Introduction

Nitrogen (N) is an essential macronutrient in all living organisms. In the human diet, N ultimately originates from plants. The reduction of molecular atmospheric N to its biologically accessible forms is highly energy consuming, and is either accomplished by N-fixing archaea and bacteria (biological N fixation), or through the industrial Haber–Bosch process (anthropogenic N fixation) (Erisman et al. 2008). The growth of the global human population from 2.5 billion people in 1950 to 8.0 billion people in November 2022 was achieved by a major intensification of agriculture, referred to as The Green Revolution. Highly productive crops were bred and selected, machinery and irrigation were developed, pest control was improved, and NPK fertilizers increasingly used (Khush 2001).

To date, the global N cycling is heavily influenced by human activities, which is linked to severe environmental problems: It contributes to climate change, water eutrophication, ocean acidification, ozone depletion, and results in loss of species diversity and ecosystem instability (Gruber and Galloway 2008; Stevens 1979; Midolo et al. 2019). The scientific community has repeatedly concluded that the human contribution to global N cycling has exceeded the safe operating space and the planetary boundaries (Gruber and Galloway 2008; Steffen et al. 2015; Kopittke et al. 2021).

Thus, it has become a major goal to reduce the N inputs to agricultural systems while securing the global food production. The development of more efficient fertilizers is, among other strategies, intended to increase the nitrogen use efficiency (NUE) of crop plants and thereby reduce the need for applying excess amounts of N to secure yield and quality (Kopittke et al. 2021). In the last decade, slow release nanofertilizers have gained increased attention. Hopes connected to their application include enhanced efficiency of plant nutrient uptake, reduced greenhouse gas emissions, reduced leaching, and a better timing of N release aligned with crop demands (Seleiman et al. 2021).

The Indian Farmers Fertilizer Cooperative (IFFCO) recently developed and patented a novel urea nanofertilizer (NU) addressing exactly the abovementioned issues. The producers state it is possible to replace “a bag of urea” (45 kg urea or 20,7 kg N) by a 500 ml bottle of 4% w/v NU foliar fertilizer (43 g urea or 20 g N). According to the company, average yield increases of 8% with 50% less N application are possible, thus reducing the costs farmers spend on fertilizers significantly (Kumar et al. 2021b; IFFCO 2022). The product is promoted as being able to reduce nitrate leaching, eutrophication, greenhouse gas emissions, and toxicity to flora and fauna (Kumar et al. 2021b; Sharma et al. 2022). If these claims reflect reality, the innovative product is paving the way for the “sustainable green revolution” that many scientists have called for during the last decade. Having the product approved in India in 2021 (Adidam and Kumar 2021; Press Trust of India Ltd. 2022), the first production plant was inaugurated by the Prime Minister of India, Narendra Modi, in 2022 in Gujarat, India (Parikh 2022). Further production plants were built within the same year. The company plans to expand sales to another 25 countries, mainly situated in Asia, Africa, and South America. IFFCO aims to sell 300 million bottles of NU until the end of 2024 (Press Trust of India Ltd. 2023a), and recently started to expand the product palette with Nano DAP (Di-ammonium phosphate), Nano Copper, and Nano Zinc (Kumar et al. 2020b; IFFCO 2022, 2023).

However, if the claims made by IFFCO prove to be overstated and far from reality, this might lead to large scale yield losses and have tremendous impacts on food security, livelihood of farmers, and reduce the trust into innovative sustainable products as well as the science behind them. From a plant nutritional perspective, it is rather puzzling how 20 g N from NU should be able to replace 20,7 kg of N from traditional urea (Kumar 2021b; Babu et al. 2022a, b; IFFCO 2022). In this Opinion Paper, the claims made by IFFCO are compared to the existing scientific evidence in the field of foliar nanofertilizers. We discuss, in how far the potential mismatches between claims and evidence might cause problems to farmers, societies, the environment, and not least the trust in plant and soil science.

What do we know about Nano Urea (liquid)?

The basic properties of NU particles are listed in Table 1. The Indian patent no. 400681 connected to the product states that 20 – 50 nm large spherical to rod-shaped nano particles are produced, when urea interacts with 50 – 500 nm long linear fibers derived from a naturally occurring glucose polymer at elevated temperatures (Raliya 2020, 2022). The authors do not describe which chemical processes lead to this remarkable change in size. The surface to volume ratio of NU particles is increased by factor 1000 (Babu et al. 2022b) to 10 000 (e.g., Babu et al. 2022a) compared to conventional urea.

