Technically, sand and clay in the wrong (or right, depending on your point of view) proportions will make adobe, not concrete.

The reason many sites recommend against adding sand to clay is that it can take a lot of sand to help matters, and until enough is added, it can make things worse.

The advantage of adding sand to clay is that if you add enough, the change it makes is permanent, but when you use organic matter, you need to replenish it on a regular basis.

The advantage of adding organic matter is that if you add a little, it will still help, just not as much, but if you don't add enough sand, it can make things worse.

If you have a fair amount of humus, you could get away with adding less sand, but you'd need to keep adding organic matter to keep the humus levels up.

Where I live, there's almost no organic matter in the soil, and it would take a huge amount of sand to help, so the recommendation is usually to avoid adding sand due to the large quantities it would take and the problems that occur when too little is added.

Soil is a living substance, and getting everything into the equation can be a real trick. Where is the line between too much sand and not enough? What kind of drainage is under the mixed layer? Is the area in a natural low spot? Has the area had livestock on it? Does it have any earthworms? Has the existing soil been tested, so you can determine if certain nutrients are missing? How are the trace minerals?

People tend to think that there is some perfect-for-every-spot formula to improve the soil, and there isn't. Farmers have been told for over half a century that there are only three important ingredients that need to be added to soil: nitrogen, phosphorus and potash, which is usually untrue.

Some soils are short in some things, and fine in others, but the shortages are throwing the equation off. Places of high rainfall (like the PNW) tend to have soil that is low in calcium and magnesium and some trace minerals. Getting the soil balanced is necessary, and often rather tricky.

One thing that people should be doing is planting cover crops, probably using a combination of plants. Some can add bulk to the soil with their relatively massive root system, others can improve drainage with their deep tap roots, others can lie down in the winter and protect the soil from erosion and wind.

You can have people who add just sand and some compost, but the soil that is there is deficient in some ingredient, so their results are poor. Others can add soil, and plant a cover crop that works with the existing soil even if it isn't perfect, and they see improvement.

And then many people get it working fairly well (not great, just better than it was), and then they start "mining" it by taking the crops (even hay is a crop) off the soil and returning nothing. Their neighbor takes all the same steps except he just mows the pasture and lets the organic matter lie there and be taken into the soil by the soil fauna. Both have taken exactly the same steps but one crucial one (whatever it may be) and that makes all the difference.

The more I understand about improving the soil, the more I realize how complicated the whole machine is, combining soil in its natural form, remineralizing it to balance missing ingredients and provide better conditions for the microherd and fungus, cover crops to add organic bulk and drainage, possibly adding lime to bring up the pH, etc.

The soil is like any other living organism, and it has to be fed. In most cases, Mother Nature can do it, but Man keeps interfering with his plow, his cattle, and too many horses pastured in too small an area.

Stepping back and asking yourself how Nature would do it can often put a person on the right road to soil improvement, but you do have to be paying attention.

Sue

Someone commented similarly in another sand/clay thread I posted to. It may have been bpgreen on that one too. The recommendation there was as bp said quantity of sand is key. I ammended my clay heavily with sand (at least around 50/50 to allow more faster water penetration instead of runoff and it worked beautifully. No adobe/cement problems at all. It was a good quick fix and now I will follow up by adding organic matter to it yearly as I was advised. Large quantities of sand by the truck load are usually MUCH cheaper than compost or other forms of organic material and most folks can only work with what they can afford. Sharp construction grade sand was very affordable at around $30 per ton delivered in our area. I've started my own composting in the meantime but it is not nearly on a large enough scale to improve my beds as quickly as I'd like. Maybe Santa will bring me a truck full of compost next Christmas.

Good luck with your garden. Sincerely, Danielle

I'm sure I read a gov't of Australia horticulture publication on the web 8 months ago, but I can't find it now, saying to mix in 20% sand to the clay, but I read other places saying 50%. If sand is cheap, then use sand. I do. I routinely mix clay with sand, or sand with clay, depending on which part of my yard I'm working on, and what I can get for free. I hauled off a lot of clay from my neighbour's lot when he was excavating, and I actually went for the 80% clay, 20% sand proportion, because all of a sudden I had unlimited amount of free clay, so I helped myself. :)
The best topsoil around here, is Goulais River farm topsoil, which happens to be 100% silt. Mix this in with sand and clay, and keep adding leaves or wood chips or whatever else you can get for free, grow cover crops and dig them in, or haul in some cow manure (available here for 50$ a pickup truck full about 1.5 yards) and you get 100% clay silt sand loam. Magic. The power of oil and gasoline. Get it while you can. 10 years from now, we'll be hauling with ox team or team of horses if we're well off, or by the sweat of our brow if we're not. We tried to escape Adam's fate, but he ate that apple, and we're not getting away that easy.
Oil will run out, and we'll be back to biblical times, with famines of biblical proportion. Repent! And learn to live off the land. Paul Mozarowski.

