Pearl millet (
Pennisetum glaucum L.) is one of the most resilient cereals cultivated across arid and semi-arid regions of Africa and Asia. It is valued for its exceptional tolerance to drought, high temperatures and nutrient-poor soils, making it a vital food and fodder crop for millions of resource-poor farmers (
FAO, 2021). India is the world’s largest producer, contributing nearly 40% of global pearl millet production, where it plays a major role in food security and dryland farming systems (
ICRISAT, 2020;
Directorate of Millets Development, 2022). Despite its adaptability, the productivity of pearl millet is strongly influenced by nitrogen (N) availability, as N drives tillering, chlorophyll formation, photosynthetic capacity and grain development
(Singh et al., 2018).
Conventional urea remains the predominant N source used by farmers; however, its low nitrogen use efficiency (NUE) often below 35% limits its effectiveness
(Ladha et al., 2016). Significant N losses through volatilization, leaching and denitrification reduce crop uptake and increase production costs while contributing to environmental degradation
(Snyder et al., 2009). These limitations underline the need for more efficient N delivery systems, particularly in dryland cereals such as pearl millet which are frequently grown under moisture and nutrient constraints.
In recent years, innovative nitrogen sources such as nano urea and urea-ammonium nitrate (UAN) have emerged as potential alternatives to conventional granular fertilizers. Nano Urea, developed as a foliar-applied nanotechnology-based fertilizer, aims to enhance nutrient absorption and reduce losses through controlled and targeted N delivery
(Prasad et al., 2017; IFFCO, 2021). UAN, a liquid fertilizer containing urea, ammonium and nitrate forms of nitrogen, offers more uniform application, reduced volatilization compared to prilled urea and better synchronization with crop demand (
Gordon, 2014). Both fertilizers represent promising tools for improving NUE and productivity in climate-stressed cereal systems.
However, despite increasing interest, comparative assessments of Nano Urea and UAN in pearl millet remain limited particularly in the context of semi-arid agroecosystems where nutrient and water stresses frequently coexist. Understanding their relative efficiency, physiological impacts, agronomic performance and environmental implications is crucial for guiding fertilizer recommendations and optimizing nitrogen management strategies for smallholder farmers.
This review article was prepared through a systematic and comprehensive survey of published scientific literature focusing on nano urea and urea-ammonium nitrate (UAN) in millet-based production systems, particularly pearl millet.
A structured literature search was conducted using major academic databases including Scopus, Web of Science, Google Scholar, ScienceDirect and AGRIS. The search covered publications from 2000 to 2025, with emphasis on recent developments in nanotechnology-based fertilizers and liquid nitrogen sources.
Keywords used included:
• “Nano urea”.
• “Urea ammonium nitrate (UAN)”.
• “Pearl millet nitrogen management”.
• “Nitrogen use efficiency in millets”.
• “Nano fertilizers in dryland agriculture”.
• “Liquid nitrogen fertilizers”.
Only peer-reviewed journal articles, research reports and credible institutional publications were included. Non-scientific reports and non-peer-reviewed news sources were excluded to maintain scientific rigor.
A total of approximately 120 relevant articles were critically analysed. The selected studies were categorized into:
1. Nitrogen dynamics in pearl millet.
2. Nano urea mechanism and agronomic performance.
3. UAN chemistry and field applications.
4. Environmental and economic implications.
The findings were synthesized thematically to provide a structured comparative assessment.
Pearl millet: Growth characteristics
Pearl millet (
Pennisetum glaucum) is widely known for its remarkable ability to survive in harsh, dry and nutrient-poor environments. The crop has a deep and extensive root system, often reaching 1.5-2 m deep, which allows it to extract water from deeper soil layers
(Ramesh et al., 2020). This root architecture is one of the biggest reasons it performs better than many cereals in drought-prone regions.
Physiologically, pearl millet uses the C4 photosynthetic pathway, which means it can maintain high photosynthetic efficiency even under high temperatures (40-45°C) and limited water availability
(Sinha et al., 2019). Its leaves have efficient stomatal regulation, helping the plant reduce water loss during peak heat stress. Additionally, pearl millet can resume growth quickly after drought breaks, a trait known as drought recovery resilience
(Kumar et al., 2021).
This combination of morphological strength, efficient carbon fixation and stress-adaptive physiological traits makes pearl millet one of the most climate-resilient cereals suitable for the semi-arid tropics.
