Never Add Clay to Sand or Sand to Clay

Adding sand to clay soil, in any amount, has been proven by the University of California Agricultural Extension and the U.S. Department of Agriculture to be about the worst thing anyone can do for their garden soil. Adding organic soil amendment, such as horse manure, to name just one possible amendment, is a worthy suggestion, especially if the manure has been aged or composted for three months by layering it with rotted alfalfa hay. However, if the manure is salty this can be a negative factor in clay soil since it’s difficult to wash out the salts. In general, the safest method for amending alkaline clay soil is to work in all the pre-nitrolized or fully composted organic amendments one can get one’s hands on or, when impossible to dig them in, then use them as mulch on top of the ground. Plant roots will also eventually help break up soils.

Southwest gardeners need to know that when poor drainage in clay soil is caused by alkalinity, one of the best things one can do is to apply soluble gypsum according to package directions. Organic gardeners prefer using rock gypsum which has not been altered by chemical processing. However, the honest truth is the soluble type works best. Gypsum breaks up clay soils that are alkaline by releasing soluble calcium which replaces some of the sodium on the clay particles and thereby produces a more open soil structure. Work approximately half a coffee can full of gypsum into the earth on the bottom of each planting hole and also broadcast gypsum on the ground surrounding plants in the established landscape so it looks as if a light snow had fallen. Do this at least once every three years and water it into the ground. The action of gypsum does not last forever, so repeated applications are necessary. Gypsum is a relatively inexpensive amendment. It will not hurt your garden soil and may help a great deal.

All soil experts will tell you one of the first rules of amending soil is “never tamper with the structure of your soil.” A soil’s “structure” means the arrangement of the sizes of particles in it and their relationship to one another. All soils are categorized according to their texture and the 4 basic soil types are clay, sand, silt, and loam. Clay is a firm, fine-grained earth containing a large amount of tiny mineral particles less than 0.002 millimeters in size that have negatively charged surfaces. Clay feels slippery and sticky when wet. Sand is a loose earth mainly composed of tiny particles of rock between 0.05 and 2 millimeters in size. It feels scratchy or gritty to the touch. Silt is a fine-grained earth made up of rounded, weathered particles. Loam is a combination of the other three (clay, sand, and silt) in varying ratios, the optimum being equal quantities.

Since loam is a naturally occurring mixture of clay, silt, and sand and is widely considered the best garden or agricultural soil, many people, sadly including some misinformed garden writers, have erroneously supposed that all you need to do to get loam is to mix the other three together, or perhaps only clay and sand, and that—Voila!— you will end up with loam. Unfortunately, this doesn’t work. The fact is that adding sand to clay soil in any amount is an extremely dangerous thing to do. You end up with something akin to concrete. The same is true of adding clay to sand. In either case, the fine clay particles will fill in the larger spaces between the sand particles, thus permanently destroying drainage. In a garden of clay soil, it would be impossible to add sand in sufficient quantities to approximate a natural soil made up of a mixture with a ratio that is more sand than clay.

Gardeners often confuse lack of drainage due to clay soil with the word hardpan, but actually these are different condition. Clay is a type of soil, but hardpan is a condition in which a layer of soil that does not drain lies on top of the soil or is buried under topsoil that drains. Hardpan is a layer of hard, compacted soil of any type cemented together by minerals and almost impenetrable to roots or water. Hardpan is often made of clay but it is not the same as clay soil.

Caliche is yet another type of soil sometimes referred to as hardpan by gardeners. Caliche is a crust of calcium carbonate, usually white or gray in color that forms on the stony soils of arid regions such as found in the Southwest. Hardpan and caliche may be buried under the ground, as in housing developments where it is sometimes covered with a layer of topsoil, or it may lie on top of the ground. In some cases the hardpan may be only be a foot thick in which case one can use a crow bar to break through it to a lower layer that drains, thus creating drainage in the bottom of planting holes. In other cases hardpan may be many feet thick. The best way to combat a clay-based hardpan is to plant in raised beds and terraces filled with amended topsoil, or to make frequent additions of soluble gypsum and a variety of organic soil amendments as explained above.



  1. Hello
    I read your article after a number of others that reccommend mixing in sand to clay soil to achieve better quality.
    You specifically warn against that. Does that apply to the Northeast as well?
    I was about to buy additives . . .
    Thank you.
    Karen in Montpelier VT

    • Dear Karen:

      It is unfortunate that some websites, books, and TV shows give the wrong advice about adding sand to clay soil in a futile attempt to lighten it up and make it drain. Adding sand to clay soil is one of the most dangerous practices imaginable since you will end up with something akin to cement. This advice applies nationwide.

      The natural arrangement of particles in soil is called “soil structure.” Never monkey with the natural structure of your soil. (See pages 21 of my organic book for a full explanation.) The only safe way to make clay soil drain better is to increase its humus content by adding organic materials, such as well-rotted bagged, trucked, or homemade compost or aged manure. In the west we also add soluble gypsum to our clay soils to make them drain better, but this technique only works when the cause of bad drainage in clay soil is it’s alkalinity and this is unlikely to be the case in your east coast garden. See page 20 for full explanation of pH.)

      In the days of my youth I lived on a Bucks County Pennsylvania organic farm. We had hard red clay soil, but clay soil is nutritious and full of minerals. We amended our garden and agricultural soils with chicken, cow, and sheep manure from our own animals, and our red clay soil became amazingly productive. In our pastures the soil became black with organic matter.

      If you were to add sand to clay or clay to sandy soil the process is similar to filling a room with basket balls. Then you add tennis balls and shake it down. This fills all the space between the basket balls, then you add ping-pong balls, and shake it down again and this fills up all the spaces between the tennis balls. Finally you add marbles and once again shake. By then all the spaces between the original balls are filled up and you have less drainage than before.

      Experiments done by Agricultural Scientists at the University of California many years ago demonstrated conclusively that adding sand to native clay soil in the attempt to make it drain or adding clay to sand with the hope of making it more water-retentive does not work and harms soils in a devastating way. It is a drastic mistake because it is irreversible, and this is true nationwide. It is impossible to add enough sand to clay to create a natural soil that drains well, as occurs in nature. (The textures of soils and types of soils are discussed in detail on pages 21 and 22 of my book.) The only safe substances to add to clay soil are organics. Adding organic matter to soil is your path to a highly productive garden. I can promise you it works.

