Advances in Controlled Release Fertilizers: Cost‐Effective Coating Techniques and Smart Stimuli‐Responsive Hydrogels

To meet the needs of a rapidly expanding global population, farmers will need more fertilizers than ever before to maintain a steady supply of affordable, nutritious food. The formulation of controlled release fertilizers (CRF) to synchronize nutrient release according to the demand of plants has emerged as a viable solution to the current problems associated with the poor nutrient usage efficiency of fertilizers. Yet, the greatest obstacle that still stands in the way of broad use of CRF in agriculture is their expensive manufacturing costs. The first section of this analysis focuses on broad topics related to CRF. Afterward, the differences between several cost‐effective raw materials and some of the production techniques used to make CRF are examined. Furthermore, the emerging field of “smart” coating materials, such as stimuli‐responsive coatings, which can accurately tailor nutrients delivery to the demands of the vegetation, is discussed, and the most important research work that could lead to their extensive use in agriculture is pointed out. The purpose of this review is to provide a strong assessment of CRF's development over the past several years by highlighting innovations and providing in‐depth analysis of prevailing patterns to better understand the future of agriculture.


Introduction
Nutrients are required for plants to grow and complete their life cycle.Those nutrients are present in soil naturally, but their quantity will fade with time.Synthetic fertilizers are crucial in this DOI: 10.1002/adsu.202300149situation since they operate as a direct source of nutrients for the soil. [1]Chemical fertilizers are now extensively applied by farmers all over the world since they can supply all the vital elements that plants require for development. [1]These nutrients are categorized into macronutrients and micronutrients, based on how much they are needed for plant growth.Because they are required in larger quantities, macronutrients, namely nitrogen (N), potassium (K), and phosphorus (P), have been the focus of manufacturing efforts by the fertilizer industry. [2]espite the increase in agricultural productivity enabled by intensive agricultural practices, chemical fertilizers still have a nutrient use efficacy (NUE) of less than 30%, with phosphorus fertilizers having an even lower NUE of less than 20%. [3]The excess fertilizer that is not quickly absorbed by plants will become unavailable for plant use due to microbiological, chemical, or physical reactions (Figure 1). [4]This poor NUE can lead to major environmental consequences.For example, leaching of nitrogen as nitrous oxide and nitrates, caused by the excess use of nitrogen fertilizers, leads to the eutrophication of aquatic ecosystems and the emission of greenhouse gases that can accelerate global climate changes. [3]Those climate changes Figure 1.A,B) Loss of excess conventional fertilizers in soil.In (A) excess N fertilizer is lost in the environment through run-off, ammonia volatilization, leaching of nitrates, and nitrous oxide emissions.In (B) excess P fertilizer is either lost by run-off and leaching, immobilized into organic form, adsorbed into inorganic form, or precipitated into mineral form.
in turn affect agriculture through a decrease in crops yield, a reduction in their nutritional quality, or an increased frequency of extreme weather events detrimental to plants' growth. [5]The development of new techniques to help increase fertilizers' efficiency, both in terms of food production and environmental protection, is thus critical to achieve more sustainable agricultural practices.
The low NUE observed for plants in agricultural soils is in part due to the nature of conventional fertilizers which, once applied, will release a large amount of nutrients in the early stage of plant growth, leaving too little at later stages. [4]To overcome this issue, controlled release fertilizers (CRF) have been explored as an approach to deliver fertilizers in a way that match the rate required by the plant, which would avoid losses caused by leaching, evaporation, and other factors related to the weather. [6]However, to accurately control the release of such fertilizers, a number of challenges and factors that determine their applicability and performance still need additional research and investigation.For example, there is a pressing need for more research into the processes that can trigger and govern the release rate of nutrients by CRF, as well as the primary environmental factors (such as temperature, moisture, microbes, acidity, and soil type) that influence them. [7]n this review, we present the most recent research on the development of CRF in an effort to highlight the most promising approaches and materials that may be exploited to coat fertilizers and achieve controlled release.We will highlight the techniques and practices that can provide greater performance with less coating material, as well as any potential difficulties or issues related with their application.We will also explore the promising field of stimuli-responsive hydrogels, which can better match nutrient release to the needs of the crops and identify the key areas that still require research before they can be successfully applied in the agricultural sector.This paper will give recommendations for future research on next generation fertilizers that, by reducing fertilizer use and increasing agricultural yield, can play an important role in achieving several of the sustainable development goals defined by the European Union, such as reducing hunger and malnutrition, promoting responsible production and consumption in the agricultural field, and decrease the greenhouse gas emissions associated with fertilizer production and application.

Controlled Release Fertilizers: Classification, Synthesis, and Mechanisms of Release
The CRFs are specially designed products that release active fertilizing nutrients in a controlled, slow fashion in synchronization with the progressive demands of plants for nutrients. [8]The initial efforts in the development of CRFs started in the 1960s and were primarily focused on sulfur and polyethylene as coating materials.Since then, many polymer materials and natural coating agents were explored, along with a wide range of advanced materials such as multifunctional super-absorbents or nanocomposite structures. [9]In this section we will discuss the distinct categories of CRF, present their synthesis methods, and highlight the different mechanisms for the release of nutrients.

Classification of CRF
The different categories of CRF, as described by Shaviv et al., [8] are presented in Figure 2.These categories are: 1) low solubility organic-N compounds, which can be further classified as either biologically decomposing products like urea-formaldehyde (UF) or chemically (mostly) degrading compounds like isobutyledenediurea, 2) low-solubility inorganic compounds, which are commonly found in forms such as metal ammonium phosphates (MgNH 4 PO 4 ) or partially acidulated phosphates rock, and 3) CRF with a physical barrier that controls the release, which can be in the form of a continuous matrix where the soluble active ingredient is spread, or nutrient granules surrounded by a hydrophobic polymer.
Compared to low-solubility macronutrient formulations, which simply slows down the release kinetic to increase their retention in soils, using an external barrier to control the release also provides an opportunity to develop functional coatings that control the release within the specific bounds that match the plant's nutritional needs. [4]Therefore, this review will be focusing on CRF materials using a physical barrier, a matrix, or an external coating to control the release.Coated CRF can be divided into two subgroups: inorganic, mineral-based coatings such as sulfur and organic coatings using polymers, thermoplastics, or resins. [4]These coatings can be either hydrophobic, such as polyolefin or rubber, or gel-forming polymers, also known as hydrogels. [9]The nature of the physical barrier determines how nutrients will be released from the CRF and therefore dictates its potential applications in the field.

