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.
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?
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.
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.
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.
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.
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.