Organic farming practices regulated by the USDA successfully limit soil risk to colonization by pathogenic bacteria, according to a recent American study published in the journal Frontiers in Sustainable Food Systems. This study found that the mandated wait period, imposed by the USDA’s National Organic Program, between harvest and application of unprocessed manure was effective in curtailing the pathogenic bacteria Listeria monocytogenes. After 120 days none of the tested produce or irrigation water samples contained L. monocytogenes, and less than 5% of soil samples tested positive.

This study also shows that, while even shorter wait periods may be sufficient to mitigate food safety risks, environmental factors make management nuanced.  The scientists recommend additional measures that can be taken to further reduce the risk of bacterial exposure to fresh produce crops, like applying untreated manure in the winter or summer, rather than spring.

Organic farmers abstain from the use of synthetic fertilizers—chemical additives used to supply nutrients to crop plants. Instead, organic farmers rely on biological soil amendments like compost and manure to naturally add nutritional factors that boost plant success. There is concern that biological soil amendments of animal origin (BSAAOs) may contain harmful bacteria like Listeria monocytogenes, contaminate the soil and crops planted in the treated soil, and cause food poisoning for the consumer

USDA requires 90-120 days, depending on the crop, between unprocessed manure application and harvesting in order to eliminate the risk of pathogenic exposure. These long waiting periods can prove to be burdensome on farmers who plant fast-growing plants like leafy greens and can get in multiple cropping rounds in a single growing season (e.g. in California). It’s important to find the most safe and feasible wait period for farmers.

While studies have been conducted in non-organic farms on additional factors that determine L. monocytogenes population in manure, this is the first in organic systems, where soil amendments tend to be manure-based, and microbial and soil ecosystems are different. By determining supplementary methods to limit L. monocytogenes population lifespans, the USDA can update standards to continue to reduce the risk of pathogenic exposure to the consumer and possibly decrease manure waiting periods for farmers.

This study was completed over the course of two years in 19 organic-certified farms in four regions across the US. Different regions experience different climatic conditions that are expected to impact the risk of bacterial spread like rainfall and temperature. Researchers sampled soil monthly for 180 days after manure application and additionally tracked soil metrics, precipitation, and farm demographics. L. monocytogenes and E. coli populations were tracked through genetic detection.

L. monocytogenes populations dwindled over time, with positivity of exposure in the soil decreasing by 80% after 60 days. However, further waiting yielded no additional benefit in reducing bacterial populations.

Farm management practices involving the presence of animals had significant effects on pathogenic exposure in soil. Prior use of the field for livestock and presence of domesticated animals on the field each decreased odds of L. monocytogenes positivity by over 70%. However, the use of feedlots on the fields increased the positivity rate threefold. Increased soil moisture (by 10%) was associated with a 40% reduction in bacteria presence. Modifying the method of manure storage had no effect on the pathogen population.

Researchers found a significant correlation between L. monocytogenes and E. coli positivity. These findings are consistent with studies conducted on non-organic farms, indicating that methods that curb L. monocytogenes populations are additionally effective in limiting other pathogenic bacteria that cause food poisoning.

This study demonstrates that current USDA regulations are effective at decreasing the risk of exposure to pathogen bacteria in organic food. However, the mandated wait periods between manure application and harvest may be too stringent and field management practices may be more impactful than longer wait periods. Strategic modifications to organic regulations could create convenience to farmers and further decrease food poisoning risk in consumers.

Photo by Manfred Antranias Zimmer; https://pixabay.com/users/antranias-50356/

https://pixabay.com/photos/work-transport-wheelbarrow-cart-1201543/