Mechanisms of Release of CRF
Each CRF granule has a distinct three-stage sigmoidal and nonlinear release profile (Figure 3); a lag phase where there is no evidence of release, a stage of continuous release, and a decay phase in which the release rate declines considerably.In the first step, soil water, primarily as vapor, wets the coating, resulting in the formation of fissures as the water penetrates to the core and allows the fertilizer to dissolve.No fertilizer is discharged at this stage due to the vapor pressure differential.The lag may be caused by the time needed to fill the internal voids with the necessary volume of water or by a steady state between water intake and solute exiting.In the following step, as water keeps entering, more material fertilizer is dissolved and the osmotic pressure in the core increases, allowing fertilizers to be gradually released through gaps in the polymer coating (Figure 4).Diffusion of nutrients to the soil is maintained if the granule's solution remains saturated.Once the osmotic pressure surpasses a set level, the coating layer collapses and the fertilizer bursts out.In the decay point, most nutrients have been dissolved and released, which reduces the osmotic equilibrium and thus the release rate. [10]ore specifically, in the case of coated CRF, a multi-stage diffusion model was suggested. [11]After spreading coated fertilizers in the soil, irrigation water penetrates the coating and condenses on the solid fertilizer core, causing the dissolution of nutrients.As osmotic pressure rises, the granule swells and exhibits two reactions.When osmotic pressure exceeds the resistance of vulnerable coatings (e.g., sulfur or modified sulfur), the coating will burst and the core nutrient immediately liberated.On the other hand, if the layer resists the increasing pressure, the core fertilizer is gradually released, driven by a concentration or pressure gradient, or a combination of both. [11]The solubility and availability of the nutrient in the soil determine the speed at which fertilizers will diffuse through the polymer layer and into the soil.The rate of dissolution of the nutrient is controlled by the concentration difference between the soil and the CRF's core. [12]Microorganisms can also play an important role in the fertilizer release process for several CRFs.Microorganisms in the soil can produce enzymes or acids that degrade the polymeric coating layer, gradually releasing the nutrients into the soil. [13]Therefore, the coating layer must be strong enough to prevent the core from expanding because of absorbing water and be able to resist microbial degradation in order to maximize the longevity of the fertilizer granule inside the soil.

Coated CRF: Definition, Coating Techniques, and Cost-Effective Coating Practices
Coated CRFs are conventional soluble fertilizer products that, after granulation, prilling, or crystallization are provided a waterinsoluble covering layer to regulate moisture absorption and hence solubilization rate and nutrient release. [7]Coating materials may be broken down into two groups: inorganic materials like sulfur, bentonite, or phosphogypsum; and organic polymers, which can be either synthetic polymers such as polyurethane, polyethylene, and alkyd resin or natural polymers, such as starch, chitosan, or cellulose. [10]owever, sulfur coated CRF can cause soil acidity if sulfur is converted into sulfuric acid after application. [14]Moreover, while CRF created from natural materials decomposes rapidly in the soil, CRF made from synthetic materials such as polyurethane and polyethylene generate volatile organic compounds and have a detrimental effect on the soil as a result. [15]This kind of polymer coatings can also break down into microplastics over its lifetime, generating another contaminant of emerging concern due to their persistence.Therefore, although the use of CRF has many positive effects in agriculture, such as lower fertilizer use and decreased nutrient pollution, the choice of the material must be made carefully as to not impact negatively the soil quality over time. [16]

Coating Techniques
Polymer-coated CRF can be produced using several types of commercially available coating processes.Examples of such techniques are the rotating drum, pan coater, and fluidized bed coater As a result of their burst release, the conventional fertilizers release most of the nutrients during the first growth stages of the plant.The CRF release follows a sigmoidal curve involving three phases: an initial lag period, followed by a linear release, and last, a decay release phase.
(Table S1, Supporting Information).Rotary pan or drum coating is one of the most common techniques used to make commercial CRF due to its simplicity: the rotary pan technique involves rotating a pan or a drum containing fertilizer granules and spraying them with a polymer solution using nozzles, which will form a coating upon drying.Warm air can be blown over the wet coatings to reduce the drying time.This simple process has been in use for producing CRF commercially; however, it is known to have a low coating efficiency and, as a result, a lot of coating material is wasted to obtain a good film on the surface of the granules. [17]To reduce solution losses during the coating process, CRF can be produced using a fluidized bed technique.Since fluidization allows for precise regulation and adaptability, it improves the efficiency of the operation.Spraying the coating material into a supporting air stream causes the coating material to create a fluidized bed.Once the fertilizer is added, the coating substance covers the granule, resulting in a new compound with gradual nutrient release. [18]A schematic representation of the main coating processes used for CRF production is shown in Figure 5.

Cost-Effective Coating Materials and Practices: Making Innovation Affordable Again
Industry is increasingly focusing on new ways to augment the productivity of agricultural systems while decreasing production costs, which has led to the adoption of novel alternative materials in agriculture. [19]Agricultural fertilizers are being used on a massive scale, which provides a strong financial reason for innovative, cost-effective formulations to be developed.For large scale applicability, it is important to use coating materials that are easy to apply, inexpensive, and environmentally benign to ensure the broad use of coated CRF.

Economic Benefits of Coated CRF
Coated CRF are designed to ensure that the late nutrient release is matching in time with the nutrient demands of crops, [18] making a single application able to meet the nutrient requirements for the entire season.That way, the use of CRF can reduce spreading costs by allowing for an earlier application that avoids the "annual spring rush," when access to the field is challenging.Likewise, single basal application of CRF can lower the demand for additional short-season top dressing applications during critical periods, such as for rice paddies, which reduces labor expenses. [8]ince financial benefit is the main driver for farmers when selecting a management practice, Zhang et al. demonstrated that a single application of blending urea (BU) which is composed of both coated controlled-release urea and uncoated common urea saved half of the labor expenses, which balanced the higher fertilizer N prices.Thus, the farmer's financial earnings were not affected by the implementation of BU and could potentially been in fact improved. [20]Through a 2 years field trials using aromatic brown rice, Shivay et al., showed that 0.5% boron-coated urea, 5.0% sulfur-coated urea, and 2.5% zinc-coated urea provide better yields than non-coated prilled urea. [21]These treatments maximized returns and benefit-cost ratios, as shown in the Figure 6.However, despite these proven benefits of CRF fertilizers, they are not commonly used in Europe outside of gardening and for the fertilization of lawns and ornamental plants. [18]