      • Hi, Thanks very much for this information. A local man here in montpelier vermont named Ray Hickory stated that I needed to buy a truckload of sand to add to my yard’s soil because of the clay soil conditions, he told me. I didn’t go along with his trucking in sand because he was not willing to write up an estimate for a job here (a bulkhead) and also I wondered about the safety of importing sand — how does one know what’s in it, for example. But I did not know that the effect of adding sand to clay soil makes for diminished drainage (cement). I wonder who else this Hickory guy has given such poor “information” to. He’s been in bizniz around here for a long time, so I can’t help wondering if he doesn’t know better about trucking in sand.

  2. Hello,
    I have been searching for a solution to gardening in clay soil since last year when I moved to Arlington, Texas. Almost everything says to add sand to amend the clay. I am wanting to plant a couple of fruit trees and had I not found your article I would have used sand (in fact I was already figuring out how many bags I needed). I am planning on putting my trees(miniature peach and apple grafted onto M-111)in a raised bed. Do you have any recommendations?

    • Thank God you did not add sand to your clay soil! I am certainly glad you looked at my blog first. Adding sand to clay soil or clay to sand results in something akin to concrete, stopping drainage completely and the problem can never be fixed. I once knew someone who did this to his perennial flower beds and drainage became ever worse as he added more sand annually. Eventually the guy, an avid gardener, died so I never heard what happened to that garden later. The opening chapter in my current organic book, and also the (conventional gardening) edition published in year 2000 and now out of print and out of date also, but still obtainable on the Internet, explain soil problems and solutions more fully. These books go into soil problems and drainage problems in detail. But it’s best now to be an organic gardener, so if you spring for one of these books, buy the current organic one.

      Clay soil is nutritious. Plants will grow in it, but you are right a raised bed can be a great help when first planting. Luckily it does not need to be very high. For a deciduous fruit tree you could build a raised bed that measures 5×5 and is at least 4 inches high and fill it with a good grade of top soil.
      What is important here however is that you must dig up the clay beneath the raised bed to about a foot deep and mix gypsum into that and also mix some of your top soil into the top layer of the native soil below the raised bed. This is so you do not create a hard horizon between two soils which will stop roots from penetrating.

      If you create a hard horizon between friable soil above and hard clay below you are making hardpan, which happens when builders in housing schemes throw a layer of top soil on top of bulldozed ground. Plant roots tend to stay up in the top layer and never penetrate the lower layer, and then the hard lower layer stops drainage. This is a genuine problem for many gardeners in new homes.

      Despite having explained the system above which will help prevent rotting roots in clay soil by use of a raised bed, my belief is that you can garden in clay soil and plant straight into it, just as it is since I have seen it done many times. The surprising thing is once you begin digging into clay soil and mulching the top of it, eventually the very act of gardening in it and the roots getting into it will help break it up and plants will grow. Clay is minerally rich and plants will often thrive in it better than poor sandy soils that drain well but do not retain nutrients.
      The wrong thing to do is to dig planting holes and fill them with organic soil amendment since that creates pockets of soggy ground that fill up with water and rot roots. Also roots will think they are in a container and go around and around inside the amended plant hole and never get out into the surrounding soil.

      Many scientific experiments have been done to show that the best way to deal with clay soil is to plant directly into the native soil, though plant a little high, and then continually mulch the top of it. The roots of plants have to get out there eventually so the sooner the better. Rough up the sides of planting holes, instead of making a smooth hard edge. Even clean unsalty horse manure can help break up clay when laid on top like mulch before the rains, but I strongly emphasize must not be salty. Manure brings earthworms and they also help break up the ground and make it drain. (When using horse manure, be sure your tetanus booster is up to date.)

      One important factor is that if the clay is compacted due to alkalinity, adding gypsum can aid in breaking it up and making it drain, but you should add the gypsum every two years and a half-coffee can full of it dug into the bottom of every planting hole. Gypsum will not correct drainage problems caused by compaction from walking on the ground or heavy equipment but it will greatly help soil to drain when the compaction is caused by alkalinity.

      I have known a few gardeners who have hauled away truckloads of pure sandy or clay soil in gardens built on sand or clay and replaced with top soil but this is not necessary and is very expensive. (Again, one must be sure not to create a hardpan layer beneath the top soil. More details on all this are in my book.)

  3. I was so pleased to find your blog. I am researching for a science activity with 3-6th graders, and this is exactly the information I was looking for. We will actually be doing exactly what you advise NOT to do, but the purpose is to show why taking our clay soil in Texas and mixing in sand would actually be detrimental to the soil.

    However, I would like to give them something to think about as positive alternatives for soil enrichment that can be done in their yards. Many of us have yards that are likely placed over thick, compacted layers of clay. (I can tell by the runoff when the sprinklers run in our neighborhood.) How do we add organics to the yard without tearing up the grass that is there?

    • Thank you for your question and for the work you are doing with kids. I think it’s a great idea to replicate experiments that were done years ago by the UC Extension and Department of Agriculture in California. These experiments could be done in 1-gallon nursery cans, I would think, though the University of California might have used larger containers. I am not sure. Presumably one could mix various measured percentages of sand with clay and then fill the nursery cans to about 2 inches from the top with the mixtures and subsequently apply measured amounts of water to the soil in the cans and then measure how much comes out the bottom of the cans and at what rate.

      The important thing to impress on the kids is that in a garden it would be impossible to add an equal amount of sand to clay. In a practical example, years ago I knew of a garden in Fallbrook California where the soil was heavy clay. Instead of adding organics to improve his soil, the gardener added wheelbarrows full of sand to the clay because an English garden book gave that advice. He kept adding more sand every year to a long flower bed filled with perennials. Eventually the soil was so ruined that drainage was nil and many plants died from root rot.

      To answer your question: There are a couple of ways that organic materials can be added to existing lawns. One way is to rent an aereating machine that goes all over the lawn and makes round holes in it. The next step is to rake up and throw away or compost the plugs that the machine pulls out of the lawn. After that spread ground bark onto the lawn and rake it into the holes. Follow up by fertilizing and watering the lawn. The best time to do this job is fall. September or October would be ideal. This will help increase drainage.

      Please note that while answering the first gardener who wrote to me on this subject I recommended adding soluble gypsum to help break up clay soil. When clay soil fails to drain due to its alkalinity, applying gypsum according to package directions can aid drainage. One can mix it with the ground bark recommended above and rake it into the holes along with the bark or apply it separately once or twice a year on top of the ground and water it in.

