Is your Garden Soil Safe for Vegetable Gardening?
By George Giltner, Adv. MG, Biology MS
Most of our gardens are safe and natural ecosystems, which grow wholesome, nutrient dense foods. We work and handle the soil, enjoy the rich earth aroma, and appreciate the vital processes of living organisms that recycle nutrients, filter water, and produce our crops.
However, soils can be polluted just like water and air. Lead, arsenic, and cadmium are toxic heavy metals that are of concern. Once these heavy metals are introduced into soil, they persist a very long time. Knowing the history of the garden location can help to identify areas that are contaminated. Examples of contamination from the past include arsenic treated lumber residues, some fertilizers, old orchard sites where lead arsenate pesticides were used, a gun range, lead bearing paint residues, and even soil near roads in the time of leaded gasoline.
Most common fertilizers are not a significant source of heavy metals. Nitrogen and potassium fertilizers are generally free of toxic metal content, but phosphate fertilizers often contain cadmium depending on the mining site. Excess phosphorus typically is “tied-up” with insoluble compounds like calcium phosphate, therefore it lingers in the soil for years. Use soil tests to determine if additional phosphate amendments are needed. Thus, avoiding unnecessary phosphorus fertilizer applications may prevent the undesirable cadmium additions, too.
Micronutrient fertilizers have been and are still being produced from recycled toxic materials. “Ironite” has contained as much as 3600 ppm arsenic and 2900 ppm lead. No federal standards for heavy metals in fertilizers exist. Composition of fertilizers is in the control of the states. However, Washington State does require testing for 9 heavy metals with results on the web. Gardeners from other states use their postings to look up heavy metal concentrations in commercial fertilizers.
What are the negative effects of heavy metals on human health? Children bear the greatest risk as the developing brain and IQ are especially vulnerable to lead. Even the lowest detectable quantities are considered toxic to children. Children’s behavior as “Rug Rats” with mouthing and crawling on floors, exposes them to greater quantities of dirt and dust. Chromosome damage, nerve damage, cancer, etc. are among other toxic effects of heavy metals on all of us.
Vegetables are not all equal in their ability to uptake heavy metals. Some are concentrators and others are not. Leafy greens like lettuce, and root crops like carrots will have more than fruits like tomatoes. Some plants like water hyacinths are super concentrators that may have thousands of ppm of mercury from water sources. Therefore it should not be used as compost material for soil.
How do you get your soil tested? The LSU AgCenter Soil Lab can do an optional lab test for heavy metals for an additional $5. Another reliable lab is TP&S Lab (956-383-0739) which will cost around $100. Interpretation of results can be done with on-line research. Common sense guidelines – “Less is better”.
Please note the first paragraph of this article, “Most gardens are safe-“. This article is for awareness of heavy metals. It is not meant to scare or deter in any manner from the joys of gardening.