Back to overview

13. November 2025, Topic: Soil Ecotoxicology Risk Assessment

Monitoring concept for plant protection products in Swiss soils

The Ecotox Centre is working with the National Soil Monitoring Programme (NABO) / Agroscope and EnviBioSoil on a concept for monitoring pesticide residues in Swiss agricultural soils. The project is part of the federal government's action plan for risk reduction and sustainable use of pesticides. Project manager and soil ecotoxicologist Mathieu Renaud reports on the progress made so far.

Plant protection products (PPPs) are used to protect crops from pests and thus ensure high yields. However, these substances can also have unintended toxic effects on beneficial non-target organisms in the soil. In order to reliably assess and minimise these risks, PPP residues in soils should be monitored regularly. Since 2019, the Ecotox Centre has been developing a concept for monitoring PPP residues in Swiss agricultural soils, which is embedded in the federal government's action plan. Two things are important for assessing the risk of PPPs in soil and maintaining long-term soil fertility: the development of risk-based soil reference values – known as soil guidance values (SGVs) – and the selection of bioindicators that can be used to investigate the effects of PPP residues on soil organisms.

What has been achieved in the project so far?

We have already made considerable progress: we have published recommendations on risk assessment in soils and developed a method for determining SGVs. With regard to bioindicators, we have established links between the various soil organisms and ecological soil functions and compiled a list of potential field and laboratory methods for the most important organisms. In doing so, we have taken into account how stakeholders assess the importance of various ecosystem services for soil fertility and consulted with experts on suitable methods. We have also developed a concept for integrated biomonitoring that combines SGVs and bioindicators.

What were the difficulties?

The properties of soils, their diversity and the way they are used complicate matters. Agricultural soils are actively farmed and PPPs are used in a targeted manner to secure the harvest. However, their concentrations should be low enough outside the application period so that soil quality is not impaired in the long term. For farmers, the sustainable use and long-term productivity of their soils is a key concern. Overall, it is a difficult balancing act to achieve the desired effect on target organisms during application while avoiding impacts on non-target organisms outside these periods.

Time plays an important role in protecting long-term soil fertility. As the European Food Safety Authority (EFSA) recently emphasised (https://doi.org/10.2903/j.efsa.2025.9501) , depending on the group of organisms, minor or moderate effects may occur during or immediately after application and are acceptable – however, they should not last longer than six months and should be negligible thereafter. For this reason, our colleagues at NABO take their samples in winter, i.e. outside the application period. During this time, PPP residues should only have a negligible impact, allowing the soil to recover until the next season. This should prevent the accumulation of residues and their effects.

According to the action plan, risk-based reference values are to be proposed in 2025. Are there already specific values?

Yes. We have already published SGV and accompanying dossiers for six plant protection products: difenoconazole, fenpyrominat, fluazinam, pendimethalin, pirimicarb and tebufenozide. Three more are to follow by the end of 2025. These dossiers are very detailed and contain not only efficacy data but also information on the physical and chemical properties of the substances, their mode of action, their use, their emissions, their fate in the environment and their accumulation in animals (bioaccumulation). As you can imagine, this process is very valuable, but also time-consuming. Since PPP authorisations can be withdrawn or new PPPs introduced, it was important to find a less complex approach to supplement it. For this reason, we are also developing ad hoc values. These are based exclusively on data from the authorisation process and do not include complete dossiers.

How can such values be used and do they have regulatory effect?

In their current form, SGVs are not regulatory values. We propose using SGVs as screening values to ensure that potentially contaminated sites are reliably identified. If the values are exceeded, this will lead to further investigations before the risk for a site is finally assessed. For sites with concentrations below the SGV, we can be sure that the PPP residues do not impair soil fertility. The SGVs are used to interpret the PPP residues measured in the soil and indicate the concentration at which soil fertility could potentially be at risk. It is up to the responsible authorities to decide whether to incorporate the values into a regulation.

Soil animals play a central role in maintaining soil functions and fertility. These include some little-known organisms such as springtails, nematodes, mites and enchytraeids, which can also serve as valuable bioindicators of soil health (photo: Andy Murray, https://www.chaosofdelight.org/).

Another project goal is to recommend bioindicators for monitoring the effects of PPPs on soil fertility. Which organisms are particularly suitable?

This decision was not an easy one for us. It seems obvious to start by considering those organisms that contribute directly to soil fertility. However, fertility is the result of a complex network of many different species working together. In addition, certain groups of organisms are the subject of more intensive research because they have been studied more thoroughly. This can easily give the impression that they are more important than they actually are.

