Extremely Low Heavy Metal Levels in PittMoss Potting Soils

C. L. Bethke pH.D. – Horticultural Soils and Nutrition Consulting – 7/28/20

Concern over the heavy metal content of potting soils is reaching new heights, as the quantity of produce from crops grown in container medium is being consumed at greater rates than ever before.  Recently, it has been noted that some crops tend to absorb and accumulate heavy metals at a greater rate than others.  It is important that the vegetables, herbs, and medicines we consume remain low in heavy metals to avoid adverse health effects.  When ingested or absorbed, heavy metals can produce distortions of normal bodily functions. Repeated and long-term exposure to many heavy metals can lead to physical and neurological degeneration and muscular problems.  Disorders like autism, multiple sclerosis, muscular dystrophy, and Alzheimer’s have often been correlated to excessive and repeated exposure and ingestion of heavy metals.  For these reasons, it is imperative that crops be produced in growing media low in the critical heavy metal levels discussed below. 

Table 1 presents the maximum tolerance levels established by a number of state, national, and international agencies.  Arsenic, cadmium, cobalt, chromium, nickel, lead, selenium, and mercury are of primary concern for crop production. The limits for the described metals, as shown in the table below, are presented using international terms applying to manufactured or processed growing medium (compost/substrate).  Each agency has established maximum levels suitable for use in production of food crops. The Organic Materials Review Institute (OMRI) limits are for organic production, and the Rodale Institute levels refer to vegetable production. The limits set by Washington State appear to be the most restrictive in the United States.   The listed limits by the United States (EPA) are for municipal sludge that is intended for application to agricultural land. The Canadian levels are more restrictive than most US states, and in Europe, the most restrictive standards are in the Netherlands.  An array of those limits is presented in Table 1. 

The heavy metal levels for four PittMoss growing blends are presented in Table 2, with a detailed analyses performed by MicroMacro Analytical Laboratories of Athens, GA. The data is presented on a dry mass basis as parts per million (mg/kg).  It is apparent that all four blends are extremely clean, consisting of some of the lowest heavy metals content available. Note that no mercury or cadmium could be detected at levels as low as 0.2 and 0.1 ppm. Little to no lead, arsenic, selenium, and cobalt could be detected in all four PittMoss blends, with chromium and nickel levels greatly below the established tolerance levels.  With these very low heavy metal levels, PittMoss growers can be assured that their crops will be clean, safe, and healthy.