salinity and pathogen should be affected under OMF fertilization. This will provide further claims that will improve product attractivity.
Regulation should be further clarified. The recommendation should not be limited to contamination level. It should also cover important aspects such as the nature of the used feedstock (treated or untreated), the minimum nutrient content, and the organic/mineral fraction ratio.
Finally, business-wise, the OMF market needs to be better segmented concerning product composition and manufacturers' claims. There is a wide variety of products for which the agronomic target (crop, soil type, and application method) is unclear. Hence, selecting an appropriate product to meet a specific need is often complex for the consumer. Furthermore, marketing strategies based on farmer intimacy should be adopted to increase end-consumers awareness. This could be done through field demonstration and targeted communication campaign.