Current and future perspectives on the use of nanofertilizers for sustainable agriculture: the case of phosphorus nanofertilizer

Abstract

Over the last century, the demand for food resources has been continuously increasing with the rapid population growth. Therefore, it is critically important to adopt sustainable farming practices that can enhance crop production without the excessive use of fertilizers. In this regard, there is a growing interest in the use of nanomaterials for improving plant nutrition as an alternative to traditional chemical or mineral fertilizers. Using this technology, the efficiency of micro- and macro-nutrients in plants can increase. Various nanomaterials have been successfully applied in agricultural production, compared to conventional fertilizers. Among the major plant nutrients, phosphorus (P) is the least accessible since most farmlands are frequently P deficient. Hence, P use efficiency should be maximized to conserve the resource base and maintain agricultural productivity. This review summarizes the current research and the future possibilities of nanotechnology in the biofortification of plant nutrition, with a focus on P fertilizers. In addition, it covers the challenges, environmental impacts, and toxic effects that have been explored in the area of nanotechnology to improve crop production. The potential uses and benefits of nanoparticle-based fertilizers in precision and sustainable agriculture are also discussed.

Introduction

Agriculture is the most important and stable sector of the global economy because it produces and provides food for humans, both directly and indirectly. As the world population increases, agricultural production must also increase to minimize poverty and nutritionally support the growing population. Currently, farmers involved in agricultural production face challenges such as water scarcity, shrinking of cultivable land due to urbanization, climate change, decline in crop yield, sustainable use of resources, micronutrient deficiencies, reduced organic matter in soils, and environmental and logistical issues such as runoff, fertilizer accumulation, and pesticide toxicity, and labor scarcity, respectively. Many new technologies have been scientifically developed as potential solutions to increase productivity and reduce resource costs and environmental issues related to agricultural production (Chen and Yada 2011).

Nanoscience and nanotechnology have the potential to revolutionize a diverse range of fields, such as chemistry, physics, medicine, food production, and agriculture, using various applications that influence human life (Linkov et al. 2011). Nanotechnology has tremendous potential for upgrading agricultural practices with novel nanotechnological strategies that control plant disease, improve precision farming techniques, enhance the capacity of plants to absorb nutrients, support land and water conservation, increase crop yield, and develop applications for nanofertilizers (NFs) and nanopesticides (Prasad et al. 2017). Therefore, it is necessary to transform agriculture and allied fields, including aquaculture and fisheries, with modern technologies such as nanotechnology and nanobiotechnology (Singh Sekhon 2014).

Fertilizers are essential resources for plant growth and development in agricultural production (Olad et al. 2018). Due to several factors such as leaching, degradation, insolubility, and decomposition, the availability of these resources is insufficient for plants and leads to lower crop yields. Hence, farmers resort to the excessive use of conventional fertilizers (CFs). Most of the applied CFs dissipate into the atmosphere or surface water bodies where they cause serious environmental issues (Singh et al. 2015). For instance, an excess of P is fixed in soil, where it forms chemical bonds with other elements, such as Ca, Mg, Al, Fe, and Zn, and becomes unavailable for plant uptake. Similarly, N is also largely inaccessible as it forms NH₃, emissions of N₂O, or NO in runoff (Raliya et al. 2018). Approximately 80–90% of P and 40–70% of N applied to farmlands are either lost to the environment by runoff or become rocks. If these nutrient losses continue, the supply of P could perish within a few years. The excessive use of inorganic fertilizers, such as N, P, and K, has been a great challenge to agriculture as these can be washed by runoff, which can lead to eutrophication and may contribute to the accumulation of heavy metals in soil, water, and air, thereby causing serious threats to the environment and human health (Savci 2012).

P is an essential element and macronutrient that is important for photosynthesis, respiration, energy storage and transfer, cell division and expansion, and metabolic functions required for plant growth, maturity, and various other processes. An adequate supply of P supports root development, flower formation and seed production, as well as stalk growth, stem strength, and resistance to plant diseases during the early stages of crop maturity. Moreover, the availability of P can increase the N-fixing capacity of legumes and support development throughout the plant’s life cycle (Amanullah et al. 2012). However, continuous application of P fertilizers to agricultural land leads to eutrophication in surface waters (Conley et al. 2009) and thus, numerous regulations (Litke 1999), best management practices (Hoffmann et al. 2009), and many novel technologies have been applied to reduce the quantity of P fertilizer that enters water bodies. Compared with CFs, smart fertilizers or NF formulations such as controlled-release or slow-release fertilizers (CRFs or SRFs) prevent the loss of P during irrigation and reduce microbial degradation, photochemical degradation, and hydrolysis (Olad et al. 2018).

NFs are the newest and most technically advanced means of gradually releasing nutrients into the soil in a controlled manner and have the potential to break seeds and boost nutrient availability and accessibility to the branches and leaves of plants to improve plant growth, quality, and yield (Singh Sekhon 2014). For this reason, NFs have allowed considerable improvements in crop yield and quality of agricultural production in recent years by helping avoid eutrophication and contributing to agricultural sustainability.

Nanomaterials have positive effects due to their ability to cover a large specific surface area to support crop growth better than the equivalent dose of a CF or the salt form of a nutrient. Thus, the use of P NFs as an alternative to CFs to improve the surface water quality in agricultural lands would increase production, regulate the use of P, and increase the effective absorption of nutritional elements (Liu and Lal 2014). In addition, NFs can help avoid environmental pollution, increase the efficiency of different water and land resources, and increase the availability of essential compounds required for plant growth and metabolism in modern agricultural systems (Morteza et al. 2013).

There are many possible ways to encapsulate fertilizers within nanoparticles (NPs), such as nutrients contained in nanotubes and nanoporous materials that are coated with a thin polymer film, and delivered as NPs or emulsions (Derosa et al. 2010). In traditional farming systems, most fertilizers are not delivered or adequately available to crops; however, NFs release nutrients in a controlled manner that can be slow or fast depending on the environmental factors of the farmland, such as soil acidity, temperature, and moisture, to enhance plant growth more effectively than CFs (Naderi and Danesh-Shahraki 2013).

This review evaluates current agricultural opportunities provided by nanomaterial applications, especially NFs, with a special focus on P NFs. In addition, different forms, preparations, and manufacturers of NFs, along with their effects on the environment and plant growth are discussed. This review can guide future research efforts aimed to improve our understanding of NP use to boost the mineral nutrition of crops.

Chemical fertilizers or conventional fertilizers

Chemical fertilizers or CFs are synthetic, meaning that they are composed of non-organic cultivated elements, and are used by farmers in higher amounts to increase crop yield. CFs can have granular or liquid forms with the same composition, are less expensive, and work faster than organic fertilizers because of their immediate dissolution in water. However, P fertilizers, such as monoammonium phosphate (MAP), diammonium phosphate (DAP), and triple superphosphate (TSP), contain some insoluble compounds; as such, they are unable to readily dissolve in water (Hansel et al. 2014). Overfertilization is the most common problem associated with these fertilizers and leads to environmental issues since these compounds pollute water bodies, get stored in crop plants, stunt plant growth, cause burns in foliage, and consequently make plants more sensitive to pests and diseases. In addition, they fail to provide the necessary nutrients to plants by causing a depletion of soil enrichment, extraction of moisture from the soil, increased field salinity, and the loss of organic matter and important living organisms that help improve the soil quality.

Organic fertilizers

Organic fertilizers consist of minimally processed materials, such as organic matter and plant and animal waste, instead of being extracted and refined. Microorganisms found in soil, releasing essential nutrients, decompose organic matter, such as animal manure, poultry droppings, sewage sludge, and food waste. Hence, this natural fertilizer is more environmentally friendly, as it improves the texture of soil, holds water for longer periods, and increases the activity of soil bacteria and fungi. The majority of nutrients released from this matter are N, P, and K, which protect plants from pests and diseases. The main disadvantage of this type of fertilizer is the gradual release of nutrients, compared to CFs.

