Put soil first to support crop protection benefits

Henny Lowth, Senior Knowledge Transfer Manager, explores whether plant protection products impact soil life, with a focus on the soil microbiome.

Our letterbox is open 24/7, 365 days a year.

It gathers your views on knowledge gaps that may hold back the productivity of cereals and oilseeds.

When put into broad topics, crop protection, including integrated pest management (IPM), is currently the most frequently cited, followed by soil health.

In this blog, I tackle both topics, as we have received several enquiries about the impact of agrochemicals on the soil microbiome.

Soil is home to an incredible diversity (and number) of organisms:

• Macrofauna, such as earthworms, ants, millipedes, spiders and beetles
• Mesofauna, such as mites and springtails
• Microfauna, such as nematodes and protists
• Microorganisms, such as bacteria, fungi and viruses (which make up the microbiome)

Collectively, these organisms influence the biological, chemical and physical properties of soil (and regulate most of the processes within it).

It's why many want to know if and how much the influence of pesticides (fungicides, herbicides and insecticides) extends beyond the target organism cited on product labels.

Pesticides are already heavily regulated by the Health and Safety Executive (HSE).

This regulation extends far beyond the primary purpose of pesticide products (to improve crop yield and or quality).

It includes assessments of the risk to non-target organisms, as well as pesticide fate and behaviour.

HSE wants to make sure that product usage does not cause ‘significant long-term changes to a population nor to the function of the ecosystem.’

As a result, standard ecotoxicology tests consider the impact on specific indicator species, including the impact on key macrofauna (e.g. earthworms) and mesofauna species (e.g. springtails and mites).

Although microfauna and microorganisms currently lack direct measures, some tests help to provide an indirect measure of pesticide impact.

For example, microscopic organisms play a key role in nutrient cycling, so significant changes in their populations may show up in nitrate-extract test results.

Soil is complex and the testing system isn’t perfect.

As discussed, there is an incredible diversity of soil life, and we are many years away from having reliable techniques to measure all of it.

Across the UK, there are also many soil types and soil conditions (at the time of any application).

Product types and usage (e.g. dose, mixtures and timing) add another thick layer of complexity.

But one thing is clear: Plant protection products do end up in the soil to various degrees.

Naturally, this is true for herbicides (e.g. pre-emergence products), but it also applies to insecticides (e.g. autumn-applied pyrethroids to small plants) and fungicides (e.g. that escape crop canopies).

Of course, seed treatments are placed in direct contact with the soil.

If you delve into the scientific literature, you will discover hundreds of papers that document the potential negative effects of pesticides on soil biology, including the microbiome.

However, many of the research studies are done under laboratory conditions on a limited number of species.

Additionally, many extend back decades and are based on products that are no longer approved (often due to tightened regulations).

In this complex world, some study results also conflict.

However, it is generally agreed that pesticides shape microbial communities and have broader effects on soil biological functions (e.g. nutrient cycling and organic matter decomposition).

Some studies even find that their influence may be more important than site-specific factors, such as soil type (especially at high pesticide concentrations).

However, there is currently no clear evidence of a consistent difference between broad pesticide types (fungicides, herbicides or insecticides).

Pesticide impact needs to be considered on a case-by-case basis.

Scientists have even conducted reviews of multiple independent studies (meta-analyses), which often show that the data is ‘noisy’.

This makes significant differences harder to pin down.

Despite this, some potential pesticide impact indicators for the microbiome show more promise than others (see the examples in the next section).

Many studies also cite positive effects on the microbiome.

For example, some pesticide residues increase the relative abundance of hundreds of bacterial and fungal species (many associated with pesticide degradation).

Additionally, as plant protection products boost plant health, this improves some aspects of soil health.

For example, stronger root systems improve soil structure, mop-up leaching-prone nutrients and build biology around them (e.g. mycorrhizal fungi).

As science advances, the ways to potentially test the impact of management on soil health have exploded.

Many of these techniques are still in their infancy, especially the interpretation of what test results mean in practice.

There is often a big gap between such tests and the rigorous and proven ecotoxicological testing procedures used by HSE.

But targeted research is advancing the debate.

It is only a matter of time before robust new pesticide risk assessments to measure more soil biology impacts are pinned down.

The microbiome is likely to play a greater role in such assessments.

Examples include the measurement of:

• Key bacterial species (e.g. Bacillus, Firmicutes spp. and Sphingomonas spp.)
• Key fungal species (e.g. Trichoderma and Rhizophagus aggregatus)
• Various organisms associated with the oxidation of ammonia
• The presence of certain enzymes and genes

This is a complex, fast-paced and, sometimes, emotive topic.

However, I will sign off this blog with three conclusions:

1. Pesticide use can change soil communities: For better, for worse or (often) unknown impact.
2. Pesticide regulation is comprehensive and evolving, with high-profile commitments to reduce the risks of pesticide use (e.g. UK Pesticides National Action Plan 2025).
3. Soils are complex, but improving soil health will make it more resilient to stress, which may help buffer the impact of pesticides, other interventions and forces beyond farmers’ control (e.g. the weather).