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 leachingDescription The natural process by which water soluble substances are carried downward through the soil into groundwater. or run-offDescription The occurrence of surplus liquid (like rain) which originates up-slope and is collected beyond the ability of the soil to absorb it. The surplus liquid then flows away over the surface to reach the nearest surface water (pond, lake, river). Authoritative On-line References and Resources US Geological Survey's Water Science School from the degrading paraquat residues reaching the soil.
A new in-depth feature article added to the Knowledge Bank explains the fate of paraquat in soil and what the implications are for the environment and paraquat’s role in raising agricultural productivity.
Fate of paraquat in soil
Soils can deactivate paraquat equivalent to hundreds to thousands of years of application
Microorganisms can quickly degrade paraquat in solution
Tiny amounts of paraquat are slowly released from soil particles, degrading so that residues plateau, but not enough to have deleterious biological effects
Paraquat is deactivated on contact with soil, being very strongly bound to mineral particles and organic matter
Two characteristics of paraquat are fundamental to what happens in soil: how it degrades and how it is adsorbed (bound) to soil. In laboratory studies, when incubated with soil microorganisms in aqueous solution, paraquat completely degrades in less than three weeks, mainly to carbon dioxide, ammonia and simple naturally-occurring organic acids1. However, in the field only very low concentrations of paraquat are available to soil organisms because after spraying it is immediately and very strongly bound (adsorbed) to soil constituents2.
Paraquat is positively charged and attracted to negatively charged sites on soil mineral particles and organic matter3.
Given that typical paraquat application rates are a few hundred grams/hectare, the capacity of agricultural soils to deactivate paraquat is equivalent to hundreds or even thousands of years’ of paraquat applications.
Field trials have been conducted on different soil types around the world to study paraquat’s long-term fate in soil. These trials have involved long-term monitoring following repeat applications over many successive seasons, or after applying extremely high rates. In a long-term trial in North Carolina, USA, annual applications of a high recommended rate of 1 kg paraquat/ha were made for 12 successive years3. 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 in Australia, Malaysia, the Netherlands and the UK have produced similar results. Orchards in South Korea that had received annual applications of paraquat for 26 years had soil concentrations averaging less than 3% of the soil adsorption capacity4.
Long-term studies involving the application of paraquat at more than 100 times the recommended rate, even exceeding the capacity of soils 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.
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..
The practical benefits of paraquat on the farm are illustrated here.
You can read the extended in-depth article in the Knowledge Bank here.
- Ricketts D (1999). The microbial degradation of paraquat in soil. Pest Management Science, 55, 596-598
- Summers. L. A. (1980). Fate of bipyridinium herbicides. In, The Biprydinium Herbicides: EDS Academic Press: San Diego. CA.
- 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 Chemistry, 50, (13), 3623-3631
- Bromilow, R H (2003). Paraquat and sustainable agricultureDescription Management and conservation of the natural resource base and the use of technological and organizational change in a manner that ensures continued agricultural production from the land for present and future generations. Such practices conserve land, water, and plant and animal genetic resources. They are environmentally non-degrading, technically appropriate, economically viable, and socially acceptable. Sustainability rests on the principle that we must meet the needs of the present without compromising the ability of future generations to meet their own needs. Therefore, stewardship of both natural and human resources is of prime importance. Authoritative On-line References and Resources www.nal.usda.gov Information from the USDA's Alternative Agricultural Systems Information Center.. Pest Management Science, 60, 340–349
The brand name for the leading paraquat product is Gramoxone