Simple Guide to Testing for Heavy Metals in Growing Media for Cannabis Cultivators

The levels of heavy metals legally allowed in a finished cannabis product for human consumption are very different from the allowed levels in the horticultural media cannabis crops are grown in. One of the main reasons for this is that plants actually need small levels of heavy metals from soil for optimum growth.

But how do cannabis cultivators know that the growing media they’re using is safe? Testing for heavy metals in growing media isn’t as simple as adding up the metals in to calculate the metals out!

With that in mind, let’s dive into the science and best practices so you can make the best decisions for your grow operation.

Relationship Between Growing Media, Heavy Metals, and Plants

Despite what some people might think, the acceptable heavy metals concentration in growing media and finished cannabis products are entirely different and have no direct correlation to each other. In general, metals are not easily taken up by plants. Here’s why:

Total metal concentrations in soil do not necessarily correspond with metal bioavailability. The bioavailability to plants of heavy metals depends on a number of physical and chemical factors in the soil. These include soil properties, e.g. pH, organic matter content, redox potential, cation exchange capacity (CEC), sulphate, carbonate, hydroxide, soil texture and clay content (Peijnenburg and Jager 2003). Apart from these factors, metal absorption by plants is influenced by the characteristics of the plants themselves (Hund-Rinke and Kordel 2003).1

This is extremely important because small amounts of some heavy metals in soil are essential for plants:

“Beneficial heavy metals play a significant role at low concentration in the synthesis of protein, nucleic acids, photosynthetic pigment and are also involved in the structural and functional integrity of cell membranes (Oves et al., 2016).2

The goal of this guide isn’t to downplay the severity of heavy metals toxicity in cannabis products or the need for heavy metals testing. Both are crucial. Instead, the goal is to help growers understand how to test for heavy metals in growing media the right way, so they get accurate and usable results.

How to Test for Heavy Metals in Growing Media

As stated above, testing for heavy metals in growing media is not the same as testing for heavy metals in flower or other cannabis products. To get accurate results, you need to choose the right lab and make sure the lab is using the right testing protocol.

Use the Right Lab

The only valid results of heavy metals testing in soil will come from certified horticultural testing labs. Do not send soils to labs that specialize in testing cannabis. Not only do most of these labs not use the proper protocol for growing media testing, but they also compare the results against the legal limits for the actual crop, such as smokeable flower.

As explained above, there is no correlation between heavy metals in growing media and cannabis, so the test results will be inaccurate and unusable.

Use the Right Protocol

The appropriate protocol for testing soil is EPA SW-0846. No other testing protocol yields meaningful results unless the testing facility can document its method development and provide validation that its method is fit for purpose.

Additional Recommendations from bio365’s Soil Experts

Unfortunately, the mechanisms by which plants uptake heavy metals are not straightforward and are poorly understood. We do know that metals are not easily taken up by plants in general, but uptake will be greater in waterlogged (anaerobic) conditions and is affected by complexation processes in the soil.

With that in mind, following are some of our recommendations based on our scientific understanding of how management practices and inputs affect heavy metals accumulation in a crop.

Choose at Testing Lab Carefully

Only have your soil tested by a certified horticultural testing lab such as A&L Labs and Midwest Laboratories or the results will be inaccurate.

Check Publicly Available Data

Some states require testing for heavy metal content, and some of those states publicly post the results of the tests, such as the California Heavy Metals Database. Check the data before you choose a growing media.

Analyze Fertilizer Composition and pH

Acidity is a major driver of uptake for most metals. Fertilizers with ammonium are known to acidify soil.

When pH is lower, plants take up more heavy metals. However, as a result of its localized concentration, acidification in the rhizosphere due to nitrification of ammonium may not be reflected in a pour through measurement of pH that measures the average pH of the soil solution.

Bottom-line, organic fertilizers are a better option than synthetic, but if you’re using a salt-based fertilizer, our current best-understanding is that Calcium Nitrate is the safest form of fertilizer with regards to heavy metals saturation.

Consider Other Sources of Heavy Metals

There are more sources of heavy metals than soil. Most products with nutrients in them will have some degree of metals present, and the more you use, the greater the cumulative effect. Therefore, growers need to track all sources of heavy metals in their growing system.

For example, bio365’s nutrient-dense soils have organic fertilizer in them, and therefore, metals will be present in the soil. However, the metals in our soils always test within the regulatory limits, which means they’re most likely only the secondary source of metals in a grower’s system.

Instead, fertilizers, feeding practices, and other amendments that cultivators commonly use are typically the major source of metals among growers who use bio365 soil. The cumulative effect of using a fertilizer regime and/or adding other amendments plus the nutrients in our soils may lead to a heavy metals issue in resulting crops.

Heavy Metals in bio365 Soils

Regulation for inputs, including horticultural media, is generally weak. The most consistent and rigorous standards apply to fertilizers, which is one of the reasons bio365 chose to register our products as fertilizer. In addition, all bio365 nutrients are organic and not derived from ammonium.

We test all of our soils for heavy metal contents using accredited labs and appropriate protocols, and all bio365 soils are significantly under the limits for permitted heavy metal contents in fertilizer or compost-based amendments.

Our test results are published in state databases, including the California Heavy Metals Database mentioned above. We can also provide cultivators with accredited, third-party lab results for all bio365 products.

Contact the soil experts at bio365 to learn more about our growing media and testing.

REFERENCES:

1Marisa Intawongse & John R. Dean (2006) Uptake of heavy metals by vegetable plants grown on contaminated soil and their bioavailability in the human gastrointestinal tract, Food Additives & Contaminants, 23:1, 36-48, DOI: 10.1080/02652030500387554

2Arif N, Yadav V, Singh S, Singh S, Ahmad P, Mishra RK, Sharma S, Tripathi DK, Dubey NK and Chauhan DK (2016) Influence of High and Low Levels of Plant-Beneficial Heavy Metal Ions on Plant Growth and Development, Frontiers in Environmental Science, 4:69. DOI: 10.3389/fenvs.2016.00069