Nitrogen uptake patterns across key growth stages
Nitrogen (N) plays a central role in pearl millet growth because it drives tillering, leaf expansion, chlorophyll formation and grain development. However, the crop does not absorb nitrogen uniformly throughout its lifecycle.
Tillering stage (20-30 DAS)
This is when the crop produces most of its productive tillers. During this period, pearl millet typically absorbs 25-35% of its total N requirement, mainly to support leaf area expansion and early canopy establishment
(Singh et al., 2020).
Booting stage (35-45 DAS)
At booting, the developing panicle is inside the stem. The plant’s nitrogen demand sharply increases because N supports panicle initiation, stem elongation and chlorophyll activity. Studies show that 30-40% of total N uptake occurs during this stage
(Sagar et al., 2023).
Flowering to grain filling stage (50-70 DAS)
During flowering and grain filling, nitrogen is essential for pollen viability, grain setting and protein accumulation. Approximately 20-30% of total N uptake takes place here.
If N supply is limited during this period, grain weight and yield decline sharply.
Thus, timely and split application of N is critical because most pearl millet varieties reach peak N demand within a short time window. Similar findings were reported in dryland millets where split nitrogen application significantly improved nitrogen recovery efficiency and yield stability
(Sagar et al., 2024).
Nitrogen use efficiency (NUE) challenges
Despite being a hardy crop, pearl millet suffers from low nitrogen use efficiency (NUE), often in the range of 25-35% under farmers’ field conditions
(Sharma et al., 2018). Several factors contribute to this challenge:
• Sandy and low-organic-matter soils, which allow nitrogen to leach rapidly.
• High temperatures and surface application of urea, leading to volatilization losses.
• Limited irrigation, which restricts nitrogen mobility and root uptake.
• Mismatch between N application timing and the crop’s physiological demand.
Poor NUE means that a large portion of applied nitrogen is lost to the environment rather than used by the plant, resulting in lower productivity and higher fertilizer costs.
Innovative fertilizers like Nano Urea and UAN are being explored to address these limitations by enhancing N availability and reducing losses.
Yield response to nitrogen levels under rainfed vs. irrigated conditions
Pearl millet responds strongly to nitrogen fertilization, but the magnitude of yield response varies with moisture conditions.
Rainfed conditions
Under rainfed systems, water availability limits nitrogen utilization.
• Yield response typically increases up to 40-60 kg N ha
-1, beyond which the benefit declines.
• Due to drought stress, the plant cannot fully utilize higher N levels.
• NUE is often lower because N uptake declines during dry spells.
Irrigated conditions
When moisture is adequate, nitrogen uptake and metabolism improve considerably.
• Studies report yield increases up to 80-120 kg N ha
-1 under irrigated production
(Bhuvaneshwari et al., 2021).
• Higher irrigation allows better root proliferation and efficient N translocation to grains.
• Grain yield, panicle length and thousand-grain weight show significant improvement.
Conventional nitrogen fertilizers: Limitations and challenges
Conventional nitrogen fertilizers especially granular urea has played a major role in boosting cereal production for decades. However, in crops like pearl millet, the efficiency of these fertilizers is often much lower than expected. Several soil, climatic and management factors contribute to nitrogen (N) losses, ultimately reducing crop productivity and increasing production costs for farmers.
Urea volatilization losses (20-40%)
Urea is the most widely used nitrogen fertilizer in India because of its low cost and high N content. However, when urea is broadcast on the soil surface, a large portion of it is lost as ammonia gas (NH
3) before the crop can absorb it.
In drylands and coarse-textured soils conditions typical for pearl millet the losses are particularly high. Studies have consistently reported 20-40% volatilization loss from surface-applied urea, especially under high temperature and alkaline soil conditions
(Bouwmeester et al., 1985; Singh and Ryan, 2015).
This means that almost one-third of the applied nitrogen never reaches the crop, making fertilization inefficient and costly.
Leaching and denitrification in light-textured soils
Pearl millet is often grown in sandy or loamy-sand soils, which have low water-holding capacity. These soils allow nitrogen, especially in the nitrate (NO
3-) form, to move downward rapidly with irrigation or rainwater.
• Leaching losses occur when nitrate passes below the root zone reducing crop N availability and contaminating groundwater (
Di and Cameron, 2002).
• Denitrification the microbial conversion of nitrate into nitrous oxide (N
2O) and N‚ gases occurs during temporary waterlogging or heavy rainfall leading to gaseous N losses
(Aulakh et al., 1992).
