Does Sand and Clay Make Concrete?

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Robert Pavlis

What happens when you add sand to clay soil? Many people claim that this will make concrete and others say that it results in soil that is easier to dig. How can there be such large discrepancies about something that is so easy to test?

Why is this a problem? Gardeners with heavy clay find it difficult to dig, so they want to loosen it up. Sand is very easy to dig and it makes a lot of common sense to add it, to create a looser soil.

Soil texture triangle - sand and clay soil
Soil texture triangle – sand and clay soil

Sand and Clay Makes Concrete

This myth, as stated, is simple to debunk. Concrete is a mixture of sand, gravel and cement. Since neither clay soil nor sand contains cement, it can’t form concrete.

Maybe when people say concrete they really mean hard soil? Does clay become harder when you add sand to it?

Compost Science for Gardeners by Robert Pavlis

Making Adobe

Some people claim that sand and clay forms adobe, a strong material used in the Southwest US and Central America for making bricks. Adobe is made from soil that has approximately 70% sand and 30% clay. Too much clay will not make hard bricks. Heavy clay soil is around 60% clay, not 30%. Adding a bit of sand will not create soil with 70% sand, so it does not make adobe.

Regional Opinions

Most gardeners who believe the myth are from the Southwestern US. There are enough reports that I am starting to think that there might be something to their claims. People tell the story of adding some sand and ending up with soil so hard they can’t dig at all. Maybe they used the wrong sand?

On the other hand, people in Europe recommend adding sand on a regular basis. Many top gardeners like Beth Chatto use this method to loosen their clay soil. A Google search for UK websites will give you a long list of recommendations for adding sand to clay. They do caution that it should be rough builder’s sand and not smooth playground sand.

Australians also recommends adding sand to clay soil, but their problem is mostly sandy soil, in which case they add clay to it.

These regional differences suggest that the clay, sand or climate in these regions affects the results people see.

Scientific Evidence

There are numerous references to a California study, but nobody ever gives the details of the reference. I have been asking for and looking for it for several years without success. None of the people who claim it exists have produced it. If you have a reference, please post it in the comments.

Personal Experience

My first garden had very heavy clay that could be used for making sculptures. Digging in 3-4 inches of sand resulted in soil that was friable enough to dig and plants started to grow better. The soil did not get harder after adding sand.

My next two gardens had 50% and 40% clay. Adding sand in both cases produced soil that was more friable.

All of these gardens are in Southern Ontario.

Some claim that you can’t mix the sand into the clay properly and that is quite true. What I found is that the sand coats the clay clumps and prevents the clumps from joining back together. This soil now has sand channels running through it that allow more air and water into the soil. Even after 5 years, I can still see the channels when I plant something. Keep in mind that I disturb my soil as little as possible.

Soil Texture Triangle

The soil texture triangle pictured above shows the amounts of clay, silt, and sand in various types of soil. The triangle is useful for classifying soil, but I think it has led to the myth that you need to add 30 – 40% of sand, before you will have any effect on the soil. Looking at the triangle, this seems to be the case. If your soil is in the middle of the clay section you have to add a lot of sand before it becomes sandy clay or clay loam. But this is just a convenient way to label soil; it does not mean that a small amount of sand can’t make a difference. Not all of the soil in the yellow clay area has the same properties. A soil with 80% clay and one with 45% are very different, but both are still classified as clay.

You do not need large amounts of sand to change the properties of soil.

Logical Extrapolation

Since we have no science data, let’s look at this logically. Let’s say that you have clay soil and after adding some sand, it gets harder. What happens if you add more sand? If the myth is true, the resulting soil will be harder still. Add more sand and it gets even harder. At some point you will have soil that is almost pure sand, and as hard as a diamond. Does this make logical sense?

Even if there is some critical point at which adding sand makes the soil harder, most gardeners will not have soil at the critical point. Logic clearly shows that, at best, the myth is only true for some clay soils.

Clay Soil Does Not Make Clay Harder

Without some scientific evidence, it is most likely that sand does not make most clay harder.  Perhaps the clay in the Southwest is different and reacts with sand differently. After all, there are many types of clay soil.

Sand Does Not Create Good Soil

Sand may loosen soil for digging, and it might even open it up and allow more air into the soil, but it can’t make good soil and it won’t improve soil structure. Clay soil needs to have more organic matter added. This will increase microbe activity, and only then will the structure of the soil improve.

