Pesticide Registration Toolkit

Local risk assessment for soil organisms


A risk assessment for soil organisms is generally carried out in a tiered manner, where a Tier 1 analysis is intended to be a conservative screening that separates pesticides that are not expected to present a risk from those pesticides that may. Higher tiers are intended to refine the estimates of exposure, effects, and the resulting characterization of risk. This tiered risk assessment approach is shown in the figure below.

Based on the above figure, the following risk assessment framework is proposed for pesticide regulators with limited resources.

Tier 1 (worst case risk assessment):

i. Ecotoxicology: Require the 3 standard laboratory tests (see Data Required).

ii. Exposure: Estimate exposure using a generic calculator, e.g. UK PEC soil calculator (or equivalent); use worst case DT50 value.

If risk unacceptable:

Tier 2 (refinement):

i. Step 1. Refine Exposure estimate through:

Use of exposure calculator with more scenarios (e.g. Dutch calculator);


Use DT50 values determined under locally relevant conditions (soil, temperature, humidity).

If risk unacceptable:

ii. Step 2. Refine Ecotoxicology through:

For species/processes that did not pass Tier 1: Add additional locally relevant species from same group (e.g. tropical earthworms);


Conduct toxicity tests in locally relevant soil/temperature;


Add additional, locally relevant, species not covered in Tier 1.

If risk unacceptable:

Tier 3a (semi-field or field studies – bridging)

Assess whether semi-field or field studies are available that can be bridged to the local situation; i.e. conducted in similar agro-ecological situations.

If not possible or risk unacceptable

Tier 3b (semi-field or field studies – local)

Conduct semi-field or field studies under locally relevant conditions (cropping system, soil type, temperature, rainfall etc.)


In the first tier risk assessments, chronic toxicity of the pesticide to soil organisms is generally compared with its predicted environmental concentration (PEC), by calculating Toxicity Exposure Ratios (TER).

The risk assessment can be refined in Tier 2 by using additional toxicity data and/or by applying more precise exposure estimates.

If the pesticide risk is unacceptable after the first two tiers, higher tier studies may be carried out, e.g. semi-field studies or field studies. In higher tier evaluations, no TERs are calculated such as in first tiers, but risks are characterized for the specific situation studied. Higher tier assessments therefore tend to be more specific to local cropping and pesticide application conditions.

Risk mitigation measures can be considered to reduce pesticide risk at any tier. If effective risk reduction measures can be identified and applied, higher tier evaluations may not be required. Risk mitigation for soil organisms is discussed in the Risk Mitigation Module.



First tier risk assessment methods described for temperate/Mediterranean countries can in principle be applied to soil organisms in other climatic zones. This is because first tier risk assessments are carried out with surrogate species (e.g. Eisenia fetida and Folsomia candida) which are considered sensitive to pesticides.

Assessment factors (or safety factors) are used in the risk assessment. These are supposed to cover various uncertainties in the first tier risk assessment, for example that species occurring in the wild or in the tropics can react more sensitively than species examined in the laboratory, or that only a few species have been tested.

Higher tier assessments tend to be more locally specific including more sophisticated exposure models.

Various relatively simple spreadsheet-based models exist which require few data, give robust results and can be applied with limited resources. These include the UK PEC Soil calculator, or the Dutch Ctgb PEC soil calculator. These models are based on similar principles and calculate pesticide concentrations in the top layer of the soil, dependent on the application rate and frequency, the half-life of the pesticide and crop interception fractions.

The UK PEC soil model is suggested to be used as basic exposure model for first tier risk assessment. It is based on single first order kinetics only and calculates pesticide concentrations in the top 5 cm soil layer.

The basic calculations of the maximum predicted environmental concentration (PEC) in the soil, for single or multiple pesticide applications, are as follows:

PECsoil ini = AR * (1- CI) / (100 * d * ρ)

PECsoil ini, multiple = (AR * (1- CI) / (100 * d * ρ)) * MAF


Parameters for PEC soil calculations

Data source

AR = application rate in kg/ha (equivalent to mg/100 cm2)

Registration dossier; GAP table

CI = crop interception (percentage or fraction) (i.e. the fraction of the applied pesticide not reaching the soil)

Value depends on the type of crop and its growth stage.

Local knowledge or published default crop interception values

d = soil depth: 5 cm

Default value

ρ = dry bulk density: 1.5 g/cm3 (equivalent to 1500 kg/m3)

Default value

DT50 = (Disappearance Time 50%) is the time within which the concentration of the test substance is reduced by 50%; it is different from the half-life (t1/2) when transformation does not follow first order kinetics.

