Natural Polymers for Encapsulating Urea: Comparison
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Increases in food production to meet global food requirements lead to an increase in the demand for nitrogen (N) fertilizers, especially urea, for soil productivity, crop yield, and food security improvement. To achieve a high yield of food crops, the excessive use of urea has resulted in low urea-N use efficiency and environmental pollution. One promising alternative to increase urea-N use efficiency, improve soil N availability, and lessen the potential environmental effects of the excessive use of urea is to encapsulate urea granules with appropriate coating materials to synchronize the N release with crop assimilation. Chemical additives, such as sulfur-based coatings, mineral-based coatings, and several polymers with different action principles, have been explored and used for coating the urea granule. However, their high material cost, limited resources, and adverse effects on the soil ecosystem limit the widespread application of urea coated with these materials. 

  • coating materials
  • efficient uptake
  • nutrient retention

1. Introduction

In plants, nitrogen (N) deficiency leads to symptoms such as stunted growth, yellowish leaves (chlorosis), and a thin and spindly stem [1]. Nitrogen is mobile within the crops; its deficiency can be found at tip of the leaf and moves along the middle towards the base [1]. In N-deficient crops, the protein content is low and the sugar content is high because there is insufficient N to combine with all the C chains stemming from the sugars that are then used to make proteins. It is prudential to ensure high yields and a high nitrogen use efficiency (NUE) in crop production systems. Nitrogen use efficiency is reflected by the ratio of the N assimilated in plant tissues to the total input of N fertilizer applied in the soil. In agronomic research, various indices are used to assess the efficiency of applied N [2]. In field studies, the indices are calculated based on differences in the crop yield and total N uptake with the aboveground biomass between fertilized and unfertilized plots. According to Dobermann [3], the agronomic framework is useful for understanding the factors governing N uptake and the NUE, to compare the short-term NUE in different environments, and to evaluate different N management strategies or technologies. The different method is simple, cost efficient, and suitable for NUE determination in field research [3]. When the values of NUE in crop yields were decreased at higher N rates, the findings indicated that the plants were unable to absorb N; this could be due to the saturation of plant absorption mechanisms. The determination of the NUE in crop plants is an important approach to evaluating the fate of the applied N fertilizers and their role in improving crop yields.
Changing the rate of urea dissolution in the soil to synchronize the availability of inorganic N with crop requirements could be achieved by coating the urea to ascertain an improvement in the NUE. Moreover, owing to the fast release of N from the urea and the N becoming readily available in the soil, not all the N is used by plants due to factors such as the plants not efficiently absorbing 40 to 70% of the N applied from urea, which results in a low N uptake and NUE [4]. Consequently, the repeated or frequent application of urea to meet crop requirements not only causes an increased cost of crop production but also pollutes the environment and is associated with an imbalance in the amount of N that is then subjected to loss through nitrate (NO3−) leaching, ammonia (NH3) volatilization, and denitrification [5,6][5][6]. The slow-release technique from encapsulated urea is one of the appropriate methods for overcoming the rapid loss of N from urea and to provide available N for plant uptake and soil N nourishment. Bortoletto-Santos [7] suggested that the solubility of urea in water is reduced by coating it with suitable materials that slow the release of N in the soil. The mechanism involves a coated film physically creating a barrier during dissolution, which determines the dissolution rate in water when the coated urea is applied to the soil [8]. According to Thind et al. [9], the encapsulated urea prills lower the discharge rate of N in the field, which can result in lessening the leaching of harmful NO3− into the groundwater and hindering the volatilization of NH3 into the atmosphere. In another related study on encapsulated urea, Mulder et al. [10] revealed that the physical characteristics of the material used to coat the urea governed the N release process in the soil. The mechanisms of N release from urea are very important, and the way that the release of N is regulated and how it acts with a coating when it is applied in the field should be well-understood [10].

