Recently I had about 8 or 10 yards of "pond fines" delivered form the local gravel pit. This is the clay and silt particles that get washed out of the mason's sand. The gravel pit itself is also located in the carver sand, so effectively this is a way of greatly increasing the very small amount of clay and silt that is already in my soil. One area of the garden is excavated in the process of building a greenhouse and so I am underlaying new beds in that area with 6 inches to a foot of this gunky material and then putting the old soil back on top. The hope is that will make those beds nearly drought-proof. In other places whenever I dig a planting hole I will add in some small lumps of the fine material along with the other fertilizers.

I sent off a sample of this material in dried form for mineral assay, won't get the results until spring. Our rock here is glacial gravel, so hopefully the mineral range will be good, not just a massive pile of iron.

It is exceedingly rare that I recommend people try to change the texture of their soil for a myriad of reasons. Since it sounds like you may have access to gravel, do give the manufactured perched water table some thought.

As far as the original question, I think mixing silt/clay up to about 50% max should be fine. They say if you're going the other way - amending with sand - you have to get into the 35-50% range to prevent forming a type of concrete, so I would think you would want to keep it at least 50% sand for safety.

You'll end up with a saturated zone about 1 or 2 inches in depth when you irrigate so the roots can access the water but the entire rootzone does not become saturated and anaerobic. When more water is applied, the gravity of the increasing hydraulic head will push the water into the gravel so you don't have to worry about over saturating.
If you google "USGA Green Construction Perched Water Table", you'll find all sorts of reading on the concept.

If someone wants to mix soil textures I can't stop them and sometimes it does work out but regardless of what anyone says, when you mix textures there is always a chance that you will end up with wide particle size distribution, which can lead to a tendency for the soil to compact. To do it without worry requires sophisticated testing, special equipment and experience. Also, it is very difficult to uniformly blend two soil types in the ground and you often end up with balls of silt and clay in a sand matrix which has its own set of problems.

Anyway, I ain't the dirt cops and anyone can do whatever they want. I just recommend looking into a strategy other than mixing soil types before making a decision.

I have been working from exactly the opposite direction. I have good clay loam soil to start with. Still, it can be slow to dry out in the spring and could crust some. With the addition of about 30% coarse sand, 30% local sphagnum peat moss with black fines that make it very well hydrated, and 40% original topsoil, I have an extremely satisfying mix nearly a foot deep that warms up well, holds moisture well, drains and soaks up rain like a sponge, and is loose in structure. I love it.

I have never seen the dreaded concrete or adobe fear mongering happen here. Perhaps some clay with smidging amounts might not work so well.

That is a likely outcome in my case. I was thinking these would act as little moisture warehouses, so to speak, and perhaps minerals as well. Do you know any specific problems with this kind of situation? The beds where I am basically making two distinct layers are perhaps a different situation.

The perched water table thing sounds fascinating, and incredible. I will read up on it. No reason why I couldn't do a test run on a small bed here. Where it could really be a huge asset is at my florida place which is nothing but enormous amounts of excessively drained sand dozens of feet deep. Here I have the luxury of the pond fines as well as crushed glacial gravel being nearby, whereas down there there are no good sources of silt or clay sized particles in quantity that I know of. However there is crushed limestone rock which is the primary construction material. I was thinking about digging out beds and lining with some water-proof membrane, or almost water-proof, or some organic material that perks extremely slowly, but all of that would be more expensive than gravel-sized rock.

Off to read now.

Garg, how much coarser do you suppose the underlaying material must be in the perched water table system? I can get something that they label "stone dust" which is a range of sizes mixed together, freshly-crushed rock, most of which is a good deal coarser than sand. Nothing bigger than about 3/8". I already have an area where this stuff is in place and in fact i was going to try putting the sandy loam soil on top of it. Maybe I"ll try another test bed with 3/8" peastone under. Seems like anything like that would need the dust-barrier material between the layers to prevent the soil from eventually working its way into the gravel.

