Alginate-based composites as novel soil conditioners for sustainable applications in agriculture: A critical review

Abstract

The development of alginate-based composites in agriculture to combat nutrient loss and drought for sustainable development has drawn increasing attention in the scientific community. Existing studies are however scattered, and the retention and slow-release mechanisms of alginate-based composites are not well understood. This paper systematically reviews the current literature on the preparation, characterization, and agricultural applications of various alginate-based composites. The synthesis methods of alginate-based composites are firstly summarized, followed by a review of available analytical techniques to characterize alginate-based composites for the attainment of their desired performance. Secondly, the performance and controlling factors for agricultural applications of alginate-based composites are discussed, including aquasorb, slow-release fertilizer, soil amendment, microbial inoculants, and controlled release of pesticides for pest management. Finally, suggestions and future perspectives are proposed to expand the applications of alginate-based composites for sustainable agriculture.

Graphical Abstract

1. Introduction

Agriculture is the foundation of the economy, while soil acts as the basis supporting crop growth by providing a survival environment and nutrients (Aksoy et al., 2023; Cetin, 2013; Cetin et al., 2022; Cicek et al., 2022). The survey by the Food and Agriculture Organization of the United Nations (FAO) pointed out that the main reason for the rapid growth of grain output was the widespread use of fertilizers (Roberts, 2009). However, improper applications of common chemical fertilizers (such as inappropriate timing and application rates) can lead to nutrient loss. They may also result in soil degradation and water pollution, and this contradicts current environmental protection concepts (Sim et al., 2021). Since the 1920s, some developed countries have successively introduced slow-release fertilizer products, which put forward new ideas to solve the issue of low fertilizer utilization (Fertahi et al., 2021; Sim et al., 2021). At present, different soil amendments have been widely used in agricultural soil, such as lime, gypsum, organic fertilizers, and synthetic polymers (Ramtahal et al., 2019; Zhu et al., 2020). Nevertheless, some of them may pose the risk of burning the roots and killing beneficial organisms in the soil due to rapid acidifying effects (Xu et al., 2021). Consequently, the exploitation of cost-effective and environmentally friendly agricultural soil amendments has drawn much attention to sustainable development.

Recently, natural polymers have gained popularity as soil amendments in agriculture, aimed at improving the utilization of fertilizers. One such polymer is sodium alginate, a carbohydrate derived from brown algae that is a natural polymeric polysaccharide (Hu et al., 2021). It consists of two monosaccharides, which are β-D-mannuronic acid (M) and α-L-guluronic acid (G). These two monomers are connected in a straight line to form a macromolecular structure of alginate (Hecht and Srebnik, 2017). Sodium alginate can react with multivalent cations to form a gel, and the gelling process is thermally irreversible (Cao et al., 2020). It exhibits good performance in terms of biodegradability, compatibility, and adhesion, which is ideal for controlling the release rate of nutrients. Sodium alginate may also be cross-linked with additional materials to prepare a series of alginate-based composites, which are applied in sewage treatment and soil remediation (Wang et al., 2018b, 2019a, 2019b). It was found that sodium alginate mixed with other materials not only had good retention and release capacity of nutrients, but also had strong water retention capacity (Wang et al., 2018c). For instance, Knijnenburg et al. (2021) used Ca2+ and Zn2+ cross-linking to prepare alginate/polyvinyl alcohol beads embedded with nano zinc oxide, and found that the incorporation of these beads improved the water absorption and retention of the soil.

At present, a variety of controlled release technologies have been developed to prepare alginate-based composites, including chemical synthesis, loading into materials with porous or layered structures, and coating (Duan et al., 2023). Among these methods, although controlled release can be achieved, the adsorption of water required for soil regulation is a major limitation. Alginate-based composites prepared by microcapsule technology not only combine the super adsorption behavior, controlled release of nutrients, and biodegradability, but also save costs. For example, Skrzypczak et al. (2019) used microcapsule technology to prepare alginate-carboxymethyl cellulose-eggshell biocomposites. The results showed that the maximum adsorption capacity of the biocomposites for Cu2+ was 281 mg g-1, and the release rate was slow in the simulated soil solution, which was 9 wt% after 14 d. Meanwhile, alginate-based composites can also be used as the fertilizer for crops due to the degradable wall material components (Fertahi et al., 2021). As a result, alginate-based composites have been used as a pesticide stabilizer, fertilizer-forming agent, and regulator in agricultural applications (Knijnenburg et al., 2021).

Although various alginate-based composites have been prepared and used, their types and physicochemical characteristics are different, and their application fields and effects are also diverse. By searching for the keywords of “alginate” and “agriculture” on Web of Science, it was found that the amount of research related to alginate-based composites had increased year by year (Fig. 1). These studies mainly focus on enhancing soil water-holding capacity (Cao et al., 2020), improving pesticides controlled release (Vallim et al., 2022), and increasing the utilization of soil nutrients (Ge et al., 2022). For example, Chiaregato et al. (2022) described the nutrient-release behavior of alginate in water/soil media as a slow-release fertilizer. Saberi Riseh et al. (2021) introduced that alginate-encapsulated biological control bacteria were used for pest management, and diverse alginate composites showed different properties. All the above studies show that alginate-based composites have potential advantages in some aspects of agricultural applications, but the current research reports are relatively scattered. Therefore, it is significant to systematically summarize the water and fertilizer conservation and biological control of alginate-based composites for agricultural applications.

Fig. 1.

The numbers of literature on the application of alginate in agriculture from 2015 to 2022 (Data come from Web of Science).

The objectives of this review are to (1) introduce the synthesis methods of different types of alginate-based composites; (2) summarize the characterization techniques of alginate-based composites to understand their physicochemical properties; (3) discuss the agricultural applications of various alginate-based composites, including aquasorb, slow-release fertilizer, soil amendment, microbial inoculants, and pest and pesticides management; (4) propose suggestions and future prospects for the advancement of alginate-based composites.

2. Preparation of alginate-based composites for agricultural application

Alginate, a natural carbohydrate, can be utilized to make biodegradable hydrogels. Unlike synthetic polymers, it has the benefit of improving soil quality as it degrades over time (Achmon et al., 2019). Due to the different mechanisms of various doping materials, the performance, structure, and strength of alginate-based composites are different. Thus, a suitable preparation process should be chosen according to the actual requirements to expand their applications in agriculture. To improve their strength and stability, a number of synthetic technologies have been innovated to modify alginate through distinct physical, chemical, and biological methods. Current preparation methods mainly include doping urea or nutrient elements, cross-linking carbonaceous materials, embedding microorganisms, and encapsulating pesticides (Fig. 2).

Fig. 2.

Preparation of alginate-based composites for agricultural applications.

