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Paraquat in soil

Paraquat is deactivated in soil and does not leach.Paraquat is deactivated in soil and does not leach. Paraquat’s distinctive behaviour in soil means that farmers around the world confidently use it to protect their crops. Paraquat provides fast broad-spectrum weed control only through foliar contact action. There is no crop damage via the roots or any effect on seed germination. Soil fauna and microorganisms are not affected and there is no leaching or run-off from the degrading paraquat residues reaching the soil.

This article explains the fate of paraquat in soil in relation to the implications for the environment and its role in raising agricultural productivity. Two inter-linked processes are fundamental to the overall fate of paraquat in soil: how it binds (adsorbs) to soil and how it degrades in soil.

Paraquat degradation

Paraquat is intrinsically biodegradable as demonstrated in laboratory incubation studies with microorganisms in aqueous solution1. Various bacteria and fungi found in soil (eg Corynebacterium fascians, Lipomyces starkeyi, Aspergillus niger, Penicillium frequentans, Fusarium spp. and Pseudomonas spp.) have been found to readily metabolise paraquat. In experiments using cultures of microorganisms extracted from two UK agricultural soils, it was demonstrated unequivocally that radiolabelled paraquat was completely degraded in less than three weeks, mainly to carbon dioxide (detected as 14CO2). Other degradation products included ammonia and simple naturally-occurring organic acids.

However, in the field, very little applied paraquat is biologically available. When paraquat spray penetrates vegetation to reach the soil it undergoes binding (adsorption) to soil constituents, which is very strong and occurs immediately. This means that only very low concentrations of paraquat are available to any soil organisms2.

Paraquat adsorption

Paraquat is positively charged, so it is initially attracted to negatively charged sites on soil mineral particles and organic matter3.

Paraquat: a planar, positively charged compoundParaquat: a planar, positively charged compound Other forces strengthen this binding over time and ensure that release cannot occur with a change in soil conditions due to changes in cropping, soil management or even climate change.

These binding sites for paraquat adsorption also attract positively charged plant nutrients such as potassium and ammonium. Of all the various soil mineral particles clays have the most binding sites due to their high surface area. Clay soils, therefore, can adsorb the most paraquat, especially those containing so-called swelling clays that also contain inner surfaces. The flat paraquat molecule is able to access these inner surface sites within clay particles due to its size and shape characteristics. This helps to strengthen its soil binding and provide very stable or unchanging conditions for binding.

The use of the same types of binding sites as positively charged plant nutrients is not an issue for paraquat binding or nutrient cycling in soil. In normal use, paraquat can only occupy a small fraction of all these binding sites. Even in studies involving grossly exaggerated paraquat rates, paraquat did not affect the concentration of potassium in the soil solution, nor were grossly exaggerated potassium fertilizer rates able to measurably displace adsorbed paraquat.

In fact, paraquat is strongly adsorbed to all soil types, depending on the amount of different soil constituents, to give a range of capacities for deactivating paraquat when in contact with soil as shown in the table below4. Recalling that typical paraquat application rates in fields average a few hundred grams/hectare, over the years this means that the deactivation capacity of agricultural soils is equivalent to hundreds or even thousands of years’ of paraquat applications.

Soil type Soil deactivation capacity 

(mg paraquat/kg soil)*
Equivalent levels of paraquat (kg/ha) if incorporated to 20 cm depth)
Clay 500 – 5000 1500 – 15000
Loam 150 – 1500 460 – 4600
Sand 25 – 250 75 – 750
Peat 50 – 150 25 – 75

*Measured as Strong Adsorption Capacity (SAC-WB) determined by the wheat bioassay – see below and Roberts et al. (2002) for further information).

Paraquat binding is so strong that it means a distinction needs to be made between the total amount in the soil and the amount that is biologically available. The total amount can be determined if soil samples are boiled in concentrated sulfuric acid for several hours, since this completely destroys the soil without destroying paraquat. Rapid laboratory bioassays are used to evaluate the potential bio-availability of paraquat to flora, fauna and microorganisms. These bioassays use wheat roots (wheat being a very sensitive species to paraquat) and indicate how much paraquat soils can deactivate – the amount is the soil deactivation capacity. Field trials confirm that bioassays with wheat seedlings are a conservative predictor of the safety levels for the long-term use of paraquat in soils.

There are two important questions concerning the long-term fate of paraquat in soil:

  • Could the soil deactivation capacity ever be exceeded in normal agricultural practice?
  • Could the tiny amount of paraquat in soil water ever have deleterious biological effects before it is degraded by microorganisms?