Sand is a mineral. Clay is a mineral. Silt is a mineral. Mixing them together in any proportion gives you a mineral soil mixture that will not support life, it will be dead. Adding organic matter to that mixture will allow the Soil Food Web to develop, if the right organic matter is added. Adding peat moss to soil does nothing for the Soil Food Web because they will not digest that material and even if they did it has no nutrients to feed the plants.
Organic matter, added to clay or sand or loam, provides the Soil Food Web with the material it needs to get to work, in your soil, and feed the plants growing there. Adding sand to clay will not do that, adding clay to sand wil not do that, adding peat moss to either will not do that.
Sand, clay, and silt have some mineral nutrients locked up in the particles but those nutrients will not be released for plant use, ever unless there is an active Soil Food Web present and you cannot have an active Soil Food Web working without organic matter.

Mixing sand and clay has a lot of "it depends" sort of things attached to it. One of the biggest wild cards is how uniformly you can mix it. If you mix the two types together and it is not uniform, the clay particles can stick together and form little clay chunks in the soil that hold water much more tightly than the capillary action of the surrounding sand can pull it out. Those little pockets go anaerobic and/or harbor diseases pathogens. It is very difficult to uniformly blend two soil types in the ground with a shovel or rototiller.
A lot also depends on particle sizes and how evenly distributed they are. When you say sand, you can be talking about a huge range of particle sizes; anything from 2.0 mm in diameter to 0.05 mm in diameters. They are broken down into very coarse sand, coarse sand, medium sand, fine sand and very find sand. If the percentages of these, as well as silt and clay, sizes are close to equal to each other (i.e. 10% very coarse, 15% coarse, 15% medium, 15% fine, 15% very fine, 15% silt and 15% clay, the soil can lock up. (Widely distributed particle sizes is a good way to describe that situation) Imagine filling a box with basketballs (very coarse sand), then having enough tennis balls (coarse sand) to fill all the spots between the basketballs. Then throw in enough marbles to take up the smaller spaces, then ball bearings, then b.b.'s. The spaces between all those particles will fill up and the clay particles can't stick together to form good structure because the other larger particles are in the way. Once the free space is occupied and good soil structure isn't possible, water and air movement can become severely impeded in addition to the particles locking together like legos.
However, if you have 80% tennis balls, a couple of basket balls and then throw in a few b.b.'s, you should be okay. Again, assuming that the soil types are uniformly mixed.
You just don't know what is going to happen without knowing the particle size distribution of both materials and the uniformity of the final mix. So, you might get lucky and you might not. When someone says "it always works" they're wrong. When someone says "it never works", they're wrong.
"Mixing them together in any proportion gives you a mineral soil mixture that will not support life, it will be dead."
That is quite possibly the most patently bizarre thing I've ever read on this forum.

Squonnk, one of the best explanations I have read on any forum. I hereby bestow upon you the VoR designation.

Lloyd

This last posting by squonnk is probably one of the better explanations I've read on the subject. It's impossible to say "do" or "don't do" with any degree of certainty for any specific situation without knowing all the variables - what kind of clay (yes, there are many different kinds and consistancies), the actual texture/structure of the native soil, the coarseness and quantity of the sand to be added, etc. There are very few absolutes in this business of growing plants and addressing their needs and I personally tend to shy away from any advice that is dogmatic in content - it generally is not based on scientific evidence and supported primarily by anecdotal experience. Not that it should be summarily dismissed out of hand, but taken for what it is and with a large dose of salt. As with pretty much everything else to do with horticulture and gardening, the most precise answer is.......it depends :-))

Clay has one important disadvantage; both water and oxygen transfuse through it very slowly. This can be seen with two dry fist sized clods, one of mostly silt, one almost pure clay. If both are submerged in water, the silt clod will turn to mud in a matter of minutes. The clay one may take days to become wet to the core. The transfusion of oxygen is similarly retarded in clay.

Where clay exists as a large mass, there is a small surface area to volume ratio. The slow transfusion rate combined with oxygen use by roots and micro organisms results in severe oxygen deficiency. Most plant roots die in oxygen deficient soil. This leads gardeners to believe that roots can't grow into a dense clay. Wrong! The true situation is similar to why you don't see limbs growing into an area of dense shade. Any limb that by chance heads into shade soon withers, while those in sun prosper. It's not that they can't grow there because the air is too dense, it's the light deficiency.

Where clay exists as small lumps the surface to volume ratio is much better and the oxygen transfusion rate matches the rate of depletion by plants and micro organisms. Hence no oxygen deficiency.

Keep in mind that water saturated soil does not kill roots, but the oxygen deficiency that often occurs with saturation is the cause of root death.

Where small lumps of clay exist in soil such that they are surrounded by a highly permeable matrix, air at the surface of the clay will prevent oxygen deficiency. If the clay is saturated and has a well balanced mix of plant nutrients, it will act like plant candy. It's like having the perfect slow release fertilizer that works for years. Plant roots grow right through the middle of such lumps. Carefully dig a plant on a good sand / clay mix, wash away the sand, and you will have beads of clay along the roots.