Registration dossier

Molecular weight for metabolites

Registration dossier

k = ln2 / DT50 (in d-1)

Single first order kinetic, calculated in the model

n = number of applications

GAP table

i = application interval in days

GAP table

MAF = multiple application factor = (1- e-kni) / (1- e-ki)

Calculated in the model


For persistent pesticides, the PEC accumulation (sometimes referred to as PEC peak) should be calculated, which is the PEC initial plus the PEC plateau. Both proposed calculators (i.e. the UK PEC Soil calculator and the Dutch Ctgb PEC soil calculator) are able to produce PEC accumulation values.


For further refinement of the exposure estimate, especially for persistent pesticides, the Dutch Ctgb PEC calculator can be used, because it also includes other degradation kinetics (i.e. First Order Multi-Compartment (FOMC), Double First-Order in Parallel (DFOP)). It further has a larger number of scenarios compared to the UK PEC soil calculator and allows metabolite degradation to be covered in the same calculations. A table comparing the two generic calculators of predicted environmental concentrations (PEC) in soil is available.

Whenever available, DT50 values from soil types and climatic conditions relevant for the local situation should be used in the models. Because this basic calculation of the predicted environmental concentration in the soil does not involve a scenario which is per se conservative, realistic worst-case DT50 values can be used as inputs into the models. These would generally be the longest laboratory DT50 values (although these are not necessarily representative of local field conditions) or the longest field DT50 values (of representative locations; normalized if appropriate).


Data required

The following data are generally required to be able to conduct a soil organisms risk assessment:

  • Effects on earthworms – Earthworm reproduction test (endpoint NOEC)
  • Effects on springtails – Collembola reproduction test (endpoints LC50, NOEC)
  • Effects on soil microbial activity – Nitrogen turnover test
  • Good Agricultural Practice (GAP) table, or detailed directions for use
  • Route and rate of degradation in soil (DT50 and DT90 values)
  • Higher tier study results (if available/required)

For more information on data requirements for soil organisms, go to the relevant section of the Data Requirements module.

Additional toxicity tests can be conducted with other species of soil organisms. One option could be to ask for data of locally relevant species under more realistic environmental conditions, such as tropical earthworm species (e.g. Perionyx excavatus) or tropical collembolan species, in tropical or semi-arid soils. For instance, Australia has developed protocols for six collembolan species (e.g. Sinella sp.) and testing temperatures of 25°C instead of the standard 20°C. The objective of adding species is to reduce the assessment factors applied in the risk assessment. The focus should be on testing more species of the group of organisms for which the risk of the pesticide was found to be unacceptable in Tier 1.

Higher tier ecotoxicological approaches include semi-field and field studies, such as terrestrial model ecosystems (TMEs) and field enclosures. For risk assessment in tropical and hot (semi)arid climates, semi-field and field tests should preferably be conducted in similar conditions and/or ecosystems. Such studies may also be bridged, if they have been conducted in comparable agro-ecological zones.


Interpretation of the outcome

When interpreting the outcome of the risk assessment, it is important to evaluate to what extent the exposure scenarios used for the assessment represent the expected use of the pesticide in the local situation. In particular, the registrar should assess the uncertainties associated with the risk assessment and possible under- or overestimation of the expected risk.

To assess whether the risk is acceptable, the toxicity exposure ratio (TER) (for earthworms and Collembola), or effect size (for microbial activity) is compared to a trigger value (or level of concern). Note that trigger values are specific to the risk assessment model and cannot be interchanged!

Trigger values for soil risk assessment used in the European Union are the following:


Species / process

Toxicity endpoint

Exposure estimation

Risk acceptable if:



Reproduction NOEC

Top 5 cm of soil, or top 20 cm of non-permanent crop situations in case of persistent substances accumulating over the years

PECaccumulation; PECinitial plus PECplateau ; PECinitial

TER > 5


Reproduction NOEC

TER > 5

Microbial activity

Effect at 28/100 days

Effect < 25%

PECaccumulation = maximum concentration over a time series of 20 years (application each year), 40 years (application every 2 years) or 60 years (application every 3 years);

PECplateau = plateau concentration; only for persistent pesticides.

PECinitial = concentration immediately after application


If the risk is unacceptable (toxicity exposure ratio is less than the trigger value) the registrar may decide to require higher tier studies and further evaluate the risk for the specific local situation concerned.

Alternatively, the registrar can assess whether any risk mitigation measures can be applied. However the options for risk mitigation of in-field soil organisms are limited (see the Risk Mitigation Module. If risk mitigation measures are required, the registrar should also evaluate whether these are feasible and realistic under the local conditions of use of the pesticide.