Problem! Clay + sand = concrere

October 19, 2017

I've lived here for 2 years but I'm still getting to know my garden. Recently I turned my attention to the small strip of soil along the side of my house and learned someone (neighbors, former owner perhaps, maybe even the fence installers) mixed sand in with the clay in that area. I got about 6in down and it's slow going. Im practically having to scrape it out with a spoon milimeter by milimeter - very gradual and with a lot of effort. Drainage is nonexistent (filled the hole up and it didn't drain at all) and the "soil" there is more like concrete.

Is there a way to fix this kind of problem? Digging it all out can be done but I don't have the time now. Is there something that might help in the short term? Will heavy mulch eventually soften a sand+clay mix ? I have plants there but they don't get bigger than a few inches and now I know why.

Any advice is appreciated!

Comments (23)

  • glib

    tap rooted cover crops!

  • Jon Biddenback

    It won't be fast, but heavy mulch will help to loosen it up, yes. I've got compacted clay that's a lot like brick, and in areas I've been able to place and keep it, heavy mulch is helping a lot.

    Cover crops with tap roots should help with breaking it open for you, and injecting organic material beneath the soil which can be eaten and spread if you cut the tops off to kill it. I've heard great things about daikon radish as a buster, but never tried it. Dandelions might be ideally suited here, actually.

    I've found that ants are pretty incredible at working very hard soil. Not only do they excavate, they inject organic material by dragging food down into their ant pantries. If ants take an interest in the area, by all means let them work; you can drive them away with flooding later but while you're growing soil instead of crops, they can be a huge help.

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  • waynedanielson

    could there have been something there? like a sidewalk or patio?

    It's always interesting to start fuggering out a new place...especially when it comes to the soil. Many things get know, out of sight, out of mind, bury the (bad word).

    As a practical matter, there's always power. Tiller, backhoe, jackhammer...

    It would be interesting to see the results of a soil test. I'm really curious as to pH.

  • kimmq

    If you can wait a few (3 to 5) years covering that area with a thick mulch will aid in adding organic matter that will make working that soil easier. If you want to work it sooner the best solution is to till in large amounts of organic material, compost, shredded leaves, hay, straw, etc., and plant cover/green manure crops and till those in.

    This is a good example of why adding some sand to clay is not a good idea. To make much difference using sand to amend clay soils one needs something close to 45 percent sand, but then there would still be little to no organic matter in that soil.

  • lazy_gardens

    Where do you live and how much sun does that area get?
    What do you want to grow there?

    The lazy no-till way: Water it well, cover it with a thick layer (6-8 inches) of shredded branches and wood chips and leave it until next spring. You will see improvement by spring - it doesn't take 3-5 years to get adequate soil.

    I started with hard-packed desert dirt, not quite adobe making material but close. Mulching heavily in May, I saw improvement by the end of summer and from using a pick to dig planting holes the first season I was using a trowel anywhere in the bed to dig planting holes.

    Then plant something. If you are trying to develop a vegetable bed, plant a clay-tolerant annual with BIG root systems, such as okra (roots go 6 feet or more deep), but anything will work, even lettuce transplants. Make openings in the mulch for the transplants. When they die at the end of next year, slice them off at ground level and renew the wood chip mulch. The roots will rot in the soil and improve the tilth.

    If all you want is ornamental perennials, select clay tolerant varieties and plant them into the mulched area next spring. SMALL containers will be easier to plant and will establish faster.


    BTW - shovels are useless on hard clay. Use a pick to break up the hardpack.

  • Tristan

    I'm in CA in the bay area. The area in question is a strip alongside my house, and doesn't get much sun. I have mint and hyacinth bean vines growing in part of the spot where the soil wasn't compromised, they do very well and look beautiful from the windows. Then about halfway down the lane it becomes "concrete" and all my plants are about two inches tall. There is NO DRAINAGE. I have never seen anything like it. The water poured in sits there until eventually it evaporates, it's like a pond! A pick is my go-to tool in my garden, but it's not handling this area well at all.

    I have bad clay everywhere, literal bricks that even a pick won't cleave, but this is different. If you google "mix sand and clay" people all say "don't you dare do that or you'll make concrete!" but what do you do if someone else did it for you? Mulch + roots? I just don't want to deal with it right now since there are higher priority spots (understatement!) I thought there might be a good "set it and forget it" option that will help at least somewhat until I'm ready to tackle it properly. I'm sure leaving it bare is only making it worse but not even weeds grow there, my backyard would be a forest of weeds (cheeseweed/mallow and black nightshade) if I let it go, but this area, not even weeds grow.


  • rgreen48


  • Tristan
    Here's the soil I have in the rest of the garden (not in this problem area, but in the main yard.) Bad clay. The brown stuff is mulch, that wall black stuff underneath is soil. I can even sculpt with it!
  • toxcrusadr

    Heavy mulch and leave it for awhile is the least effort. If you can, plant something there as suggested to begin breaking it up. If it were me and I was busy, I might do some compost with mulch on top, leave it for a season or a year or two, then try the deep rooted cover crops. Nothing may grow in it now if it's as bad as you describe. Keep it moist - dump your dishwater on it, etc.

    If it's going to be too high in elevation after thick mulch addition and subsequent 'fluffing', consider removing some of the bad stuff and replacing with a more silty topsoil, once it gets to where you can even work it. If you equalize the particle size distribution so it's not all sand + clay but has some silt size, that will help the concrete problem. You do have to be able to mix (till) that in for it to work. I'd wait on that and see how passive mulching works though.

  • armoured

    Can’t hurt to try both. Go for the cheapest mulch you can find - leaves usually fit the bill - and don’t spend a lot on cover crop, since it may well not take early on.

  • glib

    ha! alfalfa or daikon just laugh at that clay. It is a cake to them.

