Purpose: This study evaluates the fate of Salmonella in foliar fertilizers at different storage temperatures.
Methods: Rifampicin-resistant Salmonella Typhimurium LT-2 was inoculated at 4 log CFU/ml in fertilizers diluted with groundwater to the lowest recommended spray concentrations in Florida. Fertilizers included calcium nitrate (5 lb/acre), copper sulfate (2 lb/acre), ferrous sulfate (2 lb/acre), manganous sulfate (2 lb/acre), sodium molybdate (0.25 lb/acre), Solubor (1 lb/acre), zinc sulfate (2 lb/acre), and a groundwater control. Fertilizers were stored under refrigeration (4 °C) and temperatures common in a normal Florida growing season (15 °C or 30 °C) for 7 days. Salmonella populations were enumerated on rifampicin supplemented tryptic soy and bismuth sulfite agars.
Results: In all the fertilizers at all temperatures, gradual population reductions were equal to or greater than that the control. At all temperatures, Salmonella populations fell below the limit of detection in copper sulfate, ferrous sulfate, and calcium nitrate within 24 h, and in zinc sulfate within 72 h. Salmonella populations decreased by 1.4 logs in groundwater during the 7 day storage at 4 °C. The largest decline (3.2 log CFU/ml), where Salmonella populations were still detectable, was in molybdate. Over 7 days at 15 °C, Salmonella populations in ground water decreased 1.4 logs. The largest decline (3.0 log CFU/ml), where Salmonella populations were still detectable, was in manganese. Salmonella populations decreased by 3.1 logs in groundwater during the 7 day storage at 30 °C. The largest decline (4.0 log CFU/ml), where Salmonella populations were still detectable, was in Solubor.
Significance: These results demonstrate that some fertilizers may support Salmonella survival over extended periods if left in spray tanks and may be a source for pathogen contamination in foliar application in produce fields.