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RIVM on Advanced Materials, October 2024

Environment

Nanoforms of fertilisers or biostimulants sprayed directly onto plant leaves may contribute to more sustainable agricultural practices. However, it needs to be clarified whether the current legislation and methodologies sufficiently addresses the nanospecific properties of these products when assessing the benefits and risks for crops, the environment, and human health.

What are biostimulants?

A recent review paper suggests that spraying the leaves of crops with biostimulants in nanoform can offer significant benefits for agriculture. Biostimulants can enhance crop growth and yield by stimulating natural growth processes. They are believed to help plants cope with environmental stress caused by drought, salt or heavy metal pollution. In that sense, they differ from fertilisers, which provide plants with essential nutrients such as nitrogen, phosphorus and potassium. Or from pesticides that protect plants against biological stressors. The biostimulants included in the review paper primarily consisted of nanoforms of metal and metal oxide particles.

The benefits of foliar spraying of biostimulants in nanoform

Nanoforms of biostimulants and fertilisers, like nanoforms of plant protection products, are believed to offer advantages over conventional products. For instance, nanostructures containing essential plant nutrients could enable slower or more controlled release of these substances. This means less fertiliser may be needed per hectare, reducing environmental pollution from extensive fertiliser use.

A new development in nanoforms of fertilisers and biostimulants is foliar application. The product is then applied directly onto plant leaves rather than into the soil. If nanoparticles are small enough, they can enter plants through the stomatal pores of the leaves and be transported within the plant. This potentially leads to faster and more efficient uptake by the plant. This method could also reduce fertiliser use by bypassing the soil, thus avoiding washout and losses through interaction with soil components.

Potential disadvantages of foliar fertilisers and biostimulants in nanoform

Apart from the potential benefits of foliar nanoforms of fertilisers and biostimulants, the authors mention several challenges in their review. One challenge is determining the optimal concentration of foliar nanoforms of fertilisers and biostimulants, which can vary from crop to crop. Further studies are needed to establish the best application concentration and time. Additionally, the authors briefly mention concerns about potential toxicity for plants, the environment, and human health. Overall, according to the authors, foliar nanoforms of fertilisers and biostimulants offer a promising solution for more sustainable agriculture.

Reflections by RIVM

As part of the Farm to Fork strategy, one of the pillars of the European Green Deal, the European Commission aims to reduce nutrient losses during crop production. The Commission expects that the strategy will result in a 20% decrease in fertiliser use.

The use of foliar nanoforms of fertilisers and biostimulants could help achieve this goal. However, spraying these products directly onto leaves may pose additional exposures and risks for the environment, users, workers, bystanders or local residents. Even for the consumer, additional risks may occur due to exposure to residues in crops. There is a lack of scientific studies addressing the potential risks of foliar application of nanoforms of fertilisers and biostimulants. In addition, agricultural engineers will need to optimise the use conditions, which will require more precision than with conventional fertilisers or biostimulants.

In the EU European Union (European Union ), biostimulants were previously considered Plant Protection Products. This changed with the new Fertilisers Product Regulation (EU 2019/1009) that came into effect in 2022. Biostimulants are now classified as fertilisers. This reclassification is due to their properties and intended use in stimulating plant growth and resilience. It has lead to their removal from the Plant Protection Regulation.

Under the Plant Protection Product Regulation, there is an increased focus on assessing the human and environmental safety aspects specific to nanomaterials. For fertilisers, now including biostimulants, the Fertiliser Product Regulation sets out rules for the market authorisation of CE-marked fertilisers. This also requires that fertilisers must meet established standards for human health and environmental safety. In addition, the safety of biostimulants is evaluated by so-called Conformity Assessment Bodies, which are a limited number of notified bodies on EU member state level. The assessment of hazardous properties relies primarily on data required under the REACH legislation.

It is essential for the assessment of fertilisers and biostimulants to consider the potential nanospecific effects on human health and the environment. However, it currently needs to be determined whether the regulatory framework and related methods for risk assessment adequately addresses nanospecific concerns and is prepared for these upcoming products. For example, there have been some concerns about the safety of nanotechnology (e.g. as adverse effects have been observed in plants). Furthermore, uncertainties remain in understanding the environmental fate and toxicokinetics of nanomaterials as well as in the methodologies(PDF) to assess them. Further clarity on these uncertainties is needed to determine whether the promising benefits of using nanoforms of fertilisers or biostimulants directly on plant leaves indeed exist and outweigh potential risks.