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[Michaelangelica]

Some pesticides can reduce soil fertility

04 June 2007

Some pesticides developed to boost crop yields could be doing the opposite in the long term, report US researchers.

Common pesticides block the chemical signals that allow nitrogen-fixing bacteria to function, report Jennifer Fox and colleagues at Tulane University. Over time, soils surrounding treated plants can become low in nitrogen compounds, so more fertiliser is needed to produce the same yield.

Root nodules
Soybean root nodules, each containing billions of Bradyrhizobium bacteria

© USDA
Sustainable agricultural practices often use crop rotation: growing a different crop in the same soil each year. Alternating crops that fix nitrogen in the soil - so-called leguminous crops, such as beans or clover - with crops, like wheat, that don't fix nitrogen, enables soils to replenish nitrogen levels routinely. Leguminous plants contain root nodules that use soil bacteria to fix nitrogen, a process that converts atmospheric nitrogen into useful compounds like ammonia.

Fox's team tested several common pesticides on leguminous alfalfa plants, relying on the plants' nitrogen-fixing bacteria to provide the nutrients. The insecticides methyl parathion (not used in the UK, but widely used throughout the world, and registered in at least 38 countries) and DDT (which was banned by the World Health Organization for almost 30 years, before being reinstated in 2006 as an effective intervention against malaria) showed a decrease in crop yield of about 20 per cent. Treatment with pentachlorophenol (whose use is restricted in Europe to specialist timber applications), showed a decrease in crop yield of over 80 per cent.

Some pesticides can reduce soil fertility

Soybean root nodules, each containing billions of Bradyrhizobium bacteria
© USDA