CRF Production Cost Factors
The higher production costs of CRF compared to conventional fertilizers is one of the most important factors that limit their widespread adoption.Because of the prices of coating materials and the inefficiency in the coating practices, the production costs of fertilizers may be increased by 10 to 30 times when organic polymer coatings are added. [7]However, improvements in the CRF coating processes could help mitigate this higher production cost.Production methods for CRF can be divided into chemical and physical techniques with the latter being the most used today because they are simpler and less expensive to operate.However, physical coating methods use higher amount of raw coating material in order to obtain homogeneous coating layers on the fertilizer granules, which raises the overall production cost. [17]An important aspect of the cost efficiency of CRF coating is therefore maximizing the use of coating material.Techniques such as fluidized bed coating can help minimize solution loss and make the physical coating method more affordable, but this approach usually has a high capital cost and is not ideal for granules of increasing sizes. [10]Production size also matters for the CRF costs.For the moment, CRF production is often realized in small batches (1000 to 5000 kg) with select raw granular material sizes to ensure optimal coating quality, which increases the production costs. [7]This cost is likely to be mitigated as CRF becomes more abundant in the global market and production volumes increased.This cost reduction with scale-up can also be expected for chemical coating techniques, which for CRF is mostly used to make hydrogel-based coatings for controlled release or stimuliresponsive fertilizers.For chemical coatings, the use of organic solvents during hydrogel synthesis can be an important aspect of the synthesis' costs.Reusability of the solvent should therefore be an important selection criterion for hydrogel-based CRF.The polymerization step is a critical component for this, since for some hydrogel synthesis, like solution polymerization synthesis, the solvent cannot be recovered but, for other approaches such as inverse suspension polymerization, which relies on performing the polymerization reaction in monomer droplets dispersed in a non-miscible medium such as water or oil, the solvent can be recovered and purified for reuse. [10]The use of inexpensive, biobased raw materials (discussed in the next section) as well as innovative green chemistry techniques to avoid organic solvents [22] are potential avenues of development to improve the affordability of hydrogel-based CRF coatings.

Cost-Effective Coating Materials and Production Methods
Fertilizers can be coated with a variety of materials to obtain controlled release properties.The choice of coating material, along with the production method, will be determining factors to CRF costs and eventual marketability.Moreover, using bio-based and biodegradable coatings is becoming an increasing area of interest

Castor oil-based polyurethane
Water polishing • Water polishing DAP grains lowered the angle of repose by 2.48-10.57%and the specific surface area by 5.70-48.76%.• The release rate of water polished coated DAP granules is 5.36 times slower.[29]   Castor oil-based polyurethane Wax-based surface modification • Wax surface modification lowered DAP particle surface area and repose angle.
• Prior research used 9 wt% coating material to release P for 25 days, while wax-surface-modified coated DAP granules released P for 93.4 days using 4 wt% coating material.[31]   Cassava starch modified with di-sodium tetraborate Heated pre-coat solution • N release time from uncoated urea granules in water was 63.33 s, rising with pre-coat solution temperature from 50 to 80 °C, reaching 209 s at 80 °C. [32]

Recycled polystyrene foam
Large tablets size • Same release rate as the commercial polymer coated urea (PCU) with 70-80% less coating material.• Coating large urea tablets with recycled polystyrene foam is 7-8 times cheaper than commercial PCU.[30]   Recovered lignin from industrial wastewater • Lignin retrieval from Kraft and Sulfite black liquors • Acetylation of lignin using acetic acid and sodium metabisulfite • Acetylated lignin-coated urea granules released 36.3% for Kraft lignin and 45.3% for Sulfite lignin after 7 days, while commercial sulfur-coated urea released 59%.[27]
• 40% cost reduction at lab-scale compared to conventional polymerization.[24]   Vegetable Oil polymer coating Oil-based polyurethane films • After 70 days, 80% of the fertilizer content is released when using an Oil-PCU film with an isocyanate index of 1.58 • After 180 days, the degradation percentage of five polyurethane films with different isocyanate indices ranged from 10.23% to 29.63% [26]   for fertilizer production since, in addition to regulating nutrient release, there is a need to cut down on the emission of potentially hazardous chemicals during CRF production and use. [23]Table 1 provides an overview of novel potentially cost-effective materials used for the synthesis of CRF coatings.
Residue materials from industries like agriculture are a promising source of low-cost materials to explore given the scale of agricultural production.For example, biochar, as a bio-waste, is an interesting candidate due to its low cost and widespread availability.Chemical synthesis of biochar using microwave irradiation was recently introduced as a rapid, high-efficiency, lowenergy, and low-pollution bulk heating option to create biocharhydrogel composites. [24,25]Microwave irradiation, unlike traditional polymerization synthesis, does not involve the use of organic solvents or any other byproducts.Using urea coated with a biochar-hydrogel composite, only 20.03% of N was released after 30 days.Another type of agricultural materials usable for CRF synthesis are oils, as demonstrated by Feng et al., who prepared a fully vegetable oil-based polyols using epoxidized soybean oil and oleic acid as raw materials.These polyols were then used to form polyurethane films for CRF coatings. [26]Based on the isocyanate index of the polyurethane, coating degradation varied between 10% and 30% after 180 days in soil.When used to coat urea fertilizers, the resulting CRF took up to 70 days to release 80% of its cumulative capacity, showing that these oil-polyurethane coating have good prospects for controlled nutrient release.In addition to these good performances of CRFs derived from agricultural waste, the opportunity to valorize the waste back into the agricultural cycle is interesting for a circular economy perspective.
The pulp and paper Industry are another large source of useful waste in the form of lignin.Behin and Sadeghi experimented with modified lignin as a coating for urea fertilizer particles.Kraft and sulfite black liquors, as sources of sulfate and sulfite lignin, were modified using an acetylation reaction to improve their hydrophobicity.Then, by coating urea granules with the acetylated lignin, a release of 36.3% and 45.3% after 7 days for the Kraft and sulfite lignin, respectively, compared to a release of 59% for sulfur coated urea. [27]These improved rates suggest that recovered lignin from industrial wastewater can be used to achieve good performance in CRF.Waste polymers can also be recycled into new coating materials for fertilizers.For example, Yang et al. used recycled polystyrene foam, either as is or blended with wax or polyurethane, as a coating material for the coating of urea granules.This approach was estimated to not only reduce the cost of urea coating compared to commercial polymer coated urea but also provide a novel solution to the recycling of plastic products.
Beyond the nature of the coating material, coating costs can be reduced by reducing the amount of coating needed for controlled release.A study by Tian et al. showed a surface modification technique based on abrasive particles that reduced the micro/nanoscale surface morphology of urea particles and improved the coating adherence and uniformity.By reducing the surface roughness of the urea particles, the amount of coating material needed to produce coated urea CRF was reduced by up to 28.6% for an equal duration of release. [28]Water polishing is another way to improve to reduce the roughness of fertilizer particles and therefore reduce the amount of coating material needed.Lu et al. used water polishing to treat the surface of diammonium phosphate (DAP) granules, which decreased their specific surface area by 5.70-48.76%(based on the water % used) and therefore reduced the amount of surface to cover with the coating material.When applied to maize crops, the fertilizer granules coated with a castor oil-based polyurethane after water polishing had a release rate 5.36 times slower than non-polished coated granules. [29]In a similar approach, using large fertilizer tablets also reduces the specific surface area of the fertilizer material, resulting in less coatings needed.Large urea tablet (12 mm in diameter and 8.5 mm thick) had the same release rate as a commercial polymer coated urea particles (diameter of 0.5-2 mm) but used 70% to 80% less coating material. [30]Substantial savings can therefore be gained simply by better understanding and optimizing the amount of coating needed for CRF fabrication.