Biofertilizers

Biofertilizers are natural fertilizers of living formulations consisting of advantageous microbes, such as bacteria, fungi, and blue-green algae. Biofertilizers are ecofriendly, inexpensive, renewable, and as such, a strong alternative to synthetic fertilizers, as they enrich the quality of the soil to enable to provide essential nutrients required for plant fertility and productivity. Several microorganisms act as biofertilizers by indirectly helping plants, including Azotobacter, Anabaena, and Rhizobium for N fixation, and Pseudomonas spp. that function as phosphate-solubilizing bacteria (Nosheen et al. 2021). Besides fixing N and increasing the availability of nutrients for plants, these microbes produce numerous types of bioactive compounds, growth hormones, vitamins, antagonistic compounds, and organic acids, in addition to protecting plants against some pathogens.

Nanofertilizers

Nanofertilizers are plant nutrients wholly or partially composed or engineered of nanostructured formulation(s) that can release active ingredients into the soil slowly and in a controlled manner, thereby preventing nutrient loss, eutrophication, and the pollution of water and air. Owing to their high surface area to volume ratio, the efficacy, performance, availability, and utilization of NFs are higher than those of CFs and therefore, the former offer a platform for the development of sustainable and novel nutrient delivery systems (Elemike et al. 2019). Their composition can also allow for efficient uptake by crops, restoration of soil fertility, ultrahigh absorption, increased photosynthesis, increased production, reduced soil toxicity, reduced application frequency, increased plant health, and minimized environmental pollution (Naderi and Danesh-Shahraki 2013).

In NFs, nutrients such as N, P, and K can be encapsulated in nanomaterials or coated with thin polymer films and delivered as nanoemulsions by either foliar or soil application, as shown in Fig. 1.

Fig. 1.

Nutrient-loaded nanomaterials as controlled-release nanofertilizers delivering the appropriate nutrients for enhancing crop growth, yield, and productivity by foliar and soil applications

The components of nanomaterials may include zinc oxide (ZnO), silica, Fe, titanium dioxide, aluminum oxide, cerium oxide, quantum dots, gold nanorods, ZnCdSe/ZnS core–shell, Mn/ZnSe quantum dots, and an InP/ZnS core–shell (Prasad et al. 2017). The efficacy of using nanomaterials as NFs on plant growth mainly depends on the crops and the size, concentration, composition, and chemical properties of the nanomaterials (Thakur et al. 2018).The reaction of NP suspensions containing NFs with water causes the release of nutrients into the soil, which are needed by the crops. The polymer coating of NFs or the thin coating encapsulation of NPs can prevent premature contact with water and soil to prevent undesirable losses of nutrients (Shang et al. 2019).

Types of nanofertilizers

Based on the nutrient requirements of plants, NFs are currently divided into three main categories: (1) nanoporous materials, (2) nanoscale additive fertilizers, and (3) nanoscale coating fertilizers. Encapsulation of beneficial microorganisms, such as bacteria or fungi, can enhance the availability of N, P, and K in the root zone, thereby improving plant growth. The main benefits of using nutrient-loaded nanomaterials and encapsulated fertilizers are their safety and ability to accurately release fertilizers according to plant requirements. Fertilizers coated with nanomaterials provide nutrients to plants slowly, thereby reducing nutrient loss, and are considered promising alternatives to CFs (Zhong et al. 2013).

Hydroxyapatite-NPs (HA-NPs) are major mineral constituent and nano-enabled nutrient delivery systems with excellent biocompatibility, high surface area to volume ratio, and have the potential to deliver both Ca and P to plants. Urea-loaded hydroxyapatite nanohybrids are promising nano-encapsulated fertilizers for the slow release of N (Kottegoda et al. 2017). Thermoplastic starch/urea HA-NPs had a slower release of urea and reduced NH₃ volatilization when compared with pure urea (Giroto et al. 2017). The agronomic effect of these materials was assessed in a field trial and showed increased rice yield by 8% while having only half the N demand of pure urea, which was an improvement associated with the slower urea release and a 30% increase in N agronomic use efficiency (Kottegoda et al. 2017).

Nanoclays are hydrophilic inorganic compounds composed of thin-layered silicates that are a few nanometers in thickness and can be up to several 1000 nm in length. Nanoclays are the most regularly used, contain the widest range of materials, and are separated into anionic and cationic nutrient carriers. Anion-exchange nanoclays are favorable material carriers for phosphate, nitrate, and borate (Dorante 2007; Songkhum et al. 2018; Bernardo et al. 2018), whereas cationic exchange nanoclays carry nutrients such as montmorillonite, zeolites, and kaolinite (Pereira et al. 2012; Roshanravan et al. 2015; Lateef et al. 2016). Nanoclays potentially protect and sustain nutrients for longer periods via physical barriers created through ion exchange and hydrogen bonding, which enhance plant growth and minimize environmental contamination.

Mesoporous silica NPs have the potential to improve crop quality and support sustainable agriculture through their remarkable properties, which include large surface areas, mesoporous structures, biocompatibility, and nontoxicity. Silica NPs used to reduce heavy metal toxicity, dehydration, and salinity stress; they can release nutrients on demand, which can mitigate soil pollution and plant stress. Si-NPs may directly interact with and influence plants in various ways to improve plant growth and development, including their impact on plant growth during salinity stress (Abdel-Haliem et al. 2017), survival of rice cells under cadmium toxicity (Cui et al. 2017), and alleviation of drought stress (Zhang et al. 2018).

Carbon-based nanomaterials such as fullerenes, fullerols, carbon NPs, and carbon nanotubes (CNTs) have gained importance as plant growth regulators by enhancing germination rate, chlorophyll and protein content, and shoot and root lengths. For instance, polyhydroxy fullerene accelerates barley root elongation (Panova et al. 2016) and fullerol promotes chlorophyll synthesis in Triticum aestivum (Wang et al. 2016). Similarly, enhanced root and shoot lengths were observed in wheat plants treated with carbon NPs (Saxena et al. 2014), and multi-walled CNTs enhanced fruit production in hydroponically exposed tomato plants (McGehee et al. 2017).

Polymeric NPs use natural, biodegradable, and agriculturally benign carriers, such as chitosan, as alternative fertilizer compounds (Abdel-Aziz et al. 2016). Chitosan is a promising agrochemical carrier because of its polymeric cationic characteristics and can interact with negatively charged molecules or polymers. NPK-loaded chitosan NPs enhanced wheat growth, yield (Abdel-Aziz et al. 2016), nutrient uptake, photosynthesis, and growth in coffee plants (Ha et al. 2019). Chitosan NPs produce various metabolites for better growth and production in barley plants grown under drought stress (Behboudi et al. 2018), and the foliar application of chitosan-NPK fertilizer enhances growth, productivity, and the chemical composition of tomato plants (El-Tantawy 2009).

Nanofertilizer formulations and manufacturers

NFs produced using urea, ammonia, plant waste, seaweeds, peat moss, and other synthetic materials. For instance, (1) urea combined with calcium cyanamide (2) urea mixed with other biofertilizers, (3) nanoemulsions created by adding nanoforms of colloids to emulsions, (4) peat moss, ammonium humate, and other synthetic materials are generally used to prepare NFs. NFs also developed using both mechanical and biochemical processes, i.e., bulk materials are milling, crushing or grinding to nanosized materials to prepare effective NFs that slowly release the required nutrients for longer periods.

The surface polymeric coating selected for the particles protects the nutrients against homo- and hetero-aggregation, so that the NPs are unable to attach to soil surfaces and can enter the rhizosphere and improve the efficiency of crops. A list of NFs approved for worldwide use, as well as their respective constituents and manufacturers’ are presented in Table 1.

Table 1.

