How to Neutralise Glyphosate (Roundup) Herbicide Contamination in Soil

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Glyphosate (chemical compound N-phosphonomethyl glycine), also known by the trade name of Roundup, is a non-selective, post-emergence, broad-spectrum systemic organophosphate herbicide used for control of annual and perennial plants.

This weedkiller is the largest selling herbicide chemical in the market today, and the most used herbicide in human history. From 1994 to 2014, 825.8 million kilograms of glyphosate were use globally, and since its introduction in 1974 to 2014, 8.6 billion kilograms have been sprayed worldwide. Due to its extensive agricultural use, it has become a major environmental pollutant.

By account of how ubiquitous it is, even supermarkets sell it, if any herbicide is involved in accidental plant damage from spray drift in the wind by careless local government contractors, or malicious use by nasty neighbours illegally poisoning trees along property boundaries, it’s most likely bound to be glyphosate that’s the culprit.

If glyphosate has been applied to non-target plants, trees or soil, what will happen, and what can be done to remedy the situation?

In this article we will discuss:

  • The extent of the damage caused by glyphosate to plants and trees, and whether they can be saved.
  • The issue of glyphosate soil contamination.
  • How to deactivate or neutralise glyphosate herbicide contamination in the soil.

How Glyphosate (Roundup) Weedkiller Works – Mode of Action

For glyphosate to work properly, it needs to be applied to plants that are actively growing. if it’s used on plants that are growing poorly, due factors such as heat and drought stress, disease or insect damage, the effectiveness of glyphosate will be reduced, because the herbicide works by disrupting critical growth processes.

Plants will therefore not incur the maximum potential possible levels of glyphosate damage if herbicide contamination occurs in the peak of summer when they’re  possibly water stressed, or in the peak of winter, when they’re dormant due to low temperatures.

When glyphosate is sprayed onto plants, it penetrates into the plant tissues and is absorbed. Once inside the plant, the herbicide is translocated through vascular tissues, following the same pathway as photoassimilates (compounds produced by photosynthesis, such as sugars), and is carried to  actively growing parts of the plant, such as root and shoot meristems, which are the actively growing tips of shoots and roots. Since glyphosate doesn’t cause a rapid disruption of plant tissue, the herbicide has more time to spread right through the plant, increasing it lethality.

The next two sections contain the technical explanation of how glyphosate works, if it’s too complicated, please feel free skip to the non-technical explanation below them.

The Primary Mode of Action of Glyphosate

Glyphosate is a is a substituted amino acid (a glycine molecule with other non-natural chemical groups synthetically attached to it) which exerts its herbicidal action by disrupting the shikimic acid pathway (also known as the shikimate pathway), by inhibiting the enzyme 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS), which prevents the plant from producing the aromatic amino acids L-tryptophan, L-phenylalanine, and L-tyrosine. Amino acids are the building blocks of proteins, and these critical amino acids are used by plants to synthesise various proteins, and other products such as pigments, alkaloids, hormones, and cell wall components.

The shikimate pathway is a very important important metabolic pathway in all plants, and 30% or more of photosynthetically fixed carbon is directed to this biochemical process in vascular plants (plants with water-carrying tissues, basically nearly all plants, except for mosses, hornworts and liverworts, and some algae). As a consequence, disruption of this pathway is usually fatal to plants.

Plants are inefficient at breaking down glyphosate, and are unable to prevent the herbicide reaching the tips of the roots and shoots (meristems), which show high rates of metabolism and growth, the target sites where glyphosate acts. Once glyphosate reaches the plant’s actively growing areas, it binds very tightly to the critical enzyme EPSPS, preventing it from functioning.

Secondary Modes of Action of Glyphosate

There are also secondary or indirect effects of glyphosate on plant physiology, observed in current research, which may lead to plant death.

Glyphosate is broken down by soil microorganisms fairly quickly under certain favourable conditions (or very slowly under less favourable conditions), and some plants can metabolize glyphosate to break it down, but when this happens, the major glyphosate metabolite (breakdown product) is aminomethylphosphonic acid (AMPA), which is a recognized phytotoxin (plant poison), and AMPA together with glyphosate could modify the effects on plants compared to glyphosate alone.

