Soil + sand = cement? Not so much.

They say adding sand to clay soil yields cement. Maybe sometimes. But based on a highly unscientific experiment in my back yard, not always.

Soft soil 1,

The warning about sand pops up here and there all over the internet, often coming from university Extension offices. The one in Colorado for instance, says flat out, “Don't add sand to clay soil,” predicting a consequence that would make any gardener quake in her gardening boots: “this creates a soil structure similar to concrete.” A specialist with Ohio State University goes even further, saying that the result is usually “a disaster.”

Now, I have a personal stake in this issue, because I spent more hours than I plan to confess (unless you pack me off to Guantanamo) working sand into soil last fall. I call it soil, but that’s a misnomer. It was clay, and the fact that I actually bought it—paid someone to put it into my raised beds—makes me furious. I just can’t decide who I’m madder at, myself or the idiot who sold it to me as “loam.”

Didn’t I notice that it wasn’t the highest grade stuff? Sure I did. Staring at it afterwards, I thought, this was a mistake. Staring at it with me, a good friend said, Send it back. But anything decent was beyond our budget, so, fortified with Mel Bartholomew’s original Square Foot Gardening book, I set out to “improve” it instead.

Early Efforts
Every year I dug in compost. I tried little clay pebbles, and peat moss, and gypsum. (Groan. More about that another time.) Every year in the spring I thought, this isn’t so bad! And every year I watched the “soil” harden over the summer, despite the constantly increasing worm population—the only reason it drained at all, I’m convinced. Last August, when it took all my strength to sink a trowel into it to dig out carrots (and that was using two hands), I’d had enough.

It was time to get serious. Since a good loam has at least 30% sand and my soil had way too much clay, the solution seemed clear. I would add sand.

The sand I had access to—stuff my husband picked up from an abandoned building project—contained pebbles galore, so first I screened it, tossing the pebbles on our driveway, Then the real work started. This sand didn’t merely get spaded in. It got mixed. By hand. Bucket by bucket, I stirred sand into the heavy dirt, breaking up clods of clay, squashing them flat on rocks and kneading sand into them. Sometimes I used a hammer on them. By November, I was working under a greenhouse of sorts, trying to finish the four by eight bed I’d started on. Along with the sand, I added peat moss, compost, and slow-release organic fertilizers.

Sometime during the fall, husband Steve brought home the news that sand, especially fine sand, could cause clay soil to turn to cement. Great. Yet I continued. Why? Well, first of all, because I’m stubborn, I guess. (It’s a question of character.) But frankly, I didn’t believe that doing this could backfire so completely. My guess was that most people didn’t work sand in as thoroughly as I did. (This seemed a safe guess.) Or they didn’t add organic matter as well.

When I was researching the unbelievably long lawn article several months later, I ran into the sand-causes-cement claims myself, including the two quoted above. Those aren’t the only ones. They gave me a few bad moments, but it didn’t make sense that adding sand would make rock-hard soil harder. And it didn’t seem possible that my soil could get worse. Time would tell.

It’s October now, a full year since I went full-bore on the sand amendment. Since then, I’ve added nothing to that bed save a thin layer of compost. I didn’t even mulch it (gulp), and since hail took out so many of my plants this summer, a lot of that bed has lain open to the elements for several months.

Yet it’s soft and loamy. Easy to dig, it crumbles willingly in the hand. I wish you could feel it, because that’s the only real evidence. These pictures do not do the trick, I know, but they’re all I’ve got, so here goes:

The photo at the top of this post shows soil taken from the bed that got the sand treatment. The one below shows soil from another part of the garden, where I’ve planted several times, adding other amendments, but never sand.

Hard soil 1

I’m squeezing the untreated soil sample, but it still holds together. The other, treated, soil crumbled in my hand before I could squeeze it.

Since I’m not much into conspiracy theories and can’t come up with a reason why extension agents at four or five universities across the country would lie about the effect of sand on clay soil, I conclude that sand really does make some clay soils harder. Why? And why not mine?

It could be a difference in the soils themselves, or in the sands added, but I suspect that it’s at least partly a difference in method. Perhaps sand has to be integrated very thoroughly to improve drainage in clays, or perhaps it has to be used with plenty of organic material as well. I’m not sure.

I am sure of one thing: I’m sold on sand.

