Nitrogen fertilizer has undoubtedly benefited California's agriculture and citizens. However, applying more nitrogen than can be used by plants may lead to negative impacts on the environment and human health. Finding a balanced use of nitrogen to maximize benefit and minimize harm is essential to protect California's agriculture, people and natural resources. 

As California legislators focus on nitrogen use in agriculture and its ability to contaminate groundwater, potential regulation on fertilizer use will require solid information on the amount of fertilizer used by California farmers, and the extent to which that usage contributes to environmental pollution. A new study published in California Agriculture evaluates trends in fertilizer use by California's major crops. It also shows that major deficiencies in data collection need to be addressed in order to develop effective policies regarding fertilizer use.

The publication is one of the first peer-reviewed articles to emanate from the California Nitrogen Assessment (an ongoing project at UC Davis). Assessment research documents that while there are many pathways through which nitrogen can enter the environment, inorganic fertilizer use is responsible for the largest fraction of new nitrogen introduced into California annually. Currently, over 600,000 tons of nitrogen fertilizer are sold in the state each year. 

Information on fertilizer sales, however, is not an accurate indicator of fertilizer application, and authors found that fertilizer use data is not easy to come by — either because it is not tracked at relevant scales or because data sources are inconsistent.


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"We found ourselves with very limited information to understand an issue with sweeping implications for California agriculture,” says Todd Rosenstock, the article's lead author. "We dug deep to create an accurate picture of fertilizer use in the state, but the remaining gaps will require attention.”

To estimate the amount of nitrogen applied to different crops around the state, authors aggregated data from different sources, including grower surveys and University of California studies, and show application rates for 33 of California's major crops.  

While nitrogen fertilizer use on a crop-by-crop basis has risen over the last three decades, this increase has been more modest than fertilizer sales suggest. Between 1973 and 2005, fertilizer sales increased 31 percent, but nitrogen application rates increased on 25 percent across the 33 crops studied. Both data sets reflect increased nitrogen application and a shift to growing more nitrogen-intensive crops.