Damage to plants by AMPA will still occur when plants aren’t affected by glyphosate. Glyphosate-induced injuries have been observed in genetically-modified glyphosate-resistant (GR) plants, due to the formation of AMPA from glyphosate degradation. AMPA has been shown to affect chlorophyll biosynthesis and to cause plant growth reduction.

Additionally, glyphosate has been shown to have many other effects on plant physiological mechanisms, affecting processes such as photosynthesis, carbon metabolism, mineral nutrition, oxidative events, and symbiotic plant–microorganism interactions.

A chemical property of glyphosate is that it’s a chelating agent, which can bind soil macronutrients and micronutrients, impacting their uptake and availability in plants, irrespective if they’re GMO glyphosate-resistant plants or not. In particular, the availability of micronutrients such as iron, manganese, zinc, copper, and nickel may be affected. Since macronutrients and micronutrients are essential for many vital plant processes, and for pathogen resistance, a deficiency can contribute to the reported toxic effects of glyphosate on plants and lower resistance to pathogens. Similarly, symbiotic plant–microorganism interactions,such as nitrogen fixation in leguminous plants, can also be affected by the same mechanism.

The non-technical explanation of how glyphosate works is quite simple, all proteins are made up of amino acids, and glyphosate is a synthetically-corrupted glycine amino acid, with some nasty chemicals joined on to it. When it gets into the plant, it ‘throws a spanner into the works’ of a plant’s internal chemistry, disrupting its growth and eventually killing it.

What Does Glyphosate Damage Look Like, and Will it Kill the Plant?

Plants that have been exposed to glyphosate display stunted growth, loss of green coloration, leaf wrinkling or malformation, and tissue death.

Depending on the amount of glyphosate applied, the result may be partially die back, or the plant may be completely killed, and the death of the plant may occur after 4 to 20 days.

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Damage from glyphosate can occur from a previously treated crop residue/soil or drift from nearby field applied with the herbicide. Mild yellowing can be noticed as the first symptom on smaller leaves. (Source – University of Florida, Institute of Food and Agricultural Sciences, Plant Pathology Department – U-scout, Tomato diseases, Glyphosate Damage)

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Advanced stage symptom of glyphosate damage is the bleaching of the base of the leaves towards the stem region. In severe cases, many leaves or entire plant may exhibit bleached yellow symptom. (source – University of Florida, Institute of Food and Agricultural Sciences, Plant Pathology Department – U-scout, Tomato diseases, Glyphosate Damage)

How to Treat Glyphosate-Affected Plants

What do you do if a plant has been accidentally sprayed with glyphosate herbicide, and has started showing symptoms of herbicide toxicity?

A handy suggestion from the largest rose-grower production nursery in Australia, is that if the plant that has sustained glyphosate damage is fairly large, and only some branches are affected, it may be possible to cut those branches out, as the glyphosate will be translocated to the growing ends of these branches. Removing the affected parts of the branches will remove the glyphosate at the site where it is acting.

They state that the process of cutting off affected branches as they grow can take a year or more before the plant recovers, so depending on the value of the plant, and how badly it’s affected, it might not be worth the trouble.

Remember to put the prunings of glyphosate-affected plants into landfill with the regular rubbish, don’t put them into compost or green waste bins as the glyphosate will be released from decaying plants!

Their final suggestion is that If glyphosate is used on the soil in a garden bed, then it may be necessary to replace the top layer of soil. This would only work if the soil had bound the herbicide and it hasn’t washed deeper into the ground from rain or overhead irrigation.

How Does Glyphosate Cause Environmental Contamination

Herbicides can end up in offsite locations, damaging or killing non-target plants when they are carried by spray drift through the air, or water runoff over the soil surface or by leaching through the soil into groundwater.

It is claimed that glyphosate binds to organic matter in the soil, so it has low soil mobility, and is not easily washed away and carried elsewhere. While this may be true of glyphosate on its own, most glyphosate herbicide formulations also contain a surfactant, which change its properties significantly.

Surfactants are like detergents, they reduce surface tension, improving the emulsifying, dispersing, spreading, wetting properties of liquids.