      Another idea for adding organics to lawns is to top the lawn with dry cow manure in fall or early spring. Back in the years when I had a lawn I did this job every fall but my lawn was growing on sandy soil, so it was okay. First I would cut the lawn short, then top with manure and follow up with water and the grass soon bounced back greener than before. The problem with using this technique on clay soil is that bagged cow manure is often salty and clay soil can retain salts. A better way is to use any fine-textured mulch recommended for topping lawns. By topping the lawn annually or bi-annually in spring and fall you can gradually add to the organic structure of the soil under a lawn, but not as much as if you aereated first and then raked the organics into the holes.

  4. This is Ben Cole,I want to order some CLAY SAND,that you have in your shop.I want you to get back to me with the price including taxes and I want to know do you accept Credit cards as your payment.Hope to hear from you soon.

    • Hello Ben:

      Once you select the product of interest you’ll see the prices. Shipping and taxes are then calculated based on the products you order and the ship to address.

      Please contact us with any additional questions!
      -Pat Welsh Gardening

  5. This is funny since there is a book called ‘Build Your Own Earth Oven’ by Kiko Denzer that specifies mixing sand and clay or clay, sand and manure/sawdust, to make a clay oven for baking artisan breads, pizzas and other good things. The mixture of clay and sand – hard as ‘brick’ for an oven that lasts and holds together with a large baking area inside.

    • Thanks for your amusing comment. I just returned from a trip through New Mexico and Arizona where the walls and buttresses of churches that have stood for hundreds of years are built of clay, sand, and straw. In order to keep these sun-baked buildings standing to this very day, dedicated locals apply a layer of this muddy mix (sand, clay, and straw) to the exterior walls annually. While in Taos, New Mexico, I saw a video of Maria Martinez gathering clay and then mixing into it about 20% of volcanic-based sand before shaping it into pots and firing it in an outdoor open fire of dry cow pats to create her distinctive black Santa Clara pueblo pottery. I guess my heading should have been: “Be Sure to Mix Sand into Your Clay Soil if Your Aim is to Make Pottery or Adobe Bricks Instead of Growing Vegetables and Flowers.” Do you think that would finally teach people?

  6. Hello Pat!

    Wonderful Blog. Thank You.

    I have a question.

    I usually use mulch to keep weeds from coming up in the paths between my garden beds. This year I was going to use sand; until I found your website! The reason I wanted to use sand was twofold 1) to lighten the heavy clay soil (which I now know is not correct) and 2) to keep weeds down in the paths between my garden beds.

    I don’t want to use mulch any longer because it is expensive and in 6 months has turned into dirt which the weeds love.
    The benefit of the mulch as a weed detterent is short lived but then again each season I use the soil that formed from the mulch in the paths and add it (as organic matter) to the beds.

    This is working but it is a lot of work.

    I am looking for something perhaps more permanent to keep weeds down in the paths. The beds are NOT raised so adding gravel to the paths would eventully get into the beds as I tilled and worked the soil. Any reccomendations for something to keep weeds down that isn’t mulch, sand, or gravel?


  7. Good info thanks.

    So here we are in southern Vermont about to start putting some beds into sandy fill. We don’t really know how deep it is, it could be 5 to 10ft deep. We are currently planning to dig a foot down into the stuff and then mix what we dug with compost and some peat and vermiculite. We will use this to create beds that extend below the surface one foot and above a foot for a 24inch deep bed.

    One of our biggest questions is should we be putting something at the bottom of the bed to slow the drainage of nutrients through the sandy fill at the bottom?
    Any advice would be greatly appreciated

    • I know what you are up against since I garden in sandy soil and have done so for many years. Some of the soil in my garden was almost akin to beach sand when I began gardening here. Roughly a million years ago that is probably exactly what it was. Sandy soil such as I have in my garden has some huge benefits and a few detriments. Chief among the benefits of sandy soil is the good drainage plants enjoy. Most wildflowers and many drought-resistant plants revel in good drainage, but chief among the detriments is the fact that sandy soil is “hungry soil”. It retains no nutrients and rapidly eats up any composted soil amendments with which you try to build it up. Additionally and quite the contrary of what most people would suppose, in dry weather it is often difficult to get sandy soil wet. This is due to the fact that sand crystals have flat sides and square corners and tend to line up together like soldiers’ shields in a Roman phalanx, thus stopping water from penetrating. When this happens water simply rolls off dry sand especially when it is on a slope. The best way to improve the fertility and water retention of sandy soil is to add manure to the soil every year. (In dry climates such as where I live this activity needs to be combined with regular irrigation or heavy winter rains or one’s garden soil might become salty. In Vermont, however, rainfall should be adequate to leach the salts out of the manure.) Get a truck load or two of aged horse, cow, or sheep manure and spread it on top of the soil to the depth of about 4 inches.Then dig it into the ground to the depth of about one foot. From then on continue to add aged manure on top of the ground annually. If you add it in fall you can safely use fresh manure and let it age on top of the ground and then leave it there as mulch or you can safely dig it into the ground prior to planting in spring. Following this method, you will eventually enjoy great garden soil that is black and sweet smelling and in which you will be able to grow anything. I do not recommend mounding up the soil as you apparently plan to do since mounded soil dries out too quickly. Additionally, do not try to install any kind of barrier under the top soil you will produce. Your desire should be for any salts coming from the manure or fertilizers to be freely leached away into the ground by rain and irrigation. Stopping this from happening would be detrimental to your plants. Unfortunately, there will already exist somewhat of a barrier as I have explained above. Until you have begun improving your soil by manuring it annually and kept this up for several years the dry soil down deeper will form a phalanx to some extent and prevent water from pouring down into the ground as freely as it should. But as you faithfully continue your program of regularly amending your garden with manure, your problems with dry soil will eventually vanish. The manure will dramatically increase the humus-content of your soil so that in time it will become like a fertile sponge holding just the right amount of water and nutrients to give the roots of your plants the perfect growing conditions that you desire and they require for healthy plant growth.

      • I have a sandy loam (old river bank; which hasn’t flooded for 40 years since locks were build) but it dries out too quick. I want to remove 15″ from 1 bed, and lay a 2″ layer of river silt in the bottom; then put the garden soil back, without mixing it. I like horse/cow manure, but I don’t want to have to compost the grass/weed seed from it, but that’s probably what I need to do, (and dream of having a rabbit manure connection). Thanks for you post, and shoot at my silt layer idea.

  8. I Rototill gardens in the spring,I’ve countered gardens that were so heavily compacted from clay that a root wouldn’t penetrate then anyway,anything a gardener can do to loosen the soil will help. personally I would tell them to have it removed, and put in some good dirt I never known any plant that liked a clay soil..

