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Article

Controlled-Release Urea–Hydroxyapatite Nanohybrid for Foliar Nitrogen and Phosphorus Delivery Enhances Biomass and Grain Yield in Wheat (Triticum aestivum L.)

by
Mayyas Al-Rimawi
1,
Riyad Al-Sharif
2,
Ayman Ayesh
2,
Naem Mazahrih
2,
Iyad Musallam
2,
Abdel Razzaq Al-Tawaha
2,*,
Sami Awabdeh
2,
Bayan Al Rjoub
2,
Eva Raya
2 and
Saad Awamleh
2
1
Faculty of Pharmacy and Medical Sciences, University of Petra, P.O. Box 961343, Amman 11196, Jordan
2
National Agricultural Research Center (NARC), P.O. Box 639, Baq’a 19381, Jordan
*
Author to whom correspondence should be addressed.
Nitrogen 2025, 6(3), 72; https://doi.org/10.3390/nitrogen6030072
Submission received: 22 June 2025 / Revised: 18 August 2025 / Accepted: 22 August 2025 / Published: 26 August 2025
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Abstract

Efficient use of nitrogen and phosphorus is crucial for achieving sustainable wheat production. Slow-release nano-fertilizers offer a targeted strategy to minimize nutrient losses, reduce excessive fertilizer application, and improve crop yield. This study introduces urea–hydroxyapatite (n-UHA) nanohybrid as a slow-release fertilizer synthesized to enhance nitrogen (N) and phosphorus (P) delivery efficiency in wheat (Triticum aestivum L.). Physical characterization techniques, including Scanning Electron Microscopy (SEM), Energy-Dispersive X-ray Spectroscopy (EDS), Zetasizer, and Fourier Transform Infrared Spectroscopy (FTIR), confirmed the formation of spherical n-UHA with a particle size of 106 nm. FTIR results indicated the formation of physically bound urea as a coating layer on the particle surface. Foliar application of n-UHA at 2500 and 5000 ppm N significantly increased tiller intensity and grain yield compared to conventional urea. The highest biological yield, approximately 16 t ha−1, was achieved with 5000 ppm n-UHA plus supplemental soil phosphorus (P), representing a 4-fold increase over the control. Conventional urea treatments, in comparison, only doubled yield. Notably, increasing conventional urea concentration from 2500 to 5000 ppm N did not significantly increase the yield even with additional P-soil supplement, while applying 5000 ppm N from n-UHA with supplemental P provided an approximate 25% yield increase compared to 2500 ppm n-UHA without P. The n-UHA’s slow-release mechanism supported prolonged tiller intensity, enhanced protein content, and higher biomass yield and chlorophyll content. This study showed that the slow-release mechanism of urea in the monohybrid due to hydrolysis resulted in localized acidity from carbonic acid production on the leaf surface area and contributed to dissociating phosphate ions from hydroxyapatite, making phosphorous more accessible. The enhanced performance of n-UHA is due to its controlled nutrient release, enabled by the physical binding of urea with hydroxyapatite nanoparticles. This binding ensures a synchronized supply of nitrogen and phosphorus aligned with plant demand. The nano-hydroxyapatite composite (N/Ca 6:1) supplies balanced nutrients via efficient stomatal absorption and gradual release. As an eco-friendly alternative to conventional fertilizers, n-UHA improves nitrogen delivery efficiency and reduces N-evaporation, supporting sustainable agriculture.

