Ammonia fertilizer may solve pesticide problem

The soil fumigant methyl bromide kills crop-damaging nematode worms and fungi. Unfortunately, it also rips holes in the ozone layer and threatens farm workers health. Now researchers have shown in laboratory experiments that ammonia fertilizer can degrade this pesticide. Although the study is preliminary, the researchers think that the method could prevent methyl bromide emissions from crop fields.

When planting crops such as strawberries, many farmers first fumigate their fields by injecting methyl bromide into soil and tenting the soil with plastic tarps. The United Nations Montreal Protocol placed restrictions on methyl bromide, along with other ozone-destroying chemicals, starting in the 1990s. As a result, methyl bromide emissions from soil fumigation have declined 90% worldwide. The U.S. Environmental Protection Agency planned to phase out the neurotoxic pesticide by 2005, but farmers pressured the agency to grant special exemptions to use methyl bromide on certain crops, such as strawberries. The farmers consider methyl bromide to be the most effective fumigant available.

Earlier studies found that methyl bromide degrades quickly under alkaline conditions to harmless methanol and bromide. Farmers use several alkaline substances on their fields, such as agricultural lime and ammonia fertilizer, so Scott Yates, a soil scientist at the U.S. Department of Agriculture, and his colleagues wondered if these bases could enhance degradation of methyl bromide.

In the laboratory, the scientists measured how fast methyl bromide degraded in solutions of ammonia, calcium hydroxide, and potassium carbonate. Ammonia was the most effective, breaking down the pesticide about 16-times faster than calcium hydroxide did. The researchers then tried a proof-of-principle experiment that simulated field conditions by applying methyl bromide to soil samples in the lab at the same concentration that farmers use and sealing the soil samples under a virtually impermeable plastic film. The ammonia degraded more than 99.5% of the methyl bromide after just 8 hours.

The study shows that farmers could cut methyl bromide emissions so effectively that the end result would appear as if the pesticide had been banned, says Robert Rhew, a biogeochemist at the University of California, Berkeley. But he wonders if the process will work in the fields. Rhew and Susan Kegley, principal scientist at the Pesticide Research Institute, a company that consults on pesticide use, point out that the plastic tarps used on fields do not provide the tight seals available in a lab because the tarps are frequently blown off by the wind or pierced by roving deer and birds.

Yates admits that more research is needed to see if his teams method is feasible on farms. He also thinks that the technique will work to degrade other halogenated fumigants, such as methyl iodide.