For the above reason, one should never blend a sand / clay mix to a uniform homologized mess.

I'm trying to find the citation, but I am recalling that the humic acid in compost causes the silt and clay particles to clump, allowing air and water to penetrate better.

Ah, found it. In the Zamzow's web site, it talks about how humic acid amends soil, and the wiki entry talks about the detergent (surfactant) properties of humic acid.

So, lots of compost, peat, and straight humic acid, will modify the soil structure of clay.

And, of course, sand contains the trace minerals that plants need.

And, according to the wiki entry, humic acids last a very long time. So, even though compost breaks down, its chemistry persists in the soil for a long time. BTW, this also makes the case for the use of peat.

Way to go squonk! I've seen that analogy and experiment used in many ways, but this might be the best one yet.

Someone said there might not be any differences in clay. The study of clays is nearly a science all by itself. It is highly variable.

I have never understood why artificially mixing sand and clay was a problem - I just accepted that it was. Shoot, everybody knows that mixing sand and clay is a mistake. Now I'm wondering if it is a myth perpetuated by our ancestors. Does anyone know of any research done on the topic?

I do agree that the mixture does not make concrete or cement. I further say that it does not make adobe either; not all by itself.

Cement is made by grinding a certain kind of limestone to dust and baking the moisture out of it.

Concrete is made by mixing sand, water, gravel, and cement together in the right proportion. When cement gets wet, a chemical reaction takes place that hardens the cement and concrete forms.

Adobe is made by mixing clay and water, then pounding the air out of the mix, and letting it dry. Usually fibers are added to keep the brick from breaking apart if it cracks. Sand may be added but it doesn't do much for the adobe. Driving the air out keeps the microbes from living in it and breaking it apart from the inside. Adobe absorbs water much like a sponge and will wash away easily with rain or a hose.

http://www.coopext.colostate.edu/4DMG/New/colosoil.htm
http://hcs.osu.edu/hcs/webgarden/Land/LAND_Oct96.html
Those are links to articles from Colorado State and Ohio State Universities and I know that are other articles by researchers at Rutgers, Cornell, the University of Wisconsin, Purdue, The University of Washington, Iowa State University, the University of Illinois all about how adding sand to clay is not a good thing to do. There probably is a lot more research out there, along with articles from people that will state emphatically that adding sand to clay, or clay to sand, will improve that soil, something not supported by good, solid research.

"I have never understood why artificially mixing sand and clay was a problem - I just accepted that it was. Shoot, everybody knows that mixing sand and clay is a mistake. Now I'm wondering if it is a myth perpetuated by our ancestors. Does anyone know of any research done on the topic?"

I don't have the URLs, but the Utah State extension service did some research into it and they recommend against adding sand. They do say that if you add enough sand that it can help, but that it takes so much that most people who try it end up worse off than before because they don't add enough.

I also remember reading on the Rocky Mountain forum the experience of somebody who added sand to clay in one area and couldn't get a pickax into it.

plx8xx-
The problem with those little balls is that they do harbor disease causing pathogens. I've seen many situations where pythium, phytophthora, armillaria and other root diseases that are favored by saturated soil conditions are plated out of rotting roots in situations just like you describe. The people often times don't understand how that can possibly be, since their soil drains so darn well. And most of it does. However, those little lumps stay saturated and it provides thousands of opportunities for infection throughout the root zone. At that point many folks stop listening and ask what fungicide to drench with. Oh well, I can't force people to understand.
If by "uniform homologized mess" you mean a uniformly favorable growing medium with good water and nutrient holding capacity as well as proper drainage and aeration characteristics throughout, then I'll take that mess any day. At that point all you need is good cultural practices, including proper nutrient and irrigation management. I prefer that over pockets of saturation.
Do I recommend that people attempt to mix sand and clay in the field to reach that point? No, I never do. That is because in my mind, the risk of something going wrong (which was described above and is exceedingly difficult to correct) far outweighs the chance that it might go right.

Squonnk posted on Sun, Dec 30, 07 at 9:26:

"Imagine filling a box with basketballs (very coarse sand), then having enough tennis balls (coarse sand) to fill all the spots between the basketballs. Then throw in enough marbles to take up the smaller spaces, then ball bearings, then b.b.'s. The spaces between all those particles will fill up and the clay particles can't stick together to form good structure because the other larger particles are in the way. Once the free space is occupied and good soil structure isn't possible, water and air movement can become severely impeded in addition to the particles locking together like legos."

Good post Squonnk, thank you. As others have said, well-written.

Understanding the different physical sizes of the various particles which make up the mineral (rock mineral) part of soil is sometimes difficult for new gardeners. And grasping how organic materials improve soils of very different properties and textures can also be hard to understand. This metaphor helped me see it better:

Imagine a single particle of sand is the size of a beachball.

A single particle of silt would then be roughly the size of a softball.

A single 'particle' of clay would then be about the size of a pea -- a pea squashed flat.

