Nitrogen Fertilization in Central Valley Crops: Answering the question “Are we doing it Right?”
East San Joaquin Water Quality Coalition
Nitrate is a major contaminant in Central Valley groundwater and elevated levels are attributed primarily to leaching of nitrogen fertilizers past the root zone. Growers who belong to Central Valley Water Quality Coalitions (CV Coalitions) are under new requirements per the Irrigated Lands Regulatory Program to keep “on farm” a Nitrogen Management Plan (NMP) to track nitrogen fertilizer applications. A key component of the NMP is reporting nitrogen consumption during the growing season with the assumption that the remaining nitrogen is lost to groundwater. Determining crop consumption is one of several requirements of the Management Practices Evaluation Program (MPEP) that five CV Coalitions are cooperatively implementing (East San Joaquin Water Quality Coalition, Westside San Joaquin River Watershed Coalition; San Joaquin County and Delta Water Quality Coalition; Sacramento Valley Water Quality Coalition; Westlands Water Quality Coalition). The MPEP has specific objectives including identifying management practices that are protective of groundwater quality, determining whether newly implemented management practices are improving or may result in improving groundwater quality, developing an estimate of the effect of grower’s discharge of nitrate on groundwater quality and utilizing the results to determine whether practices need to be improved. There are data gaps in understanding the effectiveness of management practices on reducing the amount of nitrate transported through the root zone of walnuts. Numerous research projects are about to get under way in 2016 that will document the amount of nitrogen applied and the movement and distribution of nitrate from the point of application through the root zone in multiple fields planted to either annual or perennial crops. The projects will evaluate the movement of nitrogen through the root zone during rain and irrigation events over a two to three year period.Fields will be identified with the assistance of the cooperating CV Coalitions and commodity organizations. Management practices implemented by growers will include split fertilizer applications (based crop load and UC/industry expertise on optimal timing), and testing of soils/irrigation water/petiole-leaf to better understand crop nitrogen need and the amount of nitrogen and nutrients needed for optimal production. Measurements will be collected over two to three years (two storm seasons and two irrigation seasons). Additional management practices beyond those listed will be determined once cooperators have been identified. The BMPs will be implemented for at least two years allowing for changes in yields as a result of the BMPs and full evaluation of leaching potential. Management practices for nitrogen fertilizer applications and irrigation timing will be identified for fields prior to the implementation of the study. Throughout the two to three year studies, practices performed by the grower such as nitrogen applications and irrigation events will be recorded. Total yield and root zone nitrate results will be compared over the two years to account for the effect of the implemented BMPs on the amount of nitrate leaching and changes (if any) in yield.