Microbial Risk Assessment of Soil Amendments in Organic Romaine Lettuce, California's Low Desert, 2023 Season
Applied Research
Cuong Huu Nguyen
Food Safety and Organic Production Advisor
UC ANR
Holtville
Abstract
The use of biological soil amendments of animal origin (BSAAO) and organic liquid fertilizers in leafy green production raises concerns about the potential growth or survival of pathogenic Escherichia coli. While BSAAO has been extensively studied, the microbial food safety implications of sugar-based organic liquid fertilizers, such as corn steep liquor (CSL), remain largely unknown. This study aimed to evaluate the survival of rifampicin-resistant E. coli TVS 353 in soils amended with heat-treated poultry pellets (HTPP), seabird guano pellets (SBG), and CSL used for growing organic romaine lettuce in California's Imperial Valley during the 2023 growing season. Eight treatments were evaluated: Control (no amendment), HTPP-1 (one application), HTPP-2 (two applications), SBG-1, SBG-2, CSL, CSL+HTPP, and CSL+SGB. Romaine lettuce transplants were planted in triplicate plots, and soils were inoculated with rifampicin-resistant E. coli TVS353 (7-8 log CFU/mL). E. coli levels were quantified via direct plating and most probable number (MPN) assays from soils and lettuce plants at harvest (42 days). Weather data, soil temperature, and moisture were recorded throughout the trial. All treatments showed a 3-5 log CFU/g reduction of E. coli in soils within 21 days and an approximately 7 log CFU reduction by day 42, with no significant differences (p>0.05) between treatments. However, CSL treatments supported higher levels of E. coli on the lettuce plants compared with BSAAOs (p<0.05). Increased E. coli reduction in soils correlated positively with higher soil temperatures and longer sunlight exposures during the growing season. These findings suggest that while BSAAO and CSL amendments may initially support E. coli growth, environmental factors, such as temperature and sunlight exposure, play a crucial role in reducing E. coli levels in soils over time. However, the potential for CSL to support higher E. coli levels on lettuce plants compared to BSAAO warrants further investigation to ensure the microbial safety of organic leafy green production systems.
Poster has NOT been presented at any previous NACAA AM/PIC
This poster is being submitted for judging. It will be displayed at the AM/PIC if not selected as a State winner. The abstract will be published in the proceedings.
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Authors: Cuong Huu Nguyen
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Nguyen, C. Food Safety and Organic Production Advisor, University of California, Agriculture and Natural Resources, California, 92250