Garden Myths - Learn the truth about gardening

Sand and Clay Don’t Make Concrete

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?

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.

 

Robert Pavlis
Editor of GardenMyths.com
I live in southern Ontario, Canada, zone 5 and have been gardening a long time. 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!

I hope you find Garden Myths an educational site that helps you understand your garden better.

32 Responses to 'Sand and Clay Don’t Make Concrete'

  1. Robert,

    Regarding your April 19th reply to martinwelbank you state,

    “I have discussed this very point with Linda, who I have known for several years, on The Garden Professors Facebook Group. She was not able to provide any scientific evidence to support her statement “To significantly alter a clay soil, sand must be incorporated to about 50% of the total soil volume.” As far as I have been able to tell, this statement is a complete myth.”

    In fact, Art Spomer in a classic Journal of Environmental Horticulture article, “Physical Amendment of Landscape Soils” (http://www.hrijournal.org/doi/pdf/10.24266/0738-2898-1.3.77) explains and illustrates precisely why adding coarse sand to clay reduces soil pore space. (Versions of this article appeared in many professional turf, landscape and arboriculture publications in the early 1980s.)

    Interestingly it is possible for a 100% sandy soil to have less total pore space than a clay soil! However, because the pore spaces in sandy soil are much large (macro versus micro pores), the sandy soil is much “looser” and drains much more readily.

    • These diagrams have been published a lot. However, they are theoretical diagrams – they are not actually measuring soil properties.

      They also assume that ‘soil” consists of uniform, equally spaced small particles. Clay is actually a much different structure. There is no reason to think that these diagrams apply to clay soils.

      The property of making the soil more friable is never discussed. Nor do the diagrams imply that adding sand to clay will harden it.

  2. Bris Vegas says:

    There’s a very old saying “Clay to sand – money in hand. Sand to clay – throw money away”.

    Adding sand to clay would have to be one of the most expensive, labour intensive and impractical ways to break up clay imaginable. It simply creates a very expensive clay/sand mixture

    By far the best method is to use gypsum or a water-based clay breaking polymer. These change the electrical charge on the clay particles causing them to clump. You can then add organic material to create a friable soil.

    • Sand is very cheap, easy to spread and works. The sand stays put for ever.

      Gypsum should only be used in sodic soils.

      from a review by Dr. Linda Chalker-scott:

      This myth falls into the category of agricultural practices misapplied to ornamental landscapes.
      Gypsum effectively changes the structure and fertility of heavy clay soils, especially those that are heavily
      weathered or subject to intensive crop production. Gypsum also improves sodic (saline) soils by
      removing sodium from the soil and replacing it with calcium. Therefore, one can see improvement in
      clay soil structure and fertility, and desalinization of sodium-rich soils, by using gypsum.
      What other effects will gypsum have on soil and plant health? There are a number of scientific studies
      on gypsum usage both in the literature and on websites. Briefly, researchers have found:
      • Gypsum does not usually change soil acidity, though occasional reports of both increasing and
      decreasing pH exist;
      • Gypsum can increase leaching of aluminum, which can detoxify soils but also contaminates
      nearby watersheds;
      • Gypsum can increase leaching of iron and manganese, leading to deficiencies of these nutrients;
      • Gypsum applied to acid soils can induce magnesium deficiency in plants on site;
      • Gypsum applied to sandy soils can depress phosphorus, copper and zinc transport;
      • Gypsum can have negative effects on mycorrhizal inoculation of roots, which may account for
      several reports of negative effects of gypsum on tree seedling establishment and survival;
      • Gypsum is variable in its effects on mature trees;
      • Gypsum will not improve fertility of acid or sandy soils;
      • Gypsum will not improve water holding capacity of sandy soils; and
      • Gypsum’s effects are short-lived (often a matter of months)

  3. martinwelbank says:

    One of the links on your site took me to Linda Chalker-Scott’s site, where she has the following article :
    https://puyallup.wsu.edu/wp-content/uploads/sites/403/2015/03/soil-amendments-2.pdf
    It contains an interesting juxtaposition of ‘sand’, ‘clay’ and ‘concrete’

    • I have discussed this very point with Linda, who I have known for several years, on The Garden Professors Facebook Group. She was not able to provide any scientific evidence to support her statement “To significantly alter a clay soil, sand must be incorporated to about 50% of the total soil volume.” As far as I have been able to tell, this statement is a complete myth.