2.1. Doping urea or nutrient elements

To reduce the loss of urea or nutrient elements in farmland soil, it is particularly important to find a technology that can stably release fertilizer. In recent years, polymer controlled-release agents have shown significant advantages in maintaining farmland soil fertility and reducing soil pollution (Chen et al., 2018). Sodium alginate has a strong adsorption capacity, which opens a new door for the advancement of slow-release fertilizers (Vu and Wu, 2022). Currently, there are many methods to prepare sodium alginate-urea/nutrient composites. It is more common to glue urea/nutrients with sodium alginate and other substances, and then drop them into a calcium chloride coagulation bath. For example, Wang et al. (2021b) fully mixed the carboxylated cellulose solution, sodium alginate solution, and urea, and dropped the prepared uniform solution into CaCl2 solution by syringe to form a spherical hydrogel. Sathisaran and Balasubramanian (2020) prepared chitosan-alginate beads by the same method and found that high concentration urea-doped chitosan-alginate beads had better fertilizer encapsulation performance. The varying urea concentrations could affect the performance of gels in the preparation process. Furthermore, artificial dripping often leads to the uneven size of gel microspheres. Nowadays, peristaltic pumps are mostly used to replace syringes to effectively control the infusion speed (Feng et al., 2022b). However, the drop distance, nozzle diameter, and solution viscosity still affect the size of the gel, especially when the sodium alginate solution is higher than 5% (w/v), and the preparation is usually difficult (Bennacef et al., 2021). Moreover, when sodium alginate and CaCl2 solution are mixed, the reaction between sodium alginate and Ca2+ can be completed too quickly to control the cross-linking rate (Uyen et al., 2020). Therefore, the gel strength is usually not uniform due to the uneven distribution of Ca2+. To solve this problem, researchers proposed an in-situ gel release method (Cuomo et al., 2019). Ca2+ was introduced into the solution as calcium disodium ethylenediaminetetraacetic acid (Ca-EDTA) or calcium carbonate (CaCO3). D-gluconate (GDL) was added as the weak acidic medium. At pH = 7.0, both Ca-EDTA and CaCO3 displayed high chelation constants, leading to the complete complexation of Ca2+. As GDL decomposed into gluconic acid at room temperature, the pH gradually decreased and Ca2+ was also released, allowing it to react with sodium alginate to form a gel. At pH = 4.0, almost all of Ca2+ was released, resulting in the formation of a uniform calcium alginate dispersed gel.

The application of alginate-based composites can efficaciously alleviate the rapid release of urea or nutrients and improve soil water-holding capacity. It can be used as an efficient soil conditioner in the agricultural field. However, alginate-based composites also release inorganic salt ions such as sodium and calcium that are contained in the raw algae (Fertah et al., 2017). If it is not used properly, the soil salt content will increase. Given this deficiency, the effect of sodium alginate on nutrients should be further studied. The film-forming mechanism and nutrient release mechanism of alginate-based composites can be improved to control nutrient release better.

2.2. Cross-linking carbonaceous materials

The mechanical characteristics of alginate-based composites are closely associated with their water content and cross-linking structure (Su and Chen, 2018). The stability and mechanical resistance of alginate-based composites can be enhanced by cross-linking alginate with carbonaceous materials (Wang et al., 2018a). Salesa et al. (2020) incorporated hydrophobic carbon nanofibers or hydrophilic graphene oxide into alginate films to enhance physical and antibacterial properties. It was found that the antibacterial properties, water diffusion, wettability, and mechanical performance of the composite films were enhanced. At present, doped carbonaceous materials usually include mesoporous carbon, carbon nanomaterials, and biochar (Gao et al., 2016; Kim et al., 2018b). Alshehari et al. (2021) added multi-walled carbon nanotubes (MWCNTs) to the mixture of polyethylene oxide/sodium alginate films via the casting way. The tensile strength increased from 24.1 MPa to 47.5 MPa, and the elastic modulus increased from 1.2 GPa to 3.9 GPa (MWCNTs content was 0.40 wt%). Llorens-Gámez et al. (2020) also observed a similar phenomenon. After adding 2% w/w carbon nanofibers, the compressive modulus and tensile strength of calcium alginate films in the hydrated and dry states increased by 6 times and 3 times, respectively.

Alginate-based composites are easily affected by the properties of matrix materials and cross-linking agents during the preparation process, leading to subsequent changes in their mechanical performance and sphericity (Bennacef et al., 2021). Cross-linking carbonaceous materials can generally enhance the elasticity and tensile strength of alginate-based composites, making them more resistant to deformation and reducing nutrient leaching (Llorens-Gámez et al., 2020). However, carbon nanotubes, graphene, activated carbon, and other carbonaceous materials have high costs. In the future, more economical and effective carbonaceous materials (such as biochar) should be selected to prepare alginate-based composites.

Biochar is a carbon-rich material with a wide source of feedstocks and simple preparation. It has been entrapped with sodium alginate by many researchers with good results in removing pollutants (Wang et al., 2018a, 2019a), improving water holding, and retention of nutrients (Wang et al., 2018c). For instance, Feng et al. (2022a) prepared the calcium alginate-rice straw biochar composites (CA-MRB) to study the retention and slow-release of nutrients. The findings indicated that the addition of 0.5 wt% CA-MRB300 led to a reduction of 48.05%, 28.76%, 36.63%, and 9.08% in the cumulative leaching loss of PO43−, NO3−, NH4+, and K+. This shows that biochar is indeed a better choice for many kinds of carbonaceous materials. In addition, biochar has a significant impact on soil quality and crop production, which can improve nutrient cycling and retain soil moisture (Osman et al., 2022). However, biochar is directly added, and the nutrient may be immediately released into the soil (Zhao et al., 2016), which is not conducive to crop utilization. To solve this problem, Wang et al. (2018c) ultrasonically treated sodium alginate and ball-milled biochar to prepare a uniformly dispersed suspension, then added the suspension droplets to the CaCl2 solution to obtain CA-BMB microspheres. Subsequent research along this line established an integrated dripping method by combining ultrasound, stirring, and a syringe pump (Wang et al. (2021a). Humic acid, activated carbon, and sodium alginate were used as feedstocks to prepare adjustable gel beads (CSGCHs) for controlling the release of humic acid.

2.3. Embedding microorganisms

Beneficial microorganisms can improve soil quality, facilitate nutrients uptake by plants, and prevent the invasion of plant pathogens (Covarrubias et al., 2012). However, indigenous microorganisms may outcompete those introduced by direct application (Wang et al., 2022b). Therefore, it is imperative to find an appropriate substrate for inoculation to prolong microbial life and provide the best agricultural conditions. It is reported that alginate can be used as an inoculation carrier for plant growth-promoting bacteria, which can increase the survival rate of microorganisms and reduce transportation and storage costs (John et al., 2011). Agricultural environment or storage conditions affect the viability and biological activity of microorganisms, so it is particularly important to choose the appropriate embedded technology. The mixture of alginate solution and liquid bacterial culture solution was sprayed into the CaCl2 solution through a small nozzle at low pressure, it could be hardened into beads with a diameter of 100–200 μm. These beads could be used either wet or dry (Bashan et al., 2002). Felizatti et al. (2021) proved that the spray drying method can optimize the encapsulation of conidia of Beauveria basiana, and its effect on the biological control of Spodoptera cosmioides was better than that of the ion gel method. This method exhibited better water dispersion and smaller storage capacity. However, although the spray drying method has high embedding efficiency and rapid process, the size of the prepared composites is susceptible to gas and liquid velocity, and the yield is low (Bennacef et al., 2021). Pour et al. (2019) used an emulsification method to embed microorganisms, which was better than the spray drying method and did not require any additional equipment. The sodium alginate solution, gelatin, carbon nanotubes, and CaCO3 nanoparticles were made into a uniform mixed solution. Soybean oil was added as a continuous phase to the mixed solution to prepare an emulsion for encapsulating Pseudomonas fluorescens (VUPF5) to prevent Fusarium solani. But the operation is complex and the cost is high (Bennacef et al., 2021).

Alginate-embedded microorganisms can not only improve the total number of rhizosphere soil bacteria and soil enzyme activities, but also alleviate adverse effects of crop water shortage or biological stress (Ma, 2019). However, the directional screening and cultivation of microorganisms are complex and time-consuming, and there are certain limitations in practical agricultural applications. In addition, some microorganisms may be lost during the preparation of alginate-embedded microorganisms (Martinez-Cano et al., 2022). Therefore, attention should be paid to the survival rate and physical protection of microorganisms during long-term storage, and further research should focus on improving their practicability and acceptability by optimizing the preparation process.

