Adoption patterns of on-farm nutrient management practices and nitrogen application rates in California’s Central Valley

Nitrogen fertilizers in irrigated agriculture improve cropland productivity, but contribute to groundwater contamination, air pollution, and greenhouse gas emissions. While California has implemented agricultural water quality regulations since the 1980s, targeted efforts to address nitrate contamination through nitrogen application reporting and management have been emphasized more recently under the Irrigated Lands Regulatory Program. This study uses a unique, field-level dataset from the Kings River Water Quality Coalition in California’s Central Valley to examine nitrogen management practices, including common combinations of practices (bundling), across crop types. The analysis draws on four years of data from Irrigation and Nutrient Management Plans and details nitrogen application and management strategies. The results show that in 80% of fields across crops, between 50 and 300 pounds per acre (lbs/acre) of nitrogen is applied. Crop-level nitrogen applied minus nitrogen removed (A-R) values show substantial variation, with walnuts showing a nitrogen surplus (+65.5 lbs/acre) and alfalfa a significant deficit (−424.5 lbs/acre). Many growers adopt multiple practices, with 26% of fields utilizing six practices and 24% using five. Such bundling may provide growers with more flexibility in managing nitrogen applications. Further research is needed to evaluate the effect of various nitrogen management practices on fertilizer use and water quality effects.