Approved nanofertilizers (NFs) used worldwide, their compositions, and manufacturers

(adapted from Elemike et al. 2019)

Nanofertilizers	Constituents	Manufacturer
Nano max NPK fertilizer	Amino acids, vitamins, probiotics, organic micronutrients, organic carbon and multiple organic acids chelated with major nutrients	JU Agri Sciences Pvt. Ltd., India
Nano fertilizer (Eco Star)	Organic matter, N, K, C, and N	Shan Maw Myae Trading Co., Ltd., India
Nano ultra-fertilizer	Organic matter, N, P, K, P, K, and Mg	SMTET Eco-technologies Co., Ltd., Taiwan
Plant nutrition powder (Green Nano)	Combination of N, P, K, Ca, Mg, S, Fe, Mn, Cu, and Zn	Green Organic World Co., Ltd., Thailand
Nano calcium (Magic Green)	Combination of Ca, Mg, Si, K, Na, P, Fe, Al, S, Ba, Mn, and Zn	AC International Network Co., Ltd., Germany
Hero super nano	Combination of N, P, K, Ca, Mg, and S	World Connect Plus Mayanmar Co., Ltd., Thailand
Nano green	Extracts of corn, grain, soybeans, potatoes, coconut, and palm	Nano Green Sciences, Inc., India
Supplementary powder (The Best Nano)	Combination of N, P, K, Ca, Mg, S, Fe, Mn, Cu, and Zn	The Best International Network Co., Ltd., Thailand
PPC nano	Combination of M protein, N, P, K and diluent	WAI International Development Co., Ltd., Malaysia
TAG NANO fertilizers	Proteino–lacto–gluconate chelated with micronutrients, vitamins, probiotics, seaweed extracts and humic acid	Tropical Agrosystem India (P) Ltd., India
Biozar nano-fertilizer	Combination of organic materials, micronutrients and macromolecules	Fanavar Nano-Pazhoohesh Markazi Company, Iran
Nano capsule	Combination of N, P, K, Ca, Mg, S, Fe, Mn, Cu, and Zn	The Best International Network Co., Ltd., Thailand
IFFCO nanofertilizer	Nano N—potential to cut the requirement of urea by 50%	Indian Farmers Fertilizer Cooperative Ltd., India
	Nano Zn—10 gm would be sufficient for a hectare of land
	Nano Cu—provides both nutrition and protection to the plant

Role of nanofertilizers

A number of inorganic, organic, and composite NP-based fertilizers have been applied to various plants to assess their potential application in plant growth and productivity (Liu and Lal 2015). These improve the quality of nutrients, soil, and environment, as well as the water holding capacity and antimicrobial activity of the plants (Michalik and Wandzik 2020). Si-based fertilizers such as Si dioxide NPs can increase plant resistance to disease, nitrate reductase activity, water and fertilizer absorption capacity, and improve seedling and root development (Wang et al. 2010). Ti-based fertilizers, such as Ti dioxide NPs, increase water retention, light absorption, photo energy transmission, and plant growth (Lu et al. 2002). Zn oxide NP fertilization significantly enhances growth and biomass production (de la Rosa et al. 2013), while Ag NPs significantly increase seed germination potential (Hojjat and Kamyab 2017). Fe and Fe oxide (Fe₂O₃) NPs enhance photosynthesis rates, chlorophyll content, and biomass, and improve plant growth (Ghafari and Razmjoo 2013; Li et al. 2013; Feng et al. 2013).

Carbon-based NPs such as CNTs, multi-walled CNTs, and single-walled CNTs have been shown to promote growth, biomass, root elongation, crop yield, and seed quality (Khodakovskaya et al. 2012; Lahiani et al. 2016). Similarly, Mg oxide NPs improve biomass, chlorophyll content, and phenological growth (Raliya et al. 2014), Mn NPs improve N uptake and metabolism (Pradhan et al. 2014), and Cu-chitosan enhances seedling growth and plant biomass (Saharan et al. 2016). Ce oxide NPs improve plant yield and growth (Wang et al. 2012), In oxide NPs improve physiological and molecular responses (Ma et al. 2016), and Co ferrite NPs promote root growth (López-Moreno et al. 2016).

Some previous studies have shown that SRFs fabricated as a polymer hydrogel by combining hydroxyapatite NPs and urea increase the yield of maize, kale, and capsicum (Rop et al. 2018). Plants treated with silica NPs and NPK fertilizers combined with carboxymethyl cellulose and acrylic acid in the presence of polyvinylpyrrolidone showed an appropriate pH, salt sensitivity, and water retention in loamy sand soil (Olad et al. 2018). Similarly, hydrogels prepared from cellulose acetate as a substrate (Senna et al. 2015) and corncob biochar-based nanocomposite fertilizer are also suitable for sustainable agriculture (Lateef et al. 2019). Nano-sized zeolite is prepared as a carrier for the retention and slow release of Zn for the fertilization of crops (Yuvaraj and Subramanian 2018). Higher yield production with less fertilizer than conventional methods is achieved using NPK NFs on cotton (Sohair et al. 2018). A foliar application of nano-Zn fertilizer influenced plant growth and yield under stress conditions in cotton plants (Hussein and Abou-Baker 2018). Based on the nutrient categorization, NFs are classified as macronutrient and micronutrient NFs, with both enhancing the efficiency of the base fertilizer and improving crop growth and yield. Some nanomaterials that act as potential NFs that influence plant growth and development are summarized in Table 2.

Table 2.

Summary of different nanomaterials that influence plant growth and development

Nanomaterials	Size (nm)	Plants	Application	Benefits	References
Ag	21	Fenugreek	Seed	Promote plant growth	Jasim et al. (2017)
	22	Common bean	Foliar	Increase seed yield	El-Batal et al. (2016)
	25	P. deltoides A. thaliana	Seed	Enhance root elongation	Wang et al. (2013a)
Au	10	Cucumber	Seed	Enhance root elongation	Lin and Xing (2007)
Carbon (SWCNT)	5	Soybean	Root	Enhance seed germination and cell growth	Lahiani et al. (2015)
CeO2	25	Radish	Root	Increase fresh biomass and chlorophyll content	Gui et al. (2017)
	30	Tomato	Seed/Root	Improve growth and yield	Wang et al. (2013b)
	10–30	A. thaliana	Seed/root	Improve physiological and molecular response	López-Moreno et al. (2016)
CuO	20–30	Tomato	Root	Increase root length, chlorophyll and sugar contents	Singh et al. (2017)
	50	Maize	Root/foliar	Enhance maize growth	Adhikari et al. (2016)
	10	Corn	Seed/foliar	Enhance seedling growth and plant biomass	Saharan et al. (2016)
Fe2O3	50	Spinach	Root	Increase plant biomass	Jeyasubramanian et al. (2016)
	10–50	Peanut	Root	Increase growth and biomass	Rui et al. (2016)
	10–100	Rice	Foliar	Enhance root elongation	Alidoust and Isoda (2014)
FeS	82	Mustard	Foliar	Improve growth and yield	Rawat et al. (2017)
In2O3	20–70	A. thaliana	Seed/root	Improve physiological and molecular response	López-Moreno et al. (2016)
MgO	10	Clusterbean	Foliar	Increase fresh biomass and chlorophyll content	Pradhan et al. (2014)
MnO	20	Mungbean	Seed	Improve nitrogen uptake and metabolism	Saharan et al. (2016)
Nitrogen (HA)	200	Rice	Root	Improve yield	Kottegoda et al. (2017)
Phosphorus (Zn induced P)	10–100	Cotton	Root/foliar	Increase growth and biomass	Venkatachalam et al. (2017)
SiO2	12	Tomato	Seed	Enhance seed germination	Siddiqui and Al-Whaibi (2014)
	n/a	Cucumber	Foliar	Increase plant height, leaf size, yield and biomass	Yassen et al. (2017)
	n/a	Strawberry	Root/foliar	Increase the uptake of nutrients K, Ca, Mg, Fe, Mn and Si	Yousefi and Esna-ashari (2017)
TiO2	25	Barley	Root	Increase vegetative growth	Marchiol et al. (2016)
	30–50	Tomato	Root	Enhance vegetative growth	Tiwari et al. (2017)
	12–15	Mungbean	Foliar	Increase root length, and chlorophyll content	Raliya et al. (2015)
ZnO	35	Tomato	Root	Increase root and shoot length	Faizan et al. (2018)
	90	Lettuce	Root	Increase biomass and photosynthetic rate	Xu et al. (2018)
	23	Mungbean	Foliar	Increase plant height and photosynthetic rate	Raliya et al. (2016)

n/a values not reported

Macronutrient nanofertilizers

Macronutrient NFs (such as N, P, K, Mg, S, and Ca) have been encapsulated within specific nanomaterials to deliver an accurate amount of nutrients to plants when supplementation is required. Large quantities of macronutrient fertilizers (mainly N, P, and K fertilizers), which are known as N–P–K fertilizers or compound fertilizers, are intentionally mixed to increase the production of food, fiber, and other raw materials. The total worldwide consumption of macronutrient fertilizers (N + P₂O₅ + K₂O) is projected to increase to 263 Mt by 2050 (Alexandratos and Bruinsma 2012).

