Cinder blocks are a common building material for raised garden beds, as they are inexpensive, durable and easy to build with. Yet, what many people don’t realize is that cinder blocks are made with a toxic byproduct of coal combustion that may contain high levels of heavy metals – metals that might leach into the soil of any garden bed contained by a cinder block wall.
Cinder blocks are made from a combination of Portland cement and fly ash; it’s the latter ingredient that poses the greatest problem. Fly ash is trapped and collected as a byproduct from the burning of coal at power plants, then used to cut the costs and weight of cement blocks.
What’s in coal?
Coal is actually highly toxic, filled with heavy metals and other dangerous substances. Many of these toxins, including heavy metals, are released when coal is burned – that’s how mercury ends up in the air, and eventually in the ocean (and eventually in the fish people eat). In fact, coal burning is one of the top sources of global mercury pollution.
But mercury and other metals released from coal burning don’t just float up into the air on their own; they are released as part of the fly ash. When that ash is collected, concentrated and made into cinder blocks, the heavy metals go along with it.
That means toxic heavy metals may eventually leach into garden soil from cinder block retaining walls. And while it’s never ideal to bring heavy metals into your soil – you never know what you’ll do with that plot in the future, or if the next person who lives there will know the soil’s history – it’s especially dangerous if you’re using a cinder block-framed garden bed to grow food or medicinal plants.
Heavy metals have been linked with cancer, nervous disorders, cognitive damage and even autism.
What to do instead
This is not to say that you shouldn’t build raised beds, which are ideal for very wet climates and also for gardeners with back or knee pain. It just means that you need to be careful about what materials you use to build your retaining walls.
Cement blocks, made with Portland cements and aggregate, may also pose health concerns, albeit lesser concerns than cinder block. Any treated or scrap lumber, including railroad ties, is likely to contain toxic chemicals.
The best building materials are natural, non-treated materials. Untreated cedar is a fine choice, as it resists decay and has insect-repelling properties. Another good choice is natural rock, of the same type used to build old country fences worldwide. Even if you don’t live in an area where you can gather enough rock on your own, you may be able to strike a deal with a building contractor to gather rocks for free from a nearby site of new construction.
When planting any soil that may have heavy metal contamination, consider one or more courses of bioremediation before growing edible crops. Bioremediation refers to using living things to remove toxic agents from the environment. Sunflowers, for example, have a tendency to accumulate metals as they grow. So grow sunflowers for a season, then dispose of the plants (do not return them to your soil, even after composting). Many plants can bioremediate soil in a variety of ways, including binding heavy metals and rendering them harmless.
You know what you’ve done with your own soil, but what about the soil that most of your food actually comes from? The book Food Forensics, by Mike Adams, contains charts of which foods contain the highest levels of heavy metals. Read the book to learn about Adams’ state-of-the-art analytical laboratory, and the tests he conducted to determine the heavy metal content of groceries, fast foods, dietary supplements, spices and protein powders.
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