1. Introduction

As a cornerstone of sustaining the ever-growing global population and driving the thriving economy, agriculture assumes a vital role. In this pursuit, the indispensable use of fertilizers has emerged as an essential practice for augmenting crop yields and preserving soil fertility. Conventional fertilizers, such as urea, nitrogen, phosphorous, potassium, monoammonium phosphate, and diammonium phosphate, are widely utilized to supplement essential nutrients in the soil. However, conventional fertilizers suffer from low nutrient utilization efficiency due to leaching, leading to substantial economic losses and decreased soil fertility. The leaching of these nutrients from the soil has resulted in a significant decrease in soil fertility. This is primarily due to the relatively low nutrient utilization efficiency of conventional fertilizers, which is around 30–35% for nitrogen, 18–20% for phosphorus, and 35–40% for potassium [1] (Figure 1). The scientific community is already working on developing slow-release chemical fertilizers; for example, combining hydroxyapatite with urea has allowed researchers to develop slow-release fertilizers that gradually release plant nutrients [2]. Moreover, the environmental impact caused by releasing excess nutrients has necessitated the development of more efficient and eco-friendly fertilizers.

Nanofertilizers have emerged as a promising solution to address such challenges, offering higher efficiency and reduced environmental impacts. They can be classified based on their action, nutrient composition, and consistency. These categories include controlled-release nanofertilizers, nanofertilizers for targeted delivery, plant growth-stimulating nanofertilizers, water and nutrient loss-controlling fertilizers, inorganic and organic nanofertilizers, hybrid nanofertilizers, nutrient-loaded nanofertilizers, and various consistency-based nanofertilizers such as surface-coated, synthetic polymer-coated, biological product-coated, and nanocarrier-based nanofertilizers.

Controlled-release fertilizers (CRFs) are promising nanofertilizers with granular structures that deliver nutrients to plants over an extended period, ranging from weeks to months [3]. In addition, controlled-release fertilizers can improve the environmental sustainability of agriculture by reducing the release of nutrients into the environment (Figure 2). Nanomaterials, such as carbon nanotubes, graphene, and quantum dots, have unique properties that make them ideal for controlled-release applications [4]. Their small size, large surface area–to–volume ratio, and ability to be coated with various materials to control the release rate enhance the efficiency of nutrient delivery. These materials can also improve the granular mechanical strength of fertilizers and provide leaching resistance [5].

This review article aims to comprehensively describe the various types of nanofertilizers available and their potential applications in modern agriculture. We will discuss their effectiveness, advantages, and disadvantages, along with the materials and strategies for controlled and targeted delivery of nanoparticles (NPs). Furthermore, we will delve into the qualities of an effective nanofertilizer, potential risks related to its application, and the future outlook for this emerging technology. By evaluating and comparing different nanofertilizers, we hope to offer valuable insights for researchers, farmers, and other stakeholders in the agriculture sector.

2. Nanofertilizer Types

Nanofertilizers contain nanosized particles, which plants can absorb and improve crop yields. They are the product of a new technology with potential applications in agriculture, but their classification has some inconsistencies. However, some definitions also include other products, such as nanoscale delivery systems and nanobiosensors. The scientific community has been perplexed by the contradictory definitions of nanofertilizers. For instance, nanofertilizers are classified as a subset of nanotechnology and also as a type of fertilizer. This uncertainty has led to a lack of clarity on the definition and categorization of nanofertilizers, which may lead to misunderstanding when debating their use and possible advantages.

Nanofertilizers can also be classified based on the material used. For example, some nanofertilizers are made with carbon nanotubes, while others are made with polymers or metals. Each type of nanofertilizer has different properties and can have different effects on plants. Here in this review, they are broadly classified based on the nutrients they carry, the actions they perform, and consistency (Figure 3). Understanding the nature of nanofertilizer is essential to find the best application method. Nanofertilizers can be applied to plants through foliar, water, and soil application [6].

2.1. Action-Based

Nanofertilizers can be categorized into five types: action-based, controlled-release, targeted delivery, plant growth-stimulating, and water and nutrient loss controlling. These innovative fertilizers offer a range of benefits, such as improved nutrient utilization, controlled nutrient release, targeted nutrient delivery, enhanced plant growth, and reduced nutrient loss, making them valuable for sustainable agriculture, as described below.

2.1.1. Controlled-Release Nanofertilizers

Controlled-release nanofertilizers have emerged as a promising solution for addressing the challenges associated with conventional fertilizers, such as nutrient leaching and inefficient nutrient use [7]. These fertilizers utilize nanoparticles to control the release of nutrients, thereby improving nutrient uptake and reducing environmental impacts [8]. Controlled-release nanofertilizers encapsulate nutrients within nanoscale carrier materials composed of polymers, lipids, or inorganic substances [7]. The release of nutrients from such carriers can be influenced by environmental factors like temperature, pH, and moisture or by stimuli-responsive mechanisms, such as biodegradation or enzyme-mediated degradation [9].