2.4. Encapsulating pesticides

Pesticides have been used on a large scale to control pests and weeds to improve crop yield (Wang et al., 2022a). However, pesticides are easily degraded or lost to the environment through photolysis, leaching, or hydrolysis, resulting in relatively low efficacy. Besides, pesticide leaching poses a serious threat to the ecosystem (Zheng et al., 2022a). Research has previously shown that encapsulated pesticides promote the continuous release of active ingredients, while effectively enhancing their stability, thereby improving crop yields and reducing harmful environmental impacts (Camara et al., 2019; Walker et al., 2018). Lee et al. (2017) prepared large-scale and targeted delivery biocompatible capsules using customized nozzle spray alginate solution for encapsulation. Spray drying combines two steps of encapsulation and drying to produce alginate-based composite microspheres (Sosnik and Seremeta, 2015). The main principles include (1) dissolving or emulsifying pesticides in alginate solution; (2) using a nozzle to atomize the mixed solution feed into droplets; (3) drying the droplets and forming dry microspheres in the drying gas; (4) separating and harvesting the dry microspheres from the drying gas (Zhang et al., 2017). To minimize the overuse of pesticides in agricultural systems, Li et al. (2020) used the emulsification method to prepare an oil-in-water (O/W) emulsion for pesticide transportation from sodium alginate polymer and alkyl glycosides. However, the spray drying method and emulsification method used in the above studies may reduce the pesticide encapsulation efficiency of the composite due to the shear force, which affects the release rate of pesticides. To overcome this drawback, Zheng et al. (2022a) prepared Ca-Alg-gP(NIPAm-co-NDEAm)/SC hydrogels loaded with herbicide glyphosate by redox polymerization. In addition, Vallim et al. (2022) prepared alginate/chitosan and organophosphorus pesticide dimethoate into nano-preparations by the ionic gel method. The results indicated that the composites provided an excellent controlled-release ability, thus providing safer alternatives for agricultural applications.

Alginate as a carrier for encapsulating pesticides is an ideal choice for effectively transporting pesticides to soil/crops. However, insecticides, herbicides, and fungicides may produce a variety of toxic effects (Singh et al., 2022). Therefore, the preparation of effective pesticide formulations with long-term activity and minimum environmental risk is essential for sustainable agriculture. Choosing appropriate preparation conditions is beneficial to encapsulate pesticides and improve the application value of alginate-based composites.

To sum up, the above methods have played a positive role in soil amendment, providing nutrients for the soil, and alleviating crop diseases and pests. However, encapsulating excessive nutrients/pesticides may cause potential environmental pollution, pose challenges in controlling the release rate, affect beneficial insects in the soil, and degrade soil quality. Therefore, future research should consider the dosage and durability of alginate-based composites to prevent nutrients or residues from disrupting soil balance. It should also ensure that the use of alginate-based composites has an acceptable impact on the soil ecosystem and take appropriate measures to mitigate any potential negative effects.

3. Characterization of sodium alginate-based composites

Alginate can be used as a matrix to encapsulate various materials to obtain gel beads with different characteristics, thus acting in various agricultural fields. The physicochemical characteristics of alginate-based composites mainly include specific surface area (SSA), functional groups, surface morphology, swelling coefficient, mechanical strength, and slow-release capacity. These physicochemical properties are often intimately related to the performance of the beads. For example, the ability of the beads to adsorb water/urea is enhanced with increasing the pores and SSA. The water holding capacity of the beads could be also improved due to the increase of the swelling rate (Olad et al., 2018). Characterization of alginate-based composites by different analytical techniques can provide insight into their physicochemical properties as well as nutrient retention and slow-release properties.

3.1. Conventional characterization of physicochemical properties

The SSA and pore size are important factors affecting the adsorption properties of alginate-based composites. Brunauer-Emmett-Teller (BET) is usually used for the analysis of the SSA of materials, and quantitative information of the SSA and porous structure can be obtained (Wang and Wang, 2019). Generally, the adsorption sites increase with increasing the SSA. The addition of alginate increases the SSA of the composites so that they can adsorb more water and improve soil water-holding capacity. Kim et al. (2022) confirmed this view. CaO2 spheres were synthesized by encapsulation of calcium peroxide with 1% sodium alginate. The SSA of the spheres was 70.34 m2 g-1, which was 13 times larger than the untreated calcium peroxide (5.47 m2 g-1) according to BET. Meanwhile, the concentration of sodium alginate influences the physicochemical properties of the spheres, and the spheres with uniform texture are easier to attach to crops. Wang et al. (2020) measured the particle distribution, zeta potential, and hydrodynamic particle size of aqueous polyurethane sodium alginate (PU/SA) microcapsule loaded with indomethacin. It was found that the diameter of the microcapsule varied from 57.2 ± 0.6 nm to 70.6 ± 1.4 nm and the electronegativity changed from −45.9 ± 1.3 mV to −49.5 ± 0.4 mV as the SA concentration rose from 0 wt% to 10 wt%. Table 1 summarizes the particle sizes of different alginate-based composites, whose size and shape are significant for their applications (e.g., embedding microorganisms and encapsulating pesticides). Their microstructure and pore size parameters are often correlated with the concentration of sodium alginate.

Table 1.

Conventional physicochemical properties of alginate-based composites.

Alginate-based composites	Characterization techniques	Sodium alginate concentrations	Size of bead	Functional groups	Swelling rates	References
CSGCH	TEM, SEM, BET, XRD, FTIR	1.5 g	2 nm	C=C, C–OH, C–O, –CH, –OH	–	Wang et al. (2021a)
Sodium alginate-CaO2	XRD, SEM, TEM, ICP-OES, BET	1%	0.85 mm	–	–	Kim et al. (2022)
Polyurethane-sodium alginate	FTIR, TEM, Zeta-sizer Nano ZSE, Laser scattering	5 wt%	60.1 ± 1.9 nm	–COO–, C–O–C, –OH, –NH, C–Cl, C=O	280% (pH = 11.0)	Wang et al. (2020)
Raoultella planticola Rs-2 microcapsule	FTIR, SEM	1%	2.26 ± 0.06 mm	–OH, C–O, C–O–C, azide group	175%–198%	Wu et al. (2014)
Alginate/graphene oxide composite bead	FTIR, XRD, SEM, TGA, ICP-OES, Texture analyzer	2 wt%	2.64 ± 0.17 mm	–COO–, –OH, –CH	720%	Li et al. (2018)
Sodium alginate-g-poly (acrylic acid-co-acrylamide)/ rice husk ash superabsorbent nanocomposite	FTIR, EDX, TEM, SEM, TGA	1%	<100 nm	Na–O, –COO–, –OH, –CN, Si–O, –NH	1070 g g−1	Gharekhani et al. (2017)
Alginate-cellulose nanofiber-poly (vinyl alcohol) hydrogels	TEM, SEM, FTIR, XRD,	2.5 wt%	1.9 ± 0.1 mm	C=O, –COO–, –OH, –CH, -OCOP	112.4 g g−1	Liu et al. (2021)
Semi-IPN superabsorbent nanocomposite	FTIR, XRD, XRF, TGA, EDX, TEM, SEM	1%	–	–COO–, C–O–C, –OH, –COH–, C=O	618.92 g g−1	Olad et al. (2018)
Iron loaded calcium alginate nanocarriers	FTIR, XRD, EDX, SEM	1 g	331 nm	–COO–, C–O–C, Fe–O, –OH	5.70 ± 0.28 g g−1	Patel et al. (2017)
Neem leaf powder-alginate based beads	FTIR, SEM, EDAX, TGA	1.4%	0.85 ± 0.07 mm	–COO–, –OH	78.33 ± 2.89%	Singh et al. (2010)
Cypermethrin loaded calcium alginate nanocarriers	FTIR, FESEM, TEM, XRD, DSC, EDX	–	115–119 nm	–COO–, –OH, C=O, –CH	–	Patel et al. (2018)
Polyacrylamide/ alginate/ montmorillonite nanocomposite hydrogel	FTIR, SEM, TG, XRD, TEM	2.5%	–	–OH, CONHCO–, –COO–, –CN, –NH	–	He et al. (2019)