Table 1 Properties of NU particles as reported in different references
Full size table

The behavior of nanoparticles (NPs) as fertilizers or as potentially harmful compounds in the environment or food chain depends greatly on structural, morphological, and chemical properties of the particle itself or the detergents (solvents) used to formulate it. Therefore, it is important to investigate these characteristics thoroughly (Iavicoli et al. 2017). However, apart from a TEM image (Kumar et al. 2022), no morphological or structural analyses of NU are reported so far. Physicochemical parameters like size and surface charge of a particle critically define the stability and the ability to translocate across the foliar barriers and within the plant (Husted et al. 2023). These properties also define retention and accumulation in the environment such as in soils, water bodies, or the food chain (de Oliveira Pereira et al. 2020). The surface zeta potential of NU particles, a measure for the particle surface charge, is listed in Table 1. Remarkably, whether the values were positive or negative, was not stated in the references found. Furthermore, no studies about the dissolution dynamics or the N release from NU in laboratory experiments or in the field have been published.

Nano Urea (liquid) as a fertilizer

The foliar N uptake efficiency of urea is potentially high; it can amount to 69% in wheat (Smith et al. 1991). Foliar treatments with aqueous solutions containing > 2% urea may lead to visible and physiological symptoms associated with ammonia toxicity (de Castro et al. 2022). Controlled release N fertilizers have been proposed as a novel approach to allow for application of higher amounts of foliar N, where N is either encapsulated, coated, or present in a nano composite (Zulfiqar et al. 2019). However, IFFCO recommends a mix of NU resulting in a foliar spray containing a maximum of 0.035% w/v urea (Raliya et al. 2021). This is far below the observed limits of toxicity for conventional urea. Thus, the benefits of applying highly processed urea nanoparticles rather than dissolved urea as a foliar spray appears puzzling, unless the scientific community is unaware of unknown positive side effects of nano-sized materials stimulating plant growth.

Table 2 summarizes studies, claims, and recommendations associated with NU by IFFCO. It becomes evident that there is a significant mismatch between the control treatments used in field studies, the subsequent claims made, and the resulting recommendations to farmers. In order to state yield improvements in response to a specific form of fertilizer application, the reference point must be clear and well defined. IFFCO scientists did clearly not meet this basic requirement.

Table 2 Summary of treatments from literature in relation to NU by IFFCO and what they mean in terms of fertilizing efficiency compared to conventional fertilizers
Full size table

The major field studies cited in Table 2 were conducted in various regions in India for only one season in winter 2019/2020 (Kumar et al. 2020a, b; Tiwari et al. 2021). None of the studies shows basic physical or chemical analyses of the soils, nor weather/climate data. Most importantly, the N availability in the soils was not investigated prior to the experiments. In order to assess whether a fertilizer has a positive effect on plant growth, it is essential to ensure that the yield increase is caused by fertilizer application, and not for instance by N mineralization from residual N stocks. This is ensured by analyzing N uptake in a control treatment not receiving N fertilizer (Hochmuth et al. 2022), which was not done here. In addition, the studies lack statistical analyses of the data; the experimental set-ups are poorly described including the location of the plots and the composition of the fertilizers. It is remarkable that some text sections have been copied from one publication to another (Kumar et al. 2020a, b; Tiwari et al. 2021). Authors employed at IFFCO do generally not state any conflicts of interest (Kumar et al. 2020a, b, 2021b, c; Mishra et al. 2020; Tiwari et al. 2021).

Most recently, authors claimed that the application of 75% of the recommended N dose (in the form of conventional urea) to the soil plus two foliar sprays of NU resulted in the same yield as applying the full dose of 100% conventional urea to the soil. Based on this, the authors conclude that a 25% reduction of N usage is possible with NU. However, the conclusion falls short in stating that the 75% N treatments with and without NU resulted in the same yields in six out of eight investigated seasons. Thus, according to the results, NU could actually be omitted without a reduction in yield. Yet again, the study does not include a proper control treatment where foliar application of NU was compared to foliar application of a conventional N source (Upadhyay et al. 2023) making it impossible to isolate the effects of the nano approach.

The dry matter concentration of N in most crops is 1–5% (de Bang et al. 2021). Thus, hypothesizing that the 20 g N present in a bottle of NU was taken up by the crop 100% efficiently, a bottle of NU would provide sufficient N to grow a maximum of 0,4 – 2,0 kg plant material. A field with such a low biomass productivity would fail to produce any significant amounts of cereal grains. By contrast, a bag of conventional urea containing 20,7 kg N would provide enough N to grow 414,0 – 2070,0 kg of plant material. This difference corresponds to a reduction in biomass of more than 1000 fold.