    • There are many gardeners who would agree with your view. On the other hand there are many on the other side of the fence who say along with Alan Chadwick, the great promoter of Bio-Intensive Gardening and Double-Digging, “There is no bad soil, only neglected soil.” Many plants thrive in clay soil and will do better in clay than sand, for example, since clay is chock full of nutrients and minerals and holds moisture, while sand has nothing in it and cannot hold moisture. I point to the orange groves of California many of which thrive on clay soil and to roses that cover themselves with glory when grown in amended clay and require far less water than when grown in sandy or silty soils. I grant you that amending clay with organic matter it is a back-breaking job. Double-digging works much better than rototilling and can be accomplished with machinery also. Plowing and disking works fine. I know since I lived on a farm when I was in my early teens. Our soil was red clay with rocks in it. We plowed our fields and disked in aged chicken, sheep and cow manures. Our results were phenomenal. Everything from vegetables and fruit trees to corn, wheat and alfalfa grew vigorously and produced abundant crops. Rototilling is perhaps not the best way to treat clay soil, but incorporating manure and compost into clay soil and mulching on top of it will eventually loosen it and make it drain. In cases where clay is compacted due to alkalinity and not from mechanical effects, regular applications of gysum are extremely effective and low-cost. If you ever make the mistake of mixing sand into clay you will learn the truth of which I speak when I say— “Never mix sand into clay or clay into sand or you will end up with something akin to concrete! Never monkey with the structure of your soil. Improve all soil by adding organic matter.”

      • Your friends have never had my yard.

        I added yards of good soil twice, and sand once. I added gypsum. I had the grass aerated twice. I fertilized. I watered.

        Everything helped for a little while.

        Then the grass just died.

        I had a friend till the center portion. It was hardpan clay. Just like you would get if making sun dried brick, without the sand and hay mixed in.

        I had a lot of it removed when the came in and laid down new sod. Unfortunately, they left a lot of that ‘neglected’ soil. WORTHLESS.

        Remove the top 3 – 6 inches. THEN ADD your composting material. Otherwise the good material will just be absorbed into the bad.

        IMHO. I don’t have fancy tests. Just my one yard. YMMV, but, this has been expensive.

        PS. If adding organic matter is not monkeying with your soil’s structure, what is it doing?

        • I’m deeply sympathetic with your soil problems and I understand how frustrated you must feel. To answer your question: Adding well-rotted organic matter, such as compost or aged manure, to your native soil improves it’s texture, but not its structure. “Soil Structure” is a technical term which means the size of the particles in a native soil and how they interact with one another. Adding organic matter does not change the structure of soil because by the particles in soil agricultural scientists mean the basic components of soil, in other words the particles by which the four basic types of soil are categorized, in other words, clay (which has the smallest size particles and they have an electric charge that makes minerals attach to them as well as making them stick together. Grasp a damp handful and it holds together like a lump), sand (a larger particle with flat edges that feel gritty to the touch and will not hold together in a lump), silt (a smaller size particle than sand that has worn and rounded edges and feels soapy when wet. Grasp a damp handful and it does not stay together in a lump), and loam (which is a mixture of the first three.) People think they can make loam by mixing the other three, but they can’t. It only occurs naturally. In my books I also add decomposed granite to the above particles, since it has somewhat larger particles than sand and they are irregular in shape. Rocks are often part of the soil structure also. If you have rocky soil it’s all right to remove a few, but it’s a huge mistake to remove all of them. This can result in soil which does not drain. The best and safest way to improve poor soil is to dig in organic matter thus increasing the humus content of soil. Soils that contain plenty of humus drain better, retain water better and are more productive than soils that contain little or no organic ingredient.

  9. Two things:
    1. Using course sand will insure that you avoid making the concrete you mention above. It’s the fine sand that fails to break the clay’s bond.
    2. “it would be impossible to add sand in sufficient quantities…”
    —- Honey hush. If I’m amending the top 18 inches of clay in a 100sq ft bed, I only need to add 2 yards of sand (and 2 yards of compost). Hardly impossible!

    • In your own garden you are free to do exactly as you wish to do. This is a free country. My job is simply to report the facts. Every gardener is free to listen to the facts and accept or reject them as he or she wishes. Meanwhile, I am sincerely trying to help gardeners by giving them the facts in an honest and straight-forward way that they can understand. I would like to respectfully point out that I do not see it as my job is not to obey someone, such as you, who is saying, “Honey, hush,” which is the same as saying “shut up.” Why should I shut up and stop speaking the truth as I see it just because some guy tells me to? I based my advice in this case on scientific experiments performed during the 1970’s at the University of California, Riverside. I first covered this subject in my first book published by Chronicle Books in 1991. Vincent Lazaneo, our San Diego Farm Advisor, read every word of the introductory chapter of that book that covers this subject and he corrected any errors at that time. I’m sticking with what I said in that book, which was as accurate as I could possibly make it. The subject of not tampering with the structure of your soil is covered now in my current organic book on page 21 in the chapter called “What You Need to Know First.” I urge you to pay attention to this advice, since it did not come from me but from a “higher source”—No! Not God, but the University of California. 😉 I also had the information in my books and other writings on this subject confirmed when I learned from an excellent gardener from England who emigrated to America that he had added sand year after year to his conventional English style perennial borders in his Fallbrook garden, which had heavy clay soil located on a mesa top. This gardener added sand to clay soil, by emptying out and replanting the beds annually in the English fashion. Before replanting he would top the beds with many wheelbarrows-ful of builder’s sand mixed with compost. Eventually the beds no longer drained at all and his tulip bulbs rotted. The gardener died. Of a broken heart? No, as a matter of fact, he had AIDS, but it was sad nonetheless. Not sure what the next owners did with the non-draining soil. In the University of California percolation experiments, much smaller amounts of sand and clay were used. In some cases as small as one-gallon cans and yet the mix of sand and clay did not drain as well as the clay alone. By contrast, I remember another property that was situated 25 years ago at the top of the grade in San Marcos above Lake San Marcos. This one stubborn gardener would not sell his property to the development when it began because he had retired there. He was gardening on pure clay soil. None of the other new gardeners moving in had luck with their gardens and all of them complained about lack of drainage, but this one gardener had added truckloads of manure and his garden flourished. Also the clay drained. Surrounded by struggling gardeners having nothing but trouble, this one old-timer had planted a Garden of Eden and everything flourished. I would advise you to add compost and gypsum to your clay soil—and yes, even manure, but sorry to say my advice remains “do not add sand”, but you don’t need to follow it.