Energy dispersive spectroscopy (EDS) is used to characterize the elements and chemical composition of alginate-based composites. Since each element has a specific atomic structure, it can generate a particular set of peaks in the electromagnetic emission spectrum (Amir Afshar and Ghaee, 2016). EDS characterization can also verify the success of alginate cross-linking with other materials. Achmon et al. (2019) observed the peaks of Ca and Cl on the EDS spectrum of alginate gels, indicating that alginate was successfully cross-linked with calcium salt. As shown in Fig. 3a, Wang et al. (2018c) performed EDS characterization of calcium alginate/ball-milled bamboo biochar composite beads (Ca-BMB). It was found that the carbon content in Ca-BMB increased from 58.77 At% to 63.25 At% compared to pure calcium alginate, indicating that biochar was successfully loaded, and all contained essential elements such as C, O, and Ca.

Fig. 3.

EDS (a) and FTIR spectra (b) of sodium alginate and sodium alginate-based composites, XRD patterns (c), Surface morphologies (d) (Li et al., 2018; Liu et al., 2017; Wang et al., 2018c, 2021a).

Fourier transform infrared spectroscopy (FTIR) can recognize the functional groups of alginate-based composites. Wu et al. (2014) confirmed that C–O–C, –COOH, C–O, and –OH were beneficial for agricultural production. As shown in Fig. 3b, Li et al. (2018) prepared calcium alginate immobilized graphene oxide composite beads. The FTIR spectrum showed that the surface functional groups of calcium alginate were mainly –COOH and C–O–C, which were the most conducive to ionic cross-linking. After adding graphene oxide, the peak value of –OH shifted from 3451 cm-1 to 3408 cm-1, indicating that hydrogen bonding may exist between calcium alginate and graphene oxide.

X-ray diffraction (XRD) is used to investigate the crystal structure of materials. By comparing the XRD pattern with the original sodium alginate hydrogels, the crystal changes of alginate-based composites can be observed, and then the mechanism of cross-linking can be speculated/proved. Moradi Pour et al. (2022) characterized sodium alginate-gelatin nanocomposites by XRD. In the spectrum of sodium alginate, the characteristic diffraction peaks of sodium alginate appeared at 21.5° and 13.4° in 2θ, indicating that sodium alginate had a certain degree of crystallinity. The XRD pattern of sodium alginate-gelatin nanocomposites showed that the characteristic peak shifted to 2θ of 31.6° and 43.5°. This may be due to the existence of CaCl2, which led to the formation of crystals during the encapsulating process. At the same time, it showed that electrostatic interaction occurred between the amino group of gelatin and the carboxyl group of sodium alginate. Liu et al. (2017) investigated the microstructure of sodium alginate composite film (1 wt% silica added to sodium alginate) by FTIR and XRD. The FTIR spectrum of the composite film showed characteristic peaks similar to those of pure sodium alginate and silica. But due to the electrostatic effect, the characteristic peaks corresponding to sodium alginate were shifted from 1027 cm-1, 1405 cm-1, and 1598 cm-1 to 1024 cm-1, 1403 cm-1, and 1594 cm-1. As shown in Fig. 3c, the XRD pattern of pure sodium alginate revealed two characteristic peaks at 13.4° and 16.1° in 2θ, indicating that sodium alginate had a slight crystallinity. The XRD pattern of pure silica displayed an amorphous silica diffraction peaks at 15.8° - 25.3°. However, the absence of characteristic peaks of silica was observed in the XRD pattern of the composite film, indicating that silica was evenly distributed in the sodium alginate matrix (Hecht and Srebnik, 2017). The combination of FTIR and XRD can reflect both the changes of crystal structure and functional groups of alginate-based composites either before or after cross-linking, and also determine whether the mixture is well dispersed in the sodium alginate matrix.

3.2. Surface topography

Analyzing the surface morphology of alginate-based composites is helpful to comprehend their microstructure. The SSA and pore size seriously affect the nutrient and water in and out of the composites (Wang et al., 2019c). Therefore, the pore distribution, sample shape, and morphology changes of the composites can be analyzed by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). Liu et al. (2021) observed that the average size of cellulose nanofiber/sodium alginate wet bead was 4.5 ± 0.5 mm by SEM, and the average diameter after freeze-drying (–50 °C, 5 d) was 1.9 ± 0.1 mm. The beads presented microporous shells and macroporous cores, and many cores had diameters of 8–100 μm. As shown by SEM or TEM, most alginate hydrogels have smooth surfaces, while the overall shape after drying deviates from the ideal sphere with some collapse of the spherical structure. This phenomenon may be attributed to the bending and sinking of wet beads due to water loss during drying (Achmon et al., 2019). Wu et al. (2014) observed the subspherical surface of the Raoultella planticola RS-2 microcapsule by SEM. It was confirmed that the depression and small cracks on the capsule surface after drying could facilitate the release of bacteria from the capsule during water uptake and swelling. To improve the thermal stability and moisture retention ability of composites, the cross structure with high porosity can be formed by free radical grafting of alginate. Gharekhani et al. (2017) copolymerized alginate, acrylic acid, acrylamide, and rice hull ash into a superabsorbent nanocomposite. The SEM image showed that the polymer matrix of hydrogel was mostly spherical less than 100 nm. The TEM image showed more clearly the translucent interconnection region of the hydrogel network structure, and the superabsorbent nanocomposite formed an interconnected three-dimensional porous structure. As shown in Fig. 3d, Wang et al. (2021a) observed that the surface of the CSGCH was rough and dense by SEM, which added to the surface area. In addition, numerous pores of different sizes were observed, and this helped humic acid diffuse into the polymer network, thereby delaying its release time.

Characterizing the surface microstructure of alginate-based composites is crucial to ensure the controlled release pattern from alginate-encapsulated materials. Spherical and small-sized alginate-based composites have more efficient and sustained release capability (Jin et al., 2016). While macroporous/network structure can increase the SSA, which helps to promote fertilizer mobility as well as enhance the ability to adsorb water within/between networks (Kenawy et al., 2019; Olad et al., 2018). Therefore, it is essential to precisely control the shape, size, and pore size distribution of alginate-based composites. Different preparation technologies can regulate the morphology of alginate-based composites, and the preparation of alginate-based composites by various means should become the future trend. In addition, the solution viscosity, cross-linking agent, stirring rate, and composites also have certain effects on the morphology of alginate-based composites.

3.3. Swelling properties

Swelling is the volume expansion of polymer in solvent. The swelling kinetic model is usually used to evaluate the swelling performance of alginate gels (Günter et al., 2020). The swelling rate of alginate gels is related to their water retention. Thus the higher the swelling rate, the more obvious the effect for providing water for plants as a reservoir (Su and Chen, 2018). To enhance the swelling capacity of alginate gels, alginate can be copolymerized with a polymer to form a super absorbent nanocomposite. For example, Olad et al. (2018) utilized montmorillonite and sodium alginate-g-polyacrylic acid/polyvinylpyrrolidone to synthesize a semi-interpenetrating polymer superabsorbent nanocomposite (semi-IPN). Compared with the pure gel, the equilibrium swelling capacity of semi-IPN improved from 521.17 g g-1 to 618.92 g g-1, which helped to the improvement of the network structure of the nanocomposite. In addition, the swelling capacity of alginate gels is also affected by the type and concentration of the salt solution, and the pH of the solution (Su and Chen, 2018). Elbarbary et al. (2017) discovered that polyacrylamide/sodium alginate gels exhibited considerably lower water absorption rates in cationic solutions containing Na+, Ca2+, and Fe3+ than in distilled water, decreasing by 16%, 39%, and 63%, respectively. And as the ionic strength increased, the water absorption decreased, resulting in the lessening of their swelling capacity. This could be attributed to the reduced osmotic pressure difference between the gel network and the external solution (Khan et al., 2021). When the solution pH was raised from 1.0 to 6.0, the gel swelling capacity gradually expanded to its full swelling capacity, and then decreased and showed a downward trend. This suggested that the swelling performance of the gels was pH dependent.