 

Looking for Comments

If you have experience adding sand to clay, please let me know about your results. Be sure to include some information about where you live.

 

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Robert Pavlis

I have been gardening my whole life and have a science background. Besides writing and speaking about gardening, I own and operate a 6 acre private garden called Aspen Grove Gardens which now has over 3,000 perennials, grasses, shrubs and trees. Yes--I am a plantaholic!

73 thoughts on “Does Sand and Clay Make Concrete?”

  1. I live in the western suburbs of Chicago. We have heavy yellow clay soils. I have been adding sand to various flower beds and have never ever seen a hardening effect. It always loosens the soil. I’ve been gardening for over 60 years. Where is this idea of hardening coming from? I can only assume it may be due to the chemical composition of the clay (highly alkaline?) that causes this.

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  2. Adding sand to clay soil can cause concrete like porosity issues not make actual concrete. That is what is being claimed. For the most part, based on my experience as gardener in upstate NY I’d avoid adding sand to soil. It is much better to cultivate organic material into 6″ of clay soil than add sand. And adding sand a top a different porosity soil layer should not be done. I’ve seen gardening disasters with topsoil-sand mixes where plants will not penetrate the soil, grass won’t grow and water rides the surface. Over time it’s like concrete. If you want to improve lawn aeration use a core aerator and optionally followup with top dressing of compost (not sand). Or to break up clay soils, add compost into top 6″. Summarily, sand should not be the gardener’s go to solution for most everything and reserved only for special applications where its intention is completely understood.

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    • “Adding sand to clay soil can cause concrete like porosity issues” – prove it. As I said in the post, I have been looking for such proof for many years and nobody has provided it.

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  3. I have clay soil as well that I want to amend, it seems it takes a long time to see an improvement using compost and manure, it is about 120 square meters, and the land is a bit at a lower level, I need to raise it about 4″ with amendments, I can get coarse sand and till it in, but I am not sure about the results, so many sites and forums warn against any sand addition to clay soil, I am very confused. If compost and manure alone is better or should I add sand along with it? I need the right answer to such a very hard question.

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  4. So, I just added sand to a small raised bed because I saw it suggested for the lavenders going in it. Then I saw the no sand bit panicked thinking, after all that work getting them better soil and I ruined it. I have heavy clay soil which isn’t so bad in the summer but kills a lot of thing over the winter NY, supposedly 6A zone. Plants that should be fine just don’t like it while others are thrilled. Oh well. Then I realized that I’ve actually been adding sand for years. About once a year, either in the fall, or in the spring, I put the chicken coop leavings on my vegetable beds (if in late fall it is straight from cleaning them, if in spring with the rest of the compost having sat for a good 9 months). I use sand in my coop. (Builder’s sand, appropriate for animals) My veg beds are perfectly happy with this. Not to sandy either from what I can tell.

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  5. Here in SW Colorado we have soil that’s high in calcium carbonate as is the water we use, probably the reason for the “myth’ whichI believe involves chemistry not just soil triangle. Regardless I mixed in about 20 gallons of sand into an existing 3×12 enriched raised bed last year. I got great results and carrots for first time. I’ve also added 50% sand and grit (smallest gravel to buy in bulk) to the arid garden. My soil is a clayey loam that’s had a lot of organic added. It seems to be working very well for the flowers that need great drainage. Clay particles are mineralogical flat and attract free molecules to the surface. I suspect over time those particles will adhere to the sand grains if in solution rich in CaCO2. So; what’s your water chemistry maybe the better question.

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    • I doubt it is the water chemistry, because people report the hardness happens quickly. But clay chemistry may have an impact.

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      • It might be interesting to add a little sulfur to lower the pH and see if clay “concrete” still forms. Most plants like a slightly acid soil anyway. Here in Irvine, CA the soil behaves like clay but, mixed in a jar of water, precipitates much more quickly than real clay would.

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  6. I’ve found that, after removing the sod layer, you can mix the 1-2” layer of builder sand below that and a good amount of compost with the ultra heavy, grey anaerobic soil below it to create some very wonderful soil. Requires heavy tilling and turning of the top foot of soil initially, and then you never touch it again. Just mulch! Along the gulf coast, my soil is mineral rich but anaerobic. Other than heavy trees (oaks, holly, magnolia, etc) few things can realistically adapt to that. In a few months it’s loaded with worms and you wouldn’t believe what you can grow there.

    Reply

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