(Describing clay as 'particles' is somewhat misleading, because the physical structure of clay is more like a stack of dinner plates. Clay 'plates' naturally tend to stick to each other, unlike sand and silt particles.)

Decomposing organic materials in soil -- humus, dead roots, compost, and all the byproducts of their gradual decomposition -- the decomposing organic materials fit between the sand and silt particles and help to hold them together. The organic materials also slip between the sticky 'plates' of clay, and help to loosen and open them.

If a soil has too little decomposing organic material, water and air will easily move through sandy or silty particles. Clay will become more sticky; slow to dry when wet and very difficult to penetrate when dry.

(I don't know enough about the relative size of sand, silt, and clay to judge whether the size relationships in this metaphor are accurate. However, the metaphor helped me to see how increasing the amount of organic materials can work to help improve three different kinds of rock mineral soils.)

All the best,
-Patrick

At golf courses the soil builds up around the sand traps from golfers knocking sand onto the turf. Here in southwest Idaho, the soil is usually clay. So I am seeing sand laid on top of clay and it becomes soil.

My point it that it might work to put sand on top of clay without mixing and let the mixing happen by itself. I did something like this in my lawn; there is a low spot in my lawn and I periodically put stuff into it. At one point I laid some sand into it. I have also put sawdust, compost and soil into that low spot.

The grass is still healthy in the low spot and none of what I've added is visible.

If I had a hard clay that needed amending, I might try just putting sand on top of it before applying OM. But mostly, I'd just amend it with OM.

1) In my own experience, you do not need a lot of sand to make a significant difference. I put 1" on top and dig it in about 10". 2" would be better. Also, it does not need to be mixed well. I routinely find areas that were not mixed well as I move plants - I just stir things up a bit in the hole I am working on.

2) Squonnk said:

"If the percentages of these, as well as silt and clay, sizes are close to equal to each other (i.e. 10% very coarse, 15% coarse, 15% medium, 15% fine, 15% very fine, 15% silt and 15% clay, the soil can lock up."

Let's say the argument is valid - it certainly makes sense. It is then true that if the ratios are not all the same, this is not a problem. If clay soil is mostly clay - say 50%, you would need to add a huge amount of sand, with the right proportions of sizes, in order to reach this state.

It would also indicate that adding "at least 50% sand" as recommended in one post would be worse than adding a smaller amount, since it gets you closer to the 'lock up' state.

3) The two references listed above are not references to research. They simply restate a fact that we are discussing. Where is the evidence that these statements are based on?

A couple more comments.

Squonnk's example deals with soil texture, not structure. Since real soil aggregates, and is in fact aggregated before the sand is added, it would not follow the example given.

The second problem with the example is that clay particles stick together. They would not simply sepaarate into small individual particles and fill the void holes in the sand.

If adding sand to clay soil, results in more preferable aggregate peds, then the soil will improve.

Well, here's the thing. I was talking about both soil structure and texture. The texture part is only the percent of sand silt and clay in proportion to each other. The structure refers only to the soils ability to form channels and pours that air and water can move through. In a soil with a lot of sand, those spaces are provided simply because there is a lot of room between the sand particles. In a soil with lots of clay, the particles need to be held together by ions with a +2 charge. Calcium and magnesium are the two biggest players. At the risk of oversimplifying, I'm going to leave it at that. If you like, I can go into more detail.
So, does clay form aggregates in the soil? Yes. Those are the saturated clay chunks that I keep droning on and on about in a non-uniform mix. Does that sand around the chunks have good water movement and aeration characteristics? Yes. Do the clay chunks have good structure? Yes. Within the clay chunk (or aggregate) water and air can move around. But, water will have a hard time moving out of the aggregates because capillary action will hold it in. The channels that form in clay do allow water movement but they are exceedingly small. The sides of those channels hang on to water. Water can move through those channels because within the channel capillary action is relatively constant from point A to point B. However, once water gets to the end of a channel and is faced with the prospect of moving into the relative chasm of open space in the sand portion of the soil, it just sits there. It's like putting your thumb over a straw and pulling it out of your coke. The coke stays in the straw and won't move into the vast open space at the end of your straw because there is no capillary action to pull it out. In other words, they stay wet far longer than they should, especially with regular irrigation constantly recharging those aggregates. Again, due to capillary action, they have no problem wicking water out of the spaces between sand and holding on to it.
Okay, so that's what happens when the materials are not mixed uniformly. It's not good to have those clay aggregates surrounded by sand.
Now, when the mix is relatively uniform, everything will then depend on how much silt to sand to clay you have.
Again, does clay form aggregates? To varying degrees, yes. And there is no rule that says that each clay aggregate will be comprised of 'X' number of clay particles and be exactly such and such shape. Some are big aggregates and some are small. I found this picture that gives a better idea of particle sizes relative to one another:
{{gwi:320910}}
And yes, none of the particles are perfect spheres. Work with me ;)
So, you can see that a small aggregate of clay particles can still fit between other individual particles and take up the empty spaces. In fact, in addition to aggregates, some clay particles will exist as individual particles until they bump into other clay particles. The higher the silt and clay content, the fewer clay particles will be milling around and the more likely that is to happen.
As the percentages of all particles approach uniformity, the "lock" up situation becomes more and more likely. And there is no particular point at which everything is find and if you pass it everything suddenly locks up. It's a sliding scale. The more uniform the percentages, the more you will see the problems. Other factors, such as compaction, play into it as well.