1. Introduction

Nano-fertilizers offer a promising solution to several challenges associated with traditional fertilizers, including environmental issues [1,2,3] economic costs, low crop productivity, nutrient deficiencies [4], salt stress, and leaching losses [5,6,7,8]. Among these are metal-urea or NPK nano-fertilizers that, in particular, have been shown to effectively increase the nitrogen available for crop growth. Application of nano-nitrogen not only reduces urea loss but also enhances nutrient uptake efficiency and mitigates the environmental impact of soil, air, and water pollution [9,10].
The global population relies heavily on increased crop yields—especially for staples like wheat and rice—achieved through the use of nitrogen (N) fertilizers [11,12], a cornerstone of the so-called “Green Revolution” [13,14]. Urea [CO(NH2)2], which contains 46% N by weight, is the primary source of nitrogen fertilizer. However, its premature decomposition in the soil—driven by water, volatilization, and the urease enzyme—causes the release of ammonia before it can be efficiently absorbed by plants, presenting a significant challenge for global agriculture and threatening food security.
Phosphorus (P), another critical macronutrient for plant growth, presents its own challenges. Only 10–20% of conventional P fertilizers are absorbed by plants [15,16], with the remaining 80–90% rapidly converted to forms with low bioavailability or precipitated as insoluble inorganic compounds [17,18,19,20,21,22,23,24]. Foliar application of phosphorus can offer a more efficient route by delivering nutrients directly to plant tissues, bypassing many of these losses [25]. Numerous studies have been conducted on the use of foliar fertilization in arable crops; however, these studies yielded varying results in terms of effectiveness [26,27,28,29]. Conventional P fertilizers, when applied to the soil surface, tend to accumulate in the topsoil, reducing uptake by plants and increasing losses through leaching or runoff [30,31]. Even the use of scarcely soluble phosphorus sources, such as rock phosphate and apatite, can result in reduced plant availability [32]. Thus, foliar application, in particular urea nano-hydroxyapatite (u-NHA), represents an innovative and environmentally friendly alternative that may provide greater efficiency compared to traditional fertilizers.
Hydroxyapatite [Ca10(PO4)6(OH)2] NPs are increasingly recognized for their potential in agricultural applications as a source of phosphorus. While the majority of the existing literature focuses on the biomedical uses of hydroxyapatite (HA) due to its biocompatibility and bioactivity [33,34,35,36,37], its potential as a fertilizer remains underexplored. Due to its high dissolution rate and faster release of soluble ions, HA has been proposed as an alternative phosphorus fertilizer. During foliar application, these NPs enter the plant through nano-sized pores, such as plasmodesmata and stomata, leading to higher nutrient use efficiency and reduced phosphorus waste [38].
Despite numerous studies investigating various nanoparticle fertilizer (NPF) systems [39,40], the effects of hydroxyapatite–urea NPFs on wheat growth via foliar application remain, to our knowledge, largely unexamined. Kottegoda et al. [41] synthesized a urea–HA nano-fertilizer as a controlled-release fertilizer (CRF) for rice, showing that urea interacts with HA NPs via amine and carbonyl groups, allowing for a slow release of nitrogen over one week in water, resulting in improved crop yield at lower concentrations.
In this study, we synthesized a nanohybrid containing hydroxyapatite (HA) nanoparticles (NPs) grafted with urea in a 6:1 ratio (urea NPs). This nanohybrid demonstrated a significant slow release of nitrogen, which attributed to the weak binding of urea to HA NPs, likely due to their high surface area-to-volume ratio and surface chemistry. This slow release of urea reduces the rate of decomposition at efficient local sites on the leaf surface.
The present work is aims at the evaluation of nano-urea HA (n-UHA) via foliar spray as a source of nitrogen and phosphorus on plant growth, morphology, and protein content. We observed and documented the morphological changes in wheat crop subjected to different treatments with n-UHA and conventional urea compared to the control. Physicochemical characterization of the prepared n-UHA was also considered through the examination of morphology, FTIR, and particle size. The obtained results were discussed and well correlated with the obtained agricultural results.

2. Materials and Methods

2.1. Synthesis of Urea–Hydroxyapatite Nanohybrid (n-UHA)

Urea–hydroxyapatite nanohybrid (n-UHA) was synthesized via a one-step in situ co-precipitation method with modifications to enhance urea encapsulation. Briefly, 250 mL of 0.6 M phosphoric acid (H3PO4, 85%; sourced from a local chemical supplier) was added dropwise under continuous stirring to a suspension of 19.3 g calcium hydroxide (Ca (OH)2, 99%, VWR Chemicals, Radnor, Pennsylvania, USA) in 250 mL of deionized water. During the addition, 150 g of urea (99.5%, BIO BASIC INC., Markham, Ontario, Canada) was introduced to the reaction mixture to promote integration into the hydroxyapatite matrix. The reaction proceeded at room temperature, and the resulting suspension was maintained in aqueous form to ensure nanoparticle dispersion. Nanoparticle size distribution was analyzed using a dynamic light scattering instrument (Zetasizer Nano ZS, Malvern Panalytical GmbH, Kassel, Germany).

2.2. Structural and Morphological Characterization

2.2.1. Scanning Electron Microscopy (SEM)

The surface morphology of the synthesized n-UHA particles was examined using a Phenom XL G2 SEM (Thermo Fisher Scientific, Eindhoven, The Netherlands) operated at an accelerating voltage of 15 kV. Prior to imaging, samples were sputter-coated with a ~30 nm platinum layer using an AGAR AGB7340 high-vacuum coater (AGAR Scientific Ltd., Stansted, United Kingdom) under argon atmosphere to improve image resolution and conductivity.

2.2.2. Fourier Transform Infrared Spectroscopy (FTIR)

Chemical bonding and functional group characterization were conducted via FTIR spectroscopy using a PLATINUM-ATR ALPHA II spectrophotometer(Bruker Optik GmbH, Ettlingen, Germany). Spectra were recorded in the range of 4000–1000 cm−1 at room temperature to confirm the presence of urea and phosphate groups in the hybrid nanomaterial.

2.3. Experimental Design and Greenhouse Setup

A pot-based experiment was conducted at the National Agricultural Research Center, Al-Hussein Station (32.08523° N, 33.84416° E), to investigate the foliar application efficiency of conventional urea versus n-UHA nanohybrids in wheat (Triticum aestivum L.). The experiment was laid out in a randomized complete block design (RCBD) with seven treatment groups and four replicates per treatment (Table 1). Each treatment consisted of 15 L pots filled with homogenous clay-rich soil (54% clay content). Foliar sprays were applied at two key phenological stages: the 5–6 leaf stage and the tillering stage. Urea (conventional and nanohybrid) was administered at two nitrogen concentrations (2500 and 5000 ppm), and selected treatments were supplemented with phosphorus (P) in the form of H3PO4 (5000 ppm) to assess N–P synergistic effects. Urea–hydroxyapatite nanohybrids used in the study were characterized by nitrogen and phosphorus contents of 37% and 19.5%, respectively (Table 1), were used as a new nano-based composite and compared with a conventional urea-based fertilizer for efficient and sufficient nitrogen delivery to pot-grown wheat. The overall urea concentrations applied in the various treatments were calculated based on nitrogen content and EDS concentrations, resulting in final values of 2500 ppm and 5000 ppm (Table 1). Urea concentration (w/v %) was calculated with respect to the given nitrogen content from n-UHA solution.