Matrices CRF: Definition, and Loading Techniques
Water-soluble fertilizers with physical barriers that regulate nutrient release take the form of a matrix of active fertilizer nutrients spread throughout a coating material that impedes fertilizer dissolution.Both hydrophobic matrices, like polyolefin and rubber, and gel-forming hydrophilic matrices, commonly referred to as hydrogels, can be used as a controlled release matrix. [9]1.Hydrophilic Matrices: Hydrogels CRF Hydrogels are a form of cross-linked polymer with hydrophilic groups attached to the polymeric backbone that can absorb significant amounts of water without dissolving.[33] By itself, this high-water adsorption capacity is attractive for agriculture applications, knowing that farmers in arid and desert places across the world have limited access to water during the dry season but, at the same time, considerable precipitation is lost during this period.[34] Therefore, beyond their use to control nutrient release, hydrogels may also decrease water losses due to evaporation and reduce irrigation periodicity.[35] In the design of CRF, hydrogels are used for their high water absorption capacity, high crosslinking gel fraction, low toxicity, affordability, outstanding swelling and storage stability, and the ability to rewater after drying.[36] Moreover, the hydrogel is resistant to physical damage or other stress that would damage most coatings and result in the undesirable burst release of nutrients.[37] Research on using hydrogels as CRFs is summarized in the Table 2 below.

Cross-Linking Methods of Hydrogels
Because hydrogels are usually derived from water soluble compounds, cross-linking the hydrogel matrix is essential to preserve its structure as a coating when used in the field.The addition of cross-links between polymer chains influences the material's physical characteristics, the nature of which is determined by the degree of cross-linking as well as the crystallinity of the polymer.Several cross-linking techniques are available to be used for hydrogel formation, such as electrostatic, thermal, or chemical cross-linking.
Hydrogels can be cross-linked electrostatically through ionic interactions, complex coacervation, or hydrogen bonding. [45]oss-linking reactions using ionic interactions or complex coacervation are based on molecules or polyelectrolytes with opposing charges.Ionic cross-linking is achieved by the introduction of counterions to initiate an in situ gelation reaction, which will be influenced by factors such as ionic strength, pH, type of counterion, and functional charge density of the solution.Alginate is a common example of an ionic cross-linked hydrogel when it is mixed with divalent cations. [46]On the other hand, complex coacervation is the rapid aggregation of two polyelectrolytes with opposing charges, resulting in a liquid-liquid phase separation in an aqueous medium. [47]Some examples of this type of physical hydrogels are those made by coacervating chitosan and xanthan. [21]Polymers of the same charge can also be crosslinked through hydrogen bonding; for example, poly(acrylic acid) and poly(methacrylic acid) can make stable hydrogels in the presence of poly(ethylene glycol) by creating hydrogen bonds between the carboxyl group (─COOH) of acrylic acids and the oxygenated groups of poly(ethylene glycol). [48]Hydrogen bonding for crosslinking is of particular interest for hydrogels that possess selfhealing properties due to the reversible nature of the bonds as the microenvironment of the polymer changes. [49]hermally crosslinked hydrogels form after cooling a heated biopolymer solution.During cooling, an interchain connection is made when two or more molecules align (whether in parallel or perpendicular to the axis of the chain length), which crosslinks the biopolymer into a hydrogel.The polysaccharides agar and carrageenan are two common examples of thermally crosslinked hydrogels. [50]A similar process occurs during the crystallization process generated by freeze-thaw cycles.During the freeze-thaw process, the crystallization maximizes the density of polymer by decreasing the chain gap in the polymer, which allows the chains to align and crosslink with one another to create a network structure.After going through a series of freeze-thaw cycles, the hydrogel formed ends up with a porous morphology due to the voids left by the melting particles.Variations in polymer solution, frequency of freeze-thaw rounds, freezing interval, and freezing degree all affect the freeze-thawed hydrogel's mechanical characteristics. [51]The cryogelation of xanthan is an example of an hydrogel generated by freeze-thaw cycling. [52]These thermal cross-linking benefits from not using any added chemicals; however, the energy required for the temperature change may be prohibitive unless heating is already required to make the hydrogel solution.
Chemical cross-linking relies on covalent interaction between polymer chains to create permanent chemical hydrogels. [53]Different chemical cross-linking techniques have been used in the literature, including functional groups crosslinking, free radical polymerization, irradiation induced crosslinking, and enzyme mediated crosslinking. [48]For CRF synthesis, the primary methods used have been based on free radical polymerization synthesis and functional group cross-linking.Free radical polymerization is based on the formation of a free radical site by an initiator, which results in the accumulation of monomers in a chainlike pattern. [54]Free radical polymerization can be triggered by a variety of initiators, such as chemical redox reagents, thermal decomposition, electro-or sono-chemical activation, ionization radiation, or photopolymerization with visible or ultraviolet light.The type of initiator to be used depends on the types of monomers to be cross-linked as well as the properties of the