The yield increase reported for NU might not be a result of N fertilization, but rather of unknown growth stimulating side effects that NPs may have, e.g. scavenging reactive oxygen species to reduce oxidative damage and thereby increase plant stress tolerance (Seleiman et al. 2021). However, none of the abovementioned explanations supports a stunning increase in the NUE by more than 1000-fold. IFFCO is selling and promoting the product as a fertilizer, not as a growth stimulant (Kumar 2021b; Babu et al. 2022a; IFFCO 2022).

In another study, conventional NPKS fertilizer application was compared to application of organic manure plus two sprays of NU (Kumar et al. 2022). Both treatments occurred with the same estimated amount of nutrients. The authors claim yield increases of 24.24% (reported with two digits!) for sesame, 4.2% in pearl millet, and 8.4% in mustard, although these effects appear not to be statistically significant.

Scientific reviews on the topic repeat claims made by IFFCO since 2019, such as NU being taken up “80% more efficiently than conventional urea” (Babu et al. 2022b). However, the authors refer to a publication about a chitosan-based nano NPK fertilizer investigated for its efficiency on wheat grown in a sandy soil in Egypt (Abdel-Aziz et al. 2016), three years before NU entered the market (Kumar 2021b). Another review repeats similar statements on high efficiency and environmental compatibility of NU by IFFCO (Sharma et al. 2022), but cites an article titled “Current research on silver NPs: synthesis, characterization, and applications” (Dawadi et al. 2021). A review on the use of nanotechnology in sustainable agriculture even describes a pathway of NU particle translocation within plants including phloem translocation (Kumar et al. 2021c). However, there is not one single reference to support this statement, and to the best of our knowledge, no one has ever managed to trace urea or urea based NP translocation in the phloem. We generally observe that i) many reviews made about nanotechnology in agriculture make claims without providing proper scientific evidence, and/or ii) cite unrelated references, and/or iii) cite older reviews or statements made by companies rather than referring back to the original literature reference.

Do we know how Nano Urea interacts with plants?

IFFCO advertises NU as useful on a large variety of plants, including “cereals, pulses, vegetable, fruits, flowers, medicinal [eds. plants] and others” (IFFCO 2022). Independently of the crop concerned, two sprays are recommended: 30–35 days after germination or 20–25 days after transplanting, and 20–25 days after the first spray (Kumar 2021b; Babu et al. 2022a; IFFCO 2022; Kantwa and Yadav 2022). However, evidence suggests that uptake dynamics vary due to leaf properties such as morphology, stomata location and density, thickness and composition of the cuticular wax layer, and trichomes (Pérez-de-Luque 2017). Therefore, the efficiency of foliar uptake of any substance is expected to vary across different crops and growth stages. With respect to this evidence, there must be a more efficient way to use a foliar fertilizer than advising a one-fits-all recipe to farmers.

IFFCO reports enhanced photosynthesis and chlorophyll contents (IFFCO 2022) and refers to (Jadhav et al. 2022), where a Soil Plant Analysis Development (SPAD) study was conducted on NU sprayed maize. However, the study does not state if the basal fertilization had been reduced in the NU treatment, or whether NU was added as a top-up. The authors do not mention on which leaf position the measurements were made, how old the plants were, or how long after fertilizer application the analysis took place (Jadhav et al. 2022), all of which are critical for SPAD measurements (Kandel 2020). Among several additional flaws, the study was conducted for only one season and the conclusions are lacking statistical backup (Jadhav et al. 2022).

According to some publications, NU adheres to plant leaves and particles are taken up through stomatal openings, hydathodes and other leaf openings. Subsequently, particles are loaded into the phloem, and can pass plasmodesmata connecting individual cells. By binding to transport proteins, NU can pass aquaporins and ion channels to enter cells, or via endocytosis. NU can be stored in vacuoles until needed, and urea is released from the particles in a controlled process (Babu et al. 2022a; IFFCO 2022). It has indeed been shown that NPs may be taken up by cells via endocytosis and can be transported within the symplasm. However, it has never been shown that untargeted NPs are taken up by any transport proteins, including aquaporins and ion channels, as NPs are expected to be excluded by e.g. size (Pérez-de-Luque 2017). Thus, the uptake and transport pathway stated by IFFCO remains purely speculative.

