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The Presence and Transmission of Heavy Metals in Plant Fertilizers

Takeaway: For most, “heavy metals” summon images of skulls and crossbones—and for good reason. The consumption of heavy metals through food can lead to some serious heath issues. In this article, Lacey Macri discusses how these toxins get into plants, and how growers can reduce the presence of heavy metals in their plants.

When we as consumers hear the words “heavy metals,” our minds conjure up the notorious skull and crossbones. We prefer to avoid them at all costs, and for good reason. Heavy metals have been linked to serious illnesses and conditions, such as heart disease, cancer, neurological conditions, and birth defects.

Over time, our bodies can accumulate these toxins from the foods we eat, beverages we drink, air we breathe, and cosmetics we apply to our skin. How heavy metals enter those substances in the first place, however, is a different story. These toxins are introduced into the environment from an array of different sources.

For the sake of this article, we will focus on one of these sources: plant fertilizer. We’ll look at the presence and transmission of heavy metals in plant fertilizers, as well as how to optimize plant nourishment toward a cleaner, greener end product.

Defining Heavy Metals

Heavy metals are chemical elements known for having very high densities. In some forms, such as in golf clubs, these heavy metals are helpful as they are used to produce robust sporting equipment. In other cases, such as with accidental ingestion, these elements can act as toxins.

Heavy metals may be present in trace quantities as ions dissolved in soluble solutions, such as in fertilizer. This form allows the metal to enter the cell walls of the host. There are nine heavy metals that the Association of American Plant Food Control Officials(AAPFCO), an organization that limits and control the presence of heavy metals in fertilizers in order to make them safe for human consumers, regulates: arsenic, cadmium, cobalt, mercury, molybdenum, lead, nickel, selenium, and zinc.

The AAPFCO is setting increasingly uniform standards within the US fertilizer industry to align the practices of manufacturers with the safety of their consumers. In addition, the Department of Agriculture (DOA) of each state has its own set of standards and requirements a fertilizer must pass in order to be registered for legal commercial sale.

Origin of Heavy Metals

Many heavy metals are by-products of certain industrial processes, agricultural runoff, consumer waste, and contaminated soils. In some cases, however, these heavy metals exist naturally in the environment. Rock phosphate, a common ingredient used to produce organic phosphate-based fertilizers, is one example. Rock phosphate, in some cases, contains high levels of arsenic.

On that same thread, the actual heavy metal content in fertilizer ingredients varies depending on the region and conditions in which the materials were processed. For example, organic fertilizers derived from fish emulsion can contain higher levels of certain metals, such as mercury, depending on where the fish were raised and what they absorbed or consumed during their lifetime.

Other heavy metals, such as lead, are found in soils where they have had a chance to accumulate over time. However, humans can avoid ingesting these heavy metals by thoroughly washing soil off fruits or flowers before consuming them.

Residual Heavy Metals in Plant Tissue

So, how much of the heavy metal content in fertilizers is actually absorbed and stored in plant tissue? Well, this is a relatively controversial, poorly understood concept. One known fact is that some plants are hyperaccumulators of heavy metals and some are not. Hyperaccumulating plants absorb and store extremely high levels of heavy metals. Some examples are rapeseed, bentgrass, sunflower, and duckweed. There have been many trials to study the finer details of this concept, but many of them are inconclusive.

The good news, however, is that most researchers agree the threat of toxic heavy metal ingestion as a result of using chemical and organic fertilizers is very low. Most fertilizers, including many of the hydroponic nutrients available commercially, are sold in a concentrated form that require significant dilution before application. 

Take for example a basic organic or synthetic liquid P-K booster in which five per cent (or 50,000 ppm) of its concentrated composition is made up of available phosphate. This booster may also have trace quantities of heavy metals in the parts per billion (ppb) or parts per million (ppm) range. 

For this example, let’s say there are 5 ppm of arsenic (or 0.0005 per cent of its concentrated composition) of arsenic. The booster’s instructions suggest diluting the product with water at a ratio of 10 milliliters per gallon before feeding it to your plants. At that ratio, this nutrient solution will now contain 0.013 per cent (or 130 ppm) of available phosphate and 1.32e-10 per cent (or 0.0000013 ppm) of arsenic.

In addition, most plants are cultivated seasonally and do not have an indefinite life cycle. As such, there is a limited uptake capacity of these harmful toxins due to time restraints. What’s more, many plants are unable to uptake many of these heavy metals even when they are present.

Safety First

Even though it has been suggested that there is a very low risk of toxicity from consuming plants cultivated using fertilizers containing trace levels of heavy metals, cultivators should still aim to grow as clean as possible. After all, many states are beginning to enforce stricter guidelines when it comes to harvesting consumable crops (some will even process tissue samples from produce before allowing it to become commercially available). 

Farmers have to make sure they are knowledgeable about all of their inputs and their combined results. Choosing fertilizers that have been registered by a state’s DOA is a good first step toward peace of mind. Additional safety measures to take include making sure not to overfeed, using clean grow media, rotating crops, watering adequately, and even sending in tissue samples of your crop to a qualified lab to determine its heavy metal uptake concentration.

Cultivators can also consider using hyperaccumulating plants to cleanse grow media of toxic content. This is a particularly useful trick for soil that has accumulated heavy metals from multiple crop cycles over time, or for groundwater cleanup. This technique is referred to as phytoremediation and is considered a sustainable method of restoring resources to a point where they can be safely used again.

So, don’t let fancy lingo and trendy hype about heavy metals fool you into wheel-spinning and hypochondriasis. Acquiring as much knowledge as possible about your specific cultivation materials and techniques is key to understanding consumption safety. Integrating sustainable methods into your operation is also highly recommended to maintain the health of you, your plants, and the planet. Getting a second opinion from a qualified source is never a bad idea either.