Maize
• Low application rate of the hydrogel (<0.2%) promotes the germination rate, seedlings height, and roots height compared to higher rates (0.5% and 1%) in loam, sandy loam, and paddy soils.[44]   polymers to be achieved.For example, acrylate and methacrylate groups are frequent types of photopolymerizable chemicals while high-energy radiations, specifically gamma and electron beam, are used for the polymerization of unsaturated compounds.High-energy irradiation allows hydrogels to be formed from water-soluble polymers, with or without additional vinyl groups, through the homolytic scission of C-H links. [48]Free radical polymerization allows for a precise control on the polymer chains' length and composition, which in turns has led to its use to build a wide variety of CRF hydrogels with precise polymer properties; however, the synthesis process tends to be more complex than cross-linking pre-existing polymer chains using chemical agents.For example, crosslinkers with aldehyde groups, such as glutaraldehyde (GA), is a simple way to make covalent bonds with polymers containing hydroxyl (i.e., cellulose) and amine (i.e., chitosan) groups. [51,55]Similarly, the condensation reaction between carboxylic acids and hydroxyl or amine groups is commonly used to cross-link hydrogel polymers.Two common examples are polyesters, made by crosslinking hydroxyl with carboxylic acids, and polyamides, made by crosslinking amines with carboxylic acids. [48]Ho et al. synthesized stimuliresponsive hydrogels consisting of pH-responsive polyamide and thermo-responsive poly(-caprolactone)-b-poly(ethylene glycol)b-poly(-caprolactone) in which the pH-responsive polyamide blocks containing diamide linkages were obtained by condensation polymerization of adipic acid dihydrazide and phthalic anhydride. [56]Chemical methods tend to be easily scalable and efficient but they produce hydrogels that are typically more heterogeneous due to the potential hydrophobic aggregation of crosslinking agents during the reaction.The aggregation of crosslinking agent during the synthesis then leads to the formation of high cross-linking density, low swelling degree clusters intermixed with low cross-linking density, high swelling degree areas. [57]igure 7. Schematic presentation of the two methods of loading fertilizers into polymeric hydrogels to form controlled release hydrogel fertilizers.A) The two-step approach entails first drying the hydrogel until the equilibrium swelling is reached, and then allowing the dry gel, also known as xerogel, to absorb the aqueous solution of the fertilizer.B) The in situ approach involves adding directly aqueous solution of the fertilizer to the hydrogel mixture before drying it.

Loading Hydrogels with Fertilizers
Two approaches, described in Figure 7, can be used to load an active component into hydrogels.The first approach is to add the fertilizer directly into the reaction mixture at the start of polymerization, where it becomes incorporated into the hydrogel matrix. [58]The second approach is a two-step method, in which dried hydrogels are first soaked in a liquid fertilizer solution and then dried again.Each approach has its benefits and drawbacks.For example, synthesizing the hydrogel within the fertilizer solution can lead to lower yield and unintended side reactions with the active component during synthesis, making the first method superior for a precise control of the hydrogel properties.On the other hand, from a manufacturing perspective, synthesizing the hydrogel separately is often less desirable since it necessitates additional processes (hydrogel synthesis, drying, absorption of fertilizer, and redrying).Therefore, because it requires only one drying step, directly synthesizing the hydrogel in the active agent solution, when possible, is preferable.In these syntheses, it can be hard to extract the residual monomers from the hydrogel-fertilizer composites.Therefore, the polymerization reaction should be performed to completion to minimize the presence of residual monomers in the final product, particularly when hazardous monomers are used. [59]

Next-Generation of CRF: Stimuli-Responsive Fertilizers
Beyond slowing down the release of nutrients, external coatings can be used to provide active functionalities such as the ability to match nutrient release to specific plants or soils' stimuli.There has been a lot of progress in the biomedical field on developing stimuli-responsive nanomaterials with the goal of using them as drug delivery or diagnostic carriers.These synthetic systems can enable localized drug delivery by inducing a wide range of reactions via a series of intrinsic or extrinsic triggers, which allows for a reduced amount of drug needed and a higher treatment efficiency. [60]The same objectives are shared by the fertilizer industry, where the ability to control the delivery of nutrients can reduce phytotoxicity, leaching losses, volatilization, drift, and soil degradation, as well as enable greater safety during application.
Regulating the release of active substances using passive coatings is challenging because of their reliance on diffusion, capsule erosion, or osmotic pressure.Coatings that actively respond to minor signals or variations in the surrounding environment by altering their physicochemical characteristics to promote the release of loaded chemicals are, however, one way to achieve this controlled release. [61]Therefore, inspired by the advance and breakthroughs in the stimuli-responsive administration of therapeutic and diagnostic ingredients, more functional fertilizer coatings have been explored for the targeted delivery of agrochemicals to plant roots.Hydrogels are a common polymeric system for the design of stimuli-responsive materials.They can be engineered to have triggered reactions, such as compression or expansion, in response to changes in their surrounding environment (Figure 8).Variations in temperature, electric or magnetic fields, light, pressure, and sound are all examples of physical stimuli, whereas chemical stimuli include changes in pH, solvent composition, ionic strength, and specific molecular species.The hydrogel's response to changes in its external environment may cause swelling or de-swelling of such a magnitude that the event is known as volume collapse or phase transition. [33]he release of nutrients from hydrogel CRF occurs rapidly upon moisture exposure, with particle swelling and diffusion mechanisms instantly occurring. [62]Therefore, the design objective of stimuli-responsive systems is to match that release to a trigger associated with the need of the plant.Stimuli-responsive materials have been explored for agrochemical release in reaction to pH, temperature, redox conditions, enzymes, and light. [63] variation in these parameters can be found in soils as part of environmental perturbations or as a result of the plant to these perturbations. [64]For example, pH variations in the rhizosphere can be caused by H + or OH − release by roots to adjust for imbalanced cation-anion absorption at the soil-root interface.Roots exudation and respiration increase CO 2 concentration in the rhizosphere, forming carbonic acid that dissociates in neutral to alkaline soils and decreases the pH near the plant, which alters nutrient and toxic element bioavailability.Redox-coupled reactions by plant roots and microorganisms can also modify the pH of the rhizosphere.Table 3 compiles some examples of the pH differential between the bulk soil and the plant's rhizosphere.While these changes are very species-and soil type-dependent, the presence of a significant pH difference between the root system and the soil can provide a target pH window to engineer the release of nutrients in the roots' vicinity, which would maximize nutrient uptake by the plant.The following section provides information on the efforts to tailor the nutrient release to specific stimuli, which can inform on potential strategies to target specific physiological triggers associated with the plant's nutritional needs.