Controlled-release nanofertilizers offer several advantages that contribute to sustainable agriculture. One of these benefits is improved nutrient use efficiency, as the controlled release of nutrients allows for targeted and sustained delivery, leading to better nutrient uptake by plants and reduced fertilizer application rates [7]. Furthermore, these fertilizers help to reduce the environmental impact of agricultural practices. By minimizing nutrient leaching, controlled-release nanofertilizers decrease the contamination of water bodies and reduce eutrophication, ultimately protecting aquatic ecosystems [8,10]. Additionally, controlled-release nanofertilizers are known to enhance crop productivity. Studies indicate that their application can lead to improved crop yield, nutrient use efficiency, and overall plant health [11]. The potential applications of these fertilizers extend to various crops, including rice, wheat, corn, and soybeans, with promising results in improved crop yield and nutrient use efficiency [7]. Continued research and development efforts are crucial to fully harness the potential of these innovative fertilizers in addressing the increasing global demand for food production.

Carbon-Based

Carbon is essential for promoting life, as it is found in all organic compounds and involved in biochemical pathways. Furthermore, nanostructured carbon can play vital roles in plants [12], such as biochar, a form of leftover biomass from plants in fields. The biochar contains numerous carbon nanostructures that undergo oxidation upon exposure to air and create pores on the surface. These pores are beneficial as they can absorb micronutrients from the soil to be released in the future, as well as a large amount of water to be used during times of drought, thus helping the soil to remain wet and nourished for seed germination and plant growth [13,14].

The advent of carbon-based nanofertilizers presents a revolutionary solution that holds immense promise, encompassing a multitude of potential advantages, including improved nutrient delivery, uptake, and overall plant growth [15]. Carbon nanotubes (CNTs) are one of the most studied carbon-based nanofertilizers, known for their high aspect ratio, mechanical strength, and unique chemical properties [16]. CNTs’ application to plants improves nutrient delivery and uptake, increasing crop productivity [17]. Graphene is another type of carbon-based nanofertilizer, which has shown promise in increasing nutrient use efficiency and promoting plant growth [18].

Chitosan-Based

Chitosan, a biopolymer, is widely recognized as a leading choice among agriculture, food, and health experts. It is a deacetylated derivative of chitin derived from arthropod shells. Due to its structure, chitosan, a cationic polymer, can interact with negatively charged materials. Chitosan forms complexes with fertilizer molecules, increasing their availability to the plants. Furthermore, it has the added benefit of being adjustable in size. Chitosan NPs can serve as a vehicle for the controlled release of NPK fertilizers in agricultural applications. The process is relatively easy, as the fertilizers can be dissolved in the nanoparticle solution and stirred magnetically [19]. It is easy to modify chitosan molecules to absorb and release plant growth regulators, herbicides, insecticides, and fertilizers [20]. As a carrier and control release matrix, chitosan protects biomolecules from pH fluctuations, light, and temperature extremes and prolongs the release of active substances, safeguarding plant cells from burst release. Chitosan-based nanofertilizers improve the absorption of nutrients by plants [20].

Furthermore, chitosan’s ability to form complexes with metal ions helps to reduce its toxicity, making them safer to use. Slow-release nanofertilizers, such as chitosan NPs (CN) and potassium-incorporated chitosan NPs (CNK), are of interest for reducing soil nutrient losses and preventing land degradation associated with established fertilizer use. These NPs were characterized using Fourier-transform infrared spectroscopy (FTIR), transmission electron microscopy, a field-emission scanning electron microscope, and atomic force microscopy (AFM) techniques [21]. Membrane diffusion studies revealed the slow potassium release property of CNK. Pot trials with Zea mays plants demonstrated that soils amended with reduced potassium rates (75% CNK) significantly increased the fresh and dry biomass accumulation by 51 and 47%, respectively, concerning positive control (100% KCl) [21]. Using CNK also improves the physical properties of soil, such as porosity, water conductivity, and friability, favoring root growth and thus allowing plants to uptake higher quantities of nutrients [21]. No harmful effects of the nanoformulation were observed in the study, and the treatments showed better carbon cycling and higher soil microbial activity. It is hypothesized that CNK conditions the soil by cohesively bonding soil particles, stabilizing the soil aggregates by providing a coating, and reducing the disruptive forces that cause degradation [21]. The sustained nutrient release of CNK synchronizes with crop demands, reducing fertilizer requirements and increasing productivity.