N is a major component of plant cells required by structural, genetic, metabolic, and chlorophyll (photosynthesis) compounds. Due to their high solubility, leaching, and denitrification, a wide range of SRFs, such as montmorillonite, zeolite, bentonite, and halloysite, have been developed using synthetic polymers or biopolymers. Many approaches, such as sulfur-, neem-, and polyolefin resin-coated urea, have been used to control the release of N to reduce leaching during fertilization. Urea–hydroxyapatite nanohybrid fertilizers release N slowly and uniformly to improve plant growth and development compared to traditional bulk fertilizers (Kottegoda et al. 2011). Recently, polymer-coated urea was used as a slow-release N fertilizer to improve crop quality, yield, and water-fertilizer productivity, as well as to reduce environmental risks associated with soil N (Li et al. 2017). Porous nanomaterials, such as zeolites, clay, and chitosan, significantly reduce N loss by carrying out a demand-based release of N and by increasing plant N uptake (Abdel-Aziz et al. 2016).

More soluble P fertilizers, such as single super phosphate or triple super phosphate, pollute water bodies, which leads to eutrophication and the loss of aquatic species. Because of these negative environmental effects of extremely soluble fertilizers, a nanoscale P fertilizer has a more positive physical impact on crop development when its solubility and mobility are reduced. Recently, the uptake and concentration of P NFs in Ipomoea aquatica (Kalmi) was reported to be better than that of CFs (Rajonee et al. 2017). Similarly, the application of nano-zeolite-P to peanut crops increased crop productivity and minimized pollution hazards compared to the use of other nanomaterials (Hagab et al. 2018). Water-soluble P fertilizer and nano-hydroxyapatite incorporated into a water hyacinth cellulose–graft–poly (acrylamide) polymer hydrogel were shown to reduce leaching and toxicity in plant roots (Rop et al. 2018). Similar results from synthesized hydroxyapatite [Ca₅(PO₄)₃OH] NF indicated increased growth rates and seed yield in soybean when compared to regular P fertilizer [Ca(H₂PO₄)₂] (Liu and Lal 2014).

K is also essential for photosynthesis, photosynthate translocation, protein synthesis, ionic balance regulation, regulation of plant stomata, water use, enzyme activation, and many other processes. A recent study showed that spraying leaves with Lithovit supplemented with nano-K fertilizer enhanced the growth, yield, quality, and quantity of sweet pepper plants (El-All 2019). Similarly, the foliar treatment of wheat plants with chitosan NPs loaded with K significantly increased plant growth and productivity (Abdel-Aziz et al. 2016). K NF was prepared by incorporating K into an alginate-chitosan carrier by ionotropic pre-gelation to provide SRFs or CRFs using a two-level factorial design and the K to alginate ratio and Ca chloride to alginate ratio (Nido et al. 2019).

Mg, S, and Ca are secondary nutrients required by plants in approximately the same amounts as P. Mg is a key player in chlorophyll synthesis, protein synthesis, and enzyme activation for good crop growth. Mg NFs are prepared from zeolite nanocomposites with slow-release properties and can freely exchange Mg ions (Fansuri et al. 2008). Compared to the conventional Mg salt, Mg-NPs improved the uptake of Mg nutrients in the stems and leaves of Mg-deficient tomato plants (Solanum lycopersicum) by increasing their availability and mobility (Karny et al. 2018).

S is essential for the production of chlorophyll and the utilization of P and other essential nutrients. Moreover, S NPs could help prevent a wide range of crop diseases and increase the growth and productivity of Cucurbita pepo (Salem et al. 2015). The stability of the S nano-coated NPs reduced the rate of fertilizer decomposition and allowed for the slow, sustained release of the S-coated fertilizer (Subramanian et al. 2015; Manjunatha et al. 2016; Subramanian and Thirunavukkarasu 2017).

A biosynthesized nano Ca fertilizer increased shoot biomass and nutrient content in the roots of peanut plants compared to the application of CF (Liu et al. 2005). Similarly, the foliar application of nano-CaO fertilizer to Ca-deficient peanut plants enhanced Ca accumulation and root development when compared to bulk CaNO₃ and CaO treatments (Deepa et al. 2015). In addition, CaCO₃ NPs improved shoot and root growth and fresh biomass production in Vigna mungo plants (Yugandhar and Savithramma 2013) when compared to plants treated with conventional Ca. These macronutrient NFs reduced environmental risks to soil and irrigation water consumption while still sustaining normal growth and fruit yield, making their use extremely beneficial. Table 3 presents the different types of macronutrient NFs based on their applications and agronomic findings in comparison to bulk/salt macronutrients.

Table 3.

Effects of macronutrient NFs on crop systems compared to bulk/salt macronutrients

Nanofertilizers	Bulk/salt	Application	Benefits	References
Ca	Ca(NO3)2	Foliar	Increase shoot biomass and nutrient content	Yugandhar and Savithramma (2013)
CaCO3	CaCl2	Seed treatment	Improve root and shoot growth, and fresh biomass production	Yugandhar and Savithramma (2013)
CaO	CaO, CaNO3	Foliar	Increase Ca accumulation in Ca deficient plants, and promotion of root development	Deepa et al. (2015)
HAP	TSP	Root	Enhance growth, biomass and yield production	Bala et al. (2014)
HAP	MAP, DAP	Foliar	Enhance growth and biomass production, increase NPK contents, enhance vegetative and metabolic parameters	Soliman et al. (2016)
K	Water treatment	Soil	Increase yield and reduce K leaching	Taran et al. (2014)
K	N/A	Foliar	Increase dry matter yield and flower numbers	Amirnia et al. (2014)
Mg	MgO	Soil, foliar	Promotion of photosynthesis, growth effect and yield	Delfani et al. (2014)
MgO	N/A	Perlite infested	Protect wilt infestation	Imada et al. (2016)
N	N	Soil	Enhance plant growth and reduce N leaching	Malekian et al. (2011)
N (biosynthesis)	N/A	Soil	Enhance N and P metabolizing soil microbes, and increase biomass production	Thomas et al. (2016)
NPK-chitosan	NPK	Soil, foliar	Enhance vegetative growth, reduce crop life cycle, and increase yield	Abdel-Aziz et al. (2016)
P	Ca(H2PO4)2	Soil	Increase growth, yield and biomass	Liu and Lal (2014)
P	KH2PO4	Soil	Greater physiological efficiency of shoots and roots	Miranda-Villagómez et al. (2019)
S	N/A	Soil	Enhances the growth of roots and stems	Salem et al. (2015)
Urea	Urea	Soil	Reduction in N2O emission from urea	Kundu et al. (2016)
Urea–HAP	Urea	Soil	Slow-release of N	Kottegoda et al. (2011)
Urea–HAP	Urea	Soil	Slow-release of N, enhance grain yield, and N, K contents in leaf	Kottegoda et al. (2017)
Urea–nanoclay	Urea	N/A	Release rate of N is slowed	Pereira et al. (2012)
Urea–nanoclay–polymer	Urea	Mixed in water and extruded	Slow-release of N and reduce N2O emission from urea	Pereira et al. (2015)

n/a values not reported

Micronutrient nanofertilizers

Micronutrients include Fe, B, Mn, Zn, Cu, Cl, and Mo, which are essential nutrients required by plants in smaller amounts than N, P, and K. However, they are involved in important processes, such as the synthesis of carbohydrates, nutrient regulation, protein metabolism, reproductive growth, regulation of auxins, chlorophyll synthesis, fruit and seed development, and provide defences against harmful plant pathogens (Tripathi et al. 2015). Normally, micronutrient deficiency is a common problem for crop production, especially in semi-arid areas, where the uptake of micronutrients by roots is less due to alkaline pH, low soil moisture, coarse texture, or low soil organic matter and degraded soils (Graham 2008).

