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I'm experimenting with different soil mixes for my terraces. I'm having to build from the ground up (so to speak) since I have bright orange (red) clay and nothing else.
I bought a scoop of so-called landscape mix from my local co-op. It is equal parts topsoil, mushroom compost and sand, and it was screened. I spread it 3" deep over a 3" layer of mushroom compost which is over red clay scraped clear of grass and double-dug very choppily with a shovel. I did NOT mix it because I wanted to SEE what happens with rain. We got about 20 minutes of rain. And the mix is mud. It's sloppy. And the landscape mix is holding a LOT of water. The compost is draining just fine and staying DAMP but not SOAKED.
I don't like it. Good soil should drain, shouldn't it? I'm wondering if screening it removed all the big chunks and made it too uniform in size. I'm wondering if this is the same problem people find when they till too much and remove the structure of the soil.
The soil mixes I like best are 2 (or 3) parts mushroom compost and 1 part topsoil, mixed roughly with a shovel. The compost is chunky and has many different sizes in it. This is a good thing because air can get down into the soil. Right? Air can't move through the landscape mix at all when there is water. it's mud.
Or maybe I'm wrong on this one. I'm still learning. And I've been choosing plants that tolerate low water once established. So maybe I'm not seeing the big picture here.
It has been my experience that clay soil should be disturbed as little as possible. If you dig a hole in red clay soil and fill it with water, it drains VERY slowly.
I have found it best to leave the clay soil "as is" and build good organic soil on top. I garden on a slope, so have put in several raised beds to keep my good soil from washing down the slope when it rains. If you have level ground, this should not be a problem.
Thanks, Honeybee. Not worried about the red clay so much as the landscape mix which has the consistency of oatmeal when wet.
When I mix the chopped up red clay with mushroom compost I'm getting the best possible "mix." And since my whole place is terraced, I'm not worried about standing water... I'd pay a lot for standing water since we're on the side of a great big HILL!
>> The soil mixes I like best are 2 (or 3) parts mushroom compost and 1 part topsoil,
I think that, if you add enoguh compost, almost anything, including clay, becomes good soil. Bacteria, fungi and small insects or other living things eat the compost, and fill the soil with living microorganisms, that seem somehow to "know" how to maintain a structure that lets them thrive. Enough drainage to allow enoguh aeration, enough water retention to keep from drying out.
>> We got about 20 minutes of rain. And the mix is mud.
I don't know the secret successful method yet, but I know there is some trick to leaving newly-created soil ready for rain. Done wrong, everything melts into mud and then dries out into concrete.
What has worked best for me has been to "fluff up" whatever mix I just created, like clay + compost + bark fines & chunks + grit. Mix well, as if to add as much air as possible. Then, relying on the gritty and chunky and fibrous parts, GENTLY tamp the surface so as to make something that still has pores, voids and air channels, but is pressed together enoguh to hold onto itself and stay openj when rain comes. Of course it has to drain well enough to let the rain flow away before it becomes a deep mud puddle!
If I tamp gently, and it RESISTS a little, instead of sinking down into a solid mass with no air, I suspect it may survive rain and support some plant roots later. If there is any grating sound, I know I bought enough grit or crushed rock to support some structure.
But nothin is as reliable as using 2-3 times as much compost as I thoguht I could afford!
LOTS of organic matter (compost) seems to create a safety margin, soaking up water or soaking up fine clay particles but still holding them apart in an organic matrix. My theory is that most compost has fibers or medium-small particles that help keep the damp mix "lofted" instead of dissolving into mud.
Something like keeping clay and silt particles separated, so they don't clump together and form large, solid airless masses.
But I never have "enough" compost. You have the right idea: 50-66% compost! Yeah!! That'll work.
I try to get by with 20% compost, just so I can make more soil given a fixed budget and a very small c ompost heap. That does NOT always work, and by next year, that "20% compost" has been digested down to "1%" and the clay returns. I think that if I want to keep 12" of clay-based soil usable, I probably have to add at least 2-3" of compost every year, whether I turn it in or not. Since most of my compost comes from Lowes in 1 cubic foot bags, I don't often get enough spread. Coffee grounds from Starbucks would be great if I could get 5 times as many without getting caffiene jitters.
My theory is that, if you don't have 40-50% compost available, you have to create "mechanical structure" with solids like crushed stone, grit, or bark fibers and chunks. Or peat, if you don't mind your little bed holding 100 gallons of water at all times, and not much air ever.
MAYBE even coarse sand helps, if you have lots, and it is very coarse. I din't think that fine sand helps clay much, until after you get 30-40% compost into the clay. Medium sand, MAYBE, helps make the clay more firable, as if a big ball of snady clay is more willing to split apart when forked, like along the surfaces of sand grains, whereas pure clay sticks to itself like glue.
If that is true, sand may help you break up the clay and mix more thoroughly the things it needs: compost, grit, fibrous bark.
THANKS everybody. I think the structure of the so-called landscape mix was destroyed by the screening process and that's why it turned to oatmeal. In future I might use it -- mixed with 2-3 parts mushroom compost. Or just go back to 1 part topsoil plus 2-3 parts mushroom compost. Half the cost and I don't see the advantage of the landscape mix vs topsoil when mushroom compost has compost and structure anyway.
The mushroom compost I'm using is stall leavings from Lexington KY Horse Park mixed with a little peat. You can still see the structure of the disintegrating straw and horse leavings and it's gritty, chunky. It shrinks down a lot over a year's time. But it's been used for mushroom growth so it's no where near HOT like fresh manure would be.
Excellent point, honeybee. Except once, I'm actually bringing it home in my truck, so I get to put my hands in it before I buy it. My favorite landscape supply source got a sucky load of mushroom compost in and no one is buying it after seeing it. I'm still buying other stuff from them and they asked why no mushroom and I told them -- it's too raw, horse poop is still in big balls, its not incorporated at all, the chunky bark is still individual pieces of bark. It's not ready to be used. It's just separate ingredients in the same pile.
They offered it to me at half price, delivered free, and of course I said yes. I had them put it in a pile out of the way. After the next rain, I covered the pile with a tarp and I'll get back to it for spring planting.
When I saw my local dirt yard's "compost" from Cedar Grove (wood chips and sawdust wiuth a very faint whiff of a tiny amount of compost will diluted), I gave him the hairy eyeball.
"Yeah, we call it 'mulch'" he admitted.
This is the same place I bought some "garden soil" from, onc e. In the yard it looked and felt well-draining, even slightly gritty. What they delivered had a lot more clay and no grit or sand. Oh, wel;l, they waived the dleivery fee when I pointed out that I lived about 2 blocks away.