2.4. Agronomic Management and Growth Conditions

Wheat was sown on 18 December 2023 at a seeding rate equivalent to 56.25 kg ha−1, approximating a plant density of 111–124 plants m−2. Greenhouse conditions were maintained within a temperature range of 10–29 °C, relative humidity of 50–70%, and a photoperiod of 8–12 h natural light. Pots were irrigated to field capacity throughout the growing season.

2.5. Plant Sampling and Harvest

At physiological maturity (24 June 2024)—the stage when grain filling is complete and the crop has reached its maximum dry weight—plants were harvested manually at the soil surface to avoid soil contamination. Biomass was separated into straw and grain, then oven-dried and weighed. Yields were extrapolated to a per-hectare basis using pot dimensions. Key growth parameters, including plant height, spike length, and biomass accumulation, were measured at critical developmental stages to assess crop performance.

2.6. Protein and Spectral Analysis

Protein content in grain and straw was quantified using Near-Infrared Spectroscopy (DS2500, FOSS), employing standard calibrations based on NRI methodology. Measurements were performed in the 850–2500 nm range, with data automatically analyzed using NRI software (DS2500) to determine protein percentage. UV–Visible absorption spectra of the hybrid fertilizer formulations were obtained using a Genesys 10 UV-Vis spectrophotometer (Thermo Scientific). Samples were diluted in acetone and scanned across 200–800 nm using 1 cm quartz cuvettes; acetone served as the reference blank.

2.7. Soil Sampling and Soil FC

Two composite soil samples were collected from the homogenized clayey soil used as potting media and analyzed to determine the initial levels of nitrogen (N), phosphorus (P), and potassium (K), pH, and electrical conductivity (EC).
To determine soil field capacity (FC), empty pots were weighed (W1), filled with oven-dried soil and reweighed (W2), then saturated with water until free drainage occurred. After 24 h of drainage, the pots were weighed again (W3). FC was calculated using the formula:
Water content at FC (% by weight) = ((W3 − W2)/(W2 − W1)) × 100
where
W1: weight of the empty container;
W2: weight of the container with dry soil;
W3: weight of the container with moist soil at field capacity (FC);
W3 − W2 = weight of water retained in the soil after drainage;
W2 − W1 = weight of dry soil.

2.8. Statistical Analysis

Experimental data were subjected to analysis of variance (ANOVA) using Statistix 8.1 software to evaluate the effects of treatments on measured agronomic and biochemical parameters. Treatment means were compared using the Least Significant Difference (LSD) test at a significance level of p ≤ 0.05 to determine statistically meaningful differences among groups.

3. Results

3.1. Physicochemical Characterization of n-UHA

SEM, EDS, Zeta sizer, and FTIR results confirmed the formation of n-UHA, which is a heterogeneous nanoparticle in sphere-like morphology with urea coatings located at the nanoparticle surface. Energy-Dispersive Spectroscopy (EDS) (Thermo Fisher Scientific, Eindhoven, The Netherlands) results are listed in Table 2.
Most of the nanoparticle sizes obtained in the solution were measured to be around 106 nm (Figure 1). However, some particle sizes appeared to be around 33 nm, which suggests different aggregation mechanisms, while larger particles, around 5560 nm, are present in much smaller quantities. Furthermore, a 6:1 nitrogen/calcium ratio was previously found to be the optimal one [41].
Scanning Electron Microscopy (SEM) confirmed the presence of sphere-shaped, homogeneously distributed particles. The observed aggregation patterns suggested strong intermolecular physical bonding between the urea and hydroxyapatite components.
Figure 2 illustrates SEM—micrographs for n-UHA. The obtained results reveal the formation of different nanoparticle aggregation algorithm of urea hydroxyapatite (n- UHA). A sphere-like morphology indicates a strong U-HA interfacial physical bonding. Moreover, Zetasizer results along with SEM confirm the formation of the nanoparticles with homogeneous distribution.
Urea peaks can be clearly depicted in Figure 3 at around 3350–3500 cm−1 (could be attributed to N-H stretching from amines and O-H stretching from hydroxyl groups), 1600–1800 cm−1 (C-O stretching), and 1300–1550 cm−1 (CO3 stretching). The FTIR results confirm the existence of a urea layer on the hydroxyapatite surface with physical interfacial bonding, which aligns with Kottegoda et al. [41]. Additionally, the presence of phosphorus in the hydroxyapatite nanoparticle system can be clearly depicted in the range of about 1050–1150 cm−1. The obtained FTIR results indicate the formation of hydroxyapatite nanoparticles coated with urea that explain the slow-release mechanism through foliar application.

3.2. Field Experiment

3.2.1. Initial Soil Test

Composite soil samples (Table 3) from the pot experiment revealed a clayey texture (>57% clay, <3% sand) and slightly alkaline pH (8.2–8.3), potentially limiting micronutrient availability. Electrical conductivity (1.85–2.00 dS/m) indicated moderate salinity, which may affect salt-sensitive crops. Phosphorus (Olsen-P) levels were very low (2–3 ppm), while potassium was high (395–408 ppm), suggesting adequate reserves. Total Kjeldahl nitrogen was low (0.07–0.08%), typical of arid-region clay soils with limited organic inputs, indicating a need for nitrogen fertilization.

