Using biological soil amendments in organic farming is the most effective way to store carbon in the ground to fight climate change and boost soil health for better food production. And since synthetic nitrogen fertilizers are prohibited for use in organic farming, biological soil amendments, are heavily relied upon to manage soil fertility on organic farms. A new study published in the journal Frontiers in Sustainable Food Systems found that soil amendments of animal origin (manure-based) are not only good for the soil, but they are also safe to use in the production of crops intended to be eaten fresh. The study assessed the survival of four foodborne pathogens in soil and on the harvested crop after the use of manure-based soil amendments. Importantly, only 18% of the soil amendment samples tested positive for a foodborne pathogen, and less than 1% of harvested crop samples (two out of 527 samples) tested positive for a pathogen (in this case, Listeria monocytogenes). One of those two positive samples came from a farm where a manure-based soil amendment was not used, suggesting that the source of contamination cannot be strongly linked to the use of manure, and that soil amendments of animal origin can be safe to use when managed properly.

Care must be taken when applying manure-based fertilizers because manure commonly harbors pathogens that cause foodborne illnesses like Salmonella, Listeria and E. coli. To ensure consumer safety, the National Organic Program requires certified organic farmers to implement waiting periods of 90-120 days between manure application and harvest of the crop, depending on whether the harvestable crop comes into contact with the soil. This wait period can be burdensome when organic farmers produce crops with short growing cycles like lettuce and little science to date has tested whether this long of a waiting period is needed to effectively reduce the risk of pathogen contamination. This study found that different pathogens required different lengths of time to "die-off" with some needing only 30 days and some 60 days. A small number of samples contained Listeria after 90 and 120 days suggesting that current wait periods are sufficient in reducing food safety risks, and more research may reveal shorter wait period requirements.

The results from this work support safety of the use of biological soil amendments in organic farming, and it is also important to note that risk of contamination is complex with multiple potential sources including contaminated agricultural water (for irrigation and/or from flood/runoff), field workers who are ill and don’t take proper measures to decontaminate before handling produce, fecal deposition from domesticated or wild animals that make their way onto a farm, and contaminated farm equipment. Using soil amendments of animal origin provides an important outlet for excess manure produce in livestock operations.

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