The pH sensitivity of alginate gels is mainly attributable to the protonation of carboxyl groups in the hydrogel structure in an acidic solution. The hydrogen bond between carboxylic groups is strengthened, while the hydrogen bond is weakened between the carboxyl group and water molecules. This causes the contraction of the gel structure and the reduction of gel swelling ability. When the solution is neutral or alkaline, the carboxylic group dissociates and the carboxyl group is present mainly in the form of anions, resulting in electrostatic repulsion. Thus, water molecules are easier to enter and enhance the swelling capacity of gels than in an acidic solution (Wang et al., 2021b; Xiang et al., 2017). The swelling ability of alginate gels is directly linked to the controlled release of contents, so the influencing factors (such as the properties of the compound, the solution pH, and the type of salt solution) should be fully considered in future research.

3.4. Mechanical and thermal properties

The mechanical strength of alginate-based composites directly affects their stability and action cycle in the soil. The swelling of composites in water greatly weakens their mechanical properties and leads to a decrease in stability (Xing et al., 2017). When the structure of composites is destroyed, the balance of elastic properties and osmotic pressure is inevitably disrupted. Therefore, the mechanical performance of composites strongly depends on their swelling degrees. Inhibiting swelling is conducive to retaining the mechanical properties and maintaining the original shape of alginate-based composites (Kamata et al., 2014). Their mechanical and thermal properties can be characterized by compression test and thermogravimetric analysis, respectively (Wan et al., 2022). Sánchez-Fernández et al. (2021) obtained the mechanical properties of hydroxyapatite sodium alginate composite hydrogels through compression test. Compared with pure sodium alginate, the ultimate compressive strength of the composite hydrogels was boosted by 354.54% and Young’s modulus by 154.36% upon the incorporation of hydroxyapatite. Li et al. (2018) investigated the thermal stability of Ca-SA and Ca-SA/GO using thermogravimetric analysis (TGA) and derivative thermogravimetric analysis (DTG). The findings revealed that the TGA curves of both were similar, with significantly higher stability above 150 °C. However, the DTG curves of Ca-SA/GO displayed higher peak temperatures (indicating maximum weight loss rate) compared to those of Ca-SA, signifying that the incorporation of GO enhanced the thermal stability of SA and delayed the pyrolysis of composite beads. Nevertheless, some studies have shown that even if graphene oxide is added, its thermal properties are still affected by the preparation process. For example, Xiao et al. (2020) prepared the alginate/reduced graphene oxide composite hydrogel. Its compressive strength increased from 0.54 Mpa to 1.62 Mpa after Fe3+ cross-linking, which was much larger than that of graphene oxide/alginate aerogel prepared by freeze-drying and chemical reduction (Chowdhury and Balasubramanian, 2014). The above research illustrates that the molecular structure of alginate-based composites can influence their swelling and mechanical properties. After cross-linking with other materials, alginate can circumvent its defects and improve the action performance.

3.5. Nutrient retention and release capacity

Controlling nutrient release is a key approach to reducing nutrient leaching losses after fertilizer application (Tian et al., 2022). The slow nutrient release of biodegradable hydrogels has been extensively studied. Currently, chitosan, pectin, carboxymethyl cellulose, and other polysaccharides have been used to prepare hydrogels that can control nutrient release in the soil to enhance nutrient utilization efficiency (Chiaregato et al., 2022). The potential of alginate-based composites for nutrient retention is continuously explored. Baki and Abedi-Koupai (2018) made graft copolymer SSRF and studied the slow-release behavior of nitrogen, phosphorus, and potassium fertilizers in the soil. The findings revealed that the release rate of nitrogen was greater than that of phosphorus and potassium, and was almost completely released after 30 d. While phosphorus and potassium exhibited release rates of 83 wt% and 72 wt%, respectively within the same time frame. Some research showed that biochar significantly improved nutrient retention capacity after cross-linking with alginate because of its small pore size and high adsorption capability of nutrients. Wang et al. (2018c) found that the potassium nitrate loaded on pure calcium alginate beads (48.1%) was less than that on calcium alginate/ball-milled biochar composites (64.1%). The release kinetics of composites were consistent with the pseudo-first-order model. And the released amount of K+ and NO3− by composites was about 10% higher than that of pure calcium alginate beads after 24 h. Arafa et al. (2022) investigated the release behavior of urea in Alg/HTACC gel beads using distilled water as the slow-release medium. The results showed that the beads (50% Alg: 50% HTACC) released less than the other weight ratios of beads after three days (45%) as well as thirty days (77%), except for the first day. The rapid release on the first day may be due to the swift diffusion and dissolution of urea into solution as it is bonded to the outer surface of Alg/HTACC gel beads through weak hydrogen bonding (Tan et al., 2018).

4. Applications in agriculture

Water scarcity, soil desertification, and fertilizer overuse are the principal factors contributing to the degradation of cultivated land (Prăvălie, 2021). The application of alginate-based composites is one of the important ways to restore soil quality (Sun et al., 2019). Alginate-based composites have found broad applications in soil amendment and regulation because they can not only enhance the utilization efficiency of soil water and reduce fertilizer loss, but also have the advantages of being biodegradable, environment-friendly, and non-toxic (Knijnenburg et al., 2021). The agricultural applications of alginate-based composites have been implemented mostly in the form of aquasorb, slow-release fertilizer, soil amendment, microbial inoculants, as well as pest and pesticide management (Fig. 4).

Fig. 4.

Applications of alginate-based composites in agriculture.

4.1. Aquasorb

Aquasorb has been extensively used in agriculture because of its high water absorption multiplicity, strong water retaining capacity, and non-irritating characteristics. It has become another major agrochemical after pesticides, fertilizers, and mulching films (Islam et al., 2011; Li et al., 2023). According to the source of ingredients, aquasorb can be classified into two categories: natural polymers and chemical synthesis (Liu et al., 2020). Aquasorb derived from natural polymers like cellulose (do Nascimento et al., 2022), chitosan (Lv et al., 2023), and starch (Fang et al., 2018) offer advantages in terms of degradability, biocompatibility, and reproducibility. Nevertheless, their extraction process can be complex, their water absorption rate is low, and the required application amount is large. Chemically synthesized aquasorb like polyacrylate (PAA) (Dispat et al., 2020) and polyacrylamide (PAM) (Rodrigues Sousa et al., 2023) show benefits such as low cost, long service life, and excellent water absorption. But their non-degradable characteristics could potentially harm the environment and hinder crop growth (Xiong et al., 2018). To improve the performance of aquasorb, researchers have investigated various specific network structures including interpenetrating polymer network (IPN) (Wiwatsamphan and Chirachanchai, 2022), semi-interpenetrating polymer network (semi-IPN) (Olad et al., 2018), and copolymer network (Li et al., 2017).