Among the micronutrients, Zn and Fe mineral deficiencies are the most common, especially in plants grown under saline conditions with high pH values. Fe is an essential nutrient for all plants and is involved in many physiological processes, such as respiration, chlorophyll biosynthesis, and redox reactions. Fe₂O₃ NPs applied to the soil were found to increase the biomass, chlorophyll content, and total Fe content in peanut plants and can replace conventional Fe fertilizers (Rui et al. 2016). Similarly, Fe₂O₃ NPs significantly increase growth parameters, photosynthetic pigments, and total protein content in Catharanthus roseus plants compared to conventional Fe supplements (Askary et al. 2017). Fe NFs can support slow and continuous Fe transport from the roots to the shoots, which had a long-term effect on soybean plants (Cieschi et al. 2019). Four different legume species treated with a nano-hematite fertilizer exhibited faster growth, twice as many fruits per plant, and a longer life span compared to the control plants (Boutchuen et al. 2019). The use of nanoscale nutrients can increase the bioavailability of these elements to improve plant metabolism and thereby enhance nutritional quality, growth, and development in crop plants (Dimkpa et al. 2017a). The application of a Fe₂O₃ NF increased the chlorophyll content, number of branches, and root dry biomass in Pisum sativum (Delfani et al. 2014) and Glycine max (Cieschi et al. 2019) compared to CFs.

Mn fertilizers enhance N metabolism (Pradhan et al. 2014), increase the yield of chickpea production systems in semiarid regions (Sabaghnia and Janmohammadi 2016), and promote growth and yield parameters of peanut under sandy soil conditions (Rui et al. 2016). Foliar application of Mn NPs increased root and shoot length, the number of rootlets, and the biomass, and improved the growth and yield of mung bean (Pradhan et al. 2013) and squash plants (Shebl et al. 2019), respectively, compared to the bulk Mn sulfate (MnSO₄) treatment. However, B is involved in the biosynthesis of plant cell walls, lignification, plant growth, and several other physiological processes (Broadley et al. 2011). Hence, it is necessary to apply precise amounts of B to crops to improve yield without sacrificing quality. The foliar application of B NF showed promising results when tested with three different concentrations in pomegranate trees; low amounts of B increased fruit yield by 30% compared to the conventional fertilization method (Korkmaz and Akin 2015).

Similarly, the application of Cu oxide (CuO) and Zn oxide (ZnO) NPs to maize plants significantly increased the growth and shoot dry weight, respectively, compared with the untreated control (Adhikari et al. 2015, 2016). The uptake of N, K, Zn, and B remarkably increased when ZnO, CuO, and B oxide (B₂O₃) NPs were used as fertilizers under drought conditions (Dimkpa et al. 2017b). Root and foliar application of Zn NFs, or Zn, Cu, and B NFs increased grain yield in sorghum and soybean plants (Dimkpa et al. 2017b, 2019).

Mo is an essential component of plant enzymes such as nitrate reductase and the N-fixing bacteria enzyme nitrogenase, which are essential in legume crops (Zakikhani et al. 2014). Mo fertilizers enhance the stability of the mesophyll cell wall and membrane and promote the growth and development of Chamaecrista rotundifolia (Weng et al. 2009). Cl is also an essential micronutrient in higher plants that occurs predominantly as Cl⁻ in the soil and plants, wherein it participates in several physiological and metabolic processes. Plants require Cl in very small quantities for osmotic and stomatal regulation, photosynthesis, and disease resistance and tolerance. Cl improves the growth, development, yield, and quality of many crops, such as onions and cotton, when the soil is deficient in this nutrient; however, excessive Cl can be a major contributor to salinity stress and be toxic to plants (Chen et al. 2010). The effects of micronutrient NFs on crops in comparison with their bulk counterparts are summarized in Table 4.

Table 4.

Effects of micronutrient nanofertilizers on crops in comparisons to bulk/salt micronutrients

Nanofertilizers	Bulk/salt	Application	Agronomic findings	References
B	B	Soil	Improved grain yield	Nouraein (2019)
Cu (bacteria-mediated)	Cu	pH 4.5–5.3, Root-rot infestation, foliar	Reduction of disease incidence, enhancement of yield under disease condition	Ponmurugan et al. (2016)
Cu, CuO	Cu, CuO, CuCl2	Soil	Reduced germination, stimulated root, shoot, and biomass	Zuverza-Mena et al. (2015)
CuO	CuO	Foliar	No reduction in yield compared to control, compromised fruit quality	Hong et al. (2016)
CuO	CuSO4	Nutrient solution	Increased growth and enzymatic activities	Adhikari et al. (2016)
CuO/MnO/ZnO	CuO/MnO/ZnO and Cu, Mn	pH 6.1, Sandy loam, soil infested with Verticillium wilt fungus	Stimulation of plant defense against fungal disease, yield stimulation by CuO	Elmer and White (2016)
FeO	Fe	Nutrient solution	Increased in seed mass	Delfani et al. (2014)
FeO	Fe	Nutrient solution	Increased chlorophyll content	Ghafariyan et al. (2013)
Fe2O3	Fe-EDTA	pH 8.1, soil	Increased growth, biomass and Zn content	Rui et al. (2016)
Mn	MnSO4	Nutrient solution	Increased root, shoot and biomass	Pradhan et al. (2013)
ZnO	ZnSO4	Nutrient solution	Improved plant height, root length and dry matter	Adhikari et al. (2015)
ZnO	ZnSO4	Soil	Promotes grain yield and nutrient acquisition	Dimkpa et al. (2018)
ZnO	ZnCl2	Soil	Increased seed yield and lipid peroxidation	Yusefi-Tanha et al. (2020)
ZnO	ZnO, ZnCl2	pH 7.48, Soil	Increased soil pH, toxicity in wheat, radish, vetch, enhanced Zn in shoot	García-Gómez et al. (2015)
ZnO	ZnO, Zn salt	pH 8.2, loamy soil	Dose-dependent effect, stimulatory at 100–200 mg/kg and toxic at more than 800 mg/kg	Liu et al. (2015)
ZnO	Zn2SO4, ZnO, ZnCO3	pH 6.4, arbuscular mycorrhiza (AM), soil	Nontoxic, no effect on AM colonization of plant root, no biomass reduced	Watts-Williams et al. (2014)
ZnO	ZnO, Zn salt	pH 7.8, loamy soil	Less inhibitory, stimulates growth but not germination	Bandyopadhyay et al. (2015)
ZnO	ZnO, Zn2SO4	Suspensions, perlite	Less inhibited rapeseed	Kouhi et al. (2015)
ZnO	ZnSO4	pH 6.4, sandy clay loam soil, foliar	Promoted germination, yield and grain Zn content	Subbaiah et al. (2016)
ZnO (fungus-mediated)	ZnO	pH 8.1, sandy soil, foliar	Enhanced growth and biomass	Raliya and Tarafdar (2013)
ZnO (fungus-mediated)	ZnO	pH 8.1, sandy soil, foliar	Increased root, shoot and nodule development, increased P and Zn uptake	Raliya et al. (2016)
ZnO/surface coatings	ZnO, Zn salt	Loamy soil	Increased biomass production	Mukherjee et al. (2016)
ZnO/CuO/B2O3	ZnSO4/CuSO4/H3BO3	pH 6.8, sandy loam soil, foliar	Increased growth, yield and grain, accumulation of N, K. Zn and B under drought	Dimkpa et al. (2017b)

Controlled/slow release fertilizers

Fertilizers coated with NPs provide nutrients to plants slowly, thereby reducing nutrient loss; as such, they are considered promising alternatives to conventional fertilizing methods (Zhong et al. 2013). NPs have been developed for the precise, slow, and efficient delivery of fertilizers to plants (Tarafdar et al. 2012), increasing the availability of their nutrients to nanoscale plant pores. SRFs and CRFs have become green fertilizers as ecofriendly for agriculture and have been developed to reduce fertilizer consumption and minimize environmental contamination and pollution without compromising high yields.

