The results are worrying because farmers concerned about artificial fertiliser use are returning to traditional crop rotation methods to maintain nitrogen levels.
Legumes house nitrogen-fixing bacteria in special root nodules, providing them with an energy source in return for biologically available nitrogen.
But the researchers found that pentachlorophenol, a persistent pesticide used to treat timber, prevented nodules forming on alfalfa roots. It also slowed plant growth and the amount of nitrogen fixed.
Two other insecticides, DDT and methyl parathion - both banned in the UK - and bisphenol A, an industrial chemical with oestrogenic effects, had a similar but less dramatic effect (ENDS Report 386, pp 28-29 ).
Alfalfa plants release signalling chemicals into the soil to recruit the right bacteria species and inhibit others. The researchers think the chemicals interrupt this communication by binding with the bacteria’s receptors.
As a result, the bacterial populations do not grow as fast and produce less of their own signalling substance, limiting root nodule growth.