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Sand Mulch over clay loam creating an artificial soil horizon

 
pollinator
Posts: 436
Location: Montana
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I garden in a shallow clay loam in Ronan Montana. In soil science class years ago I learned that it takes an awful lot of sand to alter a soil. So I buy mine from a local farmer around 8 to 10 yards at a time in big loads.

This has worked out pretty well over the years and I've kept doing it and have had at least 8 loads delivered since 2011.

I tend to leave the sand mulch in place for a couple years but I got to tilling last fall and realized it can be tilled at least shallowly without destroying the sand rich layer.

I've also theorized that it holds heat better than the clay rich soil below. Thus allowing me better success with melons.

Anyone else try using sand as mulch for clay soils?
 
pollinator
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Location: Denver, CO
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I have not but am interested in this topic. Native tribes used this technique; they would garden in areas where there was a deep sand layer over clay. This allowed them to grow crops in dry areas by storing water.

Do you notice water storing effects?
 
steward
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Location: Cache Valley, zone 4b, Irrigated, 9" rain in badlands.
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My grandfather gardened in some of the heaviest clay in our area. He amended with sand on a routine basis. I love growing in clay-ish soil. Having 6 to 8 inches of sand on top of the clay would be a dream.
 
William Schlegel
pollinator
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As far as water storage goes I am uncertain. I have a well and water fairly heavily. I seed right into the sand layer and vegetables germinate just fine. In the fenced garden I built the raised beds out of the sand. I often water with soaker hoses in the paths and water until I see the wetting front reach the center of each sand bed. Sometimes I run a sprinkler as well.

I do get too much sand in some crops especially when I run the sprinkler. I may grow future greens in a sand free area because the clay soil doesn't add grit if splashing happens. The depth of my sand layer has varied from 1 to 6 inches. I looked up my soil recently on soil web and it averages about 7 inches deep for the top horizon. I suspect some areas have far less depth from their higher clay content and poorer tilth. Their is an old google earth image of my field plowed up with light spots where the soil is thin. I would like to add sand to that entire thin area over time I bet as little as an inch of sand would create a noticeable improvement in the shallowest soils even tilled in but 4 inches of sand would really clinch the deal.

One of the last things I did last fall was to till up an area that hadn't been gardened since I spread out two sand piles in 2011 The spots where the sand piles sat appeared to be pure sand even after tilling but the areas around them where I spread the sand in thinner layers seemed during and after tilling to be a sandy topsoil with great tilth to rototill in. I had a great squash patch there in 2011 and since it was recently grass I will probably grow squash or corn again.
 
William Schlegel
pollinator
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My sand supply kind of dried up as the farmer no longer can deliver. So I emailed the local gravel pit. They offered 11 yards for $120 of 3/8" minus pit sand.

Sounds like very course sand. My old sand source is basically a fine sand, sand subsoil. Thoughts on trying the new sand?
 
gardener
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I think the new stuff would be great for loosening the clay, but that's just my guess. Do you also add organic matter? Would you be tilling it in or leaving it on top?
 
William Schlegel
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James Landreth wrote:I think the new stuff would be great for loosening the clay, but that's just my guess. Do you also add organic matter? Would you be tilling it in or leaving it on top?



I usually lay it on after tilling the first season. Then in subsequent years till on the shallowest setting. If you encorporated sand to say a 12 inch depth I figure it would take ~6 inches of sand to get ~50% sand. So I buy less and encorporate it less.

I add organic material when I can.
 
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I have been told in a gardening class that you do not want to add sand as an amendment to heavy clay soils, because the soils behave like clay with as little as 20% clay size particles. The proportion of sand you would need to add to 'amend' the clay soil is large enough that usually it isn't worth doing. Instead the extension office suggests adding a lot of organic matter and stimulating your underground microbiome - the microorganisms, roots and earthworms help create larger aggregates within the clay that then helps with drainage issues, compaction issues and small pore spaces of clays. Ground covers such as daikon radish are also tremendously useful in breaking through clays.

I can't say exactly for your area, but we have a lot of clay in our region that makes gardening a challenge as well.
 
William Schlegel
pollinator
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That's why I mix the sand in as little as possible and buy it by the dump truck load. It definitely doesn't get mixed in and act like clay the way I do it and it lasts for a long time. I've been doing this on my current garden since at least 2011 but before that I tried it as far back as 1999. I have a spot from 2001 in the backyard that I spread a sand layer in and planted a wildflower meadow. The sand layer is still intact though nightcrawlers have moved a very thin layer of clay topsoil on top of it.