2. Natural Polymers for Encapsulating Urea

The materials used for coating urea can be divided into organic and inorganic coatings, as summarized in Figure 1, with the aim of controlling the fast release of ordinary or uncoated urea. The rapid release of N from urea is not necessarily good because a too-high concentration of N released into the soil might cause fertilizer burn to the crop, the contamination of ground water due to NO3− leaching, and N loss, as indicated in Figure 1 [17][11]. The most widely applicable and known coated fertilizers are sulfur-based and mineral-based coated urea. However, due to the high cost, limited resources, and soil ecosystem pollution from both sulfur-based and mineral-based coating materials, the use of organic polymers is preferable due to their abundant quantities, inexpensive cost, and biodegradable, renewable, and environmentally friendly characteristics (Figure 1). According to Fertilizer Europe [18][12], the European Union regulations set the rules for controlled-release fertilizer such that midway through 2021, the use of microplastics as coating materials for urea was restricted, and from 2026 onwards, only materials that meet the biodegradability requirements will be approved for use as coating materials [18][12]. The restriction aims to conserve the soil ecosystem and bodies of water from the pollutants associated with non-biodegradable microplastics.
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Figure 1.
Common materials used for coating urea granules to prevent rapid nitrogen loss from urea.
Starch, a naturally based coating material, is the most-researched natural polymer for coating urea due to its high availability, low cost, and environmentally friendly nature (Figure 1). Rychter et al. [19][13] prepared a starch-based, controlled-release fertilizer with urea which serves as a plasticizer to reduce moisture content, affecting the mechanical properties and crystallinity of the matrix. The use of starch in urea coating also serves as modifier, binder, or sealant for chitosan, providing the function of reducing the number of pores in the surface of the urea granule [20][14]. Chitosan–starch coatings increase the water absorption ability [20][14]. Starch blends with polyurethane and polysulfone have been used with the modification of the ratio of synthetic polymers to natural biopolymers to achieve the desired biodegradability of the coated urea in the soil [21][15]. In other studies, cassava starch has been proven to be a low-cost coating membrane for improving slow-release fertilizer while having a low negative impact on the environment. However, when corn starch was thermally processed into a coating material using disodium tetraborate and urea and coated over granular urea in a vertical bed coating reactor, the resulting coated urea was reported to have properties such as a uniform, dense, hard, and least-porous surface [22][16].
Compared with the uncoated urea granules, which released N into water in 6 min under mild shaking, the corn-starch-coated urea took approximately 32 min to completely release N [22][16].
In this revisewarch, focus is narrowed to the use of organic materials for urea coating, as summarized in Table 1. The organic materials for urea coating can be used directly, after some modification or mixing with minerals for binding, and for sealant purposes (Table 1). Organic coating materials with a high biodegradability and renewability due to their derivation from natural products encourage researchers and industrialists to search for innovative and sustainable organically based materials that can reduce the cost of coated fertilizer production while providing a high efficiency and superior properties of controlled N release from urea fertilizer. Organic materials such as biochar, resin, starch, and polyphenol (Table 1) are widely used to coat urea granules for the slow release of N, to reduce environmental pollution, and to decrease the harmful effects associated with sulfur-based and mineral-based coatings. Synthetic polymers such as polyethylene, polystyrene, and polyesters are thermoplastic materials used as urea coating materials due to their apparent benefit over natural polymers, such as a set-to-set consistency, foreseeable physico-chemical properties, and tailor-made character (Table 1). Polymer-coated urea (PCU) has great potential for increasing crop production via enhancing N fertilizer use efficiency and benefiting the ecosystem by preventing pollution from the excessive use of urea. However, most PCUs are used only in a limited market due to their high cost compared with conventional N fertilizers. To address the problem of high-cost PCUs, Yang et al. [23][17] studied a low-cost PCU and a large tablet polymer-coated urea (LTPCU) which were prepared using recycled polystyrene foam and various sealants as coating materials. The recycled polystyrene foam was the ideal coating material for the controlled-release fertilizer. The polyurethane, which was synthesized through the reaction of castor oil and isocyanate, was better than the wax as an additive for delaying the N release rate of the coated urea. The 70–80% less coating material was used for the LTPCU than for commercial PCUs with a similar N release longevity. Moreover, the cost involved in recycling the polystyrene foam to coat one ton of pure N with the LTPCU was approximately one-seventh to one-eighth of the cost of the traditional polymer used for the commercial PCU [23][17]. For resin-based materials for urea coating, acrylic resin and epoxy resin were used as a protective layer to improve the hydrophobicity of the coated urea for controlled release [24][18] (Table 1).
Table 1.
Organic materials commonly used to encapsulate urea granules.
1. Thermoplastic-Based
Material Modifier/Binder/