  10. Hi
    Very interesting article. You may have saved my soccer field! A question please – I won’t be tilling sand into clay, but was wondering if it was okay to top dress an existing lawn with sand or a sand /compost/manure mixture? Does the problem still occur if you don’t work the sand into the clay? I would have thought that you could build up a layer of sand and then top soil above the existing lawn to at least improve surface drainage, no? Any advice would be much appreciated!

    • The answer to the first part of your question is an unequivocal YES! Topping lawns and golf courses with sand, or in some cases topping them with sand mixed with organic amendment (such as nitrolized ground bark) is accepted practice. This system is often used to level bumpy golf courses and to fill holes and depressions. It is used also after de-thatching Bermuda grass lawns and golf courses. In fact, I have been recommending this method of leveling lawns after de-thatching for over 30 years in books and I demonstrated it on my television shows in the 1980’s. Once a lawn is growing and if you are simply topping it with sand, not mixing it into the ground, this is fine. In fact, it’s done all the time and in this case the results are beneficial, not negative. The grass just grows up through the sand. What is NOT a good idea is to actively mix the sand into clay soil (assuming clay is what you’ve got.) As I have also stated for over thirty years, one should never mix sand into clay or clay into sand with the hope of creating loam or improving drainage. One should never monkey with the structure of the soil one has. Instead, one should add organics to lighten it up or to make it hold more water, whichever the need might be, and in both cases to increase its humus content. This is always the best way to improve soil and I did not dream all this up. These facts were proven in scientific tests by the University of California Agricultural Extension many years ago. In 1980, after I became a magazine editor, one of our Farm Advisors whose name was Jim Breece wanted to spread the word on all this and many other subjects to home gardeners. Jim was reading my articles and columns when I first became a garden writer. He liked what I was writing but despite my best efforts to discover the truth, I didn’t get everything right. After I mentioned sumps as a way to improve drainage, Jim phoned me and came to my home several times and gave me long one-on-one lectures sitting in my garden over lunch. He explained the whole thing (and a lot more besides) to me hoping I would be his mouthpiece, and I guess I took the torch from his hand and have been carrying it for a long time. Now the Farm Advisors have Master Gardeners to whom they teach their stuff. (I was deeply touched when the San Diego Master Gardeners made me an honorary one.) But to return to your question, topping your soccer field with sand is not likely to improve drainage. Think of it this way: the sand will or should drain fine, but where will the water go? It will still be trapped beneath, thus creating a soggy situation. If your desire is to improve drainage I can offer two solutions: One is to aerate the lawn. In fall after the weather cools off cut the grass short and and then aerate it with a rented machine, rake off the debris, and then rake ground bark into the holes, follow up with fertilizer and water. You can also spread gypsum onto the lawn at the same time. Follow package directions. I strongly recommend Soil Logic Liquid Gypsum as a helpful product that can put more gypsum into your ground and give you faster results. If the lawn is a cool-season grass, growth speeds up in fall so it’s fine to thatch and aerate in fall. If Bermuda, a better time to scalp and aerate the lawn is spring because if you do the job now it may not have time to come back before cold weather. You can still use the Soil Logic or spread bagged gypsum, however, but wait to thatch it until spring when it begins to grow again.

      • Thanks for a quick reply! I am currently saving up to buy an aerator, as I reckon this baby is gonna need some work at least once or twice a season to make a tangible difference. Not only is it clay, but it’s had a roller on it a number of times. Is bark the absolute best product to use after aeration? Is sand definitely a no-no at this point? I would have thought gradually creating pockets of sand and compost/manure everywhere would eventually ‘replace’ large parts of the top layer of clay, but it seems you are sayin organic material is absolutely the way to go for aeration, followed by sand to top dress for levelling purposes only? Thanks again. Rgds

        • I think you understand what I have been saying. If you could learn the current practices for playing fields that would help also. (My expertise is for home gardening, not for commercial applications.) For soccer fields, the current protocol is to fill the holes with sand as you are planning to do. I think that is the correct thing to do with a soccer field. I am surprised that some folks do not rake off and haul away the divots, but the grass as it deteriorates adds nitrogen and the divots are usually full of roots and organic matter, not just clay, so I think that is okay. Some soccer field maintenance people aerate twice a year with the regular aerating machine leaving divots, and once a year with the solid-probe kind of machine, leaving no divots. Overseeding is also accepted practice and makes the field look better. From what I have read and heard most playing fields are clay-based. Golf courses add sand on top, but I don’t think playing fields do. It makes a very hard surface, not as resilient as clay.

  11. I truly sympathize with you, especially since I’ve had the same problem. I finally got so disgusted with the rapidly-draining sandy soil in a raised bed of mine that I have dug it all out and plan to re-fill the bed with a better quality of top soil. If I had more strength, I would have dug out only one third and replaced it with horse manure, but I can no longer go to horse ranches to get good horse manure. Cow manure is too salty in our area. Chicken manure is great but too strong, better used in smaller amounts as fertilizer. My experience is that manure builds up sandy soil better than anything else. You were wise not trying to add clay to sand, since it’s almost impossible to get the proportions just right. I do not recommend adding polymers to soil, either, since if they expand and then you work the soil, polymers change soil’s consistency to something like jelly without any air pockets, and this kills plants.

    • I’m afraid of polymers long term break-down, or I’d consider using it for a deep mosture layer, (under sandy loam).

      • Never use polymers in garden soil since polymers are not a natural material and gradually become something like jelly. Fine in containers however.

  12. It seems that the premise that adding sand to clay, or not doing it, fits what I’ve seen in Portland area landscaping for the more part, since we have clay soils. But more along the lines of whether it’s mixed WITH the clay soil.

    When it comes to mixing sand with something like sandy loam and topdressing existing clay soil with like 4 inches to 10 inches of the new blend, the results are outstandingly good.

    For amending clay, I’ve also found the organic matter to be a better alternative than sand.

    Also, regarding sand and clay, IF it were to be done, and for lawns only, I find that the less the percentage of sand that’s used, the worse it seems to be. In other words, if someone were to amend clay with sand for a lawn area, they better go with 60% to 90% sand and modify their fertilizing schedule, or just skip the sand idea.


