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].

There are three primary methods of nanofertilizer application: foliar, seed nanopriming, and soil treatment. The foliar application involves spraying nanofertilizers directly onto the leaves of plants, allowing for rapid nutrient absorption through the leaf surface [7]. The method is particularly effective when nutrients are required quickly or in regions with low soil fertility. However, foliar application is sensitive to environmental factors such as temperature, humidity, and wind, affecting nutrient uptake efficiency [190]. Seed nanopriming entails coating or soaking seeds in a solution containing nanofertilizers before planting [191]. The method promotes rapid germination, stronger seedlings, and enhanced nutrient uptake throughout the plant’s life. It is especially beneficial in areas with poor soil quality or where rapid plant establishment is necessary. However, the optimal concentration of nanofertilizers must be determined to avoid phytotoxicity [31]. Soil treatment involves incorporating nanofertilizers directly into the soil by broadcasting, banding, or localized placement [159]. The method ensures the slow and controlled release of nutrients, reducing nutrient loss through leaching or volatilization. Soil treatment is best suited for regions with high nutrient retention capacities and climates with consistent precipitation patterns. However, the application must be carefully managed to prevent nutrient imbalances or environmental pollution [8].

The appropriate method of nanofertilizer application is crucial for optimal plant growth, as it varies depending on the soil and climate type. The choice depends on soil quality, nutrient availability, and climate, which affect nutrient uptake and utilization. Understanding these factors and selecting the appropriate method can improve crop yield, reduce environmental impact, and create more sustainable agricultural practices.

The three methods of application are explained below in detail.

Foliar spray is an advanced method that directly applies liquid fertilizers to plants’ leaves or foliage, enabling rapid absorption of nutrients through the leaf surface. The method employs nanofertilizer delivery to the leaf surface for targeted, optimal, rapid, and accurate transfer to the plant. The foliar application of NPs has emerged as a promising method for delivering essential elements such as nanofertilizers, fungicides, herbicides, and preservatives to plants. This approach leverages delayed release mechanisms to enhance the effectiveness of these substances. The absorption of foliar-applied NPs can occur through stomata, endocytosis, and direct absorption, although the process heavily depends on particle size. Leaf wax and cell walls can act as barriers, inhibiting the uptake of these particles. Once absorbed, the majority of NPs accumulate in vacuoles. However, various factors influence the absorption and transport of NPs, including plant characteristics, NP physical properties, and environmental conditions.

The foliar spray offers several advantages over traditional soil applications, including a faster response, improved nutrient utilization, and reduced leaching and run-off [192]. Multiple studies have demonstrated that foliar application of nanofertilizers can significantly improve nutrient uptake, promote plant growth, and increase crop yield. Foliar application of CeO and carbon-based NPs increased wheat yield by 36.6% [193] and bitter melon yield by 28% [194]. Another study reported that foliar application of copper NPs in tomato plants increased fruit yield by 80%, with a 30% reduction in the required copper concentration compared to conventional copper-based fungicides [195].

Seed priming is a pre-sowing treatment that induces physiological changes within seeds, allowing for faster germination and promoting plant growth and development by regulating metabolic and signaling cascades. The method involves soaking seeds in nanofertilizers, which has been shown to reduce fertilizer application by half while achieving excellent results [196]. Nanobiofertilizers act as stimulants, enhancing germination and development by penetrating seed pores, dispersing within, and activating plant hormones that promote growth.

Applying nanofertilizer to seed priming increases seed germination by eliminating reactive oxygen species and regulating plant development hormones [172]. Seed priming also stimulates the expression of multiple genes during germination, particularly those related to plant resilience, resulting in enhanced resistance [7]. Conventional seed priming methods employ water, nutrients, or hormones to dissolve a seed coat. In contrast, advanced seed nano-priming techniques involve applying nanofertilizers directly to the seed surface, leaving a substantial fraction impeding pathogen penetration.

Nano-compound absorption at the cellular level reduces input and avoids molecular interactions, allowing for the production of highly resistant seeds with improved germination and seedling growth, especially under stress. Studies have shown that bean seed priming with chitosan NPs (0.1, 0.2, and 0.3%) for 3 h, followed by 100 mM NaCl treatment, enhanced seed germination and radicle length [197]. Under salt stress, proline, chlorophyll a, and antioxidant enzyme efficiencies of bean seedlings treated with 0.1% chitosan NPs increased significantly compared to untreated, salt-stressed seedlings [197]. Nanofertilizers mitigate plant stress by regulating internal hormone action in crops, strengthening antioxidants, and reducing reactive oxygen species (ROS) formation [172].

Nanofertilizers can be administered to the soil using conventional techniques such as broadcasting, side-dressing, or fertigation methods. Once in the soil, the NPs interact with plant roots through adsorption to the root surface or by penetrating root cells via endocytosis [198,199]. When applied to soil, nanofertilizers can interact with plants, soil particles, and microorganisms, which may alter their behavior and function. The controlled release of nutrients from the NPs ensures a steady supply of essential elements, which enhances plant growth and productivity [35]. This method of application, although considered reliable, suffers from uncertain long-term effects of NPs [69], higher costs [187], and regulator challenges [200].