Clay-Based

NPs composed of clay possess large surface areas and nanolayer reactivity and can be utilized to fabricate CRF formulations. Nanoclay is a crucial component in CRF synthesis because of the active surface it provides for several physicochemical and biological processes [22,23]. Organoclays are clay minerals with particle sizes less than 2 mm and demonstrate high specific surface area, hydrogel capacity, charge, and crystal-like structure. In addition to their colloidal particle size, all these qualities are advantageous for CRF production [23,24,25,26].

Layer Double Hydroxides

Layer double hydroxides (LDH) are two-dimensional layered compounds composed of intercalated anionic materials within an interlayer spacing that enable the regulated release of anions, regardless of their type, to balance the positive charge of the LDH. Furthermore, the composition and synthetic methods used to produce LDH can result in various properties, such as anion exchange and thermal stability [27]. A study proposed a route to creating a hydrotalcite-like layered double hydroxide structure ([Mg-Al]-LDH) for phosphate fertilization, which resulted in a higher phosphorus content than previously reported [28]. Another study used an Mg/Al layered double hydroxide to intercalate three anionic herbicides (2, 4-D, MCPA, and picloram). The resulting complexes were tested for controlled release in water and soil columns, and the results showed that the LDHs could be used in slow-release formulations of acid herbicides [29].

Nanocapsule-Based

Nanocapsules are microscopic capsules typically composed of organic or inorganic material used to encapsulate and deliver fertilizers [30]. They are made from biopolymers, lipids, silica, metal oxides, and carbon nanotubes [31]. The nanocapsules form a protective barrier and release the nutrients slowly over time, allowing for more efficient fertilizer use, greater control over its application, retaining nutrient stability, and providing a controlled release to the crops, thus ensuring that the nutrients are delivered correctly and at the correct times. Additionally, the slow release of nutrients reduces the risk of leaching.

Several studies have demonstrated the potential benefits of nanocapsule-based nanofertilizers in enhancing nutrient uptake and improving crop yields. For instance, it was reported that applying encapsulated zinc oxide NPs to maize plants significantly enhanced photosynthetic rate, growth, and yield under cobalt stress [32]. Furthermore, encapsulated urea nanofertilizers provide more efficient nitrogen release and improve plant growth than conventional urea [31].

Nanogel-Based

Nanogels are spongy materials composed of a polymer mixed with a liquid, which can absorb and gradually release fertilizers or other substances over time. The nanogels are impregnated with nitrogen, phosphorus, and potassium, indispensable elements for facilitating plant growth [33,34]. Nanogels have applications in fertilizers, drug delivery, and water purification. The nanogel particles can absorb and retain water in the soil, which can help to improve water retention and reduce soil erosion. They also have a high surface area–to–volume ratio, which can contain more nutrients than traditional fertilizers. Furthermore, they are biodegradable and non-toxic and thus are environmentally safe [35]. In a study comparing the effects of different fertilizers on Abelmoschus esculentus, the application of a composite fertilizer containing calcium phosphate nanogel and dipotassium hydrogen phosphate resulted in an increased germination rate of A. esculentus from 87% to 95%, as well as improved activity of the enzymes amylase, protease, and nitrate reductase, and increased weight per fruit from 59 g to 65 g [36].

Polyurethane-Based

Polyurethane-based nanofertilizers represent a cutting-edge category of fertilizers that harness the power of polyurethane NPs, which are synthesized from a polymer composed of organic units interconnected by urethane links. The polyurethane-based nanofertilizers are typically produced through ring-opening metathesis polymerization (ROMP), which involves creating a monomer from two different molecules containing a double bond, which is then polymerized. The resulting polymer creates NPs that deliver nutrients to the soil [37]. These NPs can be used to deliver essential nutrients such as nitrogen, phosphorus, and potassium to improve the efficiency of traditional fertilizer products. The polyurethane matrix of such nanofertilizers protects the particles from the environment and provides a controlled release of nutrients into the soil. The controlled release nanofertilizers offer several advantages over traditional fertilizers, including a more efficient and longer-lasting nutrient release, simultaneously releasing multiple types of nutrients, and a higher water-holding capacity that can reduce soil erosion.