SRFs/CRFs have been designed to possess all the essential properties of fertilizers, such as high solubility, stability, efficacy, controlled release, enhanced targeted activity, and protection from photolysis and hydrolysis (Qureshi et al. 2018). Several types of SRFs have been developed and tested, including biochar- (Gwenzi et al. 2018) and nano-zeolite-based SRFs (Lateef et al. 2016), and those coated with natural polymers such as chitosan, starch, and cellulose (Ramli 2019). In addition, nanoclays such as halloysite, montmorillonite, and bentonite are used as carriers to achieve the sustained release of active nutrients. However, zeolite and biochar-based materials from natural sources have attracted attention owing to the growing need for cost-effective and environmentally friendly products (Xie et al. 2015).

SRFs have a slower release of nutrients than CFs, but the rate, pattern, and duration of nutrient release are fully dependent on climate, soil temperature, and moisture conditions. Recently, SRFs prepared with cellulose-graft-polyacrylamide/hydroxyapatite composites have been shown to successfully synchronize the release of NPK fertilizers into water hyacinth (Rop et al. 2018), and NPK fertilizers encapsulated in a carboxymethyl cellulose-based nanocomposite in the presence of polyvinylpyrrolidone have been shown to have a water retention function in soil and excellent slow release of nutrients (Olad et al. 2018). Similarly, biochar-based nanocomposites improve soil fertility and provide nutrients for a longer period by reducing environmental issues such as eutrophication and runoff (Lateef et al. 2019). Together, these reports show that nano-zeolite fertilizers allow the slow release of Zn fertilizer and are considered economical, viable, and eco-friendly materials for sustainable agriculture (Yuvaraj and Subramanian 2018).

Environmental issues associated with phosphorus fertilizer

Proper management of P is an important part of agricultural systems developed to protect the environment, particularly those that use compost or manure as nutrient sources, which can disturb water bodies and soil structure (Tilman et al. 2002a, b). For example, the heavy use of N and P fertilizers is mainly responsible for the eutrophication and quality degradation of surface freshwater bodies and coastal ecosystems (Correll 1998). Eutrophication is the hyper-fertilization of lakes or streams by nutrient (P and N) enrichment, which increases algal growth and oxygen depletion in water columns and seabeds. The runoff of these nutrients into freshwater systems can stimulate a burst of algal or plankton growth and often induce a shift in the dominant algal species. In addition, biomass decomposition reduces the concentration of dissolved oxygen and leads to eventual damage to aquatic biota.

During eutrophication, P in the water column can stimulate algal growth, which can suddenly cover the surface of the water with floating algae and turn the water soupy-green with algal scum. Extreme algal growth blocks the light required for the growth of aquatic plants, such as seagrasses. When aquatic weeds and algae die, they sink to the seabed where their decomposition by microorganisms uses up the dissolved oxygen in the water, further limiting organismal growth (Weil and Brady 2017). Without sufficient dissolved oxygen in surface waters, fish and other aquatic species suffocate, and these dead aquatic species degrade water quality. Organic production systems that use manure and compost as their primary N source are likely to have excess P accumulation in the soil, which can contribute to waterway eutrophication.

P inputs into the environment have been increasing since the 1950s. Some of them originate from point sources, such as discharges from wastewater treatment plants, and non-point sources, such as runoff from soil erosion, manure from farms, rainfall erosion of agricultural fields, discharge from phosphate-containing detergents, fertilized lawns, and urban gardens. Much of the P in these effluents is soluble in water, and therefore, immediately available to aquatic animals and plants. Most of the P from soils transported to water is from eroded soil particles enriched with P (P particulates) or from excessive amounts of fertilizer or animal manure applied to soil during unsuitable growing conditions. P enters the water directly through deposited animal feces and urine that reach streams by overland water flow as well as through P-enriched soil eroded at the access points used by the animals. Additionally, cattle that enter streams can also disturb the streambed and stream-bank sediments, which can lead to the release of stored P.

Phosphorus deficiency in plants

P deficiency is widespread in tropical and subtropical soils, which are predominantly acidic and have a high P sorption capacity. Deficiency of P is more difficult to diagnose than that of N or K because of similar symptoms concerning the color, shape, and texture of the leaves. Crops usually do not display clear symptoms of P deficiency other than general stunting and the observation of thin-stemmed or spindly plants that develop dark rather than pale pigmentation that is almost bluish-green during early growth. Many plants develop purple colors in their leaves and stems as a result of P deficiency due to sugars that favor the synthesis of anthocyanin, which accumulates in the leaves of the plant (van de Wiel et al. 2016). P is a highly mobile element within plants and may be transported from old to young plant tissues when it is deficient, especially to those tissues that are actively growing (Weil and Brady 2017). Hence, early vegetative responses to P are frequently observed and as the plant matures, P is translocated into the fruiting parts of the plant where the formation of seeds and fruit requires more energy. A P-deficient plant usually presents decreased leaf number and leaf blade length, poor seed development, reduced seeds per panicle, and delayed maturity. Severe P deficiency causes yellowing and senescence of leaves of some crops, such as corn, show abnormal discoloration (Rop et al. 2019). P deficiency in maize and kale plants results in yellowish and purplish coloration in low leaves that subsequently become necrotic starting at the leaf tip (Rop et al. 2019).

Advantages of nanofertilizers over conventional fertilizers

CFs are usually applied to crops by either foliar spraying or soil dispersal. However, the factors that determine the mode of application are the final concentration or quantity of fertilizer that needs to reach the plant. Indeed, a very low concentration of the CF reaches the targeted site due to leaching of chemicals, runoff, drift, evaporation, hydrolysis by soil moisture, and photolytic or even microbial degradation (Manjunatha et al. 2019). In contrast, NFs reduce the requirement for CFs and increase soil fertility, yield, and the quality parameters of crops. They are also non-toxic and less harmful to humans, promote drought- and disease-resistant crops, and minimize crop costs while maximizing profits (Shang et al. 2019).

CFs play an important role in increasing soil fertility and crop productivity. However, excessive use of these fertilizers over long periods can reduce soil organic matter content, with a concomitant decrease in soil quality and increased soil acidification and pollution (Guo et al. 2010). Approximately 40–70% of N, 80–90% of P, and 50–90% of K content of applied CFs are lost to the environment and do not reach plants, which causes unsustainable and economic losses (Duhan et al. 2017). All these problems encourage the repeated application of fertilizers, which adversely affects the soil nutrient balance, and the native flora and fauna, and increases environmental pollution (Manjunatha et al. 2019). The optimization of the use of CFs is crucial to fulfill the requirements of crop nutrition and reduce the risk of environmental pollution. Moreover, the repeated use of CFs can also increase pathogen and pest resistance, reduce soil microflora and N fixation, and promote the accumulation of pesticides (Tilman et al. 2002b).

NFs are also nutrient fertilizers composed of a nanostructured formulation that either completely or partially deliver nutrients to plants for the uptake or slow release of active ingredients. Various engineered NPs have different sizes, morphologies, and properties that determine the level of their bio-accessibility to plants. Nanomaterials that have the same atoms as known bulk materials may have different physicochemical properties. The properties of the functionalized surface of nanomaterials differ from those of the bulk or core of the particles owing to uncoordinated bonds (Rasmussen et al. 2018). Additionally, NFs supply the plant with nutrients either by encapsulating them inside nanomaterials or nano-porous materials or by coating nutrients with a thin polymer film. Recently, NFs have been shown to fulfill plant root requirements, enhance plant growth by promoting disease resistance, improve the stability of the plants, and encourage deeper rooting of the crops, compared to CFs (Singh 2017).

Phosphorus nanofertilizers

Although P fertilizers is supplied as orthophosphate (H₂PO₄^˗), the P content is generally expressed as the weight percentage of P pentoxide (P₂O₅) or, incorrectly, as phosphoric acid. There are many commercially available P fertilizers, such as rock phosphate, calcium orthophosphates, phosphoric acid, ammonium phosphates, ammonium polyphosphate, and nitric phosphates. The use of P NFs as an alternative to P CFs to improve the surface water quality in agricultural lands would increase production, regulate the use of P, and increase the effective absorption of nutritional elements (Liu and Lal 2014). In addition, NFs also avoid environmental pollution, increase the efficiency of different water and land resources, and increase the contents of essential compounds required for plant growth and metabolism in modern agricultural systems (Morteza et al. 2013).

