Herbicides do more than kill weeds — they rewrite the soil beneath them

A new review published by Open Access Government, titled Soil health: The effects of herbicides, pulls together what agricultural science now knows about a question farmers ask every season: what do our sprayers do to the ground itself?

The short answer: quite a lot. And most of it is invisible until it isn't.

The soil is not just dirt

The review frames soil as a living system — a dense web of bacteria, fungi, earthworms and arthropods that together drive nutrient cycling, organic-matter breakdown and, ultimately, the fertility that a herbicide application is supposed to protect. When a herbicide hits the field, it doesn't stop at the weed. It reaches every organism in that web.

The report's central point is that herbicide effects on soil are cumulative, sub-lethal and easy to miss. A single pass may not collapse a soil ecosystem. But season after season, the same chemistry can shift microbial balance, slow decomposition and quietly tax the yield the pass was meant to safeguard.

Microbes: the first responders

Bacteria and fungi are the first to feel it. The review documents how repeated herbicide use can reduce microbial biomass and shift the balance between bacterial and fungal communities — a shift with consequences for nitrogen fixation, phosphorus solubilisation and disease suppression.

Glyphosate, the world's most widely used herbicide, gets particular attention. The review notes that glyphosate and its metabolite AMPA can persist in soil and disrupt the shikimate pathway — the same biochemical route it blocks in plants — in soil microbes as well. Fungi that help decompose crop residue, and beneficial bacteria that compete with pathogens, can both take a hit.

The report is careful to note the picture is not uniformly negative. Some microbial populations rebound quickly. Some even use herbicide molecules as a food source. But the review stresses that the balance of the soil community — not just its total size — is what determines whether a field's biology works for the crop or against it.

Earthworms, beetles and the bigger fauna

Larger soil organisms show effects too. The review cites evidence that certain herbicides reduce earthworm activity, particularly the surface-feeding and burrowing species that aerate soil and incorporate organic matter. Reduced earthworm populations mean poorer soil structure, slower water infiltration and more compaction risk.

Ground beetles and other arthropods that prey on weed seeds and crop pests are also affected — either directly through contact with residues or indirectly through loss of habitat and food sources when weeds are cleared. The review highlights that these predators provide a natural weed-control service, and herbicide overuse can undermine it.

Residues and the persistence question

How long do these chemicals stick around? The review breaks down persistence by chemistry:

• Glyphosate binds tightly to soil particles but can be displaced by phosphate fertilisers, potentially re-releasing it into the root zone.
• Atrazine and some older triazines can persist for months and are more prone to leaching into watercourses.
• Modern sulfonylureas and other low-dose chemistries break down faster but act at very low concentrations, meaning even trace residues can affect sensitive microbes.

The report also flags the role of formulation ingredients — surfactants and adjuvants — which can be more toxic to soil organisms than the active ingredient itself, and which are often not fully disclosed or tested.

What farmers can actually do

The review doesn't call for abandoning herbicides. It calls for using them as part of a system, not as the whole system:

1. Rotate modes of action — not just to fight resistance, but to avoid repeatedly hitting the same soil organisms with the same chemistry.
2. Integrate mechanical and cultural weed control — reduce the chemical load where the agronomy allows.
3. Feed the soil biology — cover crops, organic amendments and reduced tillage give microbial communities the resilience to buffer herbicide stress.
4. Monitor and test — soil biological indicators are increasingly available commercially and can flag problems before yields do.

The bottom line

The review's conclusion is measured but pointed: herbicides remain an essential tool, but treating soil as a passive medium rather than a living partner carries a cost. That cost shows up as reduced fertility, weaker nutrient cycling and greater dependence on inputs to compensate.

For growers, the message is that every herbicide decision is also a soil-biology decision — and the soil keeps the receipts.