Whether you’re a homesteader or a casual gardener, it’s essential to know the facts about fertilizers and potting mixes that use recycled materials. More specifically, you need to know about the heavy metal contents of the soil in which your plants are growing (and no, we don’t mean the sub-genre of rock music). To grow your understanding of what is safe and what isn’t, we’ll start by digging into what heavy metals even are.

What Are Heavy Metals?

As people start hearing the phrase “heavy metal,” in the news, they’re becoming increasingly curious about what a heavy metal actually is. If you're one of them, here are the basics: “Heavy metals” are the metallic elements with atomic weights above 50. Five of the seven essential micronutrients are “heavy metals.” They are:

  • Copper
  • Iron
  • Manganese
  • Molybdenum
  • Zinc

Albeit rare, plants can reach toxic levels of these micronutrients. However, this seldom occurs in humans or animals (note: the elements boron and chlorine are essential micronutrients and can be toxic to plants but are not metals). “Heavy metals” that are most often considered potentially dangerous include arsenic, cadmium, chromium, lead, nickel, mercury, and selenium.

These metals are common in the environment and vary in concentration from one location to another. The tolerance levels for each metal varies considerably between species of plants and animals. However, an excess of heavy metals interferes with biochemical processes by modifying, disrupting, displacing, or blocking the functions of biological systems—effectively poisoning said systems.

Most often, animal and human toxicities from heavy metals occur from consuming plants or plant products that contain an excess of the toxic elements. While it is rare, the direct human consumption of soil can cause toxicity. This issue has been reported in children with pica, an eating disorder in which people ingest non-nutritive substances.

Avoiding Heavy Metal Poisoning

To avoid these problems, many countries and states have implemented standards to minimize heavy metal levels in compost and potting soils. Belgium was one of the first countries to establish strict guidelines for composting in the 1980s. In the U.S., we now have an EPA standard known as the “EPA 503 Metal Limits Tests,” which set tolerable levels for all hazardous heavy metals.

However, compost can be contaminated depending on its source. Trash and municipal waste compost can be a significant source of “heavy metals.”

Some growers have questioned paper and printer ink as possible sources of “heavy metals.” However, in an EPA-funded study (Contract 68D60035), newspapers and advertising materials from 48 publishers were analyzed and all were considered safe and well below tolerance limits. The current production of paper and the use of soy-based ink dramatically reduces the possibility of contamination. Most recently, the soil amendment derived from recycled newspaper, “PittMoss,” was tested and passed all the 503 limits tests.

Find a Supply That is EPA Tested

Caution should be high when using local compost sites, as they are often sources of “heavy metals” and should be carefully and completely evaluated before use. Also, mined nutrients and amendments can produce heavy metals as by-products and should be used with a full understanding of their chemical composition. Usually, a supplier can provide the level of metals present in the material. Avoid using materials where no heavy metal information is available.

Growers should require the suppliers of growing media components and blend substrates to provide information and assurances that the mineral additives, composts, organic, and recycled materials are sufficiently low in “heavy metals.” Careful evaluation of compost teas and solid, liquid, “natural” or “organic” fertilizers and pesticides will help growers to avoid heavy metal contamination and ensure safe production and safe products.