Despite these challenges, we now have a better understanding of the role that different soil organisms play in soil functions and ecosystem services. On this basis, we have mapped the relationships between ecological soil functions and soil organisms. To do this, we first linked ecological soil functions – such as providing habitat, regulating processes or supporting production – with the corresponding ecosystem services and the biological processes underlying these services. We were then able to assign these processes to the corresponding organisms.

We further refined this scheme by incorporating the assessments of various Swiss stakeholders – including researchers, policy makers and farmers – on the importance of different ecosystem services for soil fertility. From this ranking, we identified the groups of organisms with the best ratings – i.e. those that are most promising as bioindicators. These include enchytraeids, springtails, nematodes and earthworms.

A number of methods had to be proposed for these key organisms. What criteria did the methods have to meet in order to be included in the toolbox?

We selected methods that are as standardised as possible or at least well established. Our concept combines ecotoxicological methods, which are applied under controlled conditions in the laboratory, with ecological field methods. For each key organism group, we held a technical workshop with national and international experts to determine the best methods.

In addition to selecting the "best" method, we also had to consider practical limitations related to sampling: Do the indicators also work in winter, when sampling is carried out to detect PPP residues? And are the soil quantities required to apply the method feasible within the scope of monitoring?

Interpreting and communicating effects can be challenging. What is the situation with regard to monitoring PPP residues?

This is because monitoring approaches covering large areas differ from the more established approaches for assessing substances or contaminated sites. Indicators are usually compared with a control or reference soil. However, it is very difficult or even impossible to find a suitable and representative control site for each monitoring site. Building on experience gained from sediment and surface water monitoring, we are currently conducting experiments to model the natural variability of our ecotoxicological indicators as a function of various soil properties. The aim is to define the normal ranges of variation and derive effect thresholds from them.

Ecological indicators are often multivariate – they reflect, for example, the abundance of many species – and are therefore more complex and variable. It is currently not possible to generate the data required to define normal ranges or effect thresholds for ecological indicators. We therefore propose monitoring the indicators over longer periods of time and using trends to draw conclusions about possible risks. Is the abundance or diversity of organisms stable, or is it increasing or decreasing?

At the same time, it is important to view the results in the right context and to take into account the existing uncertainties and limitations of the various approaches. We are currently in the applied research phase, in which many questions remain unanswered and methods need to be further validated and refined. Our goal is to eliminate as many uncertainties as possible. We therefore propose an integrated approach that encompasses the fields of chemistry (SGVs), ecotoxicology (laboratory methods) and ecology (field methods). Exceeding a single threshold value does not automatically mean there is a risk. We can only assess the risk and try to determine the causes if we consider the areas in combination.

And what are the next steps?

We will complete and publish the remaining SGVs and ad hoc SGVs by the end of 2025. We are also finalising a review article on the risk assessment of PPP mixtures, which contains corresponding recommendations.

With regard to bioindicators, we have already sampled 32 sites for the interpretation of ecotoxicological methods and 15 sites for a pilot study using the complete bioindicator toolbox. We are still working on the evaluation and publication of these studies. We will also soon be launching a pilot study on the integrated approach at several measuring points. There we will apply both the SGVs and the bioindicator toolbox.

Info: What are soil guidance values?

Soil guidance values (SGVs) are effect-based reference values for soils. They are based on ecotoxicological studies with soil organisms and indicate the concentration below which no harmful effects are to be expected – comparable to the already established environmental quality criteria for surface waters. In Switzerland, the Ordinance on Soil Pollution (VBBo) currently only specifies soil limit values for a few heavy metals and persistent organic compounds, but not for currently used plant protection products. Such values are also hardly available in other countries. There is also no internationally recognised method for deriving SGVs, so the Ecotox Centre first had to develop such a method.

Project partners

The project is being developed in collaboration with EnviBioSoil. The pesticide residues in the soil are being chemically analysed by the National Soil Monitoring Programme (NABO) / Agroscope.

More information

All publications and reports from the project can be downloaded at https://www.ecotoxcentre.ch/news-publications/news/monitoring-concept-for-plant-protection-products-in-swiss-soils

title photo: Eawag

11. November 2021

How to determine ecotoxicological reference values for soils?

The risks associated with plant protection products in Switzerland are to be reduced by half in the next decade. An important tool for this is the determination of effect-based screening values for plant protection products in soils. But which methods are available for this purpose? What points need to be taken into account? The Ecotox Center takes stock of the situation.

Read more