Surfactant compounds are combined with glyphosate to:

  • Improve wetting of the leaf surface, making thee herbicide stick better to plant foliage, and reduce evaporation to prolong contact with droplets, since herbicides are absorbed in liquid form into the leaves.
  • Produce smaller spray droplets which spread more uniformly onto plant surfaces
  • Dissolve the wax cuticle on leaves to enhance foliar retention and penetration.
  • Enhance the movement of herbicide from the leaf surface into the plant tissue to increases effectiveness, as surfactants are absorbed by the plant along with herbicides.

Glyphosate often contains a surfactant unless stated otherwise, and is usually polyoxyethyleneamine or polyethoxylated tallow amine (both abbreviated POEA), or polyether modified polysiloxane (polysiloxane surfactant).

Studies indicate that the surfactant POEA used in glyphosate formulations is much more toxic to animals than glyphosate itself, and both POEA and polysiloxane surfactants are highly toxic to fish and aquatic organisms, and have high soil mobility, readily contaminating waterways.

Herbicide sprays containing surfactants produce much smaller droplets, which are more prone to spray drift which can be carried by the wind to offsite location onto non-target plants.

In agricultural fields, when glyphosate is sprayed on plant foliage, some of the spray lands on the soil surface. While it is claimed that glyphosate binds to soil particles, is quickly broken down by soil micro-organisms in a few days, and does not affect plants when applied to the soil, this is not what has been observed by researchers in field trials and contamination testing, and sounds more akin to green-washing and marketing spin than actual objective science.

What does the research show?

The addition of surfactants increases soil mobility of glyphosate by decreasing its ability to adhere to soil. This allows it to be washed away by rain and irrigation water into waterways, non-target areas, and deeper into the soil, where it resists breakdown, and is more prone to leaching.

The contamination of waterways has been identified as a route via which glyphosate is transferred to adjacent agricultural fields, especially when water is pumped from bodies of surface water (as opposed to underground water bores) to irrigate fields.

Another important source of glyphosate exposure is exudation from roots of living sprayed plants, glyphosate is translocated from the leaves into the roots and the surrounding soil. Glyphosate is also released from dead plants back into the soil. Recent studies do suggest that glyphosate rhizosphere transfer does in fact occur and presents a  risk of glyphosate toxicity to non-target plants.

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Diagram showing potential effects of glyphosate in crop production, including crop uptake from soil. (Source: Kanissery R, Gairhe B, Kadyampakeni D, Batuman O, Alferez F. Glyphosate: Its Environmental Persistence and Impact on Crop Health and Nutrition. Plants. 2019 (20))

Once glyphosate is in soil, if it is not degraded by microbes to the phototoxic by-product AMPA (which is more mobile in the soil than glyphosate), it may be adsorbed onto soil particles, or migrate deeper into the soil via soil pores or root canals. This doesn’t mean that it’s permanently bound though, as some agricultural practices, such as the addition of phosphorous fertilizers may re-solubilise glyphosate in soils, making it available for leaching, and active in the rhizosphere where it can once again affect non-target plants.

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Fate and movement of glyphosate in different pools, it doesn’t stay bound in the soil! (Source: Kanissery R, Gairhe B, Kadyampakeni D, Batuman O, Alferez F. Glyphosate: Its Environmental Persistence and Impact on Crop Health and Nutrition. Plants. 2019 (20))

How Long Does Glyphosate Persist in the Soil?

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Farm near Tübingen, Germany 2006, showing partial desiccation of cover crops sown after the previous wheat crop which was accidentally treated with a double application rate of glyphosate of 4L/ha. (Source – Dynamics of Glyphosate in the Rhizosphere: A Possible Threat to Crop Plants? T.Tesfamariam, S.Bott. G.Neumann, I.Cakmak,V. Römheld)

The labels on gardening products for consumers can be, well, how do we say it, very liberal with the truth. Looking at an unnamed formulation of glyphosate sold in Australia, it claims to be non-residual and as a precaution, not to disturb sprayed weeds for a period of two weeks. Really?