3.2.2. Spike Length

Foliar application of n-UHA significantly improved spike length compared to traditional urea, in particular at 5000 ppm (Table 4). Foliar application of n-UHA at 2500 ppm and 5000 ppm significantly (p ≤ 0.05) increased spike length compared to traditional urea. At 5000 ppm, n-UHA produced a spike length of 74.05 mm, significantly higher than 69.36 mm with traditional urea. The highest spike length (78.24 mm) was observed with 5000 ppm n-UHA plus phosphorus. Overall, n-UHA treatments showed a higher mean spike length (70.56 mm) than traditional urea (64.30 mm), confirming the positive performance of n-UHA at p ≤ 0.05 (Table 4).

3.2.3. Tiller Intensity and Plant Height

Foliar application of urea–hydroxyapatite nanoparticles (n-UHA) at 2500 and 5000 ppm N significantly increased tiller intensity compared to conventional urea at the same concentrations (Table 5). Notably, n-UHA at 5000 ppm resulted in a 2.5-fold higher tiller intensity than conventional urea. Furthermore, n-UHA 5000 with phosphorus showed significantly higher tiller intensity (p ≤ 0.05) than n-UHA 5000 without phosphorus. In contrast, the addition of phosphorus to conventional urea had no significant effect. These results suggest that the enhanced tillering is primarily due to the n-UHA slow-release phosphorus uptake through the stomatal pathway. Treatments with conventional urea resulted in higher plant height (p ≤ 0.5) compared to those receiving n-UHA (Table 6). However, the increased height did not correspond to higher yields or protein content, indicating that plant stature alone is not a reliable indicator of nitrogen use efficiency.

3.2.4. Crop Yield

Grain yield was significantly higher in treatments that received n-UHA as a foliar spray compared to those treated with conventional urea. The application of 5000 ppm n-UHA with soil-applied phosphorus resulted in the highest grain yield (10.12 ton. ha−1), a 3.7-fold increase over the control (2.73 ton. ha−1) (Table 7). Without added phosphorus, 2500 ppm and 5000 ppm n-UHA achieved yields of 7.83 and 8.78 ton.ha−1, respectively, substantially outperforming conventional urea, which produced only 5.02 and 5.61 ton. ha−1 at the same nitrogen levels (Table 7). Surprisingly, adding soil phosphorus to 5000 ppm n-UHA significantly increased yield, whereas the same P-soil addition to conventional urea at the same N concentration did not result in additional yield (Table 7). Overall, n-UHA treatments surpassed traditional urea: specifically, 5000 ppm N from n-UHA with soil-added phosphorus increased grain yield from 7.83 to 10.12 ton. ha−1 with approximately 25% significance at p ≤ 0.05 and enhanced straw yield from 5.70 to 6.22 ton. ha−1 approximately to 9% (Table 8).
Foliar application of 5000 ppm nitrogen (N) from n-UHA significantly (p ≤ 0.05) increased biological yield 2 times compared to conventional urea (Table 9). The highest dry biomass yield, 16.34 ton. ha−1, was recorded with 5000 ppm n-UHA supplemented with soil phosphorus, which was nearly four times higher than the control (4.05 ton. ha−1). In comparison, the application of 5000 ppm N Urea resulted in only a twofold increase relative to the control.
Biomass production followed a linear trend with increasing n-UHA application rates, demonstrating improved nitrogen use efficiency compared to conventional foliar urea application. Figure 4 shows the relationship between the applied nitrogen concentration and the resulting biomass yield.

3.2.5. Protein Content and Chlorophyll Absorbance

Protein content in both grain and straw increased significantly with n-UHA treatment. Near-Infrared Spectroscopy confirmed these findings, revealing that n-UHA-treated plants exhibited nearly double the grain protein content (≈16%) compared to conventional treatments (≈9%), indicating improved nitrogen assimilation and enhanced grain quality. Additionally, Table 10 confirms that n-UHA also elevated straw protein content.
Spectral absorbance of chlorophyll a (Figure 5) indicated higher peaks around 665–670 nm in n-UHA-treated plants, suggesting enhanced photosynthetic pigment concentration.

