With every decision an organic farmer makes, there are risks and rewards to be considered. What is the risk of planting a new variety, versus the potential reward? How much can the yield improve by using a natural fertilizer, compared to the cost of application? Does the nutritive value of a soil amendment come with a hidden risk? Prior to applying new products, the benefits of soil fertility must be carefully weighed against the risks, and the first step is identifying potential sources of contamination.

While the debate continues over potential pesticide accumulation in compost sourced from conventional feedstock, National Organic Program (NOP) Guidance 5016 helps to address the requirements of unavoidable residual environmental contamination (UREC). Pesticide residue testing is now a required practice for organic certifiers. The National Organic Standards Board Crops Subcommittee recently put together a Discussion Document about contamination in inputs, and they are looking to develop new ways of addressing potential risks. They are still discussing this concept, but public comment is encouraged. Comment periods are announced on the USDA NOP website prior to each meeting. The next meeting will take place April 27-May 1, 2015 in La Jolla, CA.

Pesticides are just one type of contaminant that organic farmers must consider. The risk of pathogen contamination is another ongoing concern which, for better or worse, is being addressed by the Food Safety Modernization Act. However, heavy metal contamination hasn’t received a lot of attention other than a recent incident pertaining to arsenic contamination of rice. Since sewage sludge is explicitly prohibited by the organic standards, some people may see heavy metals as a low risk for the organic farmer. It’s important to realize that other sources of heavy metals can be introduced into an organic system.

The USDA organic regulations recognize the risk of heavy metal contamination at §205.203(c): “The producer must manage plant and animal materials to maintain or improve soil organic matter content in a manner that does not contribute to contamination of crops, soil, or water by plant nutrients, pathogenic organisms, heavy metals, or residues of prohibited substances.” However, the regulations do not otherwise provide limits for contamination, or guidance for prevention.

While certifiers require organic farmers to identify management practices that address this management requirement as part of the Organic System Plan, producers who have completed a HACCP (Hazard Analysis and Critical Control Points) assessment or other risk assessment plan may also implement heavy metals testing as a method of control. Certifiers can also request additional testing if potential contamination is suspected. But if you don’t know the risks, how can you implement the appropriate controls?

Although some heavy metals occur naturally in the soil, the risk comes when these naturally occurring elements are found in high concentrations or when their synthetic counterparts accumulate in processed waste streams. One example of this is chicken manure. Chicken manure provides high levels of nitrogen and phosphorous, but it can also be contaminated with heavy metals as a result of conventional feed additives containing arsenic. Under the organic regulations, manure from conventional animals is allowed for fertility in organic farms. And while phosphate rock provides a valuable nutrient, some sources include mineral deposits with high levels of arsenic and cadmium. For the same reasons that pesticide residues can accumulate in compost, heavy metals are also subject to concentration during composting. In fact, while some composting methods have been shown to degrade certain pesticides, heavy metals are an elemental contaminant that do not degrade into smaller, less harmful molecules during the composting process.

The Association of American Plant Food Control Officials (AAPFCO) publishes standards for heavy metal contamination in fertilizers that many state fertilizer boards have adopted. The AAPFCO standards define maximum contamination for fertilizers based on their phosphate and/or micronutrient content. Here at OMRI (the Organic Materials Review Institute), we also developed a simplified set of thresholds addressing potential contamination for input materials used in organic production. OMRI’s standards consider the typical loading rate of identified high-risk input materials. For example, compost, manure, and mulch are applied at relatively higher load rates, and therefore have lower thresholds than mined minerals and ash, which are generally applied at lower rates.

OMRI requires heavy metals testing for many types of fertilizer products. If the test reveals that heavy metals exceed a certain concentration, the product may be either OMRI Listed with a ‘Caution’ statement, or potentially considered ineligible for OMRI listing. The following table provides an overview of the threshold levels:

OMRI Heavy Metal Standards

OMRI heavy metal standards
Level 1 = level at which OMRI issues a ‘Caution’ statement for a given product on the OMRI Products List regarding the potential for long-term contamination based on elemental contaminant content

Level 2 = level at which a product is ineligible for the OMRI Products List due to risk of soil contamination

OMRI evaluates input materials to determine their suitability for use in an organic system and publishes a list of compliant products. Products that test above the established thresholds are identified with a cautionary statement in the OMRI Products List© and on the web search at OMRI.org. Growers using these products are advised to limit their use based on the potential for regular applications to result in the degradation of soils. Materials that test above the upper threshold are ineligible for OMRI listing.

The first step in risk management is to identify the risk. Although some contaminants of concern may not be visible to the naked eye, growers can look to the OMRI Products List in order to make an informed decision.

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