As aquasorb, alginate not only has the advantages of a natural polymer, but also contains rich hydrophilic carboxyl and hydroxyl groups. They can greatly enhance water retention capacity after cross-linking with other materials, so it is increasingly favored (Yang, 2022). For instance, Ca2+ cross-linking alginate nanoparticles have been prepared and their potential for sustainable water release in farmland has been evaluated. This study found that untreated soil experienced nearly complete water lost within 7 d, whereas soil mixed with alginate nanoparticles retained approximately 20% of water even after 11 d. Attributed to the enhanced water uptake of nanoparticles after cross-linking of sodium alginate with CaCl2. Alginate nanoparticles with low cross-linking may have wider pores, which could lead to the reduction of swelling rate and alleviation of rapid water release capacity (Sharma et al., 2014). Moreover, the mixture of polyacrylamide (PAM) and sodium alginate can effectively improve water absorption as well. The water absorption of the synthesized PAM/SA increased to 578 g g-1 compared to only 390 g g-1 for the original PAM (Elbarbary et al., 2017). In addition to superb water absorption capacity, alginate-based composites have high water retention, which is more meaningful in actual soil applications. One study prepared slow-release nitrogen fertilizer (SRNF) with κ-Carrageenan-sodium alginate (κC-SA) as an internal and external coating material and discussed the water evaporation behavior of SRNF in the soil. The findings revealed that when SRNF was applied at 3 wt%, the soil water retention capacity was 113.5%, and 62% of water was still retained after 10 d. Due to its application amount, the water absorption was enhanced, thus augmenting the water-holding capacity of the soil. Meanwhile, biochar has a large SSA and rich functional groups that enable it to enhance water retention capacity. For example, compared with pure alginate beads, alginate beads impregnated with ball-milled biochar had a slower water loss rate (Wang et al., 2018c).

Alginate-based composites can enhance the water-holding capacity of farmland, reduce the frequency of irrigation, and improve various physical properties of soil, such as permeability, aeration, and compaction reduction. To further improve their water retention performance, the influencing factors (cross-linking agents, initiators, and other additives) should be taken into account during their preparations.

4.2. Slow-release fertilizer

The high solubility and diffusivity of nitrogen (ranging from 40% to 70%) and phosphorus (ranging from 80% to 90%) in traditional fertilizers often result in crops being unable to absorb them (Kaur et al., 2023). In contrast, slow-release fertilizers can gradually and slowly release nutrients into the soil to meet the nutrient requirements of crops during growth. This can reduce fertilizer use, minimize pollution of soil and surface water, and avoid unnecessary labor during fertilizer application (Shin et al., 2023).

Alginate-based composites have the advantages of high fertilizer utilization, long-lasting fertilizer efficiency, and stable physicochemical properties. They can be used as an environmentally friendly strategy for slow-release fertilizers (Kim et al., 2018a). It was reported that the yield and nutrient content of sodium alginate/urea composites applied to soilless culture experiments were better than that of alginate-combined urea fertilizers during the early stage of plant growth. But they were lower during the late stage, demonstrating the continuous release effect of urea on alginate (Fan et al., 2019). Patel et al. (2017) prepared iron-loaded calcium alginate nanomaterials and used them in soil potting experiments. The findings revealed that the release of iron increased was positively correlated with both the amount of alginate and the iron content. In soil pot experiments, the application of calcium alginate nanomaterials in plants was significantly better than the direct application of trace elements in the soil. In addition, superabsorbent sodium alginate composites also had good urea slow-release performance (Kenawy et al., 2019).

Several studies have demonstrated that the incorporation of slow-release fertilizers with water retention characteristics can effectively enhance water utilization and nutrient retention in the soil (Rashidzadeh and Olad, 2014). Wu and Liu (2007) prepared a novel polymer using potassium silicate, sodium alginate, and acrylic–CO–acrylamide/kaolin. It was found that the polymer exhibited both excellent water retention capacity and gradual release of potassium silicate as a fertilizer. Rashidzadeh et al. (2014) utilized free radical polymerization of sodium alginate, clay, acrylic acid, and acrylamide to synthesize a novel slow-release fertilizer nanocomposite. The nanocomposite exhibited slow-release properties in different pH (ranging from 2.0 to 12.0) and salt solutions (including NaCl, KCl, CaCl2, and FeCl3).

The combination of sodium alginate or alginate-based composites with nutrient elements plays a crucial role in nutrient release. The release of nutrients is governed by its diffusion mechanism, while its release characteristics can be influenced by the effects of matrix swelling and erosion (Skrzypczak et al., 2022). The use of different alginate-based composites can regulate the nutrient release rate of slow-release fertilizer, so the selection of nutrient components and formula auxiliary materials is particularly important. Additionally, correct fertilization methods and recommended dosage should be followed when slow-release fertilizers are applied to avoid excessive nutrients (such as nitrogen and phosphorus) leaching into water/soil, which may cause water eutrophication and soil structure degradation.

4.3. Soil amendment

Soil amendments typically refer to materials that can improve soil structure and physicochemical properties. While organic fertilizer is a kind of fertilizer made from animal excreta, animal and plant residues, and other by-products as the main raw materials after fermentation and decomposition (Jiang et al., 2022). Generally, the main function of organic fertilizer is to increase the soil organic matter content, total nitrogen, available phosphorus, and the number of beneficial soil microorganisms (Hafez et al., 2021; Li et al., 2022). However, soil amendments are mostly composite formulations with various characteristics and functions. For instance, Kang et al. (2022) found that the stability of soil aggregates was significantly improved using biochar-based soil amendments, and the soil water retention was increased by 128.9%, promoting the growth of corn and beans. Thus the application of soil amendments is a beneficial method for enhancing soil structure, adjusting soil pH, and increasing soil fertility (Merwe et al., 2022). Nevertheless, traditional conditioners are low in efficiency and difficult to degrade in the soil. Therefore, achieving an efficient, green, and long-term soil conditioning effect has become the most critical issue of soil amendments at this stage.

Alginate-based composites exhibit remarkable pH-responsive properties and readily dissolve in alkaline conditions, resulting in faster reduction of pH and salt content in salinized soil (Song et al., 2020). For instance, Wang et al. (2021a) conducted soil amendment tests in the soil using CSGCH beads as soil amendments. It was shown that compared with blank treatment, CSGCH and pure charcoal activated carbon (CAC) treatments led to a decrease in NH4+-N content in alkaline soil after 50 d. The addition of charcoal promoted the conversion of NH4+-N to NO3--N in the soil, enhancing the biological availability of nitrogen (Nelissen et al., 2012). In salinized soil, CSGCH treatment significantly increased the contents of available phosphorus and potassium compared to pure CAC treatment. It could also increase nutrient and organic matter contents in the soil, while reducing pH and salt contents by 15.2% and 29.8%, respectively (Wang et al., 2021a).

Alginate-based composites improve the soil environment mainly by changing the physicochemical properties, including cation exchange capacity, soil pH, and electrical conductivity (Merwe et al., 2022). Alginate-based composites contain many adsorption groups, which can have a variety of physicochemical reactions with rhizosphere soil. When applied to the soil, they can change the pore size distribution and soil aggregate structure (Buchmann et al., 2020). Zhang et al. (2022) confirmed this view by analyzing the influence of hydrophilic calcium alginate (SA/Ca) and amphiphilic calcium alginate (ASA/Ca) on soil aggregation characteristics studied through aggregate stability index (ADI), rheology, and morphology. The results showed that the three-dimensional hydrogel network structure of SA/Ca and ASA/Ca hydrogel effectively promoted soil aggregation behavior. Additionally, the amphiphilicity of calcium alginate enhanced its affinity with soil, resulting in improved stability of soil aggregates. Some of the agricultural applications using sodium alginate as encapsulation material are listed in Table 2. Different materials can be embedded with sodium alginate (e.g. montmorillonite, humic acid, biochar, etc.) to suit various soil types. The improved soil quality provided by alginate-based composites is mainly due to the good physical structure and water absorption characteristic of alginate-based composites, which in turn enhance the water-holding capacity of the soil to varying degrees and reduce the leaching losses of soil nutrients. Note that the concentration of cross-linking agent and sodium alginate in alginate-based composites may influence the effect of soil amendment (Table 2). Therefore, seeking the best ratio of composites for different soil types can optimize their performance to expand their utilization.

