How oxygen and cultivation impact glyphosate breakdown
© Tim Scrivener The breakdown of glyphosate in soils is fundamentally influenced by three key factors: oxygen availability, soil biology and the presence of cultivations.
Glyphosate is an effective tool for controlling weeds and terminating cover crops, but its persistence in soil has raised concerns about environmental impacts and soil health.
Rothamsted Research microbiologist Prof Andrew Neal explains that oxygen availability within the soil is essential for effective glyphosate degradation.
See also: 7 organically approved soil amendments to boost cropping systems
Carbon-phosphorus bond
Glyphosate contains a strong carbon-phosphorus bond – a chemical bond that is rarely found in nature and requires substantial energy to break.
There is uncertainty around the number of soil microorganisms that possess the enzymes capable of cleaving this bond, but these microbes depend on high availability of oxygen to do so efficiently.
“Microscopic pores are extremely important to oxygen movement and availability. The greater the pore space and organic matter, the more oxygen can circulate.
“That’s vital for the microbes responsible for breaking down glyphosate,” he says.
In well-aerated soils rich in biological activity, glyphosate can be metabolised relatively quickly. But in compacted or oxygen-poor soils, it can persist for months.
Blackgrass sprayed with glyphosate © Tim Scrivener
On-farm trial
In an on-farm trial led by Rothamsted Research, in collaboration with the University of Nottingham, cultivated plots treated with glyphosate fell below detection limits within 12 days post-glyphosate application.
In uncultivated soils, where pore space and aeration were lower, glyphosate persisted for around 30 days.
The findings reveal that soil management has a clear effect on glyphosate longevity.
Soils with better biological activity, organic matter, and oxygen flow support faster microbial degradation.
Cultivation can enhance oxygen movement and lead to more aerated soils which
support biological function. However, repeated intensive tillage can damage the long-term pore networks.
“It shows that nothing you do to soil comes without consequence,” says Andrew.
Staffordshire farmer Tim Parton, who hosted the trial at Brewood Park Farm near Cove has long focused on biological and nutritional farming.
He has eliminated use of fungicides, insecticides, and growth regulators, instead focusing on soil and crop health.
However, Tim admits glyphosate still has a role on his farm, mainly for desiccating cover crops but this is used sparingly.
“I see glyphosate as a tool in my armoury, but I want to keep its use to an absolute minimum,” he says.
Tim refuses to use glyphosate as a pre-harvest desiccant, especially on milling wheat destined for human consumption.
“We’re living organisms ourselves. I wouldn’t want to eat food that’s been sprayed with glyphosate just before harvest,” he says. “That’s madness.”
Tim’s regenerative system revolves around cover cropping, which helps maintain living roots, improve soil structure, and feed soil microbes.
When conditions are right, covers are successfully terminated mechanically by rolling on a frost with a crimper roller.
“This maintains ground cover, suppresses weeds, and conserves soil moisture, but we all know the UK’s maritime climate doesn’t always allow for this.
“Glyphosate remains a backup when frost or timing are not in our favour,” he says.
“Everything you do to soil has side effects. The challenge is to choose the action with the least damaging impact, so the soil biology can stay alive and functioning.”
Four scenarios
To study how different management methods affect glyphosate breakdown, the trial was established in a field of spring lupins following a cover crop.
Four different treatments were compared to destroy the cover crop:
- Crimper roller (farm standard when establishing spring crops after cover crops)
- Glyphosate (2 litres/ha of glyphosate, made up to 3 litres/ha with additions of fluvic acid, citric acid and molasses)
- Tillage (two passes of carrier discs working the soil down to 10cm)
- Tillage to 10cm + glyphosate (3 litres/ha of glyphosate applied before cultivation)
Fuelling the biological breakdown
To stimulate microbial breakdown and enhance uptake into the plant, Tim adds citric and fulvic acids, and molasses to tank mixes of glyphosate.
“Citric acid lowers pH which glyphosate works more effectively at.
“Fulvic acid also helps lower pH and acts as a powerful chelating agent, enhancing glyphosate uptake into the weed for a more effective kill,” he says.
Molasses provides a readily available carbon source, to feed the microbes that help degrade glyphosate residues.
“We have to work with biology, not against it. The more we understand how our actions influence what’s happening below the surface, the better decisions we can make for our soils, crops, and the environment.”
X-ray tomography
To better understand how these management practices influenced soil structure, and in turn glyphosate breakdown, soil cores were collected and transported to the University of Nottingham to image soil structure using x-ray computed tomography.
Visualising the structure of soil following 14 years of no-till management showed a well-connected network of pores, root channels, and worm burrows – a testament to Tim’s biological farming approach.
This created the ideal soil conditions for oxygen movement and microbial activity, to breakdown glyphosate effectively.
In the tillage scenario, the soil appeared more open and “fluffy” in the top few inches with increased pore space and oxygen diffusion.
However, a week later, this fluffy structure had collapsed. In this scenario, after initial loosening, the pore space decreased, and oxygen movement became restricted again.
“The tillage area did initially have high porosity, but it soon slumped after a few weeks.
“Whereas, the direct-drilled area was maintained at the same high porosity throughout which allows the oxygen to enter the soil and breath,” says Tim.
What’s more, the tillage plot, without glyphosate was a complete disaster.
“This plot got overcome with weeds due to the excess tillage. The soil dried out far more and the crop of lupins failed.
“The direct-drilled crop did 1.2t/ha, which was good considering the dry spring,” says Tim.
Soil structure and cultivation
Soil structure had a direct impact on how long glyphosate remained in the soil. In cultivated plots, glyphosate was broken down rapidly – falling below detection limits within 12 days.
In uncultivated soils, where pore space and aeration were lower, glyphosate persisted for around 30 days.
Tim feels the results from this experiment would reveal totally different findings on a “dead” soil with very poor biology as a result of over use of nitrogen with little organic matter input and intensive tillage.
“With my soils being so microbially active, the glyphosate can be broken down very quickly. However, in a poor anaerobic soil, the glyphosate would hang around for a long time in the right conditions.
“This is why having a working soil is so important,” he says.
Therefore, managing soils for structure, stability, and aerobic microbial activity can not only accelerate the breakdown of glyphosate but supports overall system resilience.
“Healthy soils breathe. When you maintain pore space, organic matter, and biological activity, the soil can process inputs naturally – including glyphosate.
“That’s the foundation of sustainable soil management,” concludes Andrew.