Water-soluble phosphate salts, such as MAP, DAP, and TSP, are commercially available as P fertilizers, dissolve into the soil solution, and are rapidly available for plant uptake (Fageria 2008). However, these soluble phosphates often end up in surface water bodies because of runoff, where they can cause eutrophication, leaching, and ground water contamination. Rock phosphate is the raw material for the production of P fertilizer, where the phosphate locked in solid form with limited solubility, being less available to algae and less easily removed by runoff or soil erosion. Nonetheless, these rock phosphates are not efficient at delivering P to plants as they present limited mobility in the soil due to their large particle size, which prevents P from reaching and supporting crops (Liu and Lal 2014). For these reasons, rock phosphate is acidulated or treated with sulfuric, phosphoric, or nitric acid to produce P fertilizers. When a granular P fertilizer added to the soil, it quickly dissolves and releases orthophosphate ions into the soil solution. These ions react with Ca and other ions that form P compounds that are less available to plants over a long time. Hence, the development of new fertilizers is likely to assist or strengthen agricultural production and simultaneously decrease unfavorable environmental outcomes. Therefore, the utilization of nanomaterials for fertilizer delivery offers a potentially novel approach to sustainable crop cultivation. The NP-based P fertilizer has three major advantages over conventional P fertilizers: (1) the slow release of P compared with CFs, (2) it does not change soil pH upon P release, and (3) there is limited P loss from the soil. The slow and steady release of P allows plants to continuously take up nutrients as they grow, providing nutrition support throughout plant development.

Generally, an increased and efficient use of synthetic P fertilizers, such as MAP, DAP, or TSP, causes technical, economic, and environmental problems. Nanomaterials improve the productivity of crops and efficiently regulate the delivery of P nutrients to plants and targeted sites, guaranteeing the minimal usage of agrochemicals. P NFs are prepared by encapsulating P nutrients into nanomaterials or by applying a thin coating of nanomaterials on P fertilizers, and are delivered in the form of nano-sized emulsions (Iqbal 2019). Many agri-nanotechnology researchers are looking for opportunities to enhance or improve P uptake efficiency, the controlled release of P, long-term stability of plant-usable P forms, and the mobilization of native P to plants (Raliya et al. 2018). Some studies have demonstrated that P NFs can be more effective than CFs.

Recently, carboxymethyl cellulose-stabilized HA-NPs were prepared to investigate the growth rate and seed yield of Glycine max, which increased by 32 and 20%, respectively, compared to those of the plants treated with a regular P fertilizer (Liu and Lal 2014). In another study, the surfactant-modified zeolite acted as a good sorbent, carrier, and slow release system for P fertilizers (Bansiwal et al. 2006). The yields of pods, straw, and seeds of crops, and the oil content of peanut plants treated with nano-zeolite P fertilizers were higher than those of plants treated with normal zeolite P and the ordinary superphosphate fertilizer in field experiments (Hagab et al. 2018).

In a pot culture experiment with Ipomoea aquatic (Kalmi) plants, the addition of a nano-zeolite fertilizer incorporated with P increased the accumulation of P in plants, while exhibiting a better soil pH, which ranged from 6.0 to 7.0, and better moisture compared with the CF treatment (Rajonee et al. 2017). In a similar study, the uptake of P was greater in Triticum aestivum (wheat) treated with HA-NPs and TSP than in the control (bulk) treatment (Montalvo et al. 2015).

The foliar application of P NFs enhanced Gossypium barbadense L. (cotton) yield compared with the soil application method (EED et al. 2018). There are also reports of the use of nano-encapsulated slow-release P fertilizers to investigate the effectiveness of foliar application in the growth performance of Adansonia digitata, especially in sandy soil (Soliman et al. 2016). The authors reported that compared with the control (no fertilizer application), nano-encapsulated slow-release P fertilizers increased the quality and extent of plant growth, nutrition status, and the plant’s DPPH scavenger and anticancer activities (Soliman et al. 2016). Other experiments were conducted to evaluate the effect of P NFs on plant growth, photosynthesis-related parameters, P concentration, and accumulation, and found that P NFs stimulated growth and photosynthetic activity and increased the P content in rice plants (Miranda-Villagómez et al. 2019). A summary of studies investigating the efficacy of P NF on different crops is presented in Table 5.

Table 5.

Effects of phosphorus nanofertilizers on crops in comparisons to conventional fertilizers

Nanofertilizers	Bulk/salt	Plants	Agronomic findings	References
HA	Ca(H2PO4)2	Soybean	Increased biomass and yield production	Liu and Lal (2014)
HA	Commercial-P	Maize, kale, and capsicum	Higher dry matter and yields	Rop et al. (2019)
HA	MAP and DAP	Baobab	Improved growth parameters, and chemical compositions	Soliman et al. (2016)
HA	Bulk–hydroxyapatite	Wheat	More effect on shoot dry matter production and P uptake	Montalvo et al. (2015)
HA	(H3PO4-P)	Lettuce	Increased plant growth and P nutrition	Taşkın et al. (2018)
HA	Commercial-P	Okra	Increases the uptake ratio of nutrients in both soil and water	Tarafder et al. (2020)
KH2PO4	KH2PO4	Rice	Greater physiological and photosynthetic rate	Miranda-Villagómez et al. (2019)
P	Conventional-P	Egyptian cotton	Enhanced plant growth and yield	Sohair et al. (2018)
P	Commercial-P	Basil	Increased the fresh weight, P concentration, and chlorophyll content of plant tissues	Alipour (2016)
P	Commercial-P	Soybean	Increased soil organic matter, yield, nutrition and the contents of P	El-Saye et al. (2020)
P (chitosan)	Commercial-P	Wheat	Accelerating plant growth and productivity	Aziz et al. (2016)
Zeolite (porous)	Super Phosphate	Peanut	Increased nutrient contents and yield	Hagab et al. (2018)
Zeolite-KH2PO4	Fe-EDTA	Kalmi	Increased plant growth, uptake and concentration of P content	Rajonee et al. (2017)

Nanofertilizers for sustainable and precision farming

Sustainable agriculture demands minimal use of agrochemicals, and the development of an efficient plant nutrient system causing less damage to the environment is essential for this effort. Tropical and subtropical soils are mostly acidic and often greatly deficient in P with a high phosphate sorption capacity. Thus, nanotechnological and nanoengineering techniques are being used to overcome a global agricultural crisis by providing novel and advanced solutions (Kim et al. 2018) that aim to improve crop production and the efficiency of pesticide treatments, enable the development of efficient water management systems (Ram et al. 2014), and promote the use of NFs to ensure agricultural sustainability. The proper utilization of phosphate rocks as P sources can contribute to worldwide development by facilitating a sustainable agricultural supplement, particularly in developing countries that are covered with native phosphate rock resources, and help minimize pollution in countries where phosphate rocks are processed industrially.

The reaction of phosphoric acid with finely ground phosphate rocks yields TSP, and approximately 85–90% of P in superphosphates is water-soluble. Under certain tropical and subtropical soil conditions, the direct application of phosphate rocks has proved to be an economically sound alternative to the more expensive superphosphate. Nanotechnology plays an important role in plant productivity by controlling effective nutrient utilization, plant pathogen resistance, and reducing pesticide use, which supports the sustainable development of agriculture (Ram et al. 2014). NFs are designed to increase nutrient efficiency and consequently reduce the negative impact on the environment compared to CFs (Manjunatha et al. 2016). Nanocomposites and SRFs are suitable alternatives to soluble fertilizers because they release nutrients at a slower rate during crop growth, thereby reducing the requirement of excess nutrient load. A slow release of nutrients in the environment can be achieved by using zeolites, hydroxyapatite, chitosan, and metal NPs, which act as reservoirs for nutrients. (Manjunatha et al. 2016).

Hybrid materials or nanocomposites consist of a continuous (polymer) phase and a dispersed (nanofiller) phase that disperses a small amount of the nanomaterial. Nanocomposites show better performance with respect to several aspects compared to CFs due to the combination of a polymeric matrix with an inorganic nanomaterial. The improved performance of these materials is caused by the enhanced physical and mechanical properties such as greater strength, toughness and stiffness, higher pH tolerance, greater storage stability, heat distortion, and break elongation, better electrical and thermal conductivity, superior flame resistance, and a higher barrier to moisture and gases. Nanocomposites also have unique design possibilities that offer excellent advantages for creating functional materials with desired properties for specific applications. Current research focuses on the development of nanocomposites and hybrid nanomaterials that can supply essential nutrients to crops through smart delivery systems and release nutrients for crop uptake to prevent undesirable nutrient losses through soil leaching and volatilization (Bley et al. 2017).