My florida place is acres and the sand there makes the stuff here look like prarie gumbo.

gargwarb, I see that the concept was not at all what I was assuming, and I know what happens when I ass - ume. :-]

I have had the concrete effect to an extent after adding a relatively small amount of sand to a heavy clay. So I can attest it does actually happen.

I don't think having clumps or clods would be a problem, and eventually with digging, freeze/thaw and worm action, they are going to homogenize into the soil just like everything else. If I was doing this on a garden-size scale, I might consider laying it on top and then tilling the whole thing.

Alternatively - and I did this once when I got sand in early summer and couldn't use it on most of the garden right away - take a fallow bed and build a lasagne compost pile with layers of the new soil between compost materials. Next season plant into the new layer or dig/turn/till. I got a great crop of sweet potatoes the next year and that bed is still the best texture anywhere in the garden because it got the most sand added.

Anyway just a couple of thoughts, but you may be going in a different direction already.

Since you've had your clay/silt delivered already, it sounds like you're well on your way down that road. I'd be interested in hearing how it works out for you. When we first moved here I began adding copious amounts of compost and manure and encountered the same issues as you - namely, no matter how much was added, it couldn't hang onto any moisture. That's improved over the years, and I attribute the quality of my soil to two main additions: 1) pine bark fines; and 2) Turface. I was initially tipped off to these on the container forum and thought I'd experiment. I've been really pleased with the results. The benefits of these (vs. compost, manure, etc.) is they break down much slower, so they don't disappear into the sand as quickly. In fact, the Turface specs suggest a breakdown of 3% over 20 years. In other words, it's very durable. If you're not familiar with Turface, it's a calcined clay product that is used as an infield conditioner on baseball fields. It's also is similar to products used for soaking-up oil, like Napa floor dry. It can absorb/hold/release quite a bit of moisture and, with a cation exchange capacity of around 20, it will hold onto nutrients, unlike sand.

For a large area, it's relatively expensive ($8-$10/50lbs.) but it's a marvelous product and works well with sand.

Tox, I am not much worried about the clods either. In fact, when I dig planting holes whether for tomatoes or fruit trees or whatever kind of thing like that I think I will line the bottom of the hole with a thick layer of the fines to stop up the water.

Studies by soils physicists in the research area of stratified soils lead to a better actual understanding than those by horticulture professors. Unsaturated-zone hydraulics investigated by civil engineers lead to better understanding too.

But just look at the extreme, say pot-in-pot culture or other elevated air-pruning-of-roots growing method. The permeability of the air is essentially infinite. Do the elevated pots saturated at the bottom more than the trivial depth theory calls for in a transient event after each watering?

Upon more thought, the idea of putting gravel under my beds seems a lot like carrying coals to newcastle. When hilling potatoes or digging tomato holes I don't want to be bringing up gravel, I have enough of that in the subsoil as it is.

Have any studies handy? I'd like to take a look.

Are you saying that the idea of perched water--even in container culture--is transient at best? Not a challenge to your statement, just looking for clarification.

I have to say well said gargwarb, well said.

The perched water table idea is probably true, I see no reason to disbelieve it. Nevertheless, I won't spend money and large amounts of effort on importing a foot of gravel and putting it under my soil. Sooner or later the underlayment is going to get mixed into the upper layer, with gravel that would be a disaster, while with clay/silt it will likely be a benefit.

A simple nontechnical explanation for what happens and probably how the myth of a perched (i.e., saturated, reasonably persistent) layer arose. Notice how he says "'nearly' saturated." Instead the process is the slowing of an infiltration pulse but without actual saturation. See also how a clay subsoil can truly perch water.

Go to Google Scholar and search on "stratified soil" to begin the search for more.

"Coarse Sand or Gravel Subsoil
Fine soil overlaying coarse sand or gravel subsoil must become very wet before water will move down through the large pours of the subsoil. Under these conditions, the overlying soil holds up to two or three times as much water as it would if the course subsoil were not present"

And that's exactly what I was talking about. The water "perches". It's not actually 100% saturated but it is water that is perched until you apply enough for gravity to suck it down into the subsoil. With proper irrigation management, you can perch the water where you want it and use that effect to your advantage.