In such soils, it is common for farmers to lose 10-30% of applied nitrogen due to leaching or denitrification, depending on rainfall distribution.
Low nitrogen use efficiency (NUE 30-35% in pearl millet)
NUE in pearl millet is already low due to harsh environmental conditions and poor synchronization between nitrogen supply and crop demand
(Sharma et al., 2018). Under typical field conditions, only 30-35% of the applied nitrogen is taken up by the crop, while the rest is lost through volatilization, leaching, runoff, or immobilization
(Yadav et al., 2012; Rai et al., 2015).
This low NUE means:
• Higher fertilizer requirement per unit of grain produced.
• Greater production costs.
• More environmental losses.
• Less consistent yield response.
Improving NUE is therefore a major goal for sustainable millet production.
Environmental impacts: NH3 emission and groundwater pollution
Nitrogen losses are not just an economic issue; they also generate significant environmental risks.
a. Ammonia (NH3) emissions
Volatilized ammonia contributes to:
• Air pollution.
• Particulate matter formation.
• Soil acidification after redeposition.
Globally, urea is one of the largest agricultural sources of NH
3 emissions
(Bouwmeester et al., 1985; Pan et al., 2016).
b. Groundwater contamination
Leached nitrates can enter groundwater and pose health concerns such as:
• Methemoglobinemia (blue baby syndrome).
• Increased risk of digestive tract disorders
(Ward et al., 2005).
In regions with coarse-textured soils and high rainfall common for rainfed pearl millet nitrate leaching is a major environmental concern.
Economic inefficiencies for farmers
The combined effect of volatilization, leaching and low NUE means that farmers often receive a much smaller yield benefit than expected from the fertilizer they apply.
Economic implications include:
• Higher fertilizer bills due to repeated applications.
• Lower return on investment.
• Reduced profitability under rainfed conditions.
• Greater sensitivity to rising fertilizer prices.
Studies show that improving N efficiency can increase net returns by 15-30% in pearl millet systems
(Rai et al., 2015; Singh et al., 2020).
Thus, nitrogen loss is not only an agronomic problem but also a financial burden, making it necessary to explore more efficient alternatives such as Nano Urea and UAN.
Nano urea: Composition, mechanism and agronomic potential composition and nano-scale properties
Nano urea is essentially a nitrogen fertilizer formulated using ultra-small particles, usually around 20-50 nanometers in size
(Singh et al., 2021). At this scale, the particles possess a very high surface area, which makes them much more reactive than conventional urea granules. Because of this increased surface area, the fertilizer can interact more efficiently with plant tissues, allowing plants to absorb nitrogen faster and in smaller amounts
(Kumar et al., 2023).
This nano-scale design also helps nano urea penetrate leaf surfaces more effectively and reduces the need for bulk nitrogen application in the soil. In simple terms, less fertilizer can do more work, helping both the plant and the environment.
Mechanism of absorption
Foliar uptake pathways
When nano urea is sprayed on leaves, the nano-sized particles can enter the plant through stomata (tiny pores) and cuticular pathways. Their small size increases the chance of penetration and rapid absorption, compared to conventional urea that mostly relies on soil dissolution
(Yadav et al., 2020).
Slow-release and targeted delivery
Once inside the leaf, nano urea does not release nitrogen all at once. Instead, it provides a controlled or slow release, allowing nitrogen to be available gradually as the plant needs it
(Natarajan et al., 2022). This reduces nitrogen losses and makes the nutrient more synchronised with plant growth stages.
Influence on plant metabolic enzymes
Nano urea has been shown to stimulate key enzymes involved in nitrogen metabolism, especially nitrate reductase, which converts nitrate to forms that the plant can use for building proteins and chlorophyll. Several studies report increases in chlorophyll content and improved photosynthesis in crops treated with nano urea
(Sairam et al., 2024; Choudhary et al., 2023). This means the plant becomes more efficient at using sunlight and producing biomass.
Advantages of nano urea
Reduced nitrogen losses
Traditional urea suffers major losses through volatilization, leaching and denitrification-sometimes up to 40-50%. Nano urea, being foliar applied, bypasses soil-based losses and delivers nitrogen directly to plant tissues, reducing inefficiency dramatically (
ICAR, 2021).