Lastly, IFFCO states that the product works efficiently for soil applications (IFFCO 2022), but the exact application is not specified on the web page or elsewhere to our knowledge. By now, studies about NU application on soils do not exist in the scientific literature.

Is Nano Urea (liquid) non-toxic and environmentally friendly?

The environmental friendliness of NU plays a major role in the promotion of the product (Kumar 2021b). A closer inspection of the NU website: “Production of IFFCO Nano Urea is energy and resource friendly” and “It reduces excess application of bulk urea and associated volatilization as well as leaching and run off losses” (IFFCO 2022). Similar statements have been adopted or generalized in various forms in a variety of publications (e.g., Kumar et al. 2020b, 2021b; Babu et al. 2022a, b; Kantwa and Yadav 2022). However, N utilized for producing NU must originate from the Haber–Bosch process alike N in any synthetic fertilizer. According to the patent application, the N source used is urea (Raliya 2020, 2022) and any processing (e.g. mixing, cooling, heating, transport) of urea by IFFCO must increase the environmental footprint per unit of N applied, when compared to bulk urea.

N losses by leaching through the soil column in the form of NO3−, as well as N volatilization in the form of NH3 to the atmosphere can lead to a variety of negative environmental impacts, as well as economic losses for farmers (Stevens 1979). However, only the latter form of N loss from soils is a major problem for the usage of bulk urea. Therefore, urease inhibitors or fertilizer coatings are frequently used nowadays in order to prevent N loss to the atmosphere as ammonia (Klimczyk et al. 2021). It has been suggested that nanotechnology can enhance nutrient uptake by plants, and reduce their loss through leaching and volatilization (Iavicoli et al. 2017; Seleiman et al. 2021). However, this has not been investigated for NU. The N field balance for NU is not established, and the loss of N per unit N applied remains to be elucidated. One could assume lower environmental impacts of NU due to its supposedly high efficiency and lower necessity for N application. However, as discussed above, these claims are not sufficiently documented.

As stated by IFFCO, NU was tested for bio-toxicity according to the “Guidelines for Evaluation of Nano-Based-Agri-Input & Food products in India 2020” released by the Department of Biotechnology, government of India (IFFCO 2022). IFFCO provides a product safety assessment sheet meant to document the non-toxicity and biosafety of the product to a variety of organisms, as well as soil and water quality in line with several international guidelines (Kumar et al. 2021b; IFFCO 2022). However, the sources do neither contain information about the laboratories in charge for the testing, nor about the procedures used for the tests, nor about detailed test results. Risks attached to nanofertilizers and the procedures necessary to ensure their safe use have been pointed out in detail (Iavicoli et al. 2017). We must assume that such safety measures have either not been met by IFFCO, or the test results have not been published transparently to the public.

Do farmers and societies benefit financially from Nano Urea (liquid)?

The price of NU has been set strategically 10% below the price of a bag of conventional urea on the Indian market (Kumar 2021b; Kumar et al. 2021a; Baboo 2021; Kantwa and Yadav 2022) and is to date at 225 ₹ (Indian Rs.) (IFFCO 2022). While conventional urea is subsidized by the Indian government (Kishore et al. 2021), NU is excluded from the subsidies (Kumar et al. 2021a; Baboo 2021; Press Trust of India Ltd. 2023b). Potentially, this allows the Indian government to save money when conventional urea is replaced by NU (Kumar et al. 2021c; Press Trust of India Ltd. 2023b). Per unit of N sold to Indian farmers, NU is approximately 1000 times the price of conventional urea (225 ₹ for 0,02 kg N versus 242 ₹ for 20.7 kg N) (Kumar 2021b; IFFCO 2022). Therefore, farmers save expenses only as long as the efficiency of the product as a fertilizer has been proven to be at least 1045 times as high as for conventional urea. As discussed above, this is currently far from being proven. Some studies present increases in the economic returns of farmers per land unit cultivated under application of NU, but the basis for the calculations (e.g. market prices) were not specified and therefore impossible to elucidate (e.g., Kumar et al. 2020a, b; Tiwari et al. 2021).

The price of NU has already enforced a socio-economic crisis: Sri Lanka has been suffering from a severe economic crisis since 2020, which brought the government to reduce expenses spent on synthetic fertilizers by rashly shifting to organic agriculture in 2021 (Wipulasena and Mashal 2021; Torrella 2022). In combination with high fuel prices, this has lead to large scale yield losses and rising food insecurity since 2022 (UN/OCHA 2023), culminating in riots calmed only by military forces and the president fleeing from the country (Mashal 2022). Remarkably, the Sri Lankan government had purchased Nano Nitrogen by IFFCO in the course of the crisis in 2021 at a price 4 times as high as on the Indian market (Nilar 2022). Later, it turned out that the purchased Nano Nitrogen was in fact NU (Kumar 2021a; Ileperuma 2021). In 2022, these procedures were questioned by the political opposition in an audit report and required the Sri Lankan minister of external affairs to resolve the disagreements with India (Nilar 2022).