Of course, that saturated zone (not actually 100% saturation but close enough for the girl's I go with, as my grandfather use to say) Will be bigger in a fine soil over gravel due to greater capillary action but it will still be there in a sand over gravel. You might have a 5 or 6 inch layer of saturation in a heavy soil over gravel and a 1 or 2 inch layer in a sand over gravel.

In the paragraph above that, your reference states (when placing a find soil over sand):
"When water passes through a fine layer and reaches a layer of coarse sand, it stops until enough water accumulates to nearly saturate the fine soil"

Note the use of the word "stops". Again, this happens when going from a sand to a gravel due to a difference in capillary action of the two different layers. Again, this is the effect that we use to "perch" water where we want to so the roots can get to it.

As far as a clay layer goes, you're right. You can perch water that way too.

"Clay Layer
When water reaches the clay, the very fine pores of this layer resist water flow. Although water does pass through the clay, it's penetration is so slow that water tables often build up above the clay."

In that condition, note that water actually moves but slowly rather than coming to a dead stop like it does when you put a finer material over a coarser material as discussed previously. However, your zone of saturation will be larger and you can't "push" water into the subsoil when you need to by applying a bigger head of water. It will just perc as slow as the clay will allow it to move and no faster. It's harder to control soil moisture that way.

Both of these conditions become less and less important the deeper in the profile the textural interface is but when the interface exists relatively near the surface, either by accident or design, it can be an advantage or a liability depending on intent and management practices.

("Or so the legends of yore go.", I suppose I should say)

Garg, thinking about what you have said regarding my seed-starting medium, maybe a better plan is to mix in some garden soil with it. The reason I didn't want to do that was to beat weed-seed germination. I guess I could mix in some sterile bought stuff but I'd like to not spend more money.

Nope, that's the only way you can truly perch water, with a low-permeability barrier. The high-permeability zone just increases the dampness directly above it but does not perch water.

If you sincerely believe that downward water movement can not possibly be impeded in the way that I've described...well...I've explained in detail how it works and the link that you yourself posted supports what I'm saying. I'm not sure what else to tell ya there, sport.

Maybe. It depends on how heavy the clay is and how heavy the loam is. I thought we were talking about sand though.

Garg, thinking about what you have said regarding my seed-starting medium, maybe a better plan is to mix in some garden soil with it.

That could work. Especially if the "garden soil" is primarily organic material. Lots of the bagged "top soils" are mostly organic with some sand mixed in. Or are you saying some of your soil? Either way, I think that would be the route I would probably go, given the choice.

That doesn't equate to perching. This being the whole point.

I have nothing against faith-based (myth based) hydrology and the intentional or unintentional changing of the meaning of technical terms in such an inconsequential realm as a discussion board, but to soils physicists, and civil engineer and geologist hydrologists, "'perched' water table" has a meaning and "this ain't it."

If that's what's sticking in your craw, I can understand that. However, you could just say "Hey, that's not the way they use that term in the circles that I travel in."
The concepts I've described and that you and I both linked to, are not a thing of fantasy or myth. They can result in benefits or problems just as I and countless others have observed and described. To say that the idea I presented is myth in the face of so much evidence is silly when all you meant was "I don't like your use of that term to describe the condition". I'd use an alternate term for it to help you feel better, but there really isn't one. In ag and hort, that's just how people describe that condition.

I could also understand if it was just me but that's not the case. It's whole industries communicating that way. If you want to communicate fluently in multiple circles, you have to learn the languages regardless of what you feel the first and true definition is. Going back to sand and gravel, I think calling a bucket of material that is bigger than 4.0 mm across "sand" is absolutley silly. Any reasonable person would call it a bucket of gravel. But that's ASTM and, when in Rome.....

In my heavier soil I desired more looseness and permability in the upper layer.