  4. Justin Ayers says:

    Lady’s slipper orchid growers have said to watch out for sodium salt in sand. So, if sodium salt can be a contamination issue with sand and rinsing is needed it would be a good tip to mention.

  5. Matthew Koster says:

    I’m pretty sure I know why some people are reporting the addition of sand causing their soil to harden. The big retailers and landscape supply stores carry a line of products called “paver sand” or something to that extent. It is meant to be spread over the ground that pavers or flag stone will be placed to provide a level, stable base. It looks like sand, feels like sand, is called sand on the label, and it IS sand, the kicker is that it is not just sand. The product is intended to act like grout for outdoor “tiles”, spread it, lay the pavers and wet the whole area. When the sand dries it hardens. I’m not sure but the paver sand may contain cement, so in essence these people are mixing a low grade concrete into their soils. Again this is just a theory, but the myth seems so outside of logic that I would guess there is some hidden player involved.

  6. Brian Tremback says:

    “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.”

    I think what you’ve said here is the key to the beneficial use of sand. Although I would still seriously question the wisdom of using sand over using compost in the same way. Sand doesn’t facilitate the formation of soil aggregates like organic matter does. Nor does it increase fertility, soil life, or water-holding capacity.

    Clay soils are not the curse that many home gardeners consider them to be. Here in Vermont, a large proportion of agricultural production (much of it corn for silage for dairy operations) comes from soils (Vergennes series) that have clay percentages of around 75%.

    In any case, following guidelines for managing soil health (https://www.nrcs.usda.gov/wps/portal/nrcs/main/soils/health/mgnt/) is important for clay soils just as it is for any other soil texture: 1) minimize disturbance (plowing, cultivation, etc.), 2) diversify your crops, 3) keep living roots in the soil all year (perennials or winter cover crops for us in the North), and 4) keep the soil covered (mulched or fully-vegetated).

    And if you do have to disturb clay soils, pick your time wisely. You may spend years recovering soil structure after digging or tilling a clay soil when it was too wet.

    • I agree organic matter works better than sand for all the reasons you mention. The one benefit of sand is that it is long lasting. Organic matter is used up and needs to be added regularly or the effect is lost.

      • btremback says:

        A few years ago, I saw a presentation by Professor Nina Bassuk (Cornell Univ.) on amending soils to combat compaction. One method she used was referred to as “scoop-and-dump” where about 1/3 by volume of compost was worked deeply into the soil. Bulk density and soil carbon were monitored over the following 12 years and it was found that soils continued to improve over that period of time; that is, bulk density continued to decrease and carbon increased. The only input of organic matter, other than what was produced by plant roots, was maintaining a mulch of 2 to 3 inches of shredded bark. Here’s the slideshow: http://www.hort.cornell.edu/uhi/outreach/pdfs/StrategiesCompromisedSoil.pdf. There’s also a video of the operation: https://www.youtube.com/watch?list=PLHPXm2Es8aQC-A4BA87YhFLPvxKK76qpm&v=1sX5c97Mju8

        From the video, it looks like the soil is a fine sandy loam or silt loam, but I don’t know of any reason that results would be different for a more clayey texture.

  7. Lisa Barrow says:

    I live on Long Island, NY….On the n. shore it is clay like, on the S. shore it is sandy….I learned through Cornell Cooperative Extension that if your soil is to clay-like, add organic matter. If your soil is to sandy, add organic matter. Makes sense. Organic matter feeds plants and loosens up clay for more air into the soil, and sandy soil will benefit without question…the soil needs food……

  8. Roger Brook says:

    Some of your readers have alluded to zones of sand causing problems and the need to ensure thorough mixing when sand is added to clay. How true.- although your little rivulets of sand sounds intriguing
    Thin layers of sand overlying soil cause terrible drainage problems.

    • The last bed I made was for a nursery bed. It is an area where I am constantly adding small plants and removing them once they are large enough to go into the main garden. Instead of digging the sand in, I just laid it on top and started to plant. Each time I add or move plants it gets mixed up a bit. This seemed to work as well and digging it in.

  9. Bonnie says:

    Here in Central Virginia, we have lovely Virginia red clay–if you’re really lucky (like I am) you have Virginia red clay mixed with chunks of Virginia granite. However, in those areas of my yard that have clay, I have amended the soil with lots of organic matter and some sand and now have fairly good soil. The areas with granite chunks are where I put raised beds…:)

  10. I’ve been doing soil blends with poorly graded sand and clay for a number of years. Mainly for high end infield skins like the A’s, Dodgers, Padres and others. Also do sand and clay blends for horse race tracks and root zones for athletic fields. You have to be able to use the right sand cause sands are all over the place in how they are produced. It’s tricky doing it correctly but it does work extremely well when done correctly. Adding a course well graded sand with clay is not advisable and it those cases it does make bricks.

