Pre-Harvest Soil Safety

When aiming to achieve pre-harvest agricultural soil safety, the key is to maintain the level of beneficial soil microorganisms while minimizing the potential contamination of foodborne pathogens and plant disease from agricultural inputs used to produce crops, says Achyut Adhikari, PhD, associate professor and extension food safety specialist in the School of Nutrition and Food Sciences at Louisiana State University AgCenter in Baton Rouge.

Soils are often enriched with biological soil amendments of animal origin (BSAAO) to increase nutrient values, enhance water-holding capacity, and support crop growth and yield, says Kali Kniel, PhD, professor in the department of animal and food sciences at the University of Delaware in Newark. Soil amendments can be delivered to soils as raw animal manure, treated or composted manures, and compost teas.

Using animal manure as a fertilizer on agricultural farms is a common practice in the United States because it’s a good source of macro- and micronutrients required for crop production, Dr. Adhikari says. In addition, organic matter present in manure helps improve physical, chemical, and biological properties of soils. It also improves water infiltration, enhances nutrient retention, reduces wind and water erosion, and promotes the growth of beneficial organisms.

According to the Food and Agriculture Organization (FAO) of the United Nations, livestock contributes 40% of the global value of agricultural output and supports the livelihoods and food and nutrition security of almost 1.3 billion people. But, despite their benefits, BSAAOs can also contribute to food safety risks. They can be contaminated with zoonotic pathogens or enhance the growth of zoonotic pathogens in and around the growth of raw agricultural commodities, says Dr. Kniel. “It’s important that growers use soil amendments appropriately to grow healthy, efficient crops, as well as avoid the excessive use of soil amendments that could affect agricultural water if contamination occurs by runoff into produce fields,” she adds.

Potential Pre-Harvest Problems

Poor soil health can cause plants to become diseased, contaminate fruits and vegetables, and ultimately lower food production, says Dr. Adhikari. In addition to the indigenous microflora of soil, pathogens and other microorganisms can be introduced into soil from different inputs such as contaminated irrigation water, runoff water, and unfinished or improperly treated compost or raw manure application, as well as both domestic or wild grazing animals.

The persistence of bacterial and viral pathogens in raw animal manure is based on the manure type, how it’s applied and incorporated into soils, soil type, storage of manure before application onto soils, and the microbial diversity present and nutrient ratios in manure-amended soils, says Dr. Kniel. Persistence and survival of bacterial pathogens in manure-amended soils depend on geographical and environmental factors.

According to Michael Mahovic, PhD, branch chief of the division of produce safety within FDA’s Center for Food Safety and Applied Nutrition in College Park, Md., among the most commonly occurring foodborne pathogens are:

1. Salmonella spp., which can come from domesticated and wild animals and their feces as well as humans and their feces. Some strains have become resident in the environment.
2. Shiga toxin-producing Escherichia coli, which can come from domesticated and wild animals, particularly ruminant animals (e.g., cattle, sheep, goats, and deer), and their feces.
3. Listeria monocytogenes, which can be found in soil, decaying vegetation, water, and domesticated and wild animals and their feces.
4. Cyclospora cayetanensis, which can come from humans and their feces.

Using untreated or partially treated animal manure as a fertilizer in crop production may result in contaminating fresh produce with enteric pathogens, Dr. Adhikari says. Once contamination occurs, it is difficult to remove pathogens completely from fresh produce, even with chemical and physical decontamination treatments. As plants uptake water, soil-borne pathogens can enter the fruit, making it impossible to wash away, leaving heat as the only means of rendering the produce safe.