Table 2.

Soil amendment effect of different alginate-based composites.

Alginate-based composites	Cross-linking dosage	Sodium alginate concentration	Soil types	Test days (d)	Effects	References
CSGCH	1.0 wt%	1.5 g	Salinized soil	50	The salt content decreased by 15.2% and 29.8%. The maximum water-holding capacity of soil increased by 31.19%. The overall porosity of the soil was improved.	Wang et al. (2021a)
SSRF bead	–	–	Sandy soil	30	The release rate of nitrogen was higher than that of phosphorus and potassium. Soil water retention increased by 80%.	Baki and Abedi-Koupai (2018)
Iron loaded calcium alginate nanocarriers	0.5 M	0.5–2.5 g	Soil	15	The release rate of trace elements increased. The iron-release rate slowed down. Soil water holding capacity was enhanced.	Patel et al. (2017)
Amphiphilic calcium alginate	–	500 mg L−1	Farmland soil	–	The formation of soil aggregates was promoted. The migration of acetamiprid was reduced.	Zhang et al. (2022)
Lignosulfonate/konjakufl-our/sodium alginate hydrogel	1 g	5 g	Loess	30	The conductivity of saturated water and the leaching of soil nutrients were reduced. The available water capacity of the soil was increased. The harvest of tobacco plants was increased.	Song et al. (2020)
Polyvinyl alcohol-sodium alginate microbead	4%	1%	Sandy soil	30	The rhizosphere soil was improved. The adverse effects of drought and salt stress on the growth and biomass of fig seedlings were reduced.	Zheng et al. (2022b)

4.4. Microbial inoculants

The application of microorganisms, including bacteria, algae, fungi, and protozoa, has been found to enhance nutrients and water absorption in crops, leading to improve crop productivity (Wang et al., 2022b). The improvement is attributed to the ability of microorganisms to dissolve minerals, fix nitrogen, or produce plant hormones, which regulate soil nutrient cycling and enhance soil quality (Ma, 2019). However, the direct introduction of microorganisms into the soil may cause a rapid decrease in the rhizosphere microbial population. It is difficult to maintain normal physiological activities and compete with indigenous microbial communities (Pongsilp and Nimnoi, 2020). Consequently, it is essential to increase the survival rate and applicability of an introduction of microorganisms in cultivated crops. To achieve this, various preparations or carriers are used to immobilize microbial strains to form microbial inoculants to ensure the viability of microorganisms. An earlier study showed that the survival rate of cells inoculated with P. fluorescens in sodium alginate beads was higher than those directly added to the same soil (Van Elsas et al., 1992). Another study also showed that using alginate alone to immobilize phosphorus-dissolving bacteria could maximize the survival of organisms at a higher storage temperature (40 °C) (Viveganandan and Jauhri, 2000). Szopa et al. (2022) evaluated the survival rate and colonization rate of Pseudomonas putida RS-198 embedded in alginate on cotton roots under salt stress. This study found that the survival rate of embedded bacteria was 89.67%, which was an improvement of nearly 9% compared to that of free bacteria. On the 49th day of the experiment, an increase in the number of encapsulated P. putida RS-198 population was observed. These results strongly prove the effectiveness of controlled release of alginate embedded microorganisms. The main mechanism is that alginate-based composites can form a physical barrier to protect the introduced microorganisms from environmental factors (Wang et al., 2022b). Meanwhile, they can serve as a source of nutrients to enhance the post-inoculation survival of microorganisms. Then the matrix is gradually released after degradation by soil microorganisms, thus enhancing long-term effectiveness (He et al., 2016). However, Bashan et al. (2016) believed that alginate structure had low mechanical resistance, which led to uncontrolled microbial release. The irregular surface of alginate-based composites also has a negative impact on microbial release (Wong et al., 2019).

Alginate, known for its thermal insensitivity, is widely used as a biopolymer due to its suitability for diverse microorganisms (Raus et al., 2021). The main function of embedding microorganisms in alginate-based composites is to stimulate plant growth and facilitate biological control. Schoebitz et al. (2013) immobilized phosphate dissolving bacteria (P. fluorescens and Serratia sp) in sodium alginate. Then they compared the mixture of potato starch and sodium alginate to study the effects of immobilized phosphorus dissolving bacteria on wheat growth and phosphorus uptake. Immobilized phosphate dissolving bacteria were observed to dissolve 89–93 μg mL-1. The absorption effect of phosphorus was significantly higher than that of autoclaved alginate starch beads. With immobilized P. fluorescens, the phosphorus uptake by wheat plants after 60 d of growth reached about 64%. It showed that inoculating immobilized rhizosphere bacteria was a potential method to inoculate carriers to increase phosphorus levels. Pour et al. (2019) studied the alginate-gelatin entrapment of P. fluorescens (VUPF5 and T17–4 strains) and applied them to the control of Fusarium solani. When the gelatin concentration was 1.5%, the maximum entrapment efficiency of VUPF5 and T17–4 was 91.23% and 87.23%. The polymer was activated by absorbing water and the release of bacteria and nanoparticles, which enhanced the growth performance and resistance of plants and weakened the pathogenicity.

Alginate-based composites as microbial inoculants not only provide some physical protection for microorganisms, but also deliver microorganisms near crop roots, making them superior to other inoculants (Szopa et al., 2022). In addition, alginate-based composites can be biodegraded by soil microorganisms, which contribute to the controlled release of microbial communities from the inoculant into the soil. This improves their effectiveness over time and promotes successful crop cultivation. However, most of the current studies are carried out in the laboratory. In the future, more research should be done in the field to evaluate their performance in complex environments.

4.5. Pest and pesticide management

To achieve an adequate global food supply, the use of pesticides is considered a necessary agriculture practice (Wang et al., 2022a). However, pesticides can be discharged into water, leading to serious pollution problems. Alginate-based composites as a stabilizer of pesticides have the dual benefit of controlling the release of pesticides and reducing their leaching into the soil, thereby mitigating the risk of water pollution. Fernandez-Perez et al. (1999) added the herbicide diuron to the alginate-bentonite composites. It was found that the use of alginate controlled-release preparations significantly reduced the leaching of diuron compared to conventional industrial preparations. The results of the 50 d soil leaching experiment showed that the total recovery of diuron was 72.44% of the total application amount under the treatment of alginate composites, which was higher than that of the soil column treated with industrial agents (61.07%).