I think part of why it works has to do with how soils develop naturally. My soil has a clay rich layer under the top soil according to the soil survey. That means clay moves down out of the top soil. So by adding sand at the top of the soil structure as a mulch it isn't moving anywhere.

 
William Schlegel
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Ok, looking up my soil online. I have 7 inches on average of a Ap horizon that is Silty Clay Loam.

The operative words here are Silt and Loam.

I think part of why this works so well for me is I am really adding the sand to a silt rich layer that is 60 to 70 percent silt or so and probably no more than 30 to 40 % clay at most. Cut in half that could easily get below 20% clay content, I.E. my sand layer acts sandy.

The clay horizon is a B horizon from 7 to 14 inches deep. I suspect if I had pure clay B horizon and I added sand, and mixed it in when moist, the result might be similar to concrete when dry.

 https://www.southernliving.com/garden/grumpy-gardener/garden-myth-busted-dont-add-sand-to-clay

Above link seems to support my concrete hypothesis.

https://www.quora.com/What-happens-if-you-add-sand-to-garden-soil

Above suggests adding expanded shale and compost instead. Wonder what expanded shale costs per cubic yard? Wonder about other common neutral mineral amendments like Vermiculite, Perlite, and Pumice?

https://www.gardenmyths.com/sand-and-clay-dont-make-concrete/

Here is an article  suggesting that it doesn't make concrete. Could be interesting to test. Dig out some of my clay subsoil. Mix with sand.

Also saw somewhere in a scroll down that natural sand is preferable to washed sand.

My original sand source is a fine natural sand. Though I have bought washed sand before in smaller quantities for native plant gardens. It worked ok for that purpose. Not quite sure what the potential new sand source is in regards to naturalness.


Steve Solomon famously added many inches of sandy loam to his garden- but the operative word being loam. He added good sandy topsoil to his garden.


So my thoughts after just a little more googling:


What you are adding it to might matter. Silt is not clay, loam is not clay, clay is clay. Silty clay loam is a mixture of silt and clay and probably some sand.


Type of sand might definitely matter. Natural sand containing some silt and organic matter might be better (what I used to get).


Differences of opinion on what happens when you do mix real sand with clay. This could be caused by differences in what kind of clay soil. My top 7 inches isn't even in the "Clay" part of the soil pyramid. Its got more silt than clay. Also there are different types of clay (I didn't do well on that part of soil science class, but I remember there were a lot of types of clay).

There may be better alternatives depending on what your soil really is and what is available and how big an area.


Bottom line: do a small test area before committing to too much of any soil amendment. Except maybe exceptionally high quality compost. Whatever substance it is it just might not be right for your garden.


So, my thoughts, if I shell out $120 for a new load of a new kind of sand I am going to use it the same way I have been, as a surface layer, and it will be a test load. I will not buy say four or five loads of the new sand before I know if it works as good as the old sand.





 
gardener
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Location: Vilonia, Arkansas - Zone 7B/8A stoney, sandy loam soil pH 6.5
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Sounds like you are getting a good handle on your soil type William, this is the actual definition of "Loam"

a fertile soil of clay and sand containing humus.
synonyms: earth · loam · sod · ground · dirt · clay · turf · topsoil · mold · humus · marl · dust

geology
a soil with roughly equal proportions of sand, silt, and clay.
synonyms: soil · topsoil · loam · clay · silt · dirt · sod · clod · turf · ground · terrain

Having silty clay loam is what allows you to have good success by making additions of sand.

If you had pure clay you would find that adding sand made what we like to call pottery clay, a substance that turns into bricks when dried in the presence of high heat.
The difference between silt and clay is a matter of microns, Grain size is classified as clay if the particle diameter is <0.002 mm, as silt if it is between 0.002 mm and 0.06 mm, or as sand if it is between 0.06 mm and 2 mm.
Soil texture refers to the relative proportions of sand, silt, and clay particle sizes, irrespective of chemical or mineralogical composition.

Because you have enough silt, the additions of sand are doing wonderful things for your soil as you have noted. Brava ! keep up the good observation.
A coarser sand will only be helpful, it will allow for more water infiltration (think of this sand as a fine gravel and relate that to what you would see in a stream).
The larger channels will also harbor many more bacteria, fungi and the other microorganisms and those will help feed your plants by breaking the minerals out of the different particulates that make up your soil so your plants can make use of those nutrients.

Redhawk

Adding organic matter to clay allows the super fine particles places to adhere and thus clump in a manner conducive to water movement. Clay particles are so fine that when there are only other clay particles, they compact so tightly that water can take weeks to move even 1 inch from gravitational effect.
 
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