Sealant		 Research Findings References
Poly (butylene succinate) (PBS) Empty fruit bunch Irradiation of PBS prior to mixing improved the adhesion and increased the biodegradation rate compared with the non-irradiated SRF composites. This is because EFB fibres are hydrophilic in nature, while the matrix polymer is hydrophobic. The mixture of these two materials causes weak adhesion and poor fibres. [25][19]
Poly (lactic) acid - Utilizing the higher molecular weight poly (lactic acid) resulted in a slower urea release due to the decreasing permeability of the PLA. This slow-release fertilizer is promising because it does not leave residues that damage the soil structure or the nutrient balance in the soil. [26][20]
Polyester - The increasing size of the controlled-release fertilizer while using smaller urea crystals slows down the degradability and release rate. [27][21]
Polyurethane - The application of at least 50% total N as coated urea strongly reduced N leaching and improved N agronomic efficiency in comparison with traditional fertilizers, ensuring a similar fruit production in the same time. [28][22]
Mesoporous silica Filler morphology affects the release rate. [29][23]
Hydroxypropyl-terminated polydimethylsiloxane (HP-PDMS) Implementation of hydrophobic gradient layer increases urea diffusion resistance. [30][24]
Polystyrene Wax, polyurethane Wax is brittle and cannot prevent water penetrating the coating. Increasing the size slows down the release and reduces the amount of coating material required. [23][17]
Polyether sulfone Fe2O3 nanoparticles (NPs) A new class of controlled-release fertilizer. Fe2O3 NPs increase the coating thickness and reduce the release rate. They also allow the carrier to be recovered and recycled. [31][25]
2. Resin-Based
Acrylic resin N-(n-butyl) thiophosphric triamide(NBPT) and dicyandiamde(DCD) The film is integrated on the urea core surface, the coated material is uniformly distributed with the coating and closely combined with the urea core, and the surface is smooth and able to control the urea release. [32][26]
Bio-based epoxy - Urea coating with epoxy is environmentally friendly and has a superior controlled release property. [33][27]
3. Natural-Based
Cellulose Silica NP, bentonite, montmorillonite (MMT) Incorporation of filler into cellulose-based coating material promotes tortuous path and compactness which slows down diffusion. [34][28]
Ethyl cellulose (EC) as inner coating and cellulose-based superabsorbent polymer (cellulose-SAP) adsorbing biochemical inhibitors dicyandiamide (DCD) and thiourea as outer coating. It has the functions of water retention and slow release, but it also inhibits the conversion of NH4+-N to NO3−N. [35][29]
Lignin Alkenyl succinic anhydride Film-forming properties show great potential to retard nutrient release. [36][30]
- Lignin indicated as an eco-friendly material with good controlled-release capacities that potentially could be applied in agriculture and horticulture. [37][31]
Phenol-formaldehyde resin Phosphorus is partly retained inside the slightly soluble calcium sulphate matrix. [38][32]
Acetic acid/sodium metabisulfite Lignin increases the efficiency of the delivery of the bioactive material for a longer period and prevents the pollution of surface and underground water with NH4+. [39][33]
Chitosan Magnesium, rice-husk-ash, liquid natural rubber, Epsom salt Increases the yield of rice. [40][34]
- The chitosan’s resistance slows down the release rate of N, P, and K. [41][35]
Poly (acrylic acid-co-acrylamide) (P(AA-co-AM) Properties of chitosan-coated NPK compound fertilizer with controlled-release and water-retention features releases the nutrient slowly. [42][36]
Starch The presence of chitosan on the coating system is responsible for producing a porous matrix surface, while the availability of starch tends to reduce the number of pores in the surface. The swelling analysis shows that the presence of chitosan–starch coatings increases the water absorption ability. [20][14]
Biochar Bentonite Water retention and controlled-release properties improve the N utilisation efficiency and reduce the environmental impact. [43][37]
- Biochar with a high specific surface area, hydrophilic oxygen-containing functional groups, and a low pH slow down the release. [44][38]
- Porous nature and surface functional groups of biochar minimize NO3− leaching and improve the NUE. [45][39]
The use of a natural, biodegradable polymer such as starch for urea coating constantly increases because when starch-coated urea is applied to the soil, the degradation begins with the presence of bacteria, fungi, and algae enzymes which catalyze the process of chemical hydrolysis [46][40]. In addition, starch is naturally synthesized by plants; therefore, starch is completely biodegradable, able to polymerize after some processing, relatively inexpensive, and sustainably available [47,48][41][42]. The wide range of starches provides options for starch selection for coating urea granules, depending on the composition of the starch, to obtain the desired properties of the polymeric film. As summarized in Table 1, plasticization is used to obtain a more elastic film, as some studies stated that the urea solubility decreased when it was added into the matrix of a starch-based polymer [49][43] or when a combined urea coating with starch and polyacrylic acid was used [50][44].

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