The cross-linking degree of alginate-based composites can directly affect the release of pesticides. Kulkarni et al. (2000) studied the release of the natural liquid insecticide Azadirachta Indica A. Juss. (NSO) by cross-linking sodium alginate with glutaraldehyde. The findings revealed that the release rate of NSO increased with the augment of NSO loading. The increase of cross-linking degree of precipitated sodium alginate polymer resulted in a significant reduction of NSO release from beads. Another study reported the effect of crosslinker concentrations. Singh et al. (2010) used alginate to study the development of novel pesticide control preparations based on biopesticide neem leaf powder (NLP). To study the effect of bead composition on the release kinetics of thiram, NLP was mixed with thiram (fungicide) and beads were synthesized by changing the amount of NLP and cross-linking agent. The amount of NLP was taken as 0%–2% (w/v), and the concentration of cross-linking agent was 0.1–0.4 M during the synthesis of beads. It was found that as the NLP amount increased beyond the maximum limit of the formulation, thiram release also increased continuously. Conversely, an increase in cross-linking agent concentration in the formulation resulted in a decrease in thiram release. Table 3 lists the controlled release effects of alginate-based composite encapsulated pesticide applied to the soil. It shows that different materials have higher pesticide encapsulation rates, up to 98.33%, indicating that alginate-based composites can embed pesticides to the greatest extent and improve the effective utilization rate of pesticides. For example, Patel et al. (2018) loaded cypermethrin on calcium alginate nanoparticles through the lotion cross-linking method to make pesticides with a continuous release. The results showed that cypermethrin was efficiently encapsulated in calcium alginate nanoparticles without any chemical deformation (the encapsulation efficiency reached approximately 95%, and the loading rate of cypermethrin was roughly 78%). In soil pot experiments, it was noticed that the prepared nanoparticles had a superior effectiveness on plants compared to the direct application of pesticides. To enhance the efficiency of water-insoluble pesticides, montmorillonite was added to alginate-based composites. When the content of montmorillonite was 5.0%, the maximum pesticide loading rate of hydrogel was 13.32%, and the minimum pesticide release rate was 76.11% (He et al., 2019).

Table 3.

Pesticide controlled release effects of different alginate-based composites in different slow-release media.

Alginate-based composites	Cross-linking dosage	Sodium alginate concentration	Pesticides	Pesticide entrapment efficiency (%)	Pesticide release rate (%)	Slow-release medium	Release time	References
Alginate-bentonite	1.2%	1.4%	Diuron	98.33	72.44	Soil	50 d	Fernandez-Perez et al. (1999)
Cenosphere/ alginate composite hydrogel bead	0.25%	1%	Imidacloprid	80.2	32	Water	72 h	Singh et al. (2022)
Sodium alginate nanoparticle	15%	1%	Imidacloprid	98.66	–	Soil	15 d	Kumar et al. (2014)
Controlled release formulation	4.22%	1.4%	Imidacloprid	98.5	80.16	Water	14 d	Fernandez-Perez et al. (2011)
Polyurethane-sodium alginate	15 wt%	5 wt%	Indoxacarb	95.28 ± 0.46	–	Water	30 d	Wang et al. (2020)
Double-network nanocomposite hydrogel	2.5%	–	λ-Cyhalothrin	81.30	18.68	Water	87 h	Wang et al. (2019c)
Cypermethrin loaded calcium alginate nanocarrier	1.5 M	2.5 g	Cypermethrin	96.5	–	Water	15 d	Patel et al. (2018)
UiO-66-based alginate hydrogel	10 mg g−1	1%	Clothianidin	–	62.74	Soil	144 h	Feng et al. (2021)
Polymeric sodium alginate interpenetrating network bead	10%	4%	Chlorpyrifos	78.91 ± 0.11	58	30% methanol	144 h	Kulkarni et al. (2002)
Attapulgite-based hydrogel		1%	Chlorpyrifos	–	60	Water	50 h	Xiang et al. (2017)
Multi-responsive alginate-g-P(NIPAm-co-NDEAm)-based hydrogel	3%	1%	Glyphosate	–	39.3	Water	15 h	Zheng et al. (2022a)
Polyacrylamide/ alginate/ montmorillonite nanocomposite hydrogel	5.0%	2.5%	Acetamiprid	–	76.11	Water	120 h	He et al. (2019)
Interpenetrating polymer network hydrogel	10 wt%	–	Acetamiprid	–	52.12 ± 4.53	Water	54 h	Zhang et al. (2020)

Traditional pesticides are typically characterized by low utilization rates (Singh et al., 2020). The development of alginate-based composites for pesticide-controlled release technology has the potential to boost the utilization efficiency of water-insoluble pesticides. It can not only achieve the best insecticidal effect, but also avoid a variety of environmental problems caused by pesticides in the soil. As shown in Fig. 5, there are three main paths to regulating the release of pesticides in the soil. (1) The diffusion of trace pesticides may be facilitated by osmotic pressure, which arises from the disparity in pesticide concentrations between the inside and outside of the alginate-based composites. (2) Alginate-based composites swell when they encounter soil water, resulting in the loosening of the internal skeleton, and the diffusion and swelling effects further promote the diffusion of pesticides. (3) Over time, the composites are gradually biodegradable, and the pesticides embedded in the composites are gradually released. Alginate-based composites have great potential as pesticides carriers. However, current research is limited to exploring the slow-release kinetics of pesticides in water (Table 3). The biodegradability and controlled release of pesticides in the soil (such as herbicides and insect repellents) still need further exploration to demonstrate their practicality in the soil environment, especially in the actual soil environment.

Fig. 5.

Slow-release mechanisms of alginate-based composites in the soil.

5. Conclusions and future perspectives

This paper reviews the preparation, characterization, and applications of alginate-based composites in agriculture. Alginate-based composites provide synergistic effects and exhibit great potential in agricultural applications, such as soil amendments, microbial carriers, and slow-release fertilizers. To meet the requirements of practical applications, alginate-based composites with different functions are designed by selecting appropriate synthesis technologies. Generally, the application performance of alginate-based composites is affected by the type of doped materials, preparation conditions, and functionalization process. Therefore, before testing their application efficiency, it is necessary to comprehensively characterize the obtained composites. To further optimize the actual application of alginate-based composites in agriculture, we propose the following topics for future research.

1. Development and preparation of different alginate-based composites with specific functions. Alginate-based composites can improve soil fertility and promote crop growth, but the demand for nutrient elements of crops at different growth stages is different. Therefore, focusing on directionally preparing alginate-based composites according to the type and growth stage of crop will better ensure their long-term stability and slow-release properties.

2. Further exploration of migration and transformation mechanisms of alginate-based composites. At present, most studies mainly focus on the macro slow-release mechanisms of alginate-based composites. However, their micro mechanisms are still unclear. This is especially true for the migration and transformation of encapsulated materials in the soil environment. For instance, unanswered questions are how encapsulating materials interact with pests and how the encapsulation material transforms in the crops or harmful organisms. The optimal dose of embedding materials and the prevention mechanism still need to be investigated.

3. Evaluating potential environmental risks of alginate-based composites when applied to the soil. The stability and environmental risk of alginate-based composites have become the focus of current research. It is not clear if alginate-based composites cause secondary pollution to the environment due to the residue/leaching of the encapsulated materials. A question often asked is if alginate-based composites embedded with microorganisms antagonize other microorganisms in the environment after being applied to the soil. Reducing the potential toxicity of continuous overdose is a key issue to explore.

4. Cost-effectiveness evaluation of alginate-based composites. Currently, the dosage of optimal application and reaction time of alginate-based composites in the soil are still unclear. Because the release rate of encapsulated substances in alginate-based composites is affected by various factors (such as soil conditions), it is difficult to detect accurately. All these factors would make it difficult to quantitatively evaluate the environmental and economic benefits of alginate-based composites. Reduction of production costs can be achieved by selecting appropriate encapsulated materials, optimizing the synthesis conditions of composites, and refining doping technology.

5. Long-term observation research on alginate-based composites under field conditions. So far, the development of alginate-based composites as novel soil conditioners is still in its infancy. In many previous studies, the release kinetics and insect control effects of pesticides were performed under laboratory conditions which may not account for the influences of soil conditions. Therefore, establishing a composites-bioavailability-environmentally toxicology-related model and performing field experiments can inform the feasibility of alginate-based composites in agriculture. Furthermore, the field soil environment is complex and variable, and the experimental conditions cannot be controlled. It is necessary to explore research methodologies for alginate-based composites from a multidisciplinary perspective in the future.

Data availability

No data was used for the research described in the article.