Higher nitrogen use efficiency (NUE)
Because more nitrogen reaches the plant and less is lost, nano urea substantially improves Nitrogen Use Efficiency. Several field studies have shown that using nano urea can replace a portion of granular urea without reducing yields
(Singh et al., 2021; IFFCO, 2022).
Lower environmental footprint
Nano urea reduces the need for excessive fertilizer use, thereby lowering greenhouse gas emissions, soil pollution and nitrate leaching. It also aligns with sustainable agriculture initiatives aiming to reduce the environmental impact of chemical fertilizers.
Constraints/gaps
Inconsistent performance across crops and environments
Although nano urea performs well in many cereals and vegetables, researchers have observed variability in response depending on soil type, crop species, climatic conditions and management practices
(Kumar et al., 2023). Some crops respond strongly, while others show only mild improvement.
Limited long-term studies
Nano fertilizers are still relatively new and there are not enough long-term studies examining their accumulation, potential toxicity, or interactions with soil microorganisms
(Rai et al., 2022). This creates uncertainty regarding their ecological impact over many seasons.
Dosage and timing still not standardized in millets
For crops like pearl millet, research is still emerging. Optimal spray concentration, timing and frequency have not been fully standardized. Current recommendations are based largely on preliminary findings rather than multi-location, multi-year trials
(Choudhary et al., 2023).
This gap highlights the need for crop-specific guidelines, especially in dryland cereals where nitrogen dynamics differ from irrigated crops.
UAN (Urea-Ammonium nitrate): Chemistry, function and application
Composition
Urea-ammonium nitrate (UAN) is a liquid nitrogen fertilizer containing 32% N, commonly used in cereals and forage crops. What makes UAN unique is that the nitrogen is supplied in three different chemical forms, each behaving differently in the soil:
• 50% Urea-N-This portion must be hydrolyzed by the enzyme urease before it becomes available to plants, making it a more gradual source of nitrogen.
• 25% Ammonium-N (NH
4+)-This form is readily absorbed by plant roots and also binds to soil particles, reducing immediate losses (
Gastal and Lemaire, 2023).
• 25% Nitrate-N (NO
3-)-This form is instantly available to plants through mass flow, especially under good soil moisture.
Because UAN supplies fast-, medium- and slow-release nitrogen simultaneously, it supports plant growth across multiple stages, unlike conventional urea which depends heavily on soil moisture and urease activity.
Mechanism
Immediate + sustained nitrogen availability
The presence of nitrate-N ensures rapid uptake, helping plants during early vegetative growth, while ammonium-N and urea-N contribute to sustained nitrogen supply as the crop develops. This “multiple-form nitrogen strategy” improves synchrony between nitrogen release and crop demand
(Sharma et al., 2019).
Reduced volatilization losses
Compared with granular urea, UAN typically shows lower ammonia volatilization because:
1. Only half of its N is urea.
2. The solution form increases soil contact and minimizes exposure to air.
As a result, UAN can contribute to higher nitrogen use efficiency (NUE), especially in dryland conditions where urea losses are high.
Compatibility with fertigation and precision spraying
Being a liquid fertilizer, UAN is easy to apply through:
Drip and sprinkler fertigation systems.
Boom sprayers for uniform field distribution.
Variable-rate applicators in precision agriculture.
This flexibility allows farmers to apply nitrogen exactly when the crop needs it, reducing wastage and improving productivity.
Advantages
Even and uniform application
Granular fertilizers often suffer from uneven distribution, especially when wind or machinery limitations occur. UAN, on the other hand, spreads uniformly in liquid form, which ensures consistent nitrogen availability across the field.
Higher plant uptake efficiency
The simultaneous availability of ammonium and nitrate promotes better root absorption and stimulates metabolic enzyme activities, resulting in improved nitrogen recovery by the plant. Nitrate enhances root growth, while ammonium increases chlorophyll and photosynthesis together supporting vigorous vegetative development.
Less dependent on soil moisture
One major drawback of urea is its reliance on soil moisture for dissolution and conversion to ammonium. UAN, already in dissolved form, begins interacting with the soil immediately after application. This makes it particularly helpful in semi-arid crops like pearl millet, where intermittent rainfall can limit urea efficiency.
Constraints/gaps
Potential leaf scorching
Because UAN contains nitrate and ammonium salts, it can cause leaf burn if sprayed at high concentrations or during hot, dry weather. Foliar application must therefore be carefully timed ideally during cooler morning or evening hours
(Sharma et al., 2019).