  11. hb says:

    Of course sand + clay = concrete makes no sense at all. One cannot have concrete without cement.

    This garden is blessed with wonderful silty loam, so I can’t experiment with clay.

    I speculate, wonder, if the real problem for those who mix sand into clay is the “perching” problem–a sharp difference in soil textures that creates waterlogged spots, as when a layer of stone is added to the bottom of a planting container. The sand is not mixed deeply enough into the bed to be sufficiently below the root zone to allow for drainage.

    We have high levels of irrigation “salts” in our California soils because the irrigation water is very high in carbonates and has a high pH, gypsum can be a benefit.

    • A perched water table is created when the two layers of soil are very different. In pots the soilless mix and stones are very different. I doubt adding a few inches of sand will be enough to create this. If it did the upper layer would be extra wet – not hard.

  12. daryleone says:

    Start with a soil test. You can’t get to the soil mix you want unless you know where you are starting from. Throwing something, whether it is clay, sand, or organic material, at the soil you have is kind of silly unless you know why you are throwing what you are throwing. Soil is a complex material. When the four cation minerals are in proper balance, as determined by a soil test, everything else about the soil starts to fall into place, including the proper pH.

    • Standard soil tests do not measure the levels of sand, silt, and clay. You can check this yourself: https://youtu.be/AUhOBxVFcFk

      • daryleone says:

        Are you saying, Robert, that determining the volume of sand, silt, and clay is not a soil test? A soil volume test a very basic test. It is not a particularly important test. Useful, yes.
        Knowing what the mineral volumes are, as determined by a conventional soil test, is much more helpful information. A grower should be able to pick up a handful of garden soil, squeeze it, give it a poke and estimate the soil sand/clay content. You can’t squeeze the soil and tell what the ratio of calcium to magnesium is, much less whether there is enough copper in the soil to keep your hair from turning prematurely gray.

  13. gwbish says:

    We live in Armidale, NSW Australia. I had a garden bed of mostly clay. I added a really thick layer of good compost( at least 4 -6 inches) and dug it in as best I could. 12 months later we have a lovely, friable soil that will grow anything.

  14. Richard says:

    I read with interest in what Gary had to say in his reply. Does gypsum really loosen up clay soil like some say it does? I have heard it does and others say it does not. What is the truth about gypsum?

    • Gypsum only works in certain soils. It works be replacing the sodium ions in clay and it will loosen clay soils if they have high levels of sodium. It does little for clay soil that does not have high levels of sodium.

  15. David Williamson says:

    Hi, I live in Mesa AZ and we have hard packed alkaline clay soil. I added truck loads of sand and compost and now have wonderful soil in my raised beds. Adding just compost or horse manure did not break up the clay as much as adding sand and compost. When first creating the raised beds I double dig the soil down about 36 inches deep and add compost while digging. In subsequent years I add compost and sand. Without the sand the beds drop by about 6 inches per year.
    Best regards
    Dave Williamson

  16. I usually mix in even amounts of clay and sand, but in addition for the garden and planters, I add a lot of organic material. Rotting leaves, compost and other additives I can find at the nursery.
    Remember, in clay soil, I always used garden gypsum. It actually worked like little hoes in the soils breaking up the clay over a period of time. Clay actually looks like little sheets under a microscope that holds water, where sand looks like little boulders that give space for air and water to move through the soil.
    Now, most important is knowing your soil, in addition to knowing thenPH. Depending on the type of gardening you are doing the composition of the soil and the ph are very important.
    One last item which I won’t expound on is a cation exchange in the soil. If themph is out of wack, you have too many positive charged ions in the soil and it locks up the nutrients.
    Well, I won’t bleary your eyes more.
    Happy gardening.
    Just remember one important tip. ” Know What You Grow!”
    If you don’t know what is required of the plant, it might not be a successful venture and to say the least expensive!

  17. I live in the ne in New Jersey and have moderate, very rocky clay soil. I mix in compost and manure, but if I am planting something that needs really sharp drainage (for example penstemon), I will throw in a handful or two of sand into the area I’ve dug out.

Please let me know what you think - Leave a Reply