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?
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.
VERY GOOD CLARIFIER ON CLAY. sand can be a confusing, misleading additive. The moral of the Clay probelm is that Organics trump all, but you are better to be patient – adding incremental layers of organic on surface and let nature and time do its’ job. Clay has a very high electron bond that ebbs and flows with moisture levels, hydrogen ions. So Hard as a rock, plasticky paste glue, and the medium where you can break up easy enough. It never stays in that perfect balanced state.
I live in north Texas with a naturally clay soil. My *lovely* neighbor was replacing the sand in his sand volleyball court, and decided to dump ALL of his old court sand in my backyard. I haven’t had the cement issue but it does settle and compact with just basic watering once or twice a week. What can I do to fix my backyard?
add organic material
Keep mulching the area
Cement is simply clay and lime. In the Phoenix area our soil has a pH~7.8 and is ~85% clay with very high free lime. So yes, adding sand can adversely affect soil porosity if not enough organic material is also added. But once organic material is added to backfill soil, it can only be replenished top down from organic fertilizers and mulch. Often this can’t add enough organic matter back to compensate for large amounts of sand added to the soil. I never add any sand to my backfill soil, just organic matter and sulfur. And I grow very non-native things like citrus, mango and avocado.
I just read your excellent book. Knowing the theory helps me avoid wasting time and money on tilling and wrong fertilizer. Would like to grow cacti outdoors in zone 7, clay soil, 4 inches rain every month, eastern US. Some species will withstand the cold, butthey need good drainage. Am considering raised beds of 30% commercial topsoil, 70% rough sand, in truckload quantity, so am hoping to get a custom mix delivered. (Would get a sample of soil/sand first and experiment with differet %ges.) What do you think? Would this result in a perched water table when placed on clay soil (about 50% clay, 45% silt, 5% sand.)
There is always a drainage issue when the particle size is very different between two layers of soil. One way around this is to mix the top soil into the bottom creating a more gradual change in particle size.
Golf courses use perched water with a sand layer over native soil to have a moisture sink. Since capillary action requires each horizon to saturate before moving to the next one down.
This is so predictable that hydrogeology lets us do math to quantify the times and volumes
Why try growing cacti in a habitat that is clearly not meant for that? All the time, effort and expense for something that will likely fail? You’d be much better off planting native trees, flowers and shrubs for pollinators, birds and other wildlife. Your landscape will benefit and you’ll be helping the environment – win win.
I have tried mixing 1:1 red soil with river sand and put that mixture in a plastic pot. Still it doesn’t drain water.
Same here, I’ve even increased to 2/3 sharp sand and 1/3 clay soil, still really is just like a pot of concrete and the water drains even (much slower) than just my original very heavy clay soil.
What we are talking about here is starting with say 70% clay and adding maybe 5% sand.
You are taking 30% clay and 70% sand – that is the perfect recipe for adobe – as the article says.
I have experience of adding insufficient amount of fine sand to clay. It solidified my soul even worse. Then I tried to use a large amount of a pea-gravel instead together with a bit of compost. That’s when it made some difference. The next problem was the sand/gravel reduced fertility of my soil. So I abandoned gravel and used lots of compost alone. For a few years everything went great until different plants started having chlorosis. My research revealed that if you annually add more than an inch of compost to the tilled soil, your soil eventually reaches toxic levels of phosphorous. Bugger. So I switched to no-dig method. From then on I’ve been using compost alone for both the vegetable garden, and flowers. No chlorosis anymore. Plants have generally been one so healthy that I stopped using g pesticides. I even planted two apple trees and two cherry trees using no-dig. The trees are thriving. So my advice, stop tilling you’re soil. It damages it and you end up with a visions circle of eternal problems. Go No-Dig.
The California test people are referring to; is likely related to CBR (California Bearing Ratio). It is one of 2 systems used to measure the hardness of the ground.
HELP. can I grow plants in an only gravel soil? (very coarse)
yes
Lime and iron present in soil will determine hardness
Lime causes an aluminum silicate bond using the ferrite
Think hardwood ash for lime
Think river silt for ferrite
Sand is a silicate as well as clay
Determine
Silicate
Lime calcium hydroxide
Iron ferrite
You add all 3 , you get hard bonded soil
Dean Loren
Chemist Farmer ny
If you have a scientific study that shows this – I’d like to see it.
Yes! Look up native plants, trees and shrubs for your location. Become a steward of the earth and bring nature to your landscape. I’ve started doing just that and it’s amazing! I’m removing all invasive species and planting only natives. I’ve already noticed and increase in birds, bees, butterflies and other pollinators, plus wildlife too.
The increasingly popular “alternative” building material Cobb usually requires adding sand to clay soils, along with some straw or other long fibre. But here is the thing. Whereas the gardener nurtures good open structure and increased pore space, the Cobb builder aims to totally destroy voids and pores by working the material as a stiff mud so that the clay fills in all spaces between sand grains, this dries to a material of high density, and strong enough for this type of building work. Not concrete obviously, but an alternative.
If you tried to grow in this, no root or worm would be able to penetrate it.
The above info is possibly not very useful to gardeners, except as a warning: if you have clay soil, never compress it or work it when it is too wet as you will destroy its pore structure, and do long term damage.
Both soil and concrete are very complex materials. Even highly controlled concrete can result in significant differences in final product if ratios or curing conditions are changed. So, making clay soil – concrete comparisons is largely pointless.
However, it can be safely stated that clay and portland cement do in fact have many similar characteristics. One of the main differences is not so much chemical, but rather the degree of hydration due to kiln firing during portland cement making process. In other words, it is not inconceivable that certain clay-sand ratios given right wet/dry cycling and agitation could result in quite hard soil structures. Adding sufficient sand/organic materials and thorough mixing should avoid these problem ratios.