Available at: https://digitalcommons.calpoly.edu/theses/3360
Date of Award
6-2026
Degree Name
MS in Environmental Sciences and Management
Department/Program
Natural Resources Management
College
College of Agriculture, Food, and Environmental Sciences
Advisor
Charlotte Decock
Advisor Department
Natural Resources Management
Advisor College
College of Agriculture, Food, and Environmental Sciences
Abstract
Under regulatory pressures to reduce groundwater contamination by nitrate from fertilizer, such as those posed by the Central Coast Regional Water Quality Control Board’s Agricultural Order 4.0, a need exists to better understand interactions between irrigation and nitrogen (N) management. To increase N use efficiency (NUE), growers are encouraged to account for additional sources of N in their management plans, such as irrigation water N and preplant soil N. The availability of these N sources for plant uptake and how it is affected by other management conditions is not yet well documented. We conducted 6 field trials, 3 organic and 3 conventional, on commercial broccoli fields in California’s Santa Maria and Salinas agricultural valleys. Each field trial was set up as a full factorial design with two factors, irrigation (grower practice vs. evapotranspiration-based (ET) irrigation) and N rate (5 rates ranging between 0 and 150% of growers’ rate). The factorial treatment structure was replicated three times within each field trial. ET irrigation treatment is applied as recommended by CropManage, an online grower decision support tool. Yield and aboveground N uptake was determined in each trial. To study N dynamics throughout the season, soil cores to a depth of 3 feet were taken before planting, midseason, and after harvest. The ET based irrigation treatment was successful in reducing water use by 6-18% without negatively impacting yield. The lack of effect of N rate on yield in four out of six trials was attributed to high preplant soil N and/or high irrigation water N concentrations. In one of the organic trials, yield tended to increase with increasing N rate despite high inputs of irrigation water derived N. This trial occurred on a welldrained soil and irrigation rates were 140% of ET-based recommendations, indicating that N may have been flushed below the root zone. In several cases, there was significantly greater retention of N under ET irrigation when compared to GS irrigation. Our data highlights the importance of efficient irrigation and deep soil sampling for increasing nitrogen use efficiency in broccoli. Comparison between organic and conventionally managed trials indicates that N availability is lower in organic fields, implying management challenges but also considerations for additional provisions to reflect the potentially reduced leaching risk in the context of water quality regulations.