SACRAMENTO (CBS SF) — California’s Central Valley has some of the worst air quality in the nation but it turns out we may have been wrong about the source.

Beyond what comes from the automobile tailpipe, researchers at UC Davis point a finger of blame at the soil that grows our crops.

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In a study published last week, UC researchers say smog-forming nitrogen oxides (molecular abbreviation: NOx) are released from the heavily-fertilized farmland in the Central Valley.

The research indicates that fertilizer is responsible for up to 41 percent of the NOx in the valley — much higher than previous studies have showed.

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“In California it was an unrecognized source. So the farmers were not aware of it — people were not aware of it — and the state agencies were not really aware of this source of NOx,” said Dr. Benjamin Houlton, senior author of the study and a professor with the UC Davis Department of Land, Air and Water Resources.

Scientists hope their research can be used by lawmakers to reduce smog and improve air quality over much of California.

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“The only way to tackle these environmental issues that involve agriculture is to work with farmers. It is really our hope … the state, researchers and farmers come together and work proactively on the issues,” the study’s lead author, Dr. Maya Almaraz, told KPIX.