In addition, other sustainable strategies for P use management include optimizing land use, preventing erosion, maintaining soil quality, developing better methods for fertilizer replacement, improving fertilizer recommendations, selecting the most suitable crop genotypes, and exporting manure. Finally, the processes and technologies for P recovery and reuse from waste streams and sludge are key drivers for improving resource efficiency. The future perspectives for the utilization of NFs as a source of plant nutrition for sustainable crop production involve multiple factors such as effective legislation, production of novel NF products, and the development of associated risk management protocols. There is clearly an urgent need for standardizing nanomaterial formulations and conducting field and greenhouse studies for performance evaluation. Finally, for sustainable crop production, smart NFs must be formulated to release nutrients according to the requirements of the plants in temporal and spatial dimensions.

Precision agriculture or precision farming includes a range of application techniques that aim to optimize crop production and crop protection to obtain the best results with targeted fertilizers. Precision agriculture has been a long-desired plan to maximize crop yields while minimizing chemical inputs, such as those of chemical fertilizers, pesticides, and herbicides, by monitoring environmental variables and undertaking targeted action. Regarding fertilizers and pesticides, tools that can measure crop needs and that account for inter- and intra-crop variability needed to adjust the amount of product delivered to the crop or to individual plants, are required. However, the costs of product inputs, such as those of fertilizers and pesticides, are expected to increase at an alarming rate due to limited reserves (Anjum et al. 2018). To overcome these issues, precision agriculture could reduce production costs and maximize output by supporting healthier crops and higher yields than those produced using conventional methods.

The use of nanomaterials to supply essential nutrients to crops through smart delivery systems is relatively new and requires further exploration. However, nanotechnology could potentially control the release of agrochemicals and site-targeted delivery of macromolecules, such as fertilizers and pesticides, needed to improve plant disease resistance, efficient nutrient utilization, and enhanced plant growth. These modern nanotechnology-based tools and products of precision agriculture include NFs, nano-pesticides, nano-herbicides, nano-sensors, nano-scale carriers, and tools for the detection of nutrient deficiencies, all of which have the potential to address the various issues of conventional agriculture (Anjum et al. 2018). In addition to these modernized methods, satellite-based navigation systems, such as remote sensing, global positioning systems, geographical information systems, and information technologies can be applied to ensure that crops and soils receive exactly what they need for optimum health and productivity (Yousefi and Razdari 2014).

Nanofertilizer toxicity

Although the use of NPs as fertilizers to increase the availability of plant nutrients and boost agricultural production is gaining attention, there are some associated toxicity issues. Indeed, the toxicity, safety, and impact of NPs on the environment are still uncertain because smaller particles have greater penetration capabilities in biological systems and therefore, greater potential risks that still need to be evaluated (Li et al. 2016). The inherent properties of many NPs are considered to pose potential risks to human health because of their size, shape, surface area and charge, solubility, crystal phases, coatings, material types, and dosage concentrations. In addition, environmental factors such as temperature, pH, ionic strength, salinity, and organic matter could collectively influence NP behavior, transport, and toxicity. Recent studies have revealed the adverse effects of NPs on soil organic matter dynamics because of different reaction conditions, soil properties, and dosages used in the experiments (Schlich and Hund-Rinke 2015; Rahmatpour et al. 2017; Shi et al. 2018).

NPs synthesized by chemical and physical methods have greater toxicity than those produced using biological methods; however, the toxicity of biologically synthesized NPs is still under intensive investigation. Metal and metal oxide NPs are more harmful to soil microorganisms than organic nanomaterials; in particular, ZnO NPs prevent thermogenic metabolism, reduce the levels of the nutrient-fixing Azotobacter and P- and K-solubilizing bacteria, and inhibit enzymatic activities (Chai et al. 2015). CuO NPs inhibit the growth of wheat plants and affect plant photosynthetic respiratory processes at high concentrations (Lu et al. 2002). Ag NPs penetrate plant roots at higher concentrations (Rastogi et al. 2017). In conclusion, the use of NPs to deliver essential nutrients and improve agricultural production is gaining attention; however, additional research on the toxicity of newly developed NFs should be conducted to mitigate public concerns about the issues related to nanotoxicity.

Concluding remarks and future perspectives

The development of new fertilizers with greater use efficiency and capability for minimizing nutrient losses by closely matching nutrient supply to crop demand is required to increase crop production and reduce associated adverse environmental impacts. Nanotechnology has the potential to introduce extreme changes in the future of agriculture and its allied fields. Particularly, nanotechnology-based fertilizers have been shown to improve plant growth and nutrition through site-specific delivery of fertilizers and essential nutrients by NPs with controlled release formulations. Nonetheless, very little effort has been made to apply nanotechnology in the agricultural sector. The ability to alter the shape, size, and/or surface chemistry of NPs to function as nano-alloys, heterodimers, and core-shells (Gu et al. 2005; Liu and Liu 2012; Basavegowda et al. 2017) is an example of the special properties that enhance the catalytic performance of NPs when compared to normal nanomaterials. In agricultural practice, NFs must be introduced in the nursery stage of crop production and monitored carefully before being applied to farmlands. Despite the issues and challenges that could be associated with nanomaterials, understanding their modes of function would likely promote their application via regulatory frameworks developed to ensure the safe use of such NFs.

Furthermore, the selection of suitable nanomaterial types, dosages, and application systems is critical for ensuring beneficial outcomes, as the majority of nanomaterials are metallic and could potentially lead to soil metal contamination. As the environmental exposure and safety of NFs should be understood, the determination of the biocompatibility and toxicity of nanomaterials is required. It must be noted that the size of engineered NPs is smaller than that of bulk particles; hence, some of these NPs will be absorbed by plants through dietary or food chain contamination, which could lead to potential environmental risks. Further research and laboratory experiments are needed before the widespread use of agricultural nanomaterials; as such, educational programs and research projects should build a bridge between the community/users and scientists to address potential customer (i.e., farmers) concerns. Ultimately, economic feasibility, public acceptance, and regulatory compliance must all be considered to realize the goal of NF commercialization for large-scale agricultural applications.

Collectively, the evidence provided in this review strongly indicates that nanotechnology has immense potential for improving the efficiency of nutrient delivery to plants. The design and use of suitable novel NFs can provide a potential strategy for promoting plant growth, development, and productivity for global food security programs. P NFs are currently the most studied NFs because most farmlands contain low levels of available P; typically, only 10–15% of the P applied as fertilizer is absorbed by plants. A better understanding of P kinetics from soil to plant is vital for optimizing integrated P management and improving P recycling efficiency, with the aim of reducing the use of CFs. In addition, further research is required to understand the role of different P forms, such as inorganic and organic P, in soils, including P transformation within the plant, and their effects on improving P fertility to maximize crop output and minimize environmental impacts. Nanotechnology-based fertilizers with fundamental properties, such as size, surface area, crystal phase, and nanomaterial surface capping not only improve crop productivity, but also efficiently regulate the delivery of nutrients and enable the control of material behavior during application. Further, nanotechnology industries should disclose information about their products and NFs. To address the problems and challenges associated with NFs, new technologies should move on from the laboratory phase to controlled field testing. Such practices could greatly improve nutritional health, sanitation, food security, sustainability, and environmental quality. Finally, further research should focus on the toxicity of newly developed NFs to evaluate their impact on crop quality, yield, environment, and health.

Author contributions

Collection of data: NB, and K-HB; Writing—original draft preparation: NB; Writing—review and editing: K-HB. All authors have read and agreed to publish the final version of the manuscript.

Funding

This research was funded by PJ015726, Rural Development Administration, Republic of Korea.

Declarations

Conflict of interest

On behalf of all authors, the corresponding author states that there is no conflict of interest.

Ethical approval

This article does not contain any studies with human participants performed by any of the authors.