  • Tristan
    You know what laughs at my clay? The mallow and nightshade. They cover my yard. I cant keep up with weeding, there's so much. Mallow especially seems to germinate with a single drop of water and can live on that drop all year. Some of the mallow roots are humongous, perhaps even the size of diakons! But you cross this line in the sand, so to speak, and not a weed grows. Ill throw some seed down and see if I can get anything to take, and mulch heavily. Maybe ill AB test it in patches. But it's a low priority area so it won't get any serious attention/effort/resources for the next year or so.
  • lazy_gardens

    A thick layer of wood chips and ignore it until you are ready to deal with it is the least-work option. Looks tidy.

    Renew mulch as needed.

  • kimmq

    A soil that is hard, like concrete, and has very little if any of the Soil Food Web at work will not have any to digest and work any mulch material in, and wood chips with a dense structure and fairly high Carbon to Nitrogen (400 to 1) ratio would be less likely to have the appropriate biology needed to make much difference short term. Shredded leaves (a C:N ratio of about 60 to 1) would be a better choice here if one were to simply pile a mulch on the soil.

    Tilling this one time is not going to do much harm to the soil or the Soil Food Web.

  • Jon Biddenback

    Any reason you couldn't use the planting hole method for growing soil itself? Thick, heavy mulch, and at regular intervals you open it up and drop in a sample of healthy soil, lively compost, old rotting leaves or manure, just anything full of the SFW bugs and microbes you need. That way you've got everything covered, protected from baking sun and pounding rain, a home and food supply available for the partner organisms you want, and little drop ships full of colonists eager to spread out and multiply over time.

  • Tristan
    I haven't yet had time to dig down And see how far this problem extends. Just looked at it again today and its so rock like, it feels practically hopeless! It probably either needs to be replaced or it needs many many many years to be restored naturally. Im kind of curious: every thing i have read states that you cant change original soil structure, if its sand or silt or clay. So... If the original structure is sand+clay won't i always be fighting the "cement" like structure?

    Also, not planning to do this, just curious, would tilling in silt alongside organic matter turn it into something more like loam?

    I took a photo of a cross section of the hole i managed to dig. Its about 8 in deep. It looks crumbly, like it would be easy to dig into, but trust me, thats not the case.
  • rgreen48

    Tristan, after learning you're in California, I think you may benefit from researching caliche and methods of dealing with it.

  • Jon Biddenback

    You'd need an ungodly amount of silt to significantly impact the proportions. Hauling and tilling that in sounds like a backbreaker. If you can get the SFW going strong, you aren't looking at a huge amount of years to get usable results, a year or so of very heavy mulch will have noticeable results underneath. Some of it will be your mulch breaking down, some will be your soil broken up, it's enough to work with in seasons while you're waiting for change to go deeper.

    Again I gotta mention dandelions as possible helpers to get this plot into shape. They do well in hard pan clay soil (they grow out of accumulated crud in cracks in pavement, too), and drill big strong tap roots down. The leaves, roots, and flowers are edible. They attract pollinators and some beneficial predator insects. And they can seed hundreds more of themselves, which makes for very easy spread as a pioneer plant.

    You are probably going to need some very tough, stubborn, hard-working helpers for this rehabilitation; plants and bugs that put up a fight instead of rolling over and dying. It might be worth your while to look up beneficial weeds in general, and cross reference your findings against what grows in your area.

    Changing the soil structure completely can be done, natural processes and human activities both do it. "Clay" and "sand" just refer to the dominant sizes of rock fragments in the area; there is a lot more in soil than just rock. You'd be amazed at the differences that can happen when the SFW starts moving rock up and organic material down, breaking some bonds and building others. Gross shape, fine texture, air flow and drainage can all change dramatically.

  • Tristan
    How do you tell if you have caliche?? I read up a little on it and based on what I read it sounds like I maybe don't have caliche. Fruit trees do very well in my area. I haven't had any problems with stuff dying, everything I've planted has done fairly well. (well enough at least). While my clay is like mud when wet and like brick when dry, I can get down 2-3 feet with a pickaxe when the moisture is optimal. Based on my research (internetting) i *believe* I have heavy clay, but how would I tell if it is caliche? Heres a photo of how moldable my soil is.

    This *one* area is the one with drainage issues. (fill a hole with water, water ponds.) It very visably has sand, like beach sand, mixed in. You can see it in the photo I posted above.

    However, reading about caliche it sounds like it has all the same pitfalls as my sand + clay mixture . Dead (no signs of food web) . Plants cant move in it. No drainage. The advice in the articles i read on caliche range from "learn to love xeriscaping" to "replace it". Kinda makes me think my long term plan for this area will have to be something drastic!
  • Jon Biddenback

    Tristan, your duck sculpture reminds me of some of the crap under my late grandmother's property, on the edge of the North Bay Area (Napa, specifically). Lots and lots of compost can help over time, she eventually turned that stuff into as close to loam as makes no difference, using fall leaves, juniper bush prunings, lawn clippings, kitchen scraps, and time.

    Maybe you don't have as much patience, but that stuff absolutely can be transformed into good garden soil. What you might try if you want to use the area and build it at the same time is sheet composting, AKA lasagna gardening. Each season you build it up with more mulch, bringing more organic material and life to it, in the meantime you plant straight into compost and grow your plants above the problem soil.

  • kimmq

    The quickest way to find out what type of soil you have is to talk with the people at your University of California Cooperative Extension Service office.

  • rgreen48

    Yeah, even if you don't have true caliche, the problem you are experiencing is the same as those who do have that soil type. I just thought you could learn from how caliche is mitigated. I know that it's very frustrating.

  • toxcrusadr

    Love those photos! haha. At least you still have your sense of humor, so you got that going for you.

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