When paraquat residues come into contact with soil paraquat is rapidly adsorbed and strongly bound to clay and organic matter in the soil. It becomes biologically inert and as a result it cannot be taken up by plant roots or other organisms. Paraquat treated soils maintain an active soil ecosystem with no adverse effects on soil microbes, microorganisms and earthworms. Paraquat cannot be released from the soil or re-activated by the application of water or other agrochemicals.
All agricultural soils, not only those with high clay content, have a high capacity to adsorb paraquat. In most agricultural areas, applications could take place annually for more than 100 years without exceeding the adsorption capacity of just the top two inches of soil.
- In their review report for paraquat the European Commission (EC) noted that there was “very strong adsorption in all the soils tested” (EC, 2003)
- In their Reregistration Eligibility Decision (RED) document, the US Environmental Protection Agency (EPA) concluded that “due to the apparent adsorption strength of paraquat for soil clays, these bound residues do not appear to be environmentally available.” (US EPA, 1997)