  13. Pat, Thank you for writing this, and for all of your responses! I have recently been charged with the task of planting a full landscape ON the beach down in South Texas. The subdivision is aproximately 100 feet from the Gulf of Mexico. I have been adding an organic compost (a product available here called Nature’s Blend which is made up of a “blend of composted cotton burrs, composted cattle manure, alfalfa, and humate” according to the Back to Nature website). I am root tilling approximately 2″ of it into the top layer of soil and mixing it in to the holes for each plant (this varies from approximately 1:3 compost to sand for some plants and 1:1 ratio for others. I am also planting each plant (again based on the needs of the plants) with Agriform tablets. I have considered adding Pete moss or other organic materials. Do you have any recommendations/advice? I have been told that I am trying to reinvent the wheel by attempting to plant a full beautiful landscaped yard, but after reading your blog, I am confident that with proper preparation and care I can make it happen! Thank you.

    • If sounds as if you have a very good plan. When you say that you are mixing the compost into sand, I take it you mean the existing sand. If so, that is fine. In very sandy soil, planting holes need to be amended with compost because there is not enough nutrition or humus in the ground to support the plant or to retain adequate water

  14. I moved to Fallbrook, Ca 12 years ago and started an herb garden. I was having a problem growing thyme so I added sand to clay thinking it was a problem with drainage. It turned to concrete just like you said. Thanks for helping me figure this out. You definitely know what you’re talking about.

  15. I was surprised how much adding sand made the clay worse. My clay is odd. When almost dry it crumbles nicely but is like rock when wet or dry. The clay in my former garden did not do this. I think maybe sand was added to this clay previously. DG along with organic matter really seems to work. I essentially toss most of the clay every time I plant a hole. Then mix in DG and organic matter or just fill with soil from the landscape supply. By the time I am too old to garden I will have replaced most of my soil.

    • If you live in a dry western climate, where clay soil is alkaline, adding gypsum can help break up clay through chemical action. Adding organic matter is good practice. Adding DG (decomposed granite) is not good practice. You are meddling with the natural structure of the soil. This is unwise and in the long run will not help.

  16. I agree 100% on your philosophy about not adding sand to a clay soil, I’ve seen several people ruin what could have been highly productive healthy soils with that approach. In my experience it doesn’t necessarily hold true for a sandy soil though. I have sandy soils in my region and adding clay in the form of liquid clay ie dissolving the clay in water and then pouring it onto the soil is much more effective than adding endless amounts of organic matter. If too much organic matter is added to a sandy soil (which requires much more than a clay soil to provide good soil structure) it typically produce a soil that will show major extremes on a soil analysis. Also when compost amended sandy soil dries out it becomes almost impossible to re-wet. The drying out can be averted with a deep mulch but the soil only needs to dry out once and then it can be almost impossible to re-wet, we’re only human and it can be easy to not notice a dry patch until the plants growing in show signs of distress and by then it’s can be too late & the soil can be left in a state that’s nearly impossible to re-wet. In that dry state the microbiota dies all the earthworms disappear and it becomes almost useless. I’ve been treating sandy soils with liquid clay for over 20 years and after treatment the soil stays “wettable” without much for on average 2-3 years before it needs re treatment. After about ten years or so of this treatment the soil becomes remarkably transformed it holds water, drains well, PH levels stabilize and earthworms flourish. I have the best gardens in my district and annual soil tests show near perfect CEC & nutrient levels. I have tested a similar approach where the clay was turned in to the sand & it produced the concrete effect you mentioned so the liquid clay wins hands down.

    • Thank you for your comment, which makes sense to me. However, the Farm Advisors here say don’t add clay to sand. You are 100% correct in regard to the drying of sand and most organic matter. Manure, however, especially horse manure or aged cow manure (not dried from the bag) seems to have a beneficial effect and does not have the tendency to dry out. Now I have a question for you: What does “&amp” mean? You used this Internet slang three times in your comment. This term is not in the English dictionary nor is it defined in any listing of Internet slang on the Internet. Because I cannot understand what “&amp” means I cannot understand your sentence which reads: “I have tested a similar approach where the clay was turned in to the sand &” I guess you mean you have to mix the clay with water but not dig clay into sand? But I still can’t figure out the meaning of “&amp” in this sentence or in the other sentences where you used it.

  17. I don’t agree with your comment about adding sand to clay. I admit, that there seem to be a lot of people who claim that their clay became harder once sand was added. They never mention the size or roundness of the sand. Most of these comments come from the western US.

    In England and Europe it is quite a common practice to add sand to clay to open up the soil. It has been used for many years but top gardeners.

    I live in Ontario, and have added sand to clay in 3 different gardens now, and in each case the soil became more workable. the first was very heavy clay, the last was clay loam.

    There are many types of soil, and it comes in all kinds of ratios of sand to clay. It does make sense that there are soils where the addition of a small amount of sand suddenly changes the soil to become like concrete.

    I suspect that there are some other facts to this story that are not understood. Maybe, there is a different type of clay out west? Maybe your clay is 90% clay and clay in the east and Europe is more of a clay loam type? Maybe, the sand being used out west is small in size and round. I use large builders sand that is very irregular. Smaller playground sand does not work in loosening clay.

    In any event, I would be very interested in seeing your reference on this matter. I have to find a scientific study either for or against using sand.

    • Thank you for your comment. Perhaps the alkalinity of clay in the West is one reason for the problem here with adding sand to clay or clay to sand.
      I will try to track down the UC scientific tests that were done many years ago on this subject and get back to you with the actual citation. Since it’s now the Christmas season, it may take me a few days. Thanks again for writing.


    I was warned against using builders’ sand due to the risk of it hardening like cement. I was recommended to use sharp sand from the builders’ merchant which has been truly successful over the course of a year by breaking up the clay and improving drainage.

    • Unfortunately, all kinds of sand eventually have the same effect if mixed into clay. Science has proved that mixing any kind of sand into clay will eventually make the clay quit draining and turn the soil rock hard. I am not saying this out of personal opinion but proven fact. You are perhaps laying the sand on top of clay, which will be okay as long as it’s not mixed in. I fear you will most likely learn the truth for yourself eventually. Some years ago a great gardener from England moved to Fallbrook, California when he was a young man. In order to make a herbaceous border like the ones he grew in England he kept topping his beds of clay soil annually with sand. For years his system seemed to work and his garden was famous for its displays of spring bulbs. Then eventually the soil stopped draining. The gardener never admitted this to me. Very sadly, he died too young, but his partner told me later the flower beds had indeed stopped draining and all his partner’s beloved bulbs had rotted.