Oh, and by the way...
"It is then true that if the ratios are not all the same, this is not a problem. If clay soil is mostly clay - say 50%, you would need to add a huge amount of sand, with the right proportions of sizes, in order to reach this state."
Bingo! Although I would change "this is not a problem." to "this may not be a problem."
But adding less may or may not provide a whole lot of benefit. Depending on how much you add, whether it helps or hurts depends greatly on the distribution of particle size percentages, (as you correctly mentioned) and uniformity of the mix.
And remember, I'm not saying it never works. Every situation is different and there are a ton of variables. I'm pointing out potential hazards with the understanding that sometimes is does work. (It's just too much of a gamble for my taste).

And one more thing...
I didn't mean, "The higher the silt and clay content, the fewer clay particles will be milling around and the more likely that is to happen."
I meant, "The higher the sand content, the fewer clay particles will be milling around and the more likely that is to happen."

I've been gardening many years with clay. And when I say clay I mean soils that have better than a 50% clay faction.

A few years ago I mixed up a new bed and then took a sample. The mix was separated to show the make up of sand to clay as seen in the photo below.

{{gwi:33779}}

If you believe sand and clay makes adobe or concrete, or that saturated small clay lumps harbor disease causing pathogens, or that you can't be successful without the "soil food web", then you probably think such a mix is doomed to fail. For the real deal check the link below.

Here is a link that might be useful: the truth

pls8xx
I'm pointing out the potential hazards based on experience and an understanding of what is happening under ground. I too have seen what you are describing work. However, for every anecdote that I have about it working, I have five or six stories in which things went wrong in the fashion and for the reasons I described.

I think the miscommunication is right here:
"you probably think such a mix is doomed to fail."
That's precisely the opposite of my point, which I stated way back towards the beginning:
When someone says "it always works" they're wrong. When someone says "it never works", they're wrong.

And then later on in the thread:
And remember, I'm not saying it never works. Every situation is different and there are a ton of variables. I'm pointing out potential hazards with the understanding that sometimes is does work.

"The Truth" is that you said it works for you in your situation with your particular materials and variables. We believe you. There's no point that needs to be made. On that account we are on the same page. Would I recommend that everyone goes that route? When I see a strategy fail more often than succeed, and I can see why, I go a different direction.

I have lurked here for over 3 years with a post only once in a while. And I am so happy to see that this discussion about mixing sand with clay is relatively civilized. I can remember at least a couple of times previously when such an issue started lots of name calling. I can't tell you what a pleasure it is to not see it this time around.

That said, I will repost some info I found on the web that is an excerpt that is no longer out there but it does give a source/reference to Garden Gate magazine. (I have saved it to my library of garden info on my PC. ) I particularly like this info because it makes sense to me. Because there are numerous examples in nature or the sciences such as in cooking and or even plumbing where mixing two different substances is not recommended unless you have some intermediate step that can make the transition easier.

Madtripper, the original poster here, may in fact have clay that has enough humus (as he describes as a possibility) that in fact makes the transition between clay and sand gentler. Just speculating.

Gardening in Clay Soil

Because clay soil is so finely textured, it tends to stay wet and sticky for a long time. And if you work it or walk on it when it's wet, it compacts easily. Compaction creates poor drainage. And when that wet, sticky soil finally dries out, it can bake into clumps hard as rocks. This makes it tough for roots and seedlings to make any headway.

So rather than making mud pies, use this recipe for better soil from Ray McNeilan, a home horticulturalist who recently retired from the Oregon State University Extension Service in Portland: Spread 2 to 3 inches of organic matter across your soil. Bark, rotted manure, leaf mold, peat moss and compost are good choices for organic ingredients. Sprinkle in a fertilizer (such as ammonium sulfate) to give your soil a nutritious boost. Next, till, fork or dig your compost and fertilizer into the top 6 to 7 inches of the soil.

Add 4 to 5 inches of sand on top of that organic matter-clay mix for an even better-structured soil. Roughly equal portions of organic matter, clay and sand create the ideal mix for gardeners: loam. But if you thought mud pies were bad, make sure you wait until you've added the organic matter before working in sand or you may be mixing up "cement" pies.

Use this measure when you're buying sand: A cubic yard will cover an 18-by-9-foot area spread 2 inches deep.

And remember, this isn't a one-time proposition. Organic matter is at its best when it's decomposing, so keep adding compost to the mix. Plus, by consistently building your soil, over time you'll raise the nutrient level in the soil to the point where you'll rely less (or maybe even not at all) on synthetic fertilizers.

From Garden Gate Issue 14

squonnk - thanks a lot for posting.