Traditional fertilizers are coated with polymers such as polyurethane to engender slow-release fertilizers [38]. Polymeric materials, including cottonseed oil, can be synthesized using cost-effective, biodegradable, and renewable sources. The coating increases surface roughness, reduces surface energy, superhydrophobic qualities of polyurethane prevent water from coming into contact with the fertilizer in its liquid form [39]. However, not all nutrient elements can be incorporated with polyurethane. For example, sulfur’s oxidation and brittleness prevent it from being used as a slow-release fertilizer. In such cases, an alternative natural polymer is employed to absorb water. Unfortunately, the nutrient release duration of this product is shorter than 30 days. A novel approach was devised to circumvent this constraint, which entails turning natural polymers into bio-polyols by coating fertilizer with polyurethane from wheat straw and liquefying the wheat straw using solvents such as ethylene glycol/ethylene carbonate. This liquid is then mixed with polymethylene polyphenyl isocyanate and castor oil to create a bio-based polyurethane, providing a more viable solution than synthetic polymers, which are too costly. [40]. Polyurethane-based nanofertilizers are likely to become popular for use in agricultural applications in the future.

Starch-Based

Starch-based nanofertilizers comprise nanocrystals derived from starch, which can be readily dissolved in water or administered to plants in either liquid or aerosol form [41]. These nanocrystals effectively fertilize crops using a high-quality renewable energy source without creating chemical waste. A study demonstrated the effectiveness of nanotechnology in agriculture by synthesizing a polymeric formulation of polyvinyl alcohol -starch as a substrate for the slow release of Cu-Zn micronutrient-carrying carbon nanofibers (CNFs) [42]. The results showed that the plants treated with the nanofertilizer were significantly taller than the control, and the nanofertilizer was also influential in scavenging reactive oxygen species. By contrast, the metal release profile of the nanofertilizer was considerably lower than that of the CNFs [42].

Zeolite-Based

Zeolites are a class of microporous, aluminosilicate minerals that have been used for decades as a soil amendment due to their ability to absorb and retain water, nutrients, and other organic compounds [43]. Developing nanofertilizers based on zeolites involves the synthesis of zeolite NPs, which are then combined with various other compounds to create fertilizer. These nanofertilizers are small, allowing them to penetrate deeper into the soil and deliver nutrients to a crop’s root zone more efficiently [44,45]. In addition, zeolites also act as a reservoir for storing and releasing nutrients over time, providing a more consistent supply of nutrients to plants. Furthermore, zeolite-based nanofertilizers can be tailored to the specific nutrient needs of a crop, ensuring that only the required nutrients are provided, thus reducing the cost of fertilization.

Table 1 compares known controlled-release nanofertilizers. The best nanofertilizer for the application depends on soil type, crop needs, and environmental conditions. Overall chitosan-based nanofertilizers balance the advantages of controlled release, biodegradability, and ease of modification, making them a promising candidate for various agricultural applications. Moreover, they can be easily combined with other nutrients and agrochemicals, providing a versatile solution for sustainable agriculture. Regardless, it is crucial to consider the specific needs of the plants and the local environment before choosing the most appropriate nanofertilizer.

2.1.2. Nanofertilizers for Targeted Delivery

Nanoaptamers

Nanoaptamers belong to an innovative new form of fertilizer delivery system revolutionizing agricultural cultivation. Nanoaptamers target specific molecules in the soil and deliver nutrients or other molecules directly to the plant. They are tiny molecules that can bind to plant hormones and enzymes and deliver them effectively and efficiently to the plant. Nanoaptamers are reported to increase plant nutrient uptake from the soil [63,64]. Aptamers, which are small molecules made of either oligonucleotides or peptides, can be used to modify the surface of nanofertilizers. This modification enables the release of nutrients within the nanostructure once activated by signals emanating from the rhizosphere. Nanoaptamers can deliver fertilizer to plants by attaching fertilizer components to a nanoparticle, such as a gold nanoparticle or a liposome. This approach safeguards the aptamer from degradation and enables its targeted delivery to the intended plant cells. The aptamer can then bind to a specific receptor on the surface of the plant cell, allowing the nanoparticle to enter the cell and release the fertilizer payload.

Nanoaptamers are employed to regulate the dissemination of fertilizer. By forming a connection between the root system and the soil microorganisms, they enable the precise administration of the desired amount of fertilizer. Previous studies have demonstrated the effectiveness of polymer-coated controlled-release fertilizers wherein the aptamer binding to the target causes the polymer to become more permeable, delivering a payload of nutrients [65,66].

Using nanomaterials for the targeted delivery of essential nutrients, pesticides, and genetic materials could significantly enhance the agricultural industry [67,68,69]. The use of nanoaptamers in agriculture is still a relatively new concept, and much is still to be learned about their potential applications. As nanoaptamer technology develops, we expect to see even more applications for these revolutionary fertilizers soon. Future usage of nanoaptamers as a commercial fertilizer is possible. However, they are not frequently used, and there is insufficient information on the subject.