Handling and storage limitations
UAN is mildly corrosive, which means storage tanks, pumps and pipelines must be corrosion-resistant, increasing the initial investment. The solution also tends to salt-out (crystallize) at low temperatures, creating logistical challenges.
Cost fluctuations
Since UAN is a manufactured liquid fertilizer with urea and nitrate components, its price can vary depending on:
Global urea and ammonium nitrate markets.
Transportation cost.
Availability of liquid application machinery.
For small and marginal farmers, this can be a barrier unless supported by subsidies or extension guidance.
Comparative analysis: Nano urea vs UAN in pearl millet
Nitrogen use efficiency (NUE)
Nano urea is designed as a foliar-applied nano-scale nitrogen source, allowing rapid entry through stomata and cuticular pathways. This leads to improved absorption efficiency and reduced nitrogen losses, which enhances apparent NUE compared with conventional soil-applied urea
(Kumar et al., 2024; Gupta et al., 2023). Because less nitrogen is exposed to soil processes, volatilization and leaching losses are inherently lower.
Urea-ammonium nitrate (UAN) contains a mixture of urea-N, ammonium-N and nitrate-N, enabling both immediate and sustained N availability to roots. This combination often improves plant uptake efficiency and lowers ammonia volatilization relative to broadcast urea
(Ren et al., 2023; Singh et al., 2021). When applied through fertigation or injected into soil, UAN consistently improves nitrogen recovery efficiency in cereals.
Growth parameters (Tillering, LAI, chlorophyll; root vs foliar effects)
Foliar nano urea application often results in noticeable improvements in leaf greenness, chlorophyll index and leaf area index (LAI) because the absorbed nitrogen quickly activates photosynthetic enzymes and protein synthesis
(Gupta et al., 2023). These rapid physiological responses explain why nano urea is effective for mid-season crop correction.
In contrast, UAN primarily supports the root zone with balanced nitrogen forms, promoting stronger root proliferation and sustained tillering throughout the season. Its nitrate component fuels rapid metabolic activity, while ammonium supports long-term vegetative growth and root mass development
(Ren et al., 2023; Singh et al., 2021). As a result, cereals like pearl millet often show better tiller retention and extended canopy development under well-managed UAN programs.
Yield attributes (Grain, fodder, panicle length, grain weight)
Field studies on pearl millet and related cereals show that nano urea can maintain or improve grain and fodder yields when used as a supplement to partial soil-N substitution. Enhancements in plant height, LAI and chlorophyll translate into improvements in panicle length and grain weight when foliar sprays are timed at critical stages such as tillering and flowering
(Sairam et al., 2024). However, full replacement of soil-applied nitrogen with nano urea often results in yield decline, underscoring the importance of combined soil + foliar strategies.
UAN has consistently shown positive yield responses because its mixed-N form improves nitrogen availability during panicle initiation and grain filling. Several comparative studies demonstrate higher grain yield and fodder yield under UAN than under broadcast prilled urea, mainly due to better synchronization between soil N availability and crop uptake
(Ren et al., 2023; Singh et al., 2021).
Soil and environmental impact
Since nano urea is applied to leaves and largely taken up through foliar pathways, minimal nitrogen reaches the soil. This reduces the risk of nitrate leaching and limits soil accumulation of nitrogen residues, making nano formulations generally more environmentally benign. However, long-term ecological studies on nano-fertilizer residues and soil microbiome interactions are still limited
(Kumar et al., 2024).
UAN typically results in lower volatilization losses than surface-applied urea because ammonium and nitrate are less prone to gaseous transformation when applied correctly. Nevertheless, improper application such as high concentrations on dry soil can lead to localized salt buildup or leaf scorching
(Ren et al., 2023; Singh et al., 2021). Overapplication in alkaline soils may also contribute to temporary increases in soil electrical conductivity.
Economic comparison (Cost-benefit, urea reduction, ROI)
Economically, nano urea offers advantages when used to reduce a portion (not all) of soil-applied nitrogen. Studies show that replacing 25-33% of conventional urea with nano urea sprays can decrease fertilizer costs because fewer 50-kg urea bags are required while maintaining yields, improving the benefit-cost ratio
(Sairam et al., 2024; Kumar et al., 2024). However, using nano urea as a total substitute often reduces yield and net returns.
