|Fertilizer, Metals & Food Safety
The existence of metals, particularly lead, arsenic and cadmium, in some mineral-based fertilizers has been researched for more than a decade. While much scientific analysis has been and continues to be conducted, the potential impact of these metals on consumer safety is of primary concern. Foremost, are metals in fertilizer a threat to food safety?
WHAT ARE METALS?
Metals are naturally occurring elements in the earth and atmosphere with both beneficial and hazardous properties. Metals such as copper, zinc and iron, for example, are necessary to both plant and animal life as micronutrients. Metals like cadmium and lead are naturally occurring but not known to be essential to life.
Both plants and animals require common nutrients like nitrogen and potassium and certain metals like zinc and iron to grow. Just as in the human diet, many soils do not provide enough nutrients and metals for optimum plant growth and, therefore, require amendments.
HOW ARE HUMANS EXPOSED?
Most pathways of exposure to metals are man-made in nature and occur on a daily basis in minuscule amounts. Emissions from automobiles, burning of fossil fuels are considered primary sources: recycled waste (biosolids), diet and compounds used in agriculture are secondary sources. As metals are naturally occurring soil elements, some inorganic fertilizers contain traces of both essential and nonessential metals — as part of the natural composition of the materials they are manufactured from.
WHAT ARE THE HEALTH ISSUES?
Most appropriate to this question is the adage "dose makes the poison." While extreme levels of all metals are toxic, small doses are essential and/or benign.
The United States' Food and Drug Administration's (FDA) guidelines for Provisional Tolerable Daily Intake (PTDI) for lead, arsenic and cadmium are as follows:
Lead = 216 micrograms/day Arsenic = 126 micrograms/day Cadmium = 60 microgram/day
For 30-plus years, the FDA has studied the human dietary intake of metals as part of its Total Diet Study ("Journal of the Association of Official Analytical Chemists"). Dietary levels of lead, arsenic and cadmium were lower in the 1980s than in the 1970s, with evidence of an overall downward trend. Lead was consumed more than other metals due to its natural abundance in the soil and atmospheric deposition from leaded gasoline use.
Numerous studies exist on plant uptake of metals. In general, several factors impact whether a plant will absorb metals from the soil.
According to T.L. Roberts of the Canadian Potash and Phosphate Institute, cadmium is only one example where pH level, organic matter and other soil factors contribute to solubility and availability of the metals to plants.
Dr. John Mortvedt, a Colorado State University researcher and professor, reported on various commodities' uptake of heavy metals in 1995. Collected research on grain and straw of spring wheat, barley grain, potato tubers, sugar beet leaves, onion bulbs, maize (corn), soybean and timothy forage showed negligible or no impact on their cadmium levels with regard to fertilizers ("Heavy Metal Contaminants In Inorganic Fertilizers," Fertilizer Research, Fall 1995).
Dr. Richard Burau, professor emeritus of soil science and environmental toxicology at the University of California, Davis, conducted research on various types of lettuce and their uptake of cadmium. In soils with abnormally high levels of cadmium, Burau concluded that "dietary modeling of vegetable produce... suggests that it is not a significant threat to the national diet." While there are extreme cases, where an individual may reach intake limits if s/he consumes all of his/her produce grown in areas where natural cadmium soil levels are high, they are the exception, not the norm.
Restricting overall cadmium levels is an often suggested option, but, according to Roberts, one that "may have little effect on plant uptake." Roberts' research also indicates that pH is perhaps the most important factor — an issue regularly controlled by fertilizer best management practices.
A 1983 U.S. Department of Agriculture "market basket" survey showed that cadmium levels in several U.S. food crops were lower than originally reported. Overall, according to Mortvedt, "it seems doubtful that weekly cadmium intake by humans will approach the maximum recommended levels" used by the World Health Organization and the FDA.
THE ROLE OF FERTILIZERS
The use of fertilizers increased markedly beginning in the 1950s and leveled off by the mid-1980s. During this same 30-year period, as noted above, the FDA Total Diet Study (cited above) confirmed downward trends in human intake of metals.
Several factors also mitigate the cumulation of metals in the soil via fertilizers. This is especially true for phosphate fertilizers. According to Burau, "most farmers do not apply phosphate fertilizers every year; given the small rate of increase in soil level, even 50 years' application might be difficult to detect; and not all phosphate source materials are high in cadmium."
According to Dr. Allen Felsot, environmental toxicologist at Washington State University, this suggests "that soil amendments (fertilizers) have not significantly influenced dietary trends." The increased use of fertilizers during the same period of a downward trend in human consumption indicates that "soil amendments seem to be trivial factors in influencing exposure to metals... and have had neither a measurable impact on residues in food nor on dietary intake.".