Solving policy problems with field research

One in four domestic wells on Salinas Valley farms are too contaminated for safe drinking, according to sampling done in 2025. The culprit: nitrogen fertilizer spread across central California, which supplies more than 70% of America’s leafy greens. 

As fertilizer leaches into groundwater and runs off into streams, it triggers toxic algae blooms in nearby Monterey Bay and boosts nitrate in local wells past the U.S. Environmental Protection Agency’s safe drinking water limits – levels that can cause blue-baby syndrome in infants.

Cover cropping – planting crops between seasons to capture excess nutrients – is a well-known solution for keeping nitrate in the soil, yet few farms have adopted the practice. 

In 2021, the Central Coast Regional Water Quality Control Board launched Ag Order 4.0 to reverse this trend, offering payments to farmers who plant winter cover crops. Four years later, only 5-7% of acreage in the Salinas Valley utilizes cover cropping, lower than the U.S. average of about 10%. 

That gap between policy and real-world application is the focus of Sierra Castañeda, a PhD student in Earth system science professor Rob Jackson’s lab at the Stanford Doerr School of Sustainability. She works closely with fellow PhD student Anna Gomes, a member of the Soil and Environmental Biogeochemistry group led by professor Scott Fendorf.

When policy misses the mark

Cover cropping itself is not new. Decades of research confirm it reduces nitrogen pollution in water supplies while benefiting soil health. The trick is in the timing: Ag Order 4.0 only credits cover crops planted and tilled within a narrow window during winter.

“It often doesn’t align with current farmer practices and their strict planting schedules,” Castañeda said. “It poses a big management headache for them. They understand the benefits of cover crops, but the timing doesn’t always work.”

Castañeda’s research tests whether cover crops planted earlier in the season and with a shorter growing time are as effective at reducing nitrogen pollution as those currently eligible for the program. If so, the policy could be expanded to work with established farming practices.

“We don’t really know what happens if you change when the cover crop is planted, and how that timing impacts how it grows, decomposes, and releases the stored nitrogen,” she said.

Early findings suggest that cover crops planted earlier grow faster and take up more nitrogen from the soil than winter plantings. The team is now investigating what happens to the nitrogen taken up by the cover crop as it decomposes. 

From research to reality

Working with U.S. Department of Agriculture researcher Eric Brennan, the Stanford team has conducted research in controlled laboratory settings and farms – from large commercial operations to small organic growers. Last fall, Castañeda, Gomes, and Brennan presented their preliminary findings to the expert panel that recommends updates to Ag Order 4.0.

For Castañeda, whose own family includes Latino farmers, connecting science to real-world application isn’t just academic – it’s personal.

“I love being in the field and the work that I do with the farmers directly,” she said. She has conducted demonstrations for farmers and fieldworkers, providing resources in English and Spanish, both in person on Salinas Valley farms and through educational videos. 

“I really care about making this accessible for the people that it’s impacting,” she said. “My abuela would be so happy right now.”

Castañeda will graduate this fall and plans to continue researching sustainability practices that positively impact communities.

“Agriculture is one of the big problems that we have, and a lot of people have become out of touch with where food comes from and how it’s grown,” she said. “I want to work locally on a problem that can have a real policy impact.”