pH Responsive Hydrogels
When a polymer possesses acidic or basic groups that can give or accept protons in reaction to variations in pH, it is considered pH responsive.The ionizable functional groups of pH-sensitive polymers are utilized to produce pH-responsive hydrogels that either receive or release protons in response to environmental changes.This causes the hydrogel to swell in an aqueous solution. [57]If the solvent pH is adjusted, an ion concentration gradient between within and outside the gel is created, which allows mobile ions to enter and exit the hydrogel and increase the osmotic pressure  on the surface.The osmotic pressure causes the hydrogel's volume to change. [45] variety of low-cost natural polymers and biomaterials have been explored to obtain stimuli responsive CRF.For example, using in situ free-radical graft copolymerization, sodium alginate, acrylic acid, and acrylamide were combined with rice husk ash (RHA) and fertilizers to obtain a pH-responsive NPK formulation.The authors showed that as pH rises from 2 to 6, the equilibrium water absorbency of this composite also increased, reaching its highest value at pH 6.The swelling ratio was also found to be steady between pH 5 and 7 while it decreased with the pH increasing above 7. [71] Chitosan is another widely used natural pH sensitive polymer because of the number of amino groups among its chain, which makes it readily soluble at acidic pH but insoluble at alkaline pH levels. [72]Using in situ hydrogelation of chitosan with salicylaldehyde in the presence of urea, Iftime et al. developed a pH responsive hydrogel that maintained its structural integrity for up to 15 days at a pH >6, with enhanced stability with increasing pH. [73]Sodium carboxymethyl cellulose (CMC), a water-soluble anionic cellulose derivative, is also frequently used to form hydrogel, particularly in the form of gel beads. [74]To obtain a pH-responsive hydrogel from CMC for the release of NPK, Olad et al. grafted sulfonated carboxymethyl cellulose with acrylic acid in the presence of polyvinypyrrolidone and silica nanoparticles. [75]This hydrogel material was shown to swell at pH = 8 due to anion-anion electrostatic repulsion but shrink at pH = 2 due to the protonation of carboxylate groups.Additionally, this hydrogel responsive to salinity, with samples swelling in denatured water and shrinking in a 0.1 m NaCl solution.A composite CMC material made with monocalcium phosphate and zeolites was also developed by Singh et al. for P release in soils. [76]The composite hydrogel possessed a "burst release" in neutral pH but a delayed diffusion-driven release in acidic (pH = 4.2) conditions, emphasizing the pH sensitive release mechanism for this material.While production is constrained by high fixation rates and low phosphate utilization efficiency, this release profile is of particular significance for P delivery in the rhizosphere of acidic soils.
Hydrogels can be relatively fragile structures, especially in their swollen configuration.Therefore, several groups have combined hydrogels with cellulose nanofibers (CNF), which are typically characterized by high strength and stiffness, to balance the properties of the hydrogel. [77]The CNF also have moderate thermal expansion, high crystallinity, and hydrophilicity, and an easily adjustable surface chemistry, all of which can be of interest for CRF synthesis. [77]Shaghaleh et al. successfully synthesized a novel N pH-responsive/sustained release fertilizer composite based on wheat straw aminated-cellulose nanofibers and cationic poly(acrylamide-co-2-aminoethyl methacrylate hydrochloride) by direct AN fertilizer encapsulation. [78]While the results revealed that N release was lowest in a neutral pH medium, it was highest in a lower pH condition, where N supply was maintained for 90 days in a neutral-irrigated soil growing rice.In another study, CNFs were mixed with SA and PVA, and then crosslinked in the presence of NPK fertilizer. [79]This SA/CNF/PVA composite retained more water in both deionized water and mixed soil, and exhibited a pH-sensitive swelling behavior, with a rising trend in equilibrium swelling ability from 3 to 6, stability from 6 to 8, and a reduction at greater pH levels from 8 to 11.These studies emphasized the potential of CNFs as a component for designing advanced functional hydrogel fertilizers.
Combining the hydrophilic properties of acrylic polymers with the biodegradable nature of starch-based composites can allow for CRF materials that will not become persistent in the soil. [80]owever, care must be taken when altering the crosslinking of starch-based hydrogels if biodegradability is the desirable property, as extensive crosslinking and substitutions in starches can reduce their biodegradability. [81]Dudu et al. synthesized a novel N,N-dimethylacrylamide-maleic acid (MA)-Starch (St)-based superabsorbent (DMSt 1 ) and tested its abilities as a controlled release fertilizer in lettuce growing.Dudu et al. used HCl/NaOH to make negative and positive surface modifications of DMSt 1 , labeled DMSt 2 and DMSt 3 , respectively.The latter had the greatest maximum swelling rate of 37.38% in deionized water environment.The anionic and cationic characteristics of the DMSt 1 changed and the hydrogel displayed varied swelling behaviors at different pH levels after being treated with acid and base.The DMSt 1 , DMSt 2 , and DMSt 3 -based hydrogels were also shown to have excellent water uptake potential, with equilibrium swelling values of 7163.7% at pH 10, 15708% at pH 10, and 27838%, respectively, at pH 8.This water retention ability resulted, during pot experiments with lettuce plants, in a decrease in membrane damage index compared to the control sample, showing that the hydrogels, in addition to nutrient release, can also prevent water deprivation, in crops. [82]ome possibilities for customizing nutrient release to the rhizosphere microenvironment can be found by comparing the pH window of stimulus response hydrogels to the pH gap between bulk soil and plant's rhizospheres.Several investigations have reported hydrogels that are stable at near-neutral pH (about 6 to 8), but that expand more rapidly at lower pH levels. [71,73,79]These pH ranges may coincide with the soil and rhizosphere conditions seen in some crops (as noted in Table 3), indicating the potential usefulness of adopting stimuli-responsive techniques.Some crops, however, have narrow pH windows between the bulk soil and the roots, which may necessitate additional optimization of the hydrogel chemistry.In addition, a better understanding of the changes found in the rhizosphere needs to be provided before implementing such solutions for a specific crop, as the differences between the bulk soil and the pH microenvironments of the rhizosphere are likely to change with the season, soil chemistry, plant growth cycle, or physiological status.