Others

Other target-based nanofertilizers include nano-coated urea, iron oxide NPs, nano-hydroxyapatite, carbon-based nano-nutrient carriers, nano-emulsions, and nano-encapsulated micronutrients, as well as clay-based nanofertilizers [7,69]. These innovative formulations allow for the controlled release and enhanced uptake of nutrients, resulting in increased crop yields and reduced environmental impacts [31]. However, the functioning and advantages of all of these nanofertilizers overlap with other types.

2.1.3. Plant Growth-Stimulating Nanofertilizers

Certain nanofertilizers, such as carbon nanotubes (CNTs), stimulate plant growth by interacting with plant root systems and boosting hormone synthesis. They increase the amount of carbon and other nutrients in the soil. CNTs consist of rolled-up sheets of carbon atoms and possess unique properties that make them excellent fertilizers. CNTs can absorb and release nutrients, improve soil structure, increase water retention, and enhance the growth of plants. In contrast to the unfavorable effects of large-dose fertilizer experiments, low carbon nanotube concentrations may benefit seed germination, root development, and water transport with no phytotoxicity [70]. When used as a fertilizer, the CNTs blend with the soil, releasing plant nutrients. As a soil amendment, CNTs are added directly to the soil to improve its capacity to store nutrients and water. CNTs can penetrate deep into the soil, providing sustenance to the flora over a prolonged period. The diffusion of nutrients over a more extended period inhibits the occurrence of over-fertilization.

Additionally, CNTs can bolster the soil structure, improving its ability to retain water, hindering soil erosion, and augmenting its nutrient storage capacity. CNTs can also help reduce the amount of fertilizer needed, as they can be used in smaller quantities than traditional fertilizers. CNTs have tremendous tensile strength, making them one of the strongest and smallest-known fibers. They can migrate to systemic areas of plants such as fruits, leaves, and roots, indicating a significant interaction with plant cells [71]. In addition to being employed as a fertilizer or soil amendment, CNTs can also be applied as a protective coating on seeds that aids in preventing pests while simultaneously boosting the seed’s capacity to absorb nutrients and water. CNTs have been suggested to enhance the mechanical properties of plant stalks and stem. The high strength and stiffness of CNTs could potentially make these materials more resistant to breaking and collapsing under heavy loads, improving crop yields.

2.1.4. Water and Nutrient Loss-Controlling Fertilizers

Nanofertilizers contain NPs capable of controlling the rate at which fertilizers are released into the soil, allowing farmers to use lesser fertilizer while maintaining the same crop output. Several approaches are considered for designing nanofertilizers that can control the release of nutrients and reduce water loss. One method involves encapsulating the nanofertilizer in a porous matrix that can slowly release nutrients over time [72]. Another approach involves modifying the surface of the nanofertilizer to make it hydrophilic, which can increase its water-holding capacity and reduce the amount of water lost to evaporation [73].

Urea coated with NPs of iron oxide, sulfur, calcium, magnesium, zinc, copper, molybdenum, boron, ammonium sulfate, and potassium are some examples of nanofertilizer types that control water nutrient loss in soil [74,75]. Nanobeads and nanoemulsions are two prominent nanofertilizers that control soil water and nutrient loss [76].

Nanobeads

Nanobeads are minuscule particles engineered to contain slow-release nutrients, diminishing plant water loss [50]. Nanobeads are made from various materials, including iron, carbon, and other metals. The beads are so tiny that they can get into the smallest cracks in the soil and help to fertilize the plants. Nanobeads can also be used to help clean up pollution. One of the most popular nanobead-based fertilizers is commercially available NanoFert, containing macro- and micronutrients [77]. These particles are designed to dissolve in the soil quickly, allowing the nutrients to be taken up by the plant roots. NanoFert is also intended to be low in salt content, so it won’t harm beneficial soil microbes or adversely affect the soil structure [77].

Another popular nanobead-based fertilizer, N-Flex from Limagrain Europe, contains nanosized particles that slowly release nutrients over time [78]. The slow release of nutrients helps ensure the plants receive the nutrients they need for optimal growth without the risk of over-fertilization. N-Flex also contains more nitrogen than many traditional fertilizers, which helps promote healthy growth [78].

Nanoemulsion-Based Fertilizers

Nanoemulsions are tiny droplets designed to contain a mixture of water-soluble and insoluble nutrients. Nanoemulsion-based fertilizers are formulated by combining a surfactant with a liquid and using high-energy mixing to create a stable, homogenous mixture. The surfactant helps to keep the liquid droplets suspended in the water-soluble matrix, allowing them to remain evenly distributed throughout the solution. The droplets are typically smaller than 100 nanometers in diameter, making them much smaller than the particles found in traditional fertilizers. Plants readily absorb and utilize the droplets, resulting in better yields and improved crop health.