What is the way forward?

Based on the above, it seems necessary and acute that independent bodies conduct scientific research on whether NU has a positive impact on plant growth and to which extent. Its functional properties at plant physiological level (i.e. uptake and assimilation mechanisms), and assessments of environmental risks, toxicity, and life cycle assessments (LCA) must be performed and made available to the public. We propose to follow the framework listed below to prove that the nanofertilizer has the positive yield responses expected. Secondly, we refer to (Pérez-de-Luque 2017; Seleiman et al. 2021; Husted et al. 2023), as these publications describe which details must be known in order to understand uptake and assimilation of NPs as fertilizers at the mechanistic level. Thirdly, potential environmental risks associated with the agricultural use of NPs are reported in literature, and frameworks are provided in order to assess them properly (Iavicoli et al. 2017; de Oliveira Pereira et al. 2020). These guidelines should be followed, and results should be transparent to the public. In addition, LCAs may allow to justify claims on environmental friendliness of a product (Hasler et al. 2015).

Based on authoritative publications (Karamanos et al. 2014; Davis et al. 2017; Hochmuth et al. 2022), we selected four central criteria that must be met in order to document the superiority of nano based N fertilizers in the field:

  1. 1.

    The field study must be conducted on an N responsive soil

    The plant-availability of soil-derived N during the experimental period must be sufficiently low to induce deficiency and a yield response if fertilized. Preferably several levels of N additions should be used in order to reflect the yield-response curve for N at the experimental site.

  2. 2.

    Choose control treatments that fit the conclusions to be drawn

    It is essential to set and state a clear reference point (control treatment) to which the treatment is superior or mediocre to. In field testings, the control treatments are typically a plot not receiving N fertilizer and a plot reflecting best practice fertilization for a given crop cultivated on a specific soil type and location.

  3. 3.

    Describe materials and methods properly

    Materials and methods for field trials must contain proper information about which products (including detailed description of NP composition) were applied to a specific crop cultivar.

  4. 4.

    Make an adequate amount of randomized repetitions and apply statistics

    When drawing quantitative information from a field trial, it is necessary to design the experiment in a way that allows proper statistical evaluation of the obtained results. Therefore, an adequate amount of randomized repetitions (typically n>3) for each treatment and control are necessary. The experiment should span several years, and a wide range of soils and growing conditions. Statistical tests must be reported to ensure that the observed effects are indeed: i) significantly different, ii) reproducible over time and between seasons, and iii) excluding type I errors expected in nature.

Conclusion

NU is a novel N fertilizer recently marketed by India’s largest fertilizer manufacturer (IFFCO) in a way that suggests its effect and sustainability have been scientifically proven. We challenge this presentation by questioning and comparing the claims with the existing scientific literature from well-reputed journals. We conclude, that NU is a scarcely characterized product with no or poor scientifically proven effects. The product is promoted with misleading and wrong statements about its efficiency as a fertilizer, plant uptake pathways, and environmental friendliness. With the doubts about the effect of NU in mind, the price of the product seems excessively high. Reported economic benefits for farmers applying NU suffer from the absence of a solid basis for the calculations. We do not stand alone with these doubts (Saklani et al. 2022), and some critical voices have recently been raised in India as well, i.e. by N.K. Tomar (retired professor of soil science at CCS Haryana Agricultural University, India). As IFFCO aims at an expansive and large scale production, there are several serious risks associated to these commercial ambitions: Large scale yield and economic losses to farmers, food insecurity, environmental risks, social disruption and conflicts. Politics and commercial incentives might interfere with the independence of the scientists involved. We acknowledge the great benefit that nanofertilizers potentially may have, but seriously doubt the positive impact of NU for farmers, societies and the environment as it stands right now. We emphasize that independent bodies should follow a consistent framework to verify whether novel fertilizer products have indeed beneficial effects for crop growth and yield prior to marketing and applying them widely. Finally, it appears that peer review of many papers within the area of nanofertilization is of insufficient quality as the fundamental lacks described above should prevent publication in any well reputed scientific journals.