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.