Thermo-Responsive Hydrogels
Thermo-sensitive carrier systems have a lot of potential in agriculture since the temperature of the environment is always changing.The physicochemical characteristics of the hydrogel polymer can be made to vary in response to temperature and release active ingredients. [61]Hydrophilic and hydrophobic domains coexist within the structure of thermosensitive polymers.As a result of temperature fluctuations in the interaction among both hydrophilic and hydrophobic portions in the polymer and water components, a sol-gel phase transition takes place in these kinds of materials.The phase transitions of some polymers are characterized by a lower critical solution temperature (LCST), below which they are in a soluble and hydrated state but becomes hydrophobic and precipitates above this temperature (solgel transition), while the phase transitions of certain other polymers are characterized by an upper critical solution temperature (UCST), where the opposite transformation actually happens (gel-to-sol). [83]Polymer networks in temperature-sensitive hydrogels show swelling responses that can be either positive, where the gel material swells as temperature rises, or negative, where the gel material contracts as temperature rises. [57]The LCST or UCST of the hydrogels may be adjusted by tuning the ratio of hydrophilic to hydrophobic segments. [45]ydrogels having an interpenetrating polymer network (IPN) of polymer chains like polyacrylamide and PAA or poly(acrylamide-co-butyl methacrylate) crosslinked with N,N′methlenebisacrylamide are among the few known instances of UCST-type of polymers. [84]As an example, [85] examined an innovative thermo-sensitive semi-IPNs formed by incorporating cellouronic acid sodium, which is derived from bagasse pith, into a poly(acrylamide-co-diallyldimethylammonium chloride) (poly(AM-co-DAC)) network.These semi-IPN gels displayed UCST-type temperature sensitivity and excellent waterabsorbency.The UCST could be adjusted to higher temperatures by increasing the ratio of H-bonds to zwitterionic pairs and the total interchain connections between the two polymers.However, when urea and other fertilizer salts (i.e., KH 2 PO 4 , NH 4 NO 3 , and MgSO 4 ) were introduced to the solution, the UCST was found to decrease, emphasizing the need to consider the release environment's chemistry when designing the phase transition temperatures of CRF.The LCST thermosensitive hydrogels were also explored by Feng et al., who used surface-initiated atomtransfer radical polymerization to produce a "smart" fertilizer from poly(N,N-dimethylaminoethyl methacrylate) (PDMAEMA) brushes grafted onto polydopamine-coated ammonium zinc phosphate. [86]They showed that temperatures below the LCST can increase the rate of nutrient release whereas temperatures above the LCST decreased nutrient release.These findings suggested that fertilizer made with PDMAEMA brushes can be designed to possess controlled-release capabilities.
[89] One example of polyurethane used as a thermo-responsive coating material for CRF application is the study by Qiao et al., who employed polycaprolactone (PCL) with varying molecular weights (PCL500, PCL1000, PCL2000, and PCL3000) as raw materials to produce six types of polyurethane-based CRF coatings by reacting polyether polyol (PPG)/PCL blends with methylene diphenyl diisocyanate (MDI).Results demonstrated that PPG/PCL2000-based PUCF blends can copolymerize with MDI to generate a block copolymer with a two-phase structure (crystalline/amorphous).The CRFs coated with the block copolymer of PPG/PCL2000 at a molar ratio of 8 to 2 showed a sustained N release time of more than 30 days.The N release rate increased by 22.3 times per hour as the temperature rose from 32 to 33 °C, revealing a clear thermo-responsive behavior. [90]These findings emphasize the potential of using polyurethane to control nutrient release based on the temperature environment, warranting further investigation, alongside rigorous plant growth assessments, to establish its benefit as a CRF coating material.
For temperature-responsive hydrogels, targeting the seasonal shifts in soil temperature may be the main interest for the design of CRFs.Indeed, seasonal shifts in temperature are observable in many parts of the world.The soil temperature shift will also be depth dependent, being most pronounced in the soil's upper layers.By adding CRF to the soil in the off-season, and waiting for them to become active, as desired, nutrient release can be timed to coincide with a certain growing season.As a result, this stimuli-responsive CRF could be employed to cut down on costs associated with human labor during peak, high-demand periods of the growing season.For other stimuli-responsive CRFs, such as those made from pH-responsive materials, it is necessary to have a firm grasp on how soil properties evolve on a seasonal basis in a given region.

Other Stimuli-Responsive Hydrogels
Many chemical and biological processes in cells rely on enzymes, which are crucial biological catalysts.In terms of substrate specificity and selectivity, enzyme-catalyzed reactions are unmatched. [91]Making enzyme-responsive polymer complexes by covalently linking enzymatic substrates to amphiphilic copolymers is a typical approach.Since insects and their larvae have and release enzymes, most of enzyme-responsive hydrogels are being applied for the targeted delivery of insecticides.For example, [92] created a new enzyme-responsive emamectin benzoate microcapsule by cross-linking silica with carboxymethyl cellulose using epichlorohydrin.The silica-epichlorohydrincarboxymethylcellulose microcapsules demonstrated outstanding cellulase stimuli-responsive characteristics, maintained insecticidal activity against Myzus percae while protecting the active ingredient, epichlorohydrin, against light-and temperatureinduced degradation.However, as of yet, enzyme responsive hydrogels have not been investigated for nutrients delivery in soils.
It has been shown that redox-responsive hydrogels can alter their color, fluorescence reactivity, or chiral structure in response to external redox reactions or electric field stimulus by shifting the self-assembly phase of the gel component. [93]To regulate the release of agrochemicals and capture heavy metal ions at the same time, Hou et al. developed a redox-responsive hydrogel where cystamine was used as a crosslinker in a CMC matrix to produce a flexible material.A decrease in agrochemical losses following application to crops was attributed to the 3D network generated by CMC during the crosslinking process.Toxic metal leaching was decreased when redox-responsive hydrogels were applied to soils contaminated with Cu 2+ and Hg 2+ by complexing the metal ions through their thiol groups.Agrochemicals incorporated in hydrogels were delivered when the responsive networks were decomposed by the reducer. [94]oducts that can include two or more sensitive groups (temperature and pH, redox and enzyme, enzyme and pH, etc.) are called multi-responsive carriers. [93]Using salicylic acid as a standard agrochemical, Hou et al. developed redoxand enzyme-responsive macrospheres by self-assembling cyclodextrin-modified zeolite and ferrocenecarboxylic acidgrafted carboxymethyl cellulose.Salicylic acid liberation from macrospheres was increased in the presence of hydrogen peroxide (oxidant) and cellulase (enzyme), with comparable release rates of 85.2% and 80.4%, respectively, after 12 h, compared to the control, non-responsive sample (12.6% salicylic acid release).These results suggest that dual-responsive macrospheres may be a useful carrier for the controlled release of agrochemicals. [95]able 4 provides an overview of the different materials and approaches for the design of stimuli-responsive CRF or agrochemical-delivery systems.As discussed above, pH-and temperature-responsive hydrogels are the most actively explored for fertilizer delivery, due to their clear relationship with soil chemistry and its alteration by the plant's metabolic activity.Combining these types of CRFs with other functionalities to make them responsive to multiple stimuli at the same time may offer an opportunity to further refine the nutrient delivery and therefore avoid competition with the soil components in fertilizer use.This is likely to result to an increase in production cost for these more advanced stimuli-responsive CRFs, as their design become more refined and precise.However, considering that several of the materials proposed for the fabrication of stimuli-responsive CRFs are natural materials such as cellulose, chitosan, or alginate (see Table 4), there is a possibility to advance this next-generation of CRFs in a way that mitigate the costs and environmental impacts.