  19. Rachel Robson

    Dear Pat, Thank you so very much for stopping me in the nick of time from destroying 34 years of work. I’ve been an organic grower since I was 4 and I am now 67. I know I’ve put a little sand here and there. Like say in the strawberry barrels which when amended, some gets tossed. This could explain a few minor problems. I’ve got a new round of chickens as of last summer. I bought a child’s sandbox with lid to create a dust bath for them for when it rains which it finally did. They took quite awhile to accept the sand bath and so I mixed it with play sand into the horticultural sand and even some soil and potting soil until the girls liked it. Then, my daughter did not secure the lid when we suddenly got nearly 8 inches of rain here in Berkeley last month. The box is full of water and sand mix. Ugg. Had the not so bright idea to spread it to areas that do not drain well. I knew there must be some reason not to do this and so began researching and finally found you and all the explanation I need. So grateful. Thank you! Now what to do with all that wet sand in standing water-oh ick! Lid is on now. Must do something soon. Ideas anyone? Rachel

    • Thanks so much for sharing this fiasco for the edification of others. As to what to do with the water-logged sand mixture? Hmmmm….(getting creative here)—Could you turn it into a pond and grow pond plants and gold or mosquito fish in there? Nope?…. Okay, think again, Pat. What I would do is bucket out the sand, bit by bit as strength allowed, onto a tarp. When rain is expected cover it with another tarp. Let it dry out in the sun completely, then put it back into your sand box. One thing sand is good at is drying out, especially when mixed with earth as you have done. Your chickens liked it before so they will again. (But this time secure the top!)

  20. We moved to a new house in San Clemente 11 years ago. At the time a local nursery advised us to only add sand to our clay, which we did. While all the neighbors were having truckloads of compost and potting soil dumped in their driveways, ours was filled with 100% sand and they all talked about it. Initially many of their plants died and had to be replanted, but ours, maybe planted better, survived. Now it’s been 11 years (and we’ve only added bark or mulch three times, so that’s part of the problem) and today I went to dig a hole and literally stood on a shovel and tried jumping up and down and barely made a dent in the now ‘rock’ soil. We are ordering a truckload of organic mulch and adding that with gypsum. Hopefully that helps a little. I would strongly advise against adding sand to clay, it truly has turned to cement in the areas we have not amended over the years. Great advice Pat! Hopefully people listen.

    • Thank you so much for writing. I have been trying to let people know this truth (and others!) for years. Testimonials like yours really help!

  21. Hi Pat,

    This is really great info, and I learned a lot. Thank you for sharing your knowledge and hard facts you’ve acquired through your years of experience. I think people are confused by soil terms and need to understand what they are and what they mean before they mess with their soil. Let’s face it, people are lazy, and I’m so guilty of wanting to know something in 30 min. or less. However, 30 min. “experts” are not all that great, because it takes a lot more time and experience to learn valuable information.

    People who want to garden need to be teachable and learn established gardening terms like: soil structure (chemical and physical), as well as soil texture and porosity. What is compost and humus or a humectant, and how do these things all work together to make the soil you have work better? Unfortunately, there is a lot of misinformation out there. I don’t think it’s intentional, I think it’s a product of the 30 min. expert, or parroting what someone heard without checking it out – again, guilty as charged.

    Personally, I think some soils(especially sandy soil) need to be planted with native grasses and plants. This is how you mess up ecosystems, by trying to change indigenous soils to suit what you want to plant. So if you want an English cottage garden in south Florida, then you need to move, or garden with containers, a green house, and raised beds.

    People always want what they can’t have, and make a mess of things when they try to get it. =S Like you told that one cheeky fellow, you don’t have to follow expert advice and factual information. I have heard it said that a wise man learns from his own mistakes, but a genius learns from the mistakes of others. I hope to achieve genius status thanks to you ; )

    • Thank you so much for this kind and thoughtful comment on the subject of soils and how to treat them. I totally agree with you that there is a lot of misinformation out there and it is a sad fact that sometimes the misinformation comes from nursery persons, who may love plants but know very little about the more technical aspects of gardening and plant choice. Nurseries cannot require their employees to take a test in plant and gardening knowledge or they couldn’t find people to work for the usually low salaries they pay. Regarding grasses: Grasses native to each region worldwide often are found growing in sand and thus the correct native grasses in a specific region, can and often do hold dunes in place. However, when grasses lack sufficient rainfall or the right temperatures, nutrients or other requirements in the area where they are planted, they cannot survive in sand. In California where I live, people often plant ornamental, non-native grasses that unfortunately have seeds that are carried by birds or wind into wild areas where they can wreak havoc, crowding out native plants.

  22. Kim Churchman

    Please dear Pat, I hope you’re still available. I have done this very thing. I had read that an ideal soil is one third vegetation matter, one third sand and one third clay. So when I moved here, I bought 27 yards of custom blended soil and had it dumped in nice berms around the yard, framing it. Five years later, the vegetation is gone and the places where I ran a soaker hose are hard. What must I do to save it? Thank you in advance for any help you can offer. –Kim

    • I hear this kind of thing all the time. What a shame this happened. About the only thing you could do is have the berms bulldozed and hauled away, amend the native soil down below with organics and start over. Plant a thick shrubbery to take the place of the earth berm. Your idea was great. The problem was that the top soil you brought in was artificial. Many people are having problems with artificial “top soil” purchased for filling raised beds. It just doesn’t work.

  23. My back yard is in need of dramatic “facelift” – I am in Portland Oregon – I have a huge amount of moss that is battling for life in my lawn. I live in a suburban area that had up to 15 feet of topsoil stripped away from the area back in the early 80’s before they built the houses. The previous owner of the house did some magical planting of smaller plants – but there was at least 5 years of overgrowth and lack of “maintenance” to anything in the yeard – so I have tree roots that are exposed, a huge area that “floods” ( poor drainage ) when it rains. I was told that the soil is “clay”. I just had the yard aerated and I am planning on adding about 2-4 inches of dirt to the back yard. The company states it is “lawn and turf” mix, it is a blend of mulch, sand, aged barkdust – they say it is specifically formulated for new lawns. But after reading this post – I am suddenly afraid to add anything with sand – your advice?