Here are the figures from a research paper which appears to have a very pro-herbicide industry tone to it, and the figures contradict the glyphosate consumer product label claims:

“The half-life determined in a range of regulatory soil studies in the laboratory, the half-life or DT50 range 1.0–67.7 days (EFSA, 2015). In field studies conducted in North America and in Europe where the influence of climatic conditions as well as soil type can be assessed, the mean half-life for glyphosate degradation was 30 days, with a range from 5.7–40.9 days. In most conditions, over 90% of the applied amount of glyphosate dissipated within six months under aerobic conditions. Glyphosate has greater soil persistence under anaerobic conditions with a DT50 range 135 –> 1000 days (EFSA, 2015). AMPA has greater soil persistence than glyphosate with a DT50 range 40.0–301 days in laboratory studies (EFSA, 2015).”

The half-life is the time it takes for a substance to break down to half of the original amount, so keep in mind that this means there’s 50% remaining of the original applied amount after this time.

Here are the figures from the National Pesticide Information Center (NPIC), a cooperative agreement between Oregon State University and the U.S. EPA. Note that the EPA has been accused of a ‘revolving door’ arrangement between government and industry, so would be considered pro-industry, and their figures also contradicts the marketing-based ‘facts’ stated by the consumer product. Also note some of the downplaying of risk contradicted by research referenced in this article:

Environmental Fate:

Soil

  • The median half-life of glyphosate in soil has been widely studied; values between 2 and 197 days have been reported in the literature. A typical field half-life of 47 days has been suggested.Soil and climate conditions affect glyphosate’s persistence in soil.
  • Half-lives can vary widely based on environmental factors. The amount of chemical remaining after a half-life will always depend on the amount of the chemical originally applied. It should be noted that some chemicals may degrade into compounds of toxicological significance.
  • Glyphosate is relatively stable to chemical and photo decomposition. The primary pathway of glyphosate degradation is soil microbial action, which yields AMPA and glyoxylic acid. Both products are further degraded to carbon dioxide. Glyphosate adsorbs tightly to soil. Glyphosate and its residues are expected to be immobile in soil.

Water

  • The median half-life of glyphosate in water varies from a few days to 91 days.
  • Glyphosate did not undergo hydrolysis in buffered solution with a pH of 3, 6, or 9 at 35 °C. Photodegradation of glyphosate in water was insignificant under natural light in a pH 5, 7, and 9 buffered solution.
  • Glyphosate in the form of the product Roundup® was applied to aquatic plants in fresh and brackish water. Glyphosate concentrations in both ponds declined rapidly, although the binding of glyphosate to bottom sediments depended heavily on the metals in the sediments. If chelating cations are present, the sediment half-life of glyphosate may be greatly increased.
  • Glyphosate has a low potential to contaminate groundwater due to its strong adsorptive properties. However, there is potential for surface water contamination from aquatic uses of glyphosate and soil erosion.
  • Volatilization of glyphosate is not expected to be significant due to its low vapor pressure.

Air

  • Glyphosate and all its salts are very low in volatility with vapor pressures ranging from 1.84 x 10-7 mmHg to 6.75 x 10-8 mmHg at 25 °C.
  • Glyphosate is stable in air.

Plants

  • Glyphosate is absorbed by plant foliage and transported throughout the plant through the phloem. Glyphosate absorption across the cuticle is moderate, and transport across the cell membrane is slower than for most herbicides. Because glyphosate binds to the soil, plant uptake of glyphosate from soil is negligible.
  • Glyphosate accumulates in meristems, immature leaves, and underground tissues.
  • Very little glyphosate is metabolized in plants, with AMPA as the only significant degradation product.
  • Lettuce, carrots, and barley contained glyphosate residues up to one year after the soil was treated with 3.71 pounds of glyphosate per acre.
  • Glyphosate had a median half-life of 8 to 9 days in leaf litter of red alder and salmonberry sprayed with Roundup®.“

There are some dubious statements in there – “Glyphosate and its residues are expected to be immobile in soil.” Expectations are speculation, scientific observations in current researchers suggest that glyphosate does move through soil, causing contamination and affecting non-target plants.

So how long does it take for glyphosate contamination in the soil to disappear.

It’s not as simple as taking the half-life figures and doubling them. Here’s how the US EPA explains it:

“The “half-life” is the time required for half of the compound to break down in the environment.