You said:

"It is then true that if the ratios are not all the same, this is not a problem. If clay soil is mostly clay - say 50%, you would need to add a huge amount of sand, with the right proportions of sizes, in order to reach this state."
Bingo! Although I would change "this is not a problem." to "this may not be a problem."
But adding less may or may not provide a whole lot of benefit. Depending on how much you add, whether it helps or hurts depends greatly on the distribution of particle size percentages, (as you correctly mentioned) and uniformity of the mix. "

We seem to agree on this point. However, it contradicts what others claim. They claim that you NEED to use a large amount of sand to prevent adobe. Your statement, the example from pla8xx and my experience would indicate that smaller amounts of sand (I think we can agree that adding smaller amounts of sand can sometimes cause a problem and in other cases not cause a problem.

We are then still left with the original question. Why are these opinions so different?

people recommending the addition of sand usually recommend washed builders sand - fairly uniform in size - so less likely to cause a problem. maybe the original research used sand with a wider distribution?

I'd like to find an original study that actually defines the type of soils used.

How do you make Adobe?

Found this on the internet in a few places. "The best adobe soil will have between 15% and 30% clay in it to bind the material together, with the rest being mostly sand or larger aggregate."

Right in line with squonnk's numbers. However, it contradicts the advice from people saying not to use small amounts of sand???

To make Adobe you need the right kind of clay, normally found in the southwest USA and Mexico and you wet it thoroughly and add some kind of binder for strength, straw, grass, etc., put it in molds and let it sit in the sun to dry. The clays in North Carolina and other similar places can also be made into pottery, if fired, but as a rule unless there is a lot of heat to fuse those particles that clay will not make Adobe.
That numerous soil scientist, people that have studied soils, tell us that adding sand in relatively small quantities to clay will not help should tell us that adding sand to clay in small quantities does not help. That a horticulturist might tell you that adding small quantities of sand to clay may help some should tell you that this horticulturist did not pay much attention to the soil classes.
Adding more mineral to a mineral soil is not going to do much to get the Soil Food Web functioning because they need organic matter and since smaller quantities of organic matter will do more to improve clay soils why add sand which will cost money and do nothing for the Soil Food Web?

We are then still left with the original question. Why are these opinions so different?
I think the opinions are very different, because they come from people who have very different experiences. Two people may think they've done the same thing and have seen very different results. However, they got different results because they actually didn't do the same thing at all.
Example: Two people each say they added sand to their clay at a 1:1 ratio. One person improves their soil. The other person makes a drive-way.
The person who made a drive-way will forever believe that adding sand will always make a drive-way and defends that opinion to the bitter end because that was their experience. They now know that it's a bad idea.
The other person who improved their soil will forever believe that adding sand will always improve clay and defends that opinion to the bitter end because that was their experience. They now know that it's a good idea.
However, neither of them actually knows what percentage of clay their "clay" really was to begin with. Of the sand fraction, neither person knows the distribution of their sand particle sizes. Neither one knows how much silt was in their native material or the "sand" they were using. Unless they pulled soil out of the ground and actually weighed it, they're "eye-balling" how much sand to mix in for a 1:1 ratio. Some people can do that okay. Some people just think they can. When some people talk ratios, they're talking weight. When other people talk ratios, they're talking volume.
They may think that they did the same thing and got different results but they actually ended up using very different materials and/or mixing them at different ratios and that is why their results were different.
To get down to some of the specifics you were talking about:
Washed builders sand is "fairly uniform in size". How uniform is fairly uniform? Some sand providers do it well. Others do not. Even if it is a fairly uniform, oh I don't know, medium sand, what if the sand fraction of the native soil that you are amending is mostly coarse and fine sand and all it takes it a good dose of medium sand to cause a problem? Then a uniformly medium sand would do exactly that.
Will adding a lot of sand help or hurt? Will adding a little sand help or hurt? Adding a little might hurt and it might help. Adding a lot might hurt and it might help. It depends entirely on what the two materials you start with are actually made of.
I guess my big point is this: When mixing soil types it is going to be a gamble unless you-
A) have particle size distribution data for your native soil
B) have particle size distribution data for the material you would like to amend with
C) know what those data mean
D) know the final percentages you wish to achieve
E) are ready to reject a material unless adding it will give you the proportions you want
F) have the equipment and the know-how to actually mix the materials uniformly in the appropriate amounts to achieve your target distribution.
Short of that, adding "sand" to "clay" or "clay" to "sand" will be a shot in the dark. Sometimes it will work and sometimes it won't.
Neither the easy recipe for mixing soil that will always fail nor the easy recipe for mixing soil that will always succeed exists. People want to have, and lots of people even think they have, the easy answer but there is no easy answer. As gardengal48 said, "the most precise answer is.......it depends".

kimmsr - I disagree with your statement. There are many cases where 'expert scientists' have been wrong. There are also many cases where science indicates one thing, and common people, including so called experts, interprit it incorrectly. Then these people, including experts repeat it over and over again until everyone believes it. Most of what is written as 'fact' is written because someone else wrote it as fact.