UAN-based fertilization can be economically efficient where fertigation or liquid-application systems exist. Because UAN improves nitrogen recovery and often increases grain yield compared to prilled urea, return on investment (ROI) is typically higher in irrigated or semi-irrigated systems
(Ren et al., 2023; Singh et al., 2021). Farmers benefit particularly when UAN is applied in precise split doses, reducing waste and maximizing grain response.
Integration with agronomic practices
Integrating Nano Urea and UAN into a pearl millet production system requires careful alignment of when, how and how much nitrogen is supplied. Because both fertilizers behave differently in soil and plant tissues, their efficiency improves significantly when paired with optimum agronomic practices such as split scheduling, fertigation, irrigation management and balanced nutrient programs.
Timing and dosage recommendations
Pearl millet has distinct nitrogen-demand peaks during tillering, panicle initiation and flowering, making timing a crucial factor.Conventional recommendations suggest 60-90 kg N ha
-1 for rainfed and 100-120 kg N ha
-1 for irrigated systems. However, when Nano Urea or UAN is integrated, the actual quantity of granular urea can be reduced, while maintaining or improving plant performance.
Nano urea
One 500 mL bottle is considered equivalent to a top-dressing of 45 kg urea N, but must be applied as a foliar spray at critical growth stages. Most studies indicate best results when sprayed at 30-35 DAS and again at 50-55 DAS
(Prasad et al., 2023). Its rapid foliar absorption provides timely nitrogen during active vegetative growth and reduces soil N losses.
UAN (Urea-ammonium nitrate)
As a 32% N liquid, it is typically applied at 25-30% of total N at sowing, with the remaining amount distributed in one or two top-dressings at tillering and panicle initiation (
Singh and Yadav, 2021). UAN supplies nitrate, ammonium and amide forms simultaneously offering a balance of immediate and sustained release.
Key insight
When timed properly, both fertilizers can reduce dependence on bulk urea while improving nitrogen synchrony with crop needs.
Split applications and fertigation strategies
Split nitrogen applications are essential in pearl millet because the crop grows quickly and is highly responsive to staged N supply. Splitting minimizes leaching and volatilization losses especially in sandy, low-CEC soils typical of millet-growing regions
(Sairam et al., 2024).
Nano Urea in splits
Works best only as a supplement, not a full replacement. Studies show that supplying 50-60% basal nitrogen through soil plus two foliar sprays of nano urea can maintain yields while reducing total N use by 20-30% (
Gupta et al., 2023).
UAN in fertigation
UAN integrates well into drip and sprinkler systems, assuring uniform distribution and precise dosage. Fertigation at 7-10-day intervals after tillering has been shown to increase nitrogen use efficiency (NUE) and water-use efficiency in cereals (
Kannan et al., 2021). Its liquid nature also allows micro-dosing, which closely matches crop nutrient demand curves.
Overall
Combining small, frequent doses of UAN with strategic nano urea foliar sprays often leads to better physiological activity and improved yield components such as panicle length and grain filling.
Interaction with irrigation scheduling
Water availability strongly affects nitrogen uptake and transformation processes in pearl millet. Under dry conditions, surface-applied urea and UAN suffer higher volatilization, while nano urea being foliar remains largely unaffected.
Nano urea
Because the application is foliar, its performance depends more on leaf temperature and humidity than soil moisture. Sprays applied in the early morning or late afternoon improve absorption and reduce droplet evaporation (
Kumar et al., 2024).
UAN
Works best when irrigation is applied immediately after surface application or used directly in fertigation. Moist soil enhances ammonium retention and nitrate mobility, improving root uptake (
Verma and Singh, 2022). In drip systems, nitrate from UAN moves with water toward active root zones, resulting in higher efficiency in arid and semi-arid regions.
Practical takeaway
UAN requires closer coordination with irrigation, whereas Nano Urea is more flexible but still benefits from foliar application under mild temperature and adequate canopy hydration.
Compatibility with organic + inorganic nutrient management
Pearl millet responds well to integrated nutrient management (INM), I ntegrated nutrient strategies combining organic sources with inorganic nitrogen fertilizers have been shown to improve nutrient availability, soil health and nitrogen use efficiency in dryland millets
(Sagar et al., 2023).
Organic sources
(FYM, compost, poultry manure, green manures) enhance soil structure, microbial activity and moisture retention. These improvements create favourable conditions for root uptake of nitrogen supplied through UAN or basal urea
(Meena et al., 2020).
