By: Dr. Charles Bethke 8/13/18
Some have questioned whether they should be concerned about the “heavy metal” content in growing media made from recycled materials like PittMoss. Others have asked what are “heavy metals” and why are these elements important in plants that are used for consumption? Then the important question as to how to avoid them when growing plants is often asked.
“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, and Zinc and all can reach levels that are toxic to plants but seldom of concern for humans or animals. (Note that the elements Boron and Chlorine are essential micronutrients and can be toxic to plants but are not metals.) Other “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 another. Tolerance levels for each metal and between species varies considerably.
Toxicities to humans, animals, and plants are caused by a several factors. (It must be noted that “HEAVY METAL” toxicity does not come from listening to a particular music genre, at least for most people.) Excesses of “HEAVY METALS” interfere with biochemical processes by modifying, disrupting, displacing, or blocking the functions of biological systems. They effectively poison the systems. Most often animal and human toxicities from “HEAVY METALS” occurs from consuming plants or plant products that contain an excess of the toxic elements. Direct human consumption of soil is rare but is reported to occur in the disabled “pica child” who is known to eat soil. To avoid these problems many Countries and States have implemented standards to ensure that the “HEAVY METALS” levels in compost and potting soils are low enough to avoid problems. Belgium was among the first to establish strict guidelines for compost 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 “HEAVY METALS” considered hazardous.
Compost from many sources can be contaminated with the hazardous “heavy metals”. 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”. 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 soya-based ink greatly reduces the possibility of contamination. Most recently the Soil amendment derived from recycled newspaper, “PittMoss”, was tested and easily passed the all the 503 limits tests.
Caution should be high when using local composts as they are often sources of “heavy metals” and should be carefully and completely evaluated before use. Also, mined nutrients and amendments can provide 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, solid and liquid and “natural” or “organic” fertilizers and pesticides will help growers to avoid "heavy metals" contamination and insure safe production and safe products
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