How many times do you read that when planting a tree you should dig a big hole and improve the soil before adding it back in. People in the know, understand this to be poor 'soil' advice, but it is still the most poplular advice given. It will take another 20 years before that changes.

squonnk - You are probably right, but I would hope that some soil scientists have studied this, and done it properly. If they did, they would know the composition of the soil they used, and they would have tested a variety of soils. If this has not been done, then Maybe, the advice is not based on much, and should not be repeated. Or at least changed to "adding sand may work in some cases, and not others".

I don't want to give the impression that the nobody knows how to do it. It is a gamble unless you go through steps A through F mentioned above. Commercial soil providers do go through those steps and they blend soils all day long. They have the know-how, the data, pug mills, screens, weighted belts, etc. to do it right and they are quite successful. That's their whole business and some of them are very good at it. My point is not that it isn't possible or that nobody knows how. My point is, unless you go through the same process, you're just guessing. That's why I said there is no easy recipe. That is, you can't just say to someone with heavy soil in their back yard "add 50% sand" and expect it to work. That's why I never recommend that home owners try it. But if you're a soil provider with the right data on your materials and the right equipment on hand, you can make exactly the kind of blend you want that will have the physical properties that you're shooting for. It's really not that tough to do if you're set up to do it.

I disagree. I don't think there's some sort of magic involved here. I think that it's true that clay and sand that do not contain any organic material will stiffen and harden. But I also think that if you add the organic material, and lots of it, the humic acid will react with the minerals in the clay and break it up, dissolve the clay.

As well, there are surely other factors that the anecdotal evidence misses.

For example, person #1 adds sand to their soil in their front lawn. Then they drive over it for the next year with their heavy tractor once a week. This ends up compacting the sand/clay composite, driving out all air. This ends up creating a very nice adobe hard pack. At least it is nice if you are building a house.

Person #2 adds a similar amount of sand to their garden plot, which they work by hand. It gets frequent watering, plus a fair amount of organic matter added in. Careful weeding keeps it from ever packing down. It also gets turned over each year. This sand/clay mixture grows wonderful tomatoes for years afterwards.

Of course, person #2 becomes a strong advocate of adding sand to clay. Person #1 would never do so again for any amount of money.

My point is that without control of external factors, without a carefully designed and analyzed experiment, we can come to a variety of conclusions from meager data.

John

I would like to respond to all who contributed to this thread, but right now I have neither the time nor energy to do so. There has been much said that agrees with with my own experiences.

squonnk said ...

When someone says "it always works" they're wrong. When someone says "it never works", they're wrong.

I am in complete agreement with that statement. I've said something similar on several occasions myself.

Earlier in the thread I said ...
Where clay exists as small lumps the surface to volume ratio is much better and the oxygen transfusion rate matches the rate of depletion by plants and micro organisms. Hence no oxygen deficiency.

Keep in mind that water saturated soil does not kill roots, but the oxygen deficiency that often occurs with saturation is the cause of root death.

Where small lumps of clay exist in soil such that they are surrounded by a highly permeable matrix, air at the surface of the clay will prevent oxygen deficiency. If the clay is saturated and has a well balanced mix of plant nutrients, it will act like plant candy.

and squonnk responded ...
The problem with those little balls is that they do harbor disease causing pathogens. I've seen many situations where pythium, phytophthora, armillaria and other root diseases that are favored by saturated soil conditions are plated out of rotting roots in situations just like you describe. The people often times don't understand how that can possibly be, since their soil drains so darn well. And most of it does. However, those little lumps stay saturated and it provides thousands of opportunities for infection throughout the root zone.

squonnk is correct that as roots die they present an entry point for disease causing pathogens. If the roots don't die the disease process is interrupted. So we must look at when and if the roots will die.

It is well accepted that a waterlogged soil often results in root death. But the cause is not the water but the lack of oxygen.

In soils with a very uniform soil texture, saturation at any given depth equates with total saturation of all soil below that depth, certain oxygen depletion, and root death. It's no wonder that many gardeners equate saturation with root death without the consideration of oxygen.

But many good garden soils are heterogeneous and bimodal in regard to oxygen diffusion. For those of you who would dismiss my statements as antidotal, I offer the following:

From "Oxygen Transport in Waterlogged Soils, Part II.,Diffusion Coefficients
Franco Humberto Obando Moncayo1

page 5, Diffusion within the soil microstructure

.... that most structured soils are bimodal with regard to diffusion. "Most soils show heterogeneity of pore distribution. For example, soil with a highly developed natural ped structure, and cultivate soil with their crumbs and clods, have distinct zones of crumbs pores, separated by more continuous system of intercrumbs poresmeasurements of diffusion in packing of soil crumbs show that intercrumbs pores contribute more per unit of their volume to diffusion through the packing than do crumbs pores Diffusion to depth in heterogeneous soils will thus occur preferentially through intercrumbs pores, but the path to the respiring site will be completed within crumb pores that are partly or even wholly moisture-saturated.