8 Responses to Soil + sand = cement? Not so much.

  1. Hey…I got a concrete Engineer sitting in my living room so I ask him. He is my oldest son and he said the ingredient in concrete that makes it hard is Portland cement. Portland cement’s main ingredient is limestone. He said under the right conditions and amounts–it would make concrete. It’s actually a chemical reaction-I think. He was quoting lots of stuff.
    That did happen at my new house and I have a back yard full of it. They hauled in ground shell rock for backfill. The brick masons came in with sand and the land around there is clay. The landscaper arrived and mixed it all together. It rained and it’s awful.
    Lucky for you–it was a success. I would have told people to add sand also. I’m glad I read this.

  2. What a nightmare, Anna. I’m glad you read this too, and added to it. My husband had speculated about limestone, and it certainly sounds as though that’s what caused the problem in your case. Sand is an ingredient in concrete–it and gravel of some sort are the bulky things that cement binds together–but as far as I can figure out, it doesn’t play a chemical role, and why people warn about it, I don’t know. Maybe in soils rich in limestone, sand is like the proverbial straw–it’s the last, essential ingredient. Weird. Maybe your son could tell us whether this makes any sense.

  3. Trouble is . . . there are more than one kind of sand so saying do or don’t dig it in without being precise about which one might be part of the problem.
    I’ve been warned against builders sand (so I gulped part way through your post and am glad it had a happy ending ater all!).
    And sharp sand . . . you buy that at garden centres . . . for sitting bulbs on etc. so that’s clearly ok in the right place.
    And sand from the sea . . . salty. (Washable?)
    But would you post me some of what you achieved? I was having a bash at my garden the other day and found one worm. ONE! It was really exciting . . . my FIRST worm!

  4. And we thought WE had worked hard to amend our soil… can’t say either one of us has been down on the ground with a hammer working sand into our clay. We did put some coffee grounds (organic, fair trade, shade grown coffee) into the mix and planted deep rooting plants to break up the clay. When we double dig this next season we’ll see how it looks. Great job for being so patient!!

  5. Wow I’ve never heard of that before. I’ve read that it is good to add sand to clay soils. Not that I’ve done it mind you. I just put up with my clay soil and add organic matter. Lots of it.

  6. Lucy–About the different kinds of sand–yes. I, too, have read that builder’s sand is a bad idea, precisely because it isn’t “sharp”: since it’s more rounded, everything tends to glom onto it pretty easily, meaning that it isn’t as effective as sharp sand at separating other soil particals. I suspect mine worked in part because it was coarse, which has an effect similar to “sharpness.”
    About the worms–one? Only one? that is sad. Are you adding compost? They eat organic matter, so that’s the best way to boost the population: feed ’em. (You can buy a bunch to get off to a running start, if you want.) I think I had so many, despite the heavy clay, because I was adding compost every year.
    Shibaguyz–Yeah, well, I don’t get called manic for nothing. Completely and totally obsessive may be nearer the mark, but it’s a bit long and just didn’t have the ring, ya know?
    Here’s hoping you started out with better soil than I did.
    Daphne, organic stuff in large quantities will solve almost any soil problem–I just never had enough. And I do like the idea of changing the mineral content too, so that I’m not completely dependent on my compost supply.

  7. Kate is correct in her conclusions, it does have to be mixed thoroughly with organic material to produce one-third sand, one-third clay, one-third fine organic. That is a lot of sand and a lot of mixing.
    The only explanation I can come up with is that clay does act a little like cement and it will bind the sand particles together to produce something like concrete when there is just a litte percentage of sand. With a lot of sand and organic matter there is not enough clay to do the job.
    One way to think of this is if you started with one hundred percent sand and then started adding clay to it. It would crumble until the percentage of clay was high enough to bind all of the sand together.
    It is a lot of work and before starting I would suggest using the bottle of water soil test to see how much of each component is already present in the soil.
    Last summer I spread out the clay on my driveway, let it dry, and then crushed the clumps by stepping on it and with an asphalt tamper. I spread peat moss and play sand on top and mixed it together thoroughly. It does work like Kate said, but it is a lot of work and a lot of sand and I suppose the stubbornness helps.

  8. Thank you so much for weighing in, Paul. When I do an update on this topic, I’d like to draw on this comment. The proposed update, by the way, is prompted by the fact that some UK sources advise adding sand to make potting soil. However, this is a small-size operation in which it’s possible to get a high proportion of sand into the soil. Not at all like the much larger-scale operation you undertook!
    Nice to know there’s some good points about stubbornness, though…

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