Clay soil in a raised bed--how to fix the problem

ralleia(z5 Omaha, NE)May 26, 2011

A decade ago my brown-thumb father constructed nice raised bed out of timbers. The bed is 2 1/2 feet tall, about 40 feet long and eight feet deep. Then he had someone deliver a truckload of "topsoil."

When I went out to help him in his garden I was horrified to find not topsoil but some of the worst clay I have ever experienced in Midwest gardening. The stuff is rocklike when dry, dense and unworkable when wet, and let us not even talk