4. Discussion

This study demonstrated the effectiveness of nano-urea hydroxyapatite (n-UHA) as a slow-release fertilizer that enhances nutrient use efficiency and wheat productivity. Physicochemical characterization confirmed that the synthesized n-UHA had a spherical morphology with urea uniformly coated on the nanoparticle surface. SEM and EDS analyses revealed strong physical bonding and a homogeneous elemental composition, confirming the successful nanoscale integration of urea and hydroxyapatite.
The foliar application of conventional urea already improved grain and straw yield by about 50%; this finding is in agreement with Gooding and Davies [42] who also reported improved wheat yield with foliar nitrogen application. On the other hand, the application of n-UHA not only significantly enhanced plant morphological parameters—including spike length, tiller number, but also improved grain and straw yield, in addition to crude protein content—in comparison with conventional urea. These improvements can be attributed to the synergistic action of urea and hydroxyapatite within the nanocomposite, which enables a sustained and gradual nutrient release over the crop growth period.
The initial soil potassium level was relatively high (~400 mg kg−1 extractable K), indicating that potassium was not a limiting factor. Phosphorus was applied twice—once before tillering and again at the flag leaf stage—while nano-urea hydroxyapatite (n-UHA) was foliar-applied. Notably, even at 2500 ppm without supplemental P, n-UHA significantly enhanced grain protein content compared to conventional urea. At 5000 ppm, similar improvements were observed regardless of phosphorus addition, suggesting that the controlled nitrogen release, n-UHA-derived phosphorus, and high baseline potassium synergistically supported nitrogen assimilation and protein synthesis. The elevated K level promoted enzymatic activity and osmotic regulation, indirectly contributing to greater biomass and grain filling. In such K-rich soils, the synchronized release of nitrogen and phosphorus from n-UHA proved particularly effective: phosphorus drives ATP-dependent protein synthesis and root nitrogen uptake, while potassium activates over 60 enzymes involved in nitrogen metabolism and facilitates amino-N remobilization to the grain. This nutrient synergy resulted in up to twofold increases in grain protein content relative to conventional urea.
A large-scale review across 241 site-years in India, China, and North America found that supplemental potassium (60 kg K2O ha−1) raised wheat grain protein from approximately 10.6% to 11.7–11.9% at nitrogen application rates ≥ 120 kg ha−1. This improvement was attributed to enhanced nitrate transport and enzyme activation associated with protein synthesis via potassium–nitrogen interactions [43]. Duncan et al. [43] also reported that the co-application of nitrogen with phosphorus, potassium, and sulfur significantly improved nitrogen fertilizer efficiency, grain yield, and protein content in wheat, further supporting the critical role of multi-nutrient synergy in optimizing crop performance.
In wheat, at the 4th week, the plants treated with n-UHA showed a rapid increase in tiller number compared to those receiving conventional urea, along with a 22-day delay in maturity reaching 142 days, relative to conventional and control treatments. This suggests that the controlled release of nitrogen (N) and phosphorus (P) from n-UHA enhanced nutrient availability, extending the vegetative period and boosting protein accumulation.
Figure 6a,b shows the effects of foliar application on plants treated with n-UHA, conventional urea, and the control. Plants treated with n-UHA at both 2500 ppm and 5000 ppm appeared noticeably greener than those receiving conventional urea at the same nitrogen rates (Figure 6a). In addition, n-UHA-treated plants showed increased tillering and delayed senescence, retaining green foliage for 22 days longer than the control. This suggests prolonged nutrient availability and extended photosynthetic activity. While n-UHA-treated plants remained in the grain-filling stage, those treated with conventional urea and the control had already reached maturity and were ready for harvest.
Pohshna and Maillapali [44], founded that urea-doped hydroxyapatite nanomaterials (UHNs) significantly improved rice yield and nutrient use efficiency when applied to soil while reducing nutrient leaching, demonstrating UHN’s potential as an effective alternative to conventional fertilizers; however, whether applied as foliar or on soil, urea hydroxyapatite increased the yield and improved plant growth when compared to the conventional urea. Ragurai et al. [45] reported a significant increase in soil P, leaf N in plant tissues of Low Country tea (Camellia sinensis L.). Phosphorus in hydroxyapatite (HA) is tightly bound to calcium, forming a stable crystal structure. However, in nanohybrids like urea–hydroxyapatite (n-UHA), the interaction between nanoscale hydroxyapatite particles and urea enhances phosphorus bioavailability. The increased surface area allows for greater interaction with plant tissues, enhancing the dissolution rate of the HA particles. The higher dissolution rate leads to the gradual release of phosphate ions, which plants can more readily absorb. Urea in the nanohybrid (n-UHA) plays a crucial role in enhancing the availability of phosphorus: urea itself, when applied as foliar, undergoes hydrolysis, leading to the production of ammonium and a slight increase in local acidity through the production of carbonic acid (H2CO3). The localized acidic environment can promote the partial dissolution of PO4−3 bounded in the hydroxyapatite, releasing phosphate ions from the calcium-phosphate matrix, making them more accessible to the plant.
There are several studies on the utilization of urea-incorporated nano-hydroxyapatite, reporting that the hybrid fertilizer can perform slow release of both nitrogen and phosphorus nutrients, providing slow release of urea [45,46]. It is most likely that hydroxyapatite in its nano-form has a more controlled release mechanism and undergoes gradual dissolution in the presence of plant exudates or slightly acidic environments (like those near the root surface or on the leaf surface). This slow and steady release ensures that phosphorus is available over a prolonged period, improving its uptake efficiency. This observation aligns with the findings of Szamaitat et al. [47], who reported that, in hydroponically grown barley under phosphorus deficiency, nano-hydroxyapatite (nHAP) significantly increased phosphorus concentration near the plant root, which led to a significant leaf P concentration (from <2000 to >6000 ppm).
Nanoparticles with a diameter of less than 100 nm can easily penetrate through the stomatal opening and move from leaves to stems through the phloem sieve elements [48]. After entering the plant system, nanoparticles can move from one cell to another through plasmodesmata. Nitrogen hydroxyapatite nano-fertilizer, applied via foliar spray, enhances nutrient uptake, particularly phosphorus (P) and calcium (Ca). Tariq et al. [48] emphasized that phosphorus is key to enzyme activation in nutrient transport, and its deficiency causes imbalances and toxicity. Insufficient phosphorus hampers the uptake of nitrogen (N), potassium (K), and calcium (Ca), vital for cell wall development. The nano-hydroxyapatite composite (N/Ca, 6:1) not only supplies phosphorus but also delivers balanced nutrients through its nanostructure, allowing efficient stomatal absorption and gradual release in plant tissues.
Overall, the nanohybrid urea–urea-hydroxyapatite (n-UHA) fertilizer not only ensures the slow release of nitrogen and phosphorus but also promotes efficient uptake, improved yield, and enhanced protein content, supporting sustainable crop management practices.