Paraquat’s long-term fate in soil

To answer these questions, field trials have been conducted on different soil types around the world to study the impact of paraquat binding and degradation in relation to potential biological effects. These trials have involved long-term monitoring following repeated applications over many successive seasons, or after applying extremely high rates, equivalent to hundreds of years of application in a single dose.

One UK trial involved monitoring soil paraquat levels for 20 years after single initial applications of paraquat of up to 144 kg active ingredient/ha. The highest rate was over 100 times the maximum rate allowed in local farming practice and determined to be 400% of the amount of paraquat the particular soil could deactivate. Therefore, a large amount of paraquat was expected to be bioavailable. Cereal crops were grown on these plots each year after treatment. As expected, the yields were initially severely affected, but were reduced by less than 10% lower than untreated plots after 17 years due to degradation and strengthening of the binding over time.

Levels of paraquat in soil plateau as annual degradation equals application (Roberts et al, 2002).Levels of paraquat in soil plateau as annual degradation equals application (Roberts et al, 2002). In another long-term trial in North Carolina, USA, applications were made at a high recommended rate (1 kg paraquat/ha) for 12 successive years).

The actual measured residues of paraquat in the soil rose at first, but more slowly than if there were no dissipation, and then reached a plateau (see chart opposite).

Other long-term trials monitoring the soil fate of paraquat have included those on contrasting soil types in Australia, Malaysia and the Netherlands3.

Implications of paraquat’s soil properties and fate

In farming practice, target weeds intercept most of the dose applied before paraquat reaches the soil. The paraquat they intercept can be degraded by sunlight and microorganisms on leaf surfaces, and metabolised and adsorbed within plants, with only the remaining paraquat reaching the soil when the dead weeds are incorporated into it. Orchards in South Korea which had received annual applications of paraquat for 26 years had soil concentrations averaging less than 3% of the soil deacivation capacity4.

The 20-year UK experiment referred to above also involved monitoring effects on soil fauna and microorganisms3. This and other studies involving the application of paraquat at more than 100 times the recommended rate, even exceeding the capacity of the soil to adsorb and deactivate it, indicated no major long-term biological effects even at such exaggerated rates. When used at recommended rates, paraquat has no effects on earthworms, beetles and other soil fauna or on microorganisms including algae, fungi, actinomycetes and other bacteria.

Conclusions

The answers to the questions about paraquat’s long-term fate are that when paraquat is used as approved and recommended to control weeds, the amount in soil is far below the deactivation capacity. Enough is continuously released and degraded so that it does not continue to accumulate, though at amounts so small and so rapidly degraded that it does not result in deleterious biological effects.

So, there is no root uptake from paraquat in soil and it works only as a contact herbicide. Sprayed weed shoots quickly burn down leaving roots intact and seed germination is not affected. This means, first, that paraquat can be safely sprayed right up to the point when a crop emerges, and also between crop rows when directed or shielded; and, second, that weed roots and new uncompetitive seedlings can help to keep soil structure intact and prevent soil erosion.

The behaviour of paraquat in soil promoted the development of its wide and diverse range of uses. In particular, they allowed its fundamental role to help pioneer the development of workable practices in conservation agriculture, especially no-till5,6. The practical benefits of paraquat on the farm are illustrated here.

A peer-reviewed scientific publication discussing the adsorption and long-term fate of paraquat in soil is available here.

References

  1. Ricketts D (1999). The microbial degradation of paraquat in soil. Pest Management Science55, 596-598
  2. Summers, L A (1980). Fate of bipyridinium herbicides. In, The Biprydinium Herbicides: EDS Academic Press: San Diego. CA.
  3. Roberts, T R, Dyson, J S and Lane, M C G (2002). Deactivation of the biological activity of paraquat in the soil environment: a review of long-term environmental fate. Journal of Agricultural and Food Chemistry50, (13), 3623-3631
  4. Bromilow, R H (2003). Paraquat and sustainable agriculture. Pest Management Science60, 340–349
  5. Slade, P (1966). The fate of paraquat applied to plants. Weed Research6, (2), 158-167
  6. Hood, A E M, Jameson H R and Cotterell R (1963). Destruction of pastures by paraquat as a substitute for ploughing. Nature197, 4869, 381
  7. Huggins D R & Reganold J P (2008). No-Till: the Quiet Revolution. Scientific American, July 2008, pp 70–77

Notes

The brand name for the leading paraquat product is Gramoxone