For many years, people have believed that concrete can release chemicals into the soil. With this in mind, many folks have avoided using concrete blocks for vegetable gardens. Were these fears justified – does concrete leach chemicals into the soil?
Concrete does not typically leach chemicals into the soil. Old cinder blocks made from fly ash had the potential to leach chemicals, but concrete is different from cinder, and cinder isn’t typically made of fly ash anymore. The most concrete might do is raise the pH of your soil.
In this article, I’ll explain the urban legend of soil leaching and what chemicals concrete can release into it. I’ll also provide some useful tips for handling contaminated soil.
Urban Legend of Soil Leaching
If you were to search internet forums, you would likely find contradictory information about the safety of concrete in gardens. Some people believe that it is incredibly unsafe to use, while others think it’s perfectly fine.
The most common argument is around the actual terminology used. Some people use “concrete blocks,” while others will say “cinder blocks.” Ultimately, this debate can be chalked up to antiqued information.
Concrete is very different from cinder. Cinder is made of coal cinders, while concrete is made from steel, wood, or cement (for more information, check out this article: Are Concrete Blocks Safe for Gardens?
This difference, though seemingly small, is actually significant. With their varying material compositions, these materials absorb liquid and interact with soil differently.
Additionally, cinder used to be made largely of fly ash, a toxic material that could leach into the soil and be absorbed by produce grown nearby. This could create serious health issues, and, as a result, concrete was often grouped in with cinder as being dangerous for soil, despite being safe.
More importantly, cinder blocks generally don’t still include the material that made them dangerous. In the last 50 years, at least in the US, no cinder blocks have been produced with this toxin. This means that, unless using antique blocks, using these products shouldn’t be an issue.
What Chemicals Can Concrete Release Into the Soil?
While the narrative that concrete can leach dangerous chemicals into the soil is certainly not true, you can’t completely rule out the potential for chemical leaching. Concrete usually has multiple pores, making it collect and absorb water in one area and release it to another spot. During this fluid transfer, the water can absorb some chemicals from the concrete.
When concrete is dry, the only chemical that has a serious chance of leaching is lime. Lime (or calcium oxide) is a synthetic chemical often used in construction. It is not toxic to humans or plants, meaning that even if some of it were to be absorbed in the soil, it would have no negative impacts on human health. The likelihood of lime leaching is also relatively small, though possible.
The other potential for chemicals to leach into the soil from concrete is when it is first poured and is still soft or wet. When this occurs, the concrete’s general quality will be to stick to itself and not separate, though some separation is possible. In these cases, the most frequent chemicals to be leached are the following:
- Lime
- Silica (silicon dioxide)
- Alumina (aluminum oxide)
- Iron oxide
- Sulfate
Fortunately, these chemicals are non-toxic and won’t harm your plants directly. However, they might change elements of your soil that could impact plant life.
Concrete Leaching And Chemical Impact On Soil
Though it’s possible to limit a chemical’s impact on a substance, whenever a new chemical is introduced to a substance, it has the potential to interact with other chemicals and may change the composition of the substance.
This is especially true in soil. Since soil’s chemical composition is so dynamic in quality, even the smallest change can impact it. In this case, the biggest impact is a change in the soil’s pH.
pH is the measure of a substance’s acidity. Many chemicals have a particle called a hydrogen ion. These are individual hydrogen atoms that carry a slight charge because they have lost the electron that “neutralizes” the atom’s charge. The number of hydrogen ions impacts how chemicals interact, explaining the importance of measuring pH. Having many hydrogen ions makes the substance “basic,” while having fewer makes it “acidic.”
Certain chemicals, because of the number of hydrogen ions they carry, have a clear impact on pH. All chemicals that have the potential to leach from concrete have a low number of these ions, meaning that they are more likely to make the soil basic. This is probably a phenomenon you’ve observed since plants won’t grow well next to a sidewalk because of changes in acidity.
So, what does this mean for your garden? If you are using concrete, your soil can become more basic. If you have plants that can’t tolerate basic soil, this can harm them, while plants that enjoy basic soil will thrive.
The impact can’t be perfectly predicted and may or may not be noticed depending on how responsive your plants are to soil pH.
To learn more about the importance of soil ph to your plants, check out my other article: 5 Reasons Why Soil PH is So Important
How to Handle Contaminated Soil
While a change in acidity is not the worst thing that can happen to your soil, it can still damage plant life if not handled correctly. Acidic soil won’t harm humans, so you don’t need to take any special measures to protect yourself from soil that has been contaminated by cement leaching. All you need to do is correct the soil pH.
The easiest way to do this is by adding a supplement to neutralize the increased pH levels. You can do this by adding acidic materials such as:
- Sulfur
- Peat moss
- Highly acidic fertilizer
The best method is usually to add fertilizer, as this will provide your plants with additional nutrients and will help to balance future pH changes.
It’s important to note that if you want to use concrete in your garden, plan which vegetables to grow in that area with the acidity levels in mind. If you plant vegetables like asparagus, beans, or beets, these plants can handle higher pH levels and won’t face serious issues with soil leaching changes.
How to Prevent Chemical Leaching
In the previous section, I discussed that concrete is unlikely to leach chemicals into soil and, even if this happens, it’s not harmful. However, if you’re still worried about potential chemical leaching, there are some steps you can take:
Seal Concrete Blocks
An excellent way to help prevent chemicals from leaching into soil is by sealing concrete blocks. By using a waterproof sealant, the block’s pores will be clogged, preventing water from being absorbed and dispersed. This will prevent the chemicals from the concrete being released.
Use a Barrier
Using a barrier can also prevent chemicals from leaching into the soil. However, make sure that the barrier you use is non-porous and won’t rip.
Pond liner is a fantastic product to use to prevent chemical leaching. Mil Pond Black Liner from Amazon comes in a roll and can be cut to size. It is durable enough so that tearing is unlikely, and you can use it for other things in your backyard.
Use Concrete Blocks Where Contaminated Water Won’t Be Absorbed
When chemicals are leached from concrete blocks, it is often transferred through water being dispersed from the block. One way to prevent the water from contaminating soil is to prevent the water from being absorbed by the soil. Stone or sand placed over soil, for example, can help prevent the soil from absorbing chemicals.
Final Thoughts
When it comes to concrete leaching chemicals into the soil, this is a concern based on antiquated and irrelevant information. Concrete is very unlikely to leach any chemicals into the soil and if so, will only release a small number of chemicals that are non-toxic to humans and plants.
In the case that concrete does leach chemicals, prepare for an increase in soil pH and be prepared to use soil additives to lower it. Acidic fertilizer is best, or take steps so that changes in pH don’t impact your plants substantially.