5. Conclusions

Urea-coated hydroxyapatite (n-UHA) nanoparticles, synthesized in the nanometer range as confirmed by SEM and sizer analysis, exhibited a controlled-release mechanism with gradual nitrogen delivery followed by phosphorous dissociation. FTIR analysis confirmed the presence of functional groups related to urea, phosphate, and hydroxyl, validating the successful integration of nitrogen, phosphorus, and calcium within the nano-structure. This unique composition enabled efficient and sustained nutrient delivery. Foliar application of n-UHA significantly enhanced wheat biomass and yield compared to conventional urea, supporting its potential as a sustainable nano-fertilizer.

Author Contributions

Conceptualization, M.A.-R., R.A.-S. and A.A.; validation, R.A.-S. and A.A.; resources, R.A.-S.; data curation, R.A.-S., N.M., I.M., S.A. (Sami Awabdeh), B.A.R., E.R. and S.A. (Saad Awamleh); writing—original draft preparation, R.A.-S. and A.R.A.-T.; writing—review and editing, R.A.-S. and A.R.A.-T. All authors have read and agreed to the published version of the manuscript.

Funding

This research was conducted without external funding, relying solely on the resources and dedication of the authors to achieve the outlined objectives.

Data Availability Statement

The data presented in this study are available in the article.

Acknowledgments

We gratefully acknowledge the University of Petra for providing access to their advanced laboratory facilities and for the invaluable support of their dedicated scientific staff. Their generous contributions and technical assistance played a vital role in making this study possible.

Conflicts of Interest

The authors declare no conflicts of interest.