A study found that adding 1% paraffin oil nanoemulsion to the Blue-green 11 media significantly increased biomass yield, chlorophyll-a synthesis, cell numbers, CO2 absorption, and biochemical content of the freshwater microalgal strain Chlorella pyrenoidosa [79].

Nanoemulsion-based fertilizers are advantageous because their minute droplet size increases plant cell walls’ permeability, facilitating plant cells’ rapid and complete fertilizer absorption [80]. The capacity of nanoemulsion-based fertilizers to target-specific nutrients provides an additional advantage. Due to the small size of the droplets, they may be adjusted to provide specific nutrients to plants, allowing farmers to meet the requirements of their crops better. In addition, they can be applied to any conventional irrigation system in various ways. A nanostructured slow-release fertilizer system was formulated in a study by blending nanoemulsions of nano phosphate and potash fertilizer with neem cake and PGPR [81]. Nanoemulsion-based formulations effectively combat fungal pathogens in crops, which is a major cause of crop loss, as demonstrated in multiple research studies, and it can significantly impact crop yields and economic benefits [82,83].

Nanotechnology-based nanoemulsions offer a range of potential applications in agriculture, such as high surface area per unit volume, improved stability, enhanced transparency, and potent rheology. The advantages of spreadability, wettability, bioactivity, and mechanical strength make nanoemulsions especially promising for delivering plant growth-promoting rhizobacteria (PGPRs) in soil [84].

Nanoemulsions are developed by combining two immiscible phases, an aqueous phase and an oil phase, with emulsifiers such as surfactants and co-surfactants. Typical oil phases include captex 355, captex 8000, witepsol, myritol 318, capryol 90, sefsol 218, triacetin, isopropyl myristate, castor oil, and olive oil, while water is used as aqueous phase [85,86]. Emulsifiers play an essential role in nanoemulsion formulation, as they improve kinetic stability [87], interactions [88], and shelf-life [89], as well as reduce the interfacial tension between the two phases. Typical surfactants are cationic, anionic, amphoteric, and nonionic [90]. Scientists are now exploring nanoemulsified microbial-based methods that can reduce the impact of chemicals and provide sustainable solutions to current food and climate challenges. However, additional research is needed to ensure their sustainability and affordability as a strategy for soil health and crop improvement.

2.2. Nutrient Based

The increasing global population and consequent demand for food have made it essential to develop sustainable agricultural practices. Nutrient-based nanofertilizers are one such innovation that addresses this challenge by enhancing nutrient availability, uptake, and utilization in plants [7]. This section discusses various nutrient-based nanofertilizers and their benefits in sustainable agriculture.

2.2.1. Inorganic Nanofertilizers

Inorganic nanofertilizers include metals, metalloids, and non-metallic NPs, and they can provide essential nutrients, such as nitrogen, phosphorus, and potassium, to plants. These fertilizers are designed to improve the efficiency of nutrient uptake by plants and can be used to improve yields in agriculture [33,91]. The choice of material depends on the desired properties of the fertilizer. Inorganic nanofertilizers offer a unique advantage due to their ability to be customized to the specific nutrient requirements of targeted plants, thereby allowing targeted applications to enhance yield. Inorganic nanofertilizers are already being used in agriculture, and their use is expected to increase as more farmers adopt precision agriculture practices. In a study on safflower plant growth, the foliar application of silicon dioxide NPs (nSiO2) improved canopy spread, stem diameter, plant height, and ground cover [92]. The study suggests that organic fertilizers combined with nSiO2 application can improve safflower production in semi-arid areas.

There is great interest in using inorganic nanofertilizers; several companies are already producing and marketing these products. For better understanding, inorganic nanofertilizers are divided into macronutrient and micronutrient-based nanofertilizers.

Macronutrient Nanofertilizers

Macronutrient nanofertilizers offer significant advantages for plant growth and environmental sustainability. For example, nitrogen-based nanofertilizers enhance nitrogen utilization efficiency, reducing eutrophication and greenhouse gas emissions [93]. Phosphorous nanofertilizers help plants absorb nutrients and have shown promising results in soil reclamation [94]. Potassium-based nanofertilizers have higher absorption rates, are more resistant to leaching, and can improve soil physical properties. Calcium-based nanofertilizers contribute to higher crop yields, improved fruit and vegetable quality, and increased disease resistance [95,96]. Magnesium-based nanofertilizers enhance crop growth, quality, and resistance to disease and pests while benefiting various crop types [97]. Lastly, sulfur-based nanofertilizers offer slow-release options for sustained nutrient supply, minimizing the risk of soil acidification [98,99]. The details of each type are described below.