1 half-life = 50% remaining
2 half-lives = 25% remaining
3 half-lives = 12% remaining
4 half-lives = 6% remaining
5 half-lives = 3% remaining

Half-lives can vary widely based on environmental factors. The amount of chemical remaining after a half-life will always depend on the amount of the chemical originally applied. It should be noted that some chemicals may degrade into compounds of toxicological significance.”

So, if we work through the conservative EPA statement that “A typical field half-life of 47 days has been suggested”, then the actual numbers would look like this:

1 half-life (47 days) = 50% remaining
2 half-lives (94 days) = 25% remaining
3 half-lives (141 days) = 12% remaining
4 half-lives (188) = 6% remaining
5 half-lives (235 days) = 3% remaining

If we graph these US EPA figures, we see a typical exponential decay curve. In mathematics, exponential decay describes the process of reducing an amount by a consistent percentage rate over a period of time. This gives us a realistic perspective of how long we really need to wait for nature to do its work.

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Remember, these are typical figures, and best and worse-case scenario figures between 2 and 197 days respectively have been reported in the literature. With a very optimistic 2-day half-life under ideal breakdown conditions, it would take 10 days for the applied glyphosate to break down so there is only 3% left, 12 days for 1.5% remaining, and 14 days (2 weeks) for 0.75% remaining. That’s where the figure on the label of the consumer glyphosate product came from.

On the other hand, with a worst-case scenario 197-day half-life, it will take 985 days (2.7 years) for the glyphosate to break down to 3% or the original amount, and using the 0.75% remaining figure used earlier, would be 7 half-lives, or 1379 days (3.8 years).

Is Glyphosate Bound in the Soil Safe?

As much as the idea of glyphosate being immobile in soil is pushed my marketing departments to downplay the risk of contamination, what we’re seeing here is misleading.

The rate of mineralization (breakdown) of glyphosate in soils was found by researchers to be correlated with the abundance of Pseudomonas spp. of microorganisms in the soil. They also found that adding phosphate to the soil stimulated glyphosate mineralization, and that’s because the addition of phosphorous fertilizers unbinds the glyphosate from the soil, re-solubilising it, where it can once again become bioavailable to affect plants, and also leach into groundwater.

The degradation of glyphosate has been found to have a negative correlation with the soil adsorption capacity for glyphosate, and researchers suggest that this may possibly be because of low bioavailability. If glyphosate is bound up in the soil, it won’t necessarily be available to soil micro-organisms to break it down.strong, and adsorption by soil solids such as iron and aluminium oxides may prevent microbial access to the glyphosate.

It would seem that the bioavailability of glyphosate determines its fate and impact as a contaminant. When it’s bound, it persists to be released at a later time, when it becomes unbound it is accessible to bacteria to be degrades, but also available to plants to affect their health.

How to Neutralise Glyphosate in the Soil

How easy is it to get rid of glyphosate contamination in the soil?

The EPA states that “Glyphosate did not undergo hydrolysis in buffered solution with a pH of 3, 6, or 9 at 35 °C. Photodegradation of glyphosate in water was insignificant under natural light in a pH 5, 7, and 9 buffered solution.

What that means is that glyphosate doesn’t break down in water, whether it’s acidic or alkaline, nor through exposure to light, such as sunlight. It’s quite stable unfortunately. It has a melting point of 189.5 °C (373 °F) and decomposes at 230 °C (446 °F), so degrading it with heat is not an option.

There is a way to decrease the effectiveness of glyphosate herbicide, and we can find lots of information about it from agricultural extension agencies advising farmers what things to maximise the effectiveness of their glyphosate spraying. In our case, we’ll do the opposite, doing what they tell the farmers not to do, to mess up the way glyphosate works!

By doing some crafty chemistry, we can make conditions as detrimental to the activity of glyphosate as possible, to reduce the damage it causes to plants and the soil.

Water Quality and Glyphosate Effectiveness

Glyphosate products are mixed with water for spraying, but if the water is ‘hard water’, meaning it contains large amounts of dissolved salts, high levels of calcium (Ca), magnesium (Mg), sodium (Na), or iron (Fe), then the effectiveness of glyphosate may be reduced, particularly if the dissolved salts in the water are calcium and magnesium salts.