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Nitrogen 06 00072 g001
Figure 1. Size distribution of n-UHA.
Figure 1. Size distribution of n-UHA.
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Figure 2. SEM micrographs for prepared n-UHA.
Figure 2. SEM micrographs for prepared n-UHA.
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Figure 3. FTIR spectra for n-UHA (Ca10(PO4)6(OH)2).
Figure 3. FTIR spectra for n-UHA (Ca10(PO4)6(OH)2).
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Figure 4. Foliar nitrogen (N) concentration for plants receiving different concentrations via foliar application, expressed as biomass (ton. ha−1).
Figure 4. Foliar nitrogen (N) concentration for plants receiving different concentrations via foliar application, expressed as biomass (ton. ha−1).
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Nitrogen 06 00072 g005
Figure 5. UV–Visible absorbance spectrum of chlorophyll a, showing the main absorption peak in the red region around 665–670 nm, set to the arbitrary units (ABS).
Figure 5. UV–Visible absorbance spectrum of chlorophyll a, showing the main absorption peak in the red region around 665–670 nm, set to the arbitrary units (ABS).
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Nitrogen 06 00072 g006aNitrogen 06 00072 g006b
Figure 6. (a) Wheat (Triticum aestivum L.) treated with nano-urea hydroxyapatite (n-UHA) showing denser canopy and reduced plant height compared to plants treated with conventional urea and the untreated control. (b) Comparison between n-UHA-treated wheat and the untreated control, which exhibits taller plants and reduced vigor.
Figure 6. (a) Wheat (Triticum aestivum L.) treated with nano-urea hydroxyapatite (n-UHA) showing denser canopy and reduced plant height compared to plants treated with conventional urea and the untreated control. (b) Comparison between n-UHA-treated wheat and the untreated control, which exhibits taller plants and reduced vigor.
Nitrogen 06 00072 g006aNitrogen 06 00072 g006b
Table 1. Fertilizer treatments with nitrogen and phosphorus applications.
Table 1. Fertilizer treatments with nitrogen and phosphorus applications.
TreatmentNitrogen ConcentrationFertilizer TypePhosphorus Addition Per PotApplication Time of Phosphorus
T1NoneControl (no fertilizer)NoneNone
T22500 ppm NConventional UreaNone-
T35000 ppm NConventional UreaNone-
T45000 ppm NConventional Urea0.361 g P (1.2 P2O5)Split at 5–6 leaf and tillering
T52500 ppm NNano-Urea HydroxyapatiteNone-
T65000 ppm NNano-Urea HydroxyapatiteNone-
T75000 ppm NNano-Urea Hydroxyapatite0.361 g PSplit at 5–6 leaf and tillering
Table 2. Energy-Dispersive Spectroscopy (EDS).
Table 2. Energy-Dispersive Spectroscopy (EDS).
ElmentsWeight Ratio (%)
N37.80
Ca18.80
P19.50
Table 3. Initial soil test just before sawing.
Table 3. Initial soil test just before sawing.
SamplepH (Water)EC (dS/m)N(Kjeldal)
(%)		P Olsen (ppm)K-Exchangeable (ppm)Clay
(%)		Silt (%)Sand (%)Soil Texture
Sample 18.32.000.07240859.1238.352.54Clayey
Sample 28.21.850.08339557.2140.442.35Clayey
Table 4. Mean spike length (mm) under different application methods and nitrogen rates.
Table 4. Mean spike length (mm) under different application methods and nitrogen rates.
Application Method0 ppm
(±SE)		2500 ppm N-Urea (±SE)5000 ppm N-Urea (±SE)5000 ppm N-Urea + * Phosphorus (±SE)Mean
** Conventional Urea (46% N)57.47 ± 1.84 e65.64 ± 1.45 d69.36 ± 1.24 c64.74 ± 1.25 d64.30
** n-UHA (37% N)57.47 ± 1.84 e72.47 ± 0.99 bc74.05 ±1.28 b78.24 ± 0.94 a70.56
Mean57.4769.0671.4971.49
CV (%): 3.03, LSD main: 3.29, LSD submain: 2.15, Interaction: 2.10. * Phosphorus fertilizer was applied to the soil twice: once at the tillering stage and once at the flag leaf stage. ** Conventional Urea and n-UHA (37% N) were applied as foliar spray once just before tillering stage and once at flag leaf stage. Means in the same column followed by different letters are significantly different (p ≤ 0.05).
Table 5. * Mean plant tiller intensity (m2) at maturity stage.
Table 5. * Mean plant tiller intensity (m2) at maturity stage.
Application
Method		0 ppm
(±SE)		2500 ppm N-Urea (±SE)5000 ppm N-Urea (±SE)5000 ppm N-Urea + ** Phosphorus (±SE)Mean
*** Conventional
Urea (46% N)		123.5 ± 14.57 e227.5 ± 19.2 d240.5 ± 17.4 d243.75 ± 18.3 d208.68
*** n-UHA (37% N)123.5 ± 14.57 e542.75 ± 21.3 c715.00 ± 19.4 b786.5 ± 22.8 a541.91
Mean123.5385.12477.75515.125
CV (%): 6.54, LSD main: 33.11, LSD, submain: 25.77, Interaction: 36.45. * This value is multiplied by factor 12.4. A planted pot is 32 cm in diameter; the estimated area in m2: Area = π × (0.16 m)2, Area ≈ 3.14159 × 0.0256 m2 ≈ 0.0804 m2. ** Phosphorus fertilizer was applied to the soil twice: once at the tillering stage and once at the flag leaf stage.*** Conventional Urea and n-UHA (37% N) were applied as foliar spray once just before tillering stage and once at flag leaf stage. Means in the same column followed by different letters are significantly different (p ≤ 0.05).
Table 6. Mean plant apparent height (cm) at maturity stage.
Table 6. Mean plant apparent height (cm) at maturity stage.
Application
Method		0 ppm
(±SE)		2500 ppm N-Urea (±SE)5000 ppm N-Urea (±SE)5000 ppm N-Urea + ** Phosphorus (±SE)Mean
*** Conventional Urea (46%N)91.3 ± 1.11 c102.6 ± 1.23 a104.12 ± 1.34 a104.59 ± 1.43 a100.65
*** n-UHA (37%N)91.3 ± 1.11 c96.49 ± 1.29 b96.85 ± 1.27 b98.25 ± 1.45 b95.72
Mean91.399.54100.42101.42
CV (%): 1.46, LSD main: 1.91, LSD, submain: 1.5, Interaction: 2.13. ** Phosphorus fertilizer was applied to the soil twice: once at the tillering stage and once at the flag leaf stage. *** Conventional Urea and n-UHA (37%N) were applied as foliar spray once just before tillering stage and once at flag leaf stage. Means in the same column followed by different letters are significantly different (p ≤ 0.05)
Table 7. * Grain yield (ton. ha−1) under different application methods and nitrogen treatments.
Table 7. * Grain yield (ton. ha−1) under different application methods and nitrogen treatments.
Application Method0 ppm
(±SE)		2500 ppm N-Urea (±SE)5000 ppm N-Urea (±SE)5000 ppm N-Urea + ** Phosphorus (±SE)Mean
*** Conventional Urea (46% N)2.73 ± 0.093 d5.02 ± 0.073 c5.61 ± 0.18 c5.49 ± 0.22 c4.71
*** n-UHA (37% N)2.7 ± 0.093 d7.83 ± 0.16 b8.78 ± 0.14 b10.12 ± 0.19 a7.37
Mean2.736.437.207.81
CV (%): 11.2, LSD main: 0.82, LSD submain: 0.71, Interaction: 1.00. * This value is multiplied by a factor of 12.4. A planted pot is 32 cm in diameter; the estimated area in m2: Area = π × (0.16 m)2, Area ≈ 3.14159 × 0.0256 m2 ≈ 0.0804 m2. ** Phosphorus fertilizer was applied to the soil twice: once at the tillering stage and once at the flag leaf stage. *** Conventional Urea and n-UHA (37% N) were applied as foliar spray once just before tillering stage and once at flag leaf stage. Means in the same column followed by different letters are significantly different (p ≤ 0.05).
Table 8. * Straw yield (ton. ha−1) under different application methods and nitrogen treatments.
Table 8. * Straw yield (ton. ha−1) under different application methods and nitrogen treatments.
Application Method0 ppm
(±SE)		2500 ppm N-Urea (±SE)5000 ppm N-Urea (±SE)5000 ppm N-Urea + ** Phosphorus (±SE)Mean
*** Conventional Urea (46% N)1.31 ± 0.074 c2.67 ± 0.14 b2.78 ± 0.17 b2.85 ± 0.21 b2.40
*** n-UHA (37% N)1.31 ± 0.074 c5.51 ± 0.20 a5.70 ± 0.18 a6.22 ± 0.27 a4.69
Mean1.314.094.244.54
CV (%): 14.2, LSD main: 0.56, LSD submain: 0.54, Interaction: 0.76. * This value is multiplied by factor 12.4. A planted pot is 32 cm in diameter; the estimated area in m2: Area = π × (0.16 m)2, Area ≈ 3.14159 × 0.0256 m2 ≈ 0.0804 m2. ** Phosphorus fertilizer was applied to the soil twice: once at the tillering stage and once at the flag leaf stage. *** Conventional Urea and n-UHA (37% N) were applied as foliar spray once just before tillering stage and once at flag leaf stage. Means in the same column followed by different letters are significantly different (p ≤ 0.05).
Table 9. * Biological yield (ton. ha−1) under different application methods and fertilizer rates.
Table 9. * Biological yield (ton. ha−1) under different application methods and fertilizer rates.
Application
Method		0 ppm
(±SE)		2500 ppm N
(±SE)		5000 ppm N
(±SE)		5000 ppm N + ** P (±SE)Mean
*** Conventional Urea (46% N)4.05 ± 0.16 e7.69 ± 0.19 d8.40 ± 0.28 d8.35 ± 0.32 d7.12
n-UHA (37% N)4.05 ± 0.16 e13.35 ± 0.29 c14.84 ± 0.27 b16.34 ± 0.34 a12.15
Mean4.0510.5211.4412.38
CV (%): 9.48, LSD main: 0.76, LSD submain: 0.95, Interaction: 1.35. * This value is multiplied by a factor of 12.4. A planted pot is 32 cm in diameter; the estimated area in m2: Area = π × (0.16 m)2, Area ≈ 3.14159 × 0.0256 m2 ≈ 0.0804 m2. ** Phosphorus fertilizer was applied to the soil twice: once at the tillering stage and once at the flag leaf stage. *** Conventional Urea and n-UHA (37% N) were applied as foliar spray once just before the tillering stage and once at the flag leaf stage. Means in the same column followed by different letters are significantly different (p ≤ 0.05)
Table 10. Crude protein content of plant dry matter residue, grain and straw.
Table 10. Crude protein content of plant dry matter residue, grain and straw.
TreatmentGrainStraw
CP%NitrogenCP%Nitrogen
T1 control8.11.31.50.24
T2 2500 U9.391.51.750.28
T3 5000 U8.531.361.580.25
T4 5000 U + P9.441.511.890.3
T5 2500 n-UHA16.692.672.430.39
T6 5000 n-UHA15.632.52.580.41
T7 5000 n-UHA + P16.812.692.960.47
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Al-Rimawi, M.; Al-Sharif, R.; Ayesh, A.; Mazahrih, N.; Musallam, I.; Al-Tawaha, A.R.; Awabdeh, S.; Al Rjoub, B.; Raya, E.; Awamleh, S. Controlled-Release Urea–Hydroxyapatite Nanohybrid for Foliar Nitrogen and Phosphorus Delivery Enhances Biomass and Grain Yield in Wheat (Triticum aestivum L.). Nitrogen 2025, 6, 72. https://doi.org/10.3390/nitrogen6030072