    • Can you afford a landscape designer? You mention some “magical planting of smaller plants”. There are landscape designers in Portland who are skilled at this sort of thing. (Landscape architects usually, though not always, less so.) You need to get advice and find the right one. Do you have any need for a lawn? If not, I suggest re-contouring the ground into a landscape of mounds and wandering paths, bringing in rock or using whatever exists on the property already, adding drainage pipes and making the entire thing into a “magical landscape of smaller plants.” In this case, moss on the mounds can be part of the delightful scene. You can also plant small Japanese maples on some of the mounds. They grow marvelously in Portland. Tour other gardens this spring and summer when garden tours are available. Find out who designed the good ones and interview them. Go to see the Portland Botanical Garden. Talk to experts there. I have seen gardens in Portland done like I have suggested above. Many small rock garden plants grow there marvelously well.
      Yes, you are right. Whatever you do, please do not bring in sand. Perhaps in the past someone might have ignorantly done that, but most likely the drainage problem causing moss growth is from the clay soil and bedrock.

  24. The soil in our garden has a really high clay content. Are you sure that adding COARSE sand won’t help improve my soil? I’ve heard that it helps to add coarse sand to clay soil… If not, what is the best solution for improving clay soil over a large garden area? We have a very big garden that really needs help. I was looking foreward to next spring gardening…

    • I am absolutely sure that adding sand to clay, yes, even coarse sand, will turn clay into something like rock, or as I always say something akin to concrete. If you would like rock hard soil that will not drain and in which you cannot grow anything at all, go ahead and add sand to your clay and you will see as others have seen that I am speaking the truth.

      This is based on scientific experiment. You can experiment yourself if you want, but not in the ground. Instead try it in containers all the same size, like one-gallon nursery cans. You will find out that the clay mixed with sand makes excellent modeling clay but will not drain.

      The thing to add to clay to improve it is aged compost, either bagged or trucked, and gypsum (If you live in Southern California or any dry climate.) One of the best things to add to a large garden is stable manure, that is horse manure. Lay it in a layer all over the top of the ground. Let the rains wash it in and then next spring you can dig it into the ground. If you do that every year eventually you will have excellent soil. Another thing you can do is to plant a cover crop such as fava beans or scarlet clover. In spring before planting dig it into the ground. It rots quickly and improves the soil. Additionally if you live in a dry Mediterranean climate add soluble gypsum according to the directions on the bag and water it into the ground. This will make clay drain when the lack of drainage is caused by alkalinity. Keep adding gypsum annually since it doesn’t last forever. Also put gypsum in the bottom of every planting hole.

  25. I’m planning on adding sand to this clay soil to loosen it up a little. What do you think?

    (Pat is now throwing things)

    I’ve made this exact mistake. And as far as roughness of the sand, I used VERY course dry riverbed sand from behind my house. Also, a bunch of it.

    The worst part is while I was doing it I thought to myself, “concrete = cement + rocks. Clay is kinda like cement. Sand is basically little rocks. But somewhere I learned that sand breaks up clay so I’ll keep going.”

    Big mistake. People, listen to me. Pat speaks the truth. Don’t add sand to clay. It will only make it worse.

    • Thank you for backing me up! This was a nice Christmas present since your message arrived Christmas Eve. I just cannot understand why folks don’t believe me!

      Scientific studies were done on this subject but folks don’t even believe the science. Also, I took ceramics in college and learned then that adding coarse “grog” (ground particles of silica-filled, fired clay) to raw terra cotta clay makes it even stronger than clay alone.

      I am grateful for your sense of humor and good message. (But sorry to hear the problem with your soil.)

      Merry Christmas and Happy New Year!

  26. Hello Pat great article. I am having the most difficult time draining my field. I’ll attempt your soluble gypsum. I am growing inside chicken coops with hard pan clay with raised beds. I’ve been attempting to slowly compost tons of chicken manure into workable growing soil sitting at top of the clay. I’m about to buy a lot of organic compost. Is there any other way around this? I have lots of worms in manure with straw mixed into it cc email thx

    • If poor drainage of clay soil is caused by alkalinity then regular applications of soluble gypsum will eventually cure the problem, though it may take some time and also you will need to continue applications of gypsum annually or even semi-annually. However, if you are speaking of a dish-shaped piece of land, lower in the middle than on the sides, it sounds as if you might also need a system of French drains or re-grading the land in order to cure your problem of waterlogged soil. When faced with a condition like this I always think, why not capitalize on the existing situation and construct a dew pond in the low area? You would not need a pond liner but instead could use the ancient system of straw and puddled clay to create a natural pond. Once the pond is filled with water and some water plants you could then raise fish in it—bass are usually the fish chosen for this purpose, but you should consult your Farm Advisor for advice. Raising fish could give you another valuable crop with which to feed your family and also sell. If you do this, be sure not to get any gypsum into the clay you use to create the waterproof bottom of the pond, since it could ruin the process. Also you will want to keep the chicken manure out of the pond. Once it is aged and has become a natural part of the soil, however, then it should pose no hazard.

      Another factor is where you live. If you live west of the Rocky Mountains, then most likely you have alkaline soil, but if you live east of the Rocky Mountains, your soil is likely to be acid. In this case adding gypsum to your clay soil will do no harm but neither will it solve your problem. If you live in the west and your soil is compacted due to alkalinity then applications of gypsum should gradually clear up the problem, though it takes time. If the soil is compacted partly by foot traffic or heavy machinery like tractors, that is yet another problem, since the effect of heavy traffic on sticky clay is much the same as “puddling”, mentioned above, and can gradually turn the clay into a leathery mass that will not drain and is more suitable for making pots or lining the bottom of a natural, water-holding pond. Gypsum will not solve a drainage problem caused when clay has been mechanically compacted.

      Additionally, I should warn you, chicken manure contains some salt and adding it to poorly drained clay will increase the salt content of the clay. Basically there are no shortcuts. If you have alkaline clay soil, you need first to improve the drainage with regular applications of gypsum washed in by rainfall or irrigation. Then you will need to first age the manure so it will not burn and after that gradually improve the soil of your entire field with applications of aged chicken manure and well-rotted compost. Incidentally if your clay soil also contains rocks, do not remove them since rocks when they are a natural part of the structure of clay soil can actually improve and not harm the drainage. Do not however add rocks hoping to improve drainage. Far from helping, adding rocks to your soil would be likely to wreck the drainage. A cardinal rule of soil improvement is “Never monkey with the structure of your soil!” As I’ve undoubtedly mentioned elsewhere, the “structure” of soil means the percentages of various soil types (such as sand, silt and clay) that are in a specific native soil and the size and arrangement of the particles in it. Organic matter and gypsum are additives that can gradually improve clay soil safely without trying to change its basic structure.

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