Conclusion and Future Outlooks
Implementing CRF has a lot of potential as a strategy to improve NUE in agriculture.However, due to their greater production costs, their broad application is still limited.The choice of coating materials and methodology for a cost-effective CRF production remains to be defined; however significant progress in the synthesis of novel CRF materials based on improved coating protocols or using low-cost, renewable coating materials have highlighted several promising avenues to improve the performance of CRF and reduce their production costs.Furthermore, innovation in this niche sector of CRF can also be attained by stimuli responsive CRF, which not only enables passive controlled nutrient release via diffusion but also regulates this process to only occur in response to certain environmental stimuli.This strategy allows for greater control of fertilizer release to meet crop uptake.Research in this area has begun to emerge for pH-and temperatureresponsive CRF; however, other stimuli more closely related to the plant's activity, such as enzyme-responsive hydrogels, have not been explored yet.Finally, while CRF are generating a lot of interest on their ability to reduce fertilizer use, improve nutrient use by the plants, and reduce environmental impacts, more indepth financial and cost-benefit analyses are still needed to really identify the type of crops or soil environments where the switch from conventional fertilizers to CRF would be beneficial.

Figure 2 .
Figure 2. Classification of CRF into low solubility organic and inorganic compounds, and compounds with a physical barrier to control the release of the core nutrient (coatings and matrices).Text in bold to highlight the only categories covered in this review.

Figure 3 .
Figure 3. Schematic representation of the temporal development of soil nutrient availability, as supplied by CRF and conventional fertilizers.As a result of their burst release, the conventional fertilizers release most of the nutrients during the first growth stages of the plant.The CRF release follows a sigmoidal curve involving three phases: an initial lag period, followed by a linear release, and last, a decay release phase.

Figure 4 .
Figure 4. Schematic representation of the different processes through which CRF releases nutrients.A) The coating layer failed, owing to the osmotic pressure induced by water penetration exceeding its pressure resistance threshold, resulting in a burst release.B) Gradual release of nutrients from the core through the expanding pores caused by the water penetrating the coating, resulting in increased internal osmotic pressure.C) Water absorption by the hydrogel coating, which swells to slowly release the dissolved nutrients under osmotic pressure (deswelling).

Figure 5 .
Figure 5. Schematic illustration of the two most common coating techniques.A) The rotary pan/drum; B) The fluidized bed process used to form polymeric coatings on granular fertilizers.The movement of the air current causes the grains to swirl and hover around.A nozzle is used to coat fertilizer granules by spraying coating material over them, producing coated CRFs.

Figure 6 .
Figure 6.Costs of coating prilled urea with borax, sulfur, and zinc, as well as an economic assessment of net return and benefit:cost ratio per fertilizer treatment for a 1 hectare aromatic rice crop (Shivay et al. 2019) *At the rate of 7% of fertilizer prices (USD ha −1 ).Prevailing prices of fertilizer materials during 2013-14: i) Borax (12% boron) at the rate of 2.0 USD kg −1 , ii) sulfur dust (90% S) at the rate of 0.50 USD kg −1 , iii) ZnSO 4 •7H 2 O (20% Zn) at the rate of 0.50 USD kg −1 ; iv) ZnO (80% Zn) at the rate of 1.49 USD kg −1 , v) prilled urea at the rate of 0.087 USD kg −1 (Note: Prilled urea at the rate of 883 USD tonne −1 ; 24.97 USD for an application of 130 kg ha −1 ).

Figure 8 .
Figure 8. Schematic presentation of the stimuli triggering the release of active compounds from stimuli-responsive hydrogels.

Hamid
Ait Said obtained his Ph.D. from the Faculty of Sciences Semlalia at Cadi Ayyad University in 2021.He currently holds a position as a postdoctoral researcher in the Department of High Throughput Multidisciplinary Research at Mohammed VI Polytechnic University.His research encompasses a wide range of interests, with a particular focus on the development of calcium phosphate for bone regeneration.Moreover, Dr. Ait Said is actively engaged in the development of advanced inorganic and composite materials for agricultural purposes, specifically in the context of slow-release fertilizers.Hassan Noukrati is currently an assistant professor at the Faculty of Medical Sciences at Mohammed VI Polytechnic University.He holds a Ph.D. in Materials Science and Engineering from the National Polytechnic Institute of Toulouse in France and Cadi Ayyad University in Morocco.His research interests focus on the development of innovative biomaterials based on calcium phosphates, bioactive glasses, and biomolecules (biopolymers and drugs) for application in bone regeneration, cartilage repair, and drug delivery.Abdallah Oukarroum is a plant physiologist with extensive experience in environmental plant biology.He realized his Ph.D. at the University of Geneva, Switzerland, studying the alteration of the photosynthetic apparatus of plants during environmental stress caused by drought, salt, and heat.As a post-doc and researcher/lecturer at the University of Quebec in Montreal, Canada, Dr. Oukarroum studied the cellular level inhibitory effects of metals and nanomaterials in aquatic plants.Currently, Dr. Oukarroum is a full professor at the University Mohammed VI Polytechnic in Morocco.His research mainly focuses on the physiological and biochemical responses of plant to abiotic stresses.Hicham Ben youcef is currently an associate professor at Mohammed VI Polytechnic University.He holds a Ph.D. in chemistry and materials science from the Swiss Federal Institute of Technology Zurich, Switzerland.His main research interests focus on the development of smart and nano materials for different applications, such as agriculture, membrane technology, sensors, coatings, energy storage, and conversion toward competitive targets (performance, durability, and cost).He is currently leading the High Throughput Multidisciplinary Research Laboratory.François Perreault is a professor in the Department of Chemistry at the University of Quebec in Montreal and an adjunct professor in the School of Sustainable Engineering and the Built Environment at Arizona State University.He completed his Ph.D. in environmental sciences at the University of Quebec in Montreal and was a NSERC postdoctoral fellow in the Department of Chemical and Environmental Engineering at Yale University before starting as an assistant and then associate professor at Arizona State University.His research explores the interface between materials and biological systems, with a focus on environmental nanotechnology, toxicology, and water quality.

Table 1 .
Overview of cost-effective coating materials and techniques.

Table 2 .
Overview of hydrogel-based CRF studies.

Table 3 .
Root-induced change of pH between bulk soil and rhizosphere.