(a)

Nitrogen-based

Nitrogen is the most critical nutrient that restricts agricultural production on a global scale. Despite numerous endeavors, the nitrogen utilization efficiency in farming remains less than 50% [100]. In the past few decades, nitrogen over-utilization was done to achieve targeted agricultural yields, a financial and ecological concern of global relevance. [101]. Existing nitrogen fertilizers have a poor utilization efficiency (20%), leading to eutrophication and increased greenhouse gas emissions [102]. Most of the nitrogen in urea is lost owing to rapid volatilization and leaching quickly after application.

Nitrogen nanofertilizers combine nitrogen molecules with NPs such as carbon nanotubes, graphene, and metal oxides. This combination of particles helps increase the available nitrogen in the soil, allowing plants to access more nutrients. The gradual release of nitrogen into the soil from these fertilizers decreases the amount of nitrogen in aquatic systems, thereby reducing the risk of environmental damage from leaching and runoff. One study observed the nutrient release pattern of nitrogen-containing nanofertilizer formulations and demonstrated that nanofertilizer releases nutrients for up to 1200 h, but traditional fertilizer only lasts 300–350 h [103]. Another study suggests using zeolite as a nanofertilizer to increase N usage efficiency [45].

(b)

Phosphorous-based

Phosphorous is one of the critical nutrients plants need to grow and thrive. It is an essential component in photosynthesis and helps plants to absorb other nutrients, such as nitrogen and potassium. Phosphorous nanofertilizers are a relatively new type of fertilizer that has the potential to revolutionize the way food is grown. They are more efficient, cost-effective, and environmentally friendly than traditional fertilizers. Using slow-release phosphorus nanofertilizer to supply the crop with phosphorus throughout its life cycle can conserve this element [6]. In an experiment, applying nano-rock phosphate to maize plants resulted in similar phosphorous utilization as superphosphates but at a more affordable cost [104]. Recently the nanoformulations of hydroxyapatite (nHAP; Ca10(PO4)6(OH)2) were used to deliver phosphorus to plants [105].

When used in the right amount, it helps plants absorb more phosphorus and other nutrients [106]. Phosphorus nanofertilizers have been demonstrated to be efficacious in soil reclamation processes. A study found that applying rock phosphate-based nanofertilizer enhanced plant growth and yield in degraded soils, particularly when the phosphate-based nanofertilizer was encapsulated in a chitosan shell [107]. A newly developed slow-release phosphorous nanofertilizers stimulated rice growth when applied with chitosan. This nanofertilizer combined poly-beta-amino-esters, graphene oxide, chitosan, poly lactic-co-glycolic acid, and phosphorus in active and barrier layers to control the slow release of phosphorus during the first stages of rice production [108].

(c)

Potassium-based

Potassium nanofertilizers, also known as nano potassium, are a modern innovation in agricultural technology. These fertilizers are composed of microscopic particles, enabling them to penetrate deeper into the soil and reach the roots of plants. As a result, they have a higher absorption rate than traditional fertilizers, delivering essential nutrients to plants more quickly and efficiently. Furthermore, nano potassium is more resistant to leaching and is more soluble in water, making it less vulnerable to being washed away by rainfall or irrigation [109]. These features make potassium-based nanofertilizers significantly more effective in sustaining higher yields over extended periods.

(d)

Calcium-based

Calcium is essential for plants in many ways, including cell division, cell wall formation, and the transport of water and nutrients. Calcium ions are a secondary messenger in signal transduction under various stress circumstances [110]. The improvement of calcium ion levels in the cytosol due to stress signals is predicted by calcium ion-binding proteins, which cause changes in gene expression and plant acclimation to stress circumstances [111,112]. Nitric oxide (NO) was found to promote the concentration of calcium ions in the cytosol under diverse biotic and abiotic stress situations; consequently, calcium ions result in the synthesis of nitric oxide [113]. Research has indicated that using silver nanomaterials on rice roots can impact various plant processes such as responsive protein regulation, calcium ion signaling, transcription, protein degradation, oxidative stress response pathways, cell wall formation, and cell division. The effects of these nanomaterials can vary depending on the concentration used. These findings suggest that NPs can affect plant growth and development by influencing molecular mechanisms and signaling pathways.

Calcium is also necessary for forming seeds and fruit and helps protect plants from disease and pests. However, calcium is not always readily available to plants and can be challenging to apply in the right amounts. Traditional fertilizers that contain calcium, such as lime and gypsum, are often not as effective as they could be.

Multiple variants of calcium-based nanofertilizers have been developed. Some are made from calcium carbonate [114], while others are made from calcium nitrate dope in calcium phosphate [115]. Calcium-based nanofertilizers are effective at increasing the growth and yield of crops. They have also been shown to improve the quality of fruits and vegetables and increase plants’ resistance to disease and pests. In recent years, calcium phosphate NPs (CaP) have received significant attention as potential macronutrient nanofertilizers with better nutrient-use efficiency than traditional fertilizers. Their high content of macronutrients, such as phosphorus, and slow solubility in water make them useful as slow-release P nanofertilizers [116].