and

Currie (1961) presented expressions to describe the radial diffusion of oxygen towards the centers of waterlogged soil aggregates, which made it possible to estimate what fraction of a spherical aggregate would be anaerobic, if its radius, the external oxygen concentration, the diffusion coefficient, the solubility of oxygen, and the respiration rate were all known. For any given values for these parameters, there is a critical aggregate radius below which no anaerobic zone exists, ...

emphasis added

The above paper can be found at ...
http://users.ictp.it/~pub_off/lectures/lns018/25Obando2.pdf

I keep seeing gardeners that want to "break down" their clay. If they only understood that the problem of high clay soils is oxygen deficiency, they would see the error in trying to do that. Take any soil with a clay faction over 50% and add anything, sand, topsoil, or even gravel, beat it to a uniform texture, and you will probably have a disaster.

Here's the thing,
You keep saying heterogeneous soil and you're talking about heterogeneous texture, that is, two intermingled soil types.
When the paper you've quoted says "heterogeneous soil", they are talking about the heterogeneous pore and aggregate sizes. Structure, not texture. Most soils show heterogeneity of pore distribution.
Those aggregates, clods, crumbs, peds, pore spaces, etc. all exist and in different sizes in an homogeneous soil. That's the stuff we're talking about when we say "proper soil structure" that results from the correct ratio of cations as well as microbial activity (fungi and actinomycetes are pretty big players).
Uniform texture does not equal lack of aggregation.
The difference is this. In a soil that is uniformly, let's say sandy loam, the pore spaces within each aggregate are huge compared to the pore spaces between clay particles in a similarly sized clay aggregate. The capillary action through those larger pore spaces is, of course, much weaker and water can be pulled out of those aggregates by the capillary action of the similarly sized pore spaces surrounding the aggregate. The combination of aggregates, pore spaces and smaller differences between the forces that move water and gas around allows for proper drainage and aeration throughout the soil profile.

As to the point that there is a critical aggregate radius below which no anaerobic zone exists. Well sure, the flip side of that is there is a critical aggregate radius above which an anaerobic zone is likely to exist. In other words, once those aggregates reach a certain size, you're virtually guaranteed to have an anaerobic zone. That is A) one of the many variables that factor into whether or not things will work out well in a given situation...and B) why good soil structure as discussed above is so important.

Perhaps I should expand on the point I was making concerning anaerobic activity. My point was not that water saturation is magically bad for plants just because it is. Of course it is a lack of oxygen and proper gas exchange. When you have pockets of anaerobic activity in the soil and a root grows through them, roots will not be able to achieve proper gas exchange and oxygenation over the portion of the root surface in that anaerobic area. Let's say that 10% of the root surface area runs through those anaerobic spots. That will reduce proper gas exchange in the root system by approximately 10%. Will that kill the plant all by itself? I sincerely doubt it. However, a 10% reduction in proper gas exchange in the roots will stress the plant. Any thing that stresses a plant can predispose it to other problems such as diseases. The saturated spots decrease gas exchange and provide an environment that favors the pathogens ready to infect and the combination greatly increases the chances of infection by a root disease pathogen. Again, will it happen every time? No, but it is a hazard.

The root doesn't have to die to present an opportunity for infection. When favored by the environment and then presented with a host, pathogens can infect healthy tissue and cause the diseases that kill healthy tissue. Or if a soil is uniformly anaerobic that too will kill a plant due to, as you correctly stated, lack of oxygen (and exchange of other gases). It was never stated that water kills the roots all by itself.

squonnk said
As to the point that there is a critical aggregate radius below which no anaerobic zone exists. Well sure, the flip side of that is there is a critical aggregate radius above which an anaerobic zone is likely to exist.

Yes! And for any given clay, knowing what that critical radius is, determines success or failure. For my clay, years of gardening indicate that clay peds should be less than one half inch and better if near a quarter inch.

If one refers to the standard soil triangle, it can be seen that where the soil contains greater than 50% clay the label is simply "clay" with no other descriptive words used. There is no need of other names because at over 50% it doesn't mater what other particles exist in the soil, it will have all the attributes of pure clay.

When one mixes such clay with a large amount of sand by hand, it is possible to keep the clay together as lumps intermixed in a sand matrix. Such a mix has both heterogeneity of texture and heterogeneity of pore distribution.

I do agree that for some plants even a small amount of oxygen deficiency can result in a loss of plant vigor. This can happen in many soil types and is often overlooked by gardeners because it doesn't present particular symptoms. This is a more common condition where there has been some compaction of the soil.

Pure sand can not be compacted to close off macro pores. Saturated clay is already at a compacted state. The two in combination are resistant to compaction problems.

A properly formulated sand clay mix does not suffer an oxygen deficiency even in the saturated clay. Can a beginner get it right? Maybe, maybe not. I suggest that one start with a small area. If you can learn to do it the advantages are many.

I spent my first 20 years gardening with topsoil using conventional practices to overcome the many problems encountered. I've been successful now for many years with clay. I would never go back.