AMA Style

Al-Rimawi M, Al-Sharif R, Ayesh A, Mazahrih N, Musallam I, Al-Tawaha AR, Awabdeh S, Al Rjoub B, Raya E, Awamleh S. Controlled-Release Urea–Hydroxyapatite Nanohybrid for Foliar Nitrogen and Phosphorus Delivery Enhances Biomass and Grain Yield in Wheat (Triticum aestivum L.). Nitrogen. 2025; 6(3):72. https://doi.org/10.3390/nitrogen6030072

Chicago/Turabian Style

Al-Rimawi, Mayyas, Riyad Al-Sharif, Ayman Ayesh, Naem Mazahrih, Iyad Musallam, Abdel Razzaq Al-Tawaha, Sami Awabdeh, Bayan Al Rjoub, Eva Raya, and Saad Awamleh. 2025. "Controlled-Release Urea–Hydroxyapatite Nanohybrid for Foliar Nitrogen and Phosphorus Delivery Enhances Biomass and Grain Yield in Wheat (Triticum aestivum L.)" Nitrogen 6, no. 3: 72. https://doi.org/10.3390/nitrogen6030072

APA Style

Al-Rimawi, M., Al-Sharif, R., Ayesh, A., Mazahrih, N., Musallam, I., Al-Tawaha, A. R., Awabdeh, S., Al Rjoub, B., Raya, E., & Awamleh, S. (2025). Controlled-Release Urea–Hydroxyapatite Nanohybrid for Foliar Nitrogen and Phosphorus Delivery Enhances Biomass and Grain Yield in Wheat (Triticum aestivum L.). Nitrogen, 6(3), 72. https://doi.org/10.3390/nitrogen6030072

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