Additionally, their large surface area can be modified to hold other macronutrient-containing substances, such as urea or nitrate, to create nanofertilizers with improved nitrogen-releasing properties. Studies have shown that CaP nanofertilizers are more effective in agriculture than traditional fertilizers [116]. Biomimetic calcium phosphate NPs (CaP) have been used in biomedicine due to their biodegradability and biocompatibility. Despite this, less progress has been made in precision agriculture for the controlled delivery of active species to plants. A study reports a straightforward and green synthetic method to dope CaP with potassium and nitrogen to create multi-nutrient nanofertilizers (nanoU-NPK) that provide a slow and gradual release of essential plant macronutrients (NPK) and can be used to reduce the amount of nitrogen supplied to durum wheat by 40% concerning conventional treatment, without affecting the final kernel weight per plant [117]. Applying slow-release NPK nanofertilizers is a promising strategy for enhancing fertilization efficiency in precision agriculture [117].

Bio-inspired synthetic calcium phosphate NPs are emergent materials for sustainable applications in agriculture. These salts have self-inhibiting dissolution processes under saturated aqueous media, the control of which is not fully understood [118]. Comprehending the mechanisms involved in the dissolution of particles holds tremendous significance for effectively supplying macronutrients to plants and adopting a valuable synthesis-by-design strategy. Additionally, it bears relevance to the (de)mineralization of bones. In this study, authors shed light on the role of size, morphology, and crystallinity in the dissolution behavior of CaP NPs and on their nitrate doping for potential use as (P and N)-nanofertilizers. They found that morphology actively directs the dissolution kinetics [118].

Amorphous NPs manifest a rapid loss of nitrates governed by surface chemistry. NPs show slower release, paralleling Ca2+ ions, supporting detectable nitrate incorporation in the apatite structure and dissolution from the core basal faces [115]. In a further study on nitrate-doped amorphous calcium phosphate NPs with urea, the same authors found that the process leads to high levels of nitrogen payloads, is cost-effective to scale up, and slows down the release of urea. In tests on cucumber plants, authors found that these NPs promote growth and biomass formation using less nitrogen than traditional fertilizers, making them a promising option for sustainable use as a nanofertilizer [119].

(e)

Magnesium-based

Magnesium is an essential plant mineral in many vital processes, such as photosynthesis, enzyme activation, and protein synthesis. It is also a key component of chlorophyll, the pigment that gives plants green color and allows them to capture sunlight for photosynthesis. However, magnesium is not always readily available to plants and can be challenging to apply correctly. Traditional fertilizers that contain magnesium, such as dolomitic lime and Epsom salts, are often not as effective as they could be.

Several different types of magnesium-based nanofertilizers have been developed. Some are made from magnesium oxide, while others are from magnesium sulfate [120]. Magnesium-based nanofertilizers are effective in increasing the growth and yield of crops. Magnesium enhances the nutritional quality of fruits and vegetables while concurrently bolstering plants’ resilience against diseases and pests. These fertilizers benefit crops such as rice, sugarcane, tomato, and potato. One study considered using nano and common forms of iron and magnesium as foliar applications on black-eyed peas [121]. A factorial experiment with three replicates was conducted in a study using different concentrations of Fe and Mg. The elements were applied to the leaves 56 and 72 days after planting, and data were collected after 85 days. The results showed that Fe significantly affected yield, leaf Fe content, stem Mg content, plasma membrane stability, and chlorophyll content. The most significant effect was observed with two combinations of treatments: 0.5 g L?1 standard Fe + 0.5% nano-Mg and 0.5 g L?1 typical Fe + 0.5 g L?1 standard Mg. Almost all analyzed traits were improved by the foliar application of these two elements, likely due to more efficient photosynthesis [121].

Another study investigated the impact of chitosan and magnesium-nano fertilizers on sesame plants’ photosynthetic pigments, protein, proline, and soluble sugar content under drought stress [122]. The results show that chitosan foliar application improved the mean traits of chlorophyll a, b, total, carotenoid, protein, proline, and soluble sugar. By contrast, severe drought stress and no nanofertilizer application decreased chlorophyll content and plant damage. The findings suggest that co-applying chitosan and Mg nanofertilizers could effectively reduce plant damage due to drought stress [122].

(f)

Sulfur-based

Sulfur is an essential plant mineral involved in many vital processes, such as protein synthesis, enzyme activation, and the production of vitamins and hormones [