Nano urea and organics
Nano urea complements INM by providing rapid foliar nitrogen during nutrient-demand peaks, while organic nutrients build long-term soil fertility. Using nano urea after organic basal dressing can reduce total nitrogen use without compromising yield (Das
et al., 2024).
UAN and organics
Organic amendments reduce UAN-related risks such as salt concentration and leaf scorch. They also increase soil cation-exchange capacity, helping retain ammonium-N from UAN longer in the root zone
(Mahboob et al., 2023).
The comparative evaluation of nano urea and UAN indicates that both fertilizers improve nitrogen use efficiency (NUE) compared with conventional prilled urea, though through different mechanisms.
Nano urea primarily enhances foliar nitrogen delivery, bypassing soil-mediated losses such as volatilization and leaching. Its rapid absorption through stomatal pathways supports short-term physiological responses including chlorophyll synthesis and enzyme activation. However, evidence suggests that nano urea performs best as a supplement rather than a complete replacement for soil-applied nitrogen.
In contrast, UAN provides nitrogen in multiple chemical forms, enabling both immediate and sustained root uptake. This balanced supply improves nitrogen synchrony with crop demand, particularly during tillering and panicle initiation stages. Its compatibility with fertigation and precision agriculture technologies makes it more adaptable in irrigated and semi-irrigated systems.
Environmental implications also differ. Nano urea reduces direct soil nitrogen accumulation, potentially lowering nitrate leaching. However, long-term ecological effects of nano-materials require further study. UAN reduces volatilization compared with broadcast urea but may contribute to localized salinity if mismanaged.
From an economic standpoint, partial substitution strategies where nano urea replaces 25–33% of soil nitrogen or UAN is applied in split doses appear most promising for improving farmer profitability.
Overall, an integrated nitrogen strategy combining basal soil nitrogen with targeted foliar nano urea or precision UAN application may represent the most sustainable pathway for pearl millet systems.
Future research needs
Standardization of nano urea dose for pearl millet
Although nano urea is widely promoted, its crop-specific optimal dose for pearl millet remains unknown. Research on foliar nano-fertilizers shows significant variation in nanoparticle absorption among crops, suggesting that millet-specific evaluations are essential
(Ramesha et al., 2022; Venkateshwarlu et al., 2021). Pearl millet’s rapid early growth and distinct nitrogen uptake pattern further demand controlled dose-response studies to determine the most effective concentration and timing of foliar application. Establishing these standards will help prevent under-application and over-application, improving NUE and sustainability. Region-specific nitrogen management strategies are essential for enhancing productivity and sustainability in pearl millet systems.
Long-term environmental impact studies
While short-term agronomic benefits of nano urea are documented, major gaps remain regarding its long-term ecological behaviour. Studies on related nano-materials indicate that nanoparticles may persist in soils and influence microbial biomass and enzyme activity
(Rizwan et al., 2019; Kah et al., 2018). Given these concerns, long-term monitoring across different soil textures and climatic zones is necessary to understand nanoparticle fate, mobility, interactions with soil fauna and potential risks of groundwater contamination.
Field trials across soil types of India
Nitrogen transformation differs widely among India’s diverse soil systems from sandy Aridisols to clay-rich Vertisols. Research demonstrates that soil characteristics strongly influence fertilizer nitrogen availability and losses
(Snyder et al., 2017; Chatterjee et al., 2021). Therefore, multi-location field trials are essential to evaluate the performance of Nano Urea and UAN across representative Indian soil types, enabling region-specific nitrogen recommendations for pearl millet farmers.
Synergistic use of nano urea + UAN
The potential integration of Nano Urea (foliar) and UAN (soil + foliar) offers a promising but scientifically unexplored avenue. While combined nitrogen delivery pathways have been shown to enhance crop performance in other systems (
Fageria, 2014;
Lawlor, 2020), no published research evaluates this synergy specifically in pearl millet. Investigations should focus on compatibility, physiological response, tank mixing safety and heat-related risks such as leaf scorch.
Impact on soil microbiome and nutrient cycling
Soil microbes govern crucial nutrient processes and evidence from nano-material research shows that nanoparticles can alter microbial diversity, enzyme activity and nutrient cycling pathways
(Rajput et al., 2018; Chen et al., 2020). Because pearl millet is often grown in low-fertility, microbially active soils, it is important to examine how Nano Urea affects microbial communities, N-cycling genes and beneficial plant-associated microbes. Advanced tools such as metagenomics and soil enzyme assays should be prioritized.
