Metals in Hemp-Avoiding Contamination

Having trouble viewing this email? View it as a Web page. Bookmark and Share


Metals in Hemp-Avoiding Contamination

With the busy growing season behind us, it's a good time to share best practices for quality hemp production and solicit ideas for how Maine's licensing program can develop and improve. In this newsletter, we discuss metal contamination in hemp, a topic that was suggested by a grower.

Although not considered a “heavy metal” hyperaccumulator plant like some mustards (e.g., Brassica juncea L.), hemp (Cannabis sativa L.) has a high tolerance to growing in soils contaminated with metals and can be used to bioaccumulate metals and other substances to help remediate contaminated soils. By virtue of its fast growth rate and large size, hemp can take up significant quantities of lead, cadmium, arsenic, nickel, mercury, chromium, and other potentially harmful metals present in the soil or other media in which is it growing. Many metals have well-known, negative health effects in humans; some can be toxic even in minute quantities, especially for children. It is therefore of utmost importance to take steps to avoid contamination of your hemp crop. Remember, the foremost use for hemp grown in Maine is for CBD and other cannabinoids and these phytochemicals are used primarily for therapeutic purposes.

Q: How might these so called “heavy metals” enter my hemp crop?

A: Even growers who are following organic standards need to ask themselves this question. While metals may be a natural component of certain bedrock and soil types, human contributions are considerable. Elevated lead levels in soil can be attributed to lead-based paints, scrap metals (pipe solder, roof flashing), and pesticides. If you are using old orchard land, for example, be aware that lead arsenate was a heavily used insecticide into the 1940s, until it was replaced by DDT, which is also known to persist in soil. Arsenate compounds were a common livestock feed additive, which means that manure used to fertilize fields could contain arsenic. Other inputs that may contain elevated levels of metals include biosolids, industrial wood ash, fertilizers, rockwool, and irrigation water, although this is not an all-inclusive list. Uranium, a metal that is also radioactive, is another concern for Maine because it occurs naturally in certain types of granite bedrock and can contaminate well water. Uranium can be taken up by some vegetable crops through irrigation water, but we don’t know about hemp specifically.

Q: How does a grower avoid these contaminants in hemp?

A: First, test your soil. The basic and very economical soil test through the University of Maine includes a lead scan. Next, find labs that will test for other metal contaminants and seek advice from your local cooperative extension office as to whether the land under consideration is suitable for growing hemp. If you are an indoor grower, you can request analyses from the companies producing your growing medium and fertilizers or have your own tests done. If well water is used for crop irrigation, we recommend testing for metals and especially arsenic and uranium because Maine wells are at risk for containing elevated levels of these naturally occurring but potentially harmful elements.

How much metal is taken up by hemp plants and where metals end up in the plant (roots, stem, leaves, flowers) depends on the hemp strain, soil pH, soil organic matter, and probably many other factors. More research is needed. To address the risk, producers should submit flowers, extracts and/or end-stage products for testing to an ISO-accredited laboratory that has the capacity for detecting metals in Cannabis. Ideally, one representative sample (>0.5g) of extract or finished product correlating back to each hemp lot used should be analyzed.

Q: What are the concentration limits for these metals of concern in hemp?

A: Well, it depends who you ask since there is no single regulatory standard for the country and Maine has yet to establish any for hemp. Putting clean, healthful hemp into the marketplace should be everyone’s goal. If test results show that your hemp contains concentrations above detection limits for these metals, you should research where the contamination is coming from and whether those levels are of toxicological concern. If you have found information regarding tolerances for these metals in hemp, we would like to hear from you. Drop me an email at

We hope you have a peaceful holiday season.


Mary Yurlina
Hemp Program Manager