Glyphosate is a weak acid, and therefore has a weak negative charge. For easier handling and stability, glyphosate-containing products are formulated as salts. A salt is formed when the glyphosate acid is bound to a base that has a positive charge. The most common glyphosate salt formulations are potassium, isopropylamine, monoammonium, diammonium, and trimesium salts.

The salts of calcium (Ca2+) and magnesium (Mg2+) have positive charges which may bind with the negatively charged glyphosate molecule, displacing the isopropylamine or other salt used in the formulated product.

When glyphosate is not bound with the salts it was originally formulated with, but is bound with calcium or magnesium salts instead, is less readily absorbed by plants, and this reduces the effectiveness of the glyphosate.

Farmers are actually advised to add special surfactants to the tank prior to glyphosate to prevent the formation of inactive complexes between glyphosate and antagonistic calcium (Ca 2+) and magnesium (Mg 2+) cations (positively charged ions).

Water pH and Glyphosate Effectiveness

The pH is a measure of acidity or alkalinity, with a pH of 7.0 being neutral, a pH lower than 7.0 acidic, and a pH higher than 7.0 alkaline.

Acids are compounds that release hydrogen (H+) ions when dissolved in water, and weak acids are compounds that release a small amount of H+ ions.

Herbicides such as glyphosate, 2,4-D, dicamba, and many others are all weak acids, and partially dissociate (split apart) when mixed in water, only some of the herbicide molecules will dissociate and the rest will not. Herbicides are more readily absorbed by plant foliage when they are not dissociated (split apart). The pH of the water determines how much the herbicide is dissociated.

If the water mixed with the herbicide is alkaline, more of the herbicide is dissociated, becomes negatively charged, and is more susceptible to being tied up by cations (positively charged ions) such as Calcium (Ca2+), Magnesium (Mg2+), and iron (Fe2+, Fe3+), which occur in hard water, forming complexes that are not easily absorbed by plants, thus reducing the effectiveness of the herbicide.

The combination of high pH and hard water act together to reduce the effectiveness of glyphosate. High pH causes more of the herbicide to dissociate while high concentrations of cations bind with the dissociated herbicide to reduce its effectiveness.

Knowing this, we can find a soil amendment that is both high in Calcium (Ca2+), and has a high pH (alkaline) to bind up glyphosate, neutralise it and reduce its effectiveness. Since alkalinity refers to carbonate (CO₃2-) and bicarbonate (HCO₃-) levels in water, this gives us the other half of the formula.

Calcium carbonate (chemical formula CaCO₃) is a calcium salt with a fairly alkaline pH of 8-9, and is the major component of sea shells, limestone, marble and eggshells. Gardeners know this product as garden lime!

If we wanted to increase the hardness of the water to bind up glyphosate, but not change the pH of the soil, then we could use magnesium as the positively charged cation instead, and use magnesium sulphate (chemical formula MgSO4), which is commonly known as Epsom salts, and used to treat magnesium deficiencies in plants, especially citrus.

Garden Lime Application Rate

How much lime (calcium carbonate) should you added to increase soil pH?

Suggested amounts of garden lime for various soil types

  • Sands …………. 150g per square metre
  • Loams ………… 200g per square metre
  • Clay sand …… 350g per square metre

This is the aplication rate normally used to treat soils witth a pH of 5.0-5.5 to make them less acidic.

Garden lime is virtually insolouble and needs to be dug into the soil to be effective. In an empty garden bed, garden lime can also be sprinkled on the surface and raked in, then watered to allow it to work itself into the soil.

Epsom Salts Application Rate

The advantage of Epsom salts (Magnesium sulphate) is that it is very solouble in water, and can be used as a quick and effective soil drench by mixing it into a watering can and pouring it into the soil around the affected plant.

What are the recommended application rates for Epsom salts?

  • For fruit trees and large shrubs, apply 20g (4 teaspoons) of Epsom salts (Magnesium sulphate) per square metre (square yard), spreading evenly around the drip line of the tree or shrub, then water in well. Wash off any granules that have landed on plant foliage. Also, don’t apply any closer than 10cm (4”) to stems or trunks.
  • Another recommendation is  dissolving 10g (2 teaspoons) of Epsom salts (Magnesium sulphate) in a litre of water and applying at a rate of 1 litre per square metre of garden bed with a watering can.

 

Binding Glyphosate in Other Ways

Farmers spraying glyphosate are advised to use clean water, as turbidity, the amount of suspended soil and organic matter particles in the water can  reduce the effectiveness of the herbicide.

Glyphosate has a high soil organic carbon sorption coefficient (Koc) of 24,000 mL/g, a measure used to describe the binding strength of herbicides to soil, and therefore is rapidly and tightly adsorbed to soil particles and organic matter.

The figure represents the ratio of herbicide that is bound to soil particles when the herbicide is mixed with a slurry of water and soil. Herbicides with high Koc or values bind more tightly to soil particles.

Which soil materials bind glyphosate best?

Studies in Brazil have shown that adsorption of glyphosate depends on surface area for clays and amount of clays and CEC (cation exchange coefficient) for soils, while organic matter only plays a secondary role in the adsorption of glyphosate in soils. The adsorption of glyphosate on montmorillonite and kaolinite clays decreased when pH increased, but remained constant with bentonite clay.

Therefore, bentonite clay, commonly sold in garden centres as a soil amendment to increase water retention in soils, and used in agriculture to line ponds and dams, can be used for adsorption of glyphosate, and its effectiveness is not reduced in alkaline pH soils, which result after adding garden lime as suggested earlier to inactivate glyphosate.

Adding high phosphate fertilisers, such as chicken manure and blood & bone, or synthetic phosphate fertilisers (which an organic gardener should never use) such as superphosphate (which is also very acidic) will liberate glyphosate bound in the soil, but glyphosate is not easily displaced by phosphate from clays. Adding calcium bentonite clay can ensure that glyphosate bound up in the soil stays there!

In this article, we mentioned earlier that the rate of breakdown of glyphosate in soils was found to be correlated with the abundance of Pseudomonas spp. of microorganisms in the soil.With that said, the most commonly found bacteria in matured vermicompost (worm castings) are Pseudomonas, Bacillus and Microbacterium species, though the bacteria associated with the vermicompost vary depending on the food added into the worm farms. Watering in the worm casting leachate (‘worm wee’) into the soil, diluted 10:1 or to the colour of weak tea, preferably with rainwater as it’s not chlorinated like tap water, or digging in worm castings, are a good way to inoculate the soil with beneficial bacteria, and hopefully increase the Pseudomonas species which break down glyphosate.

Many studies have isolated bacteria from glyphosate-contaminated soils, and cultured them in large numbers to test their effectiveness at breaking down glyphosate, while not being affected themselves, as bacteria utilise the shikimate biochemical pathway which glyphosate inhibits. Mycobacterium brisbanense, Bacillus aryabhattai, Pseudomonas azotoformans and Sphingomonas pseudosanguinis are some of the many microorganisms tested for bioremediation of glyphosate in soil. Obtaining microorganisms for inoculating soil might not be that easy to do, but adding the liquid and solid products from a vermicomposting system is a much easier process.

It’s important to point out that most bacteria function in neutral to acidic pH environments, and if soil has been made very alkaline with limestone to bind up glyphosate, bacterial activity will be reduced. Using Epsom salts adds magnesium to the soil to bind glyphosate without changing the pH, making it more hospitable for micro-organism activity.

References:

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2 Comments

    1. That sounds rather dramatic, but there are health concerns which I haven’t discusssed in this article on soil remediation. the United Nations World Health Organization’s International Agency for Research on Cancer (IARC), declared that glyphosate, the active ingredient in the herbicide Roundup, as a Class 2A carcinogen – ‘probably carcinogenic to humans’.

      The California Environmental Protection Agency, Office of Environmental Health Hazard Assessment (OEHHA) in the US reviewed the studies discussed by the IARC, and determined that the evidence was of sufficient quality that they added glyphosate to the Proposition 65 list of cancer-causing chemicals on 7th July, 2017. Any herbicides sold in the state of California containing glyphosate must now have a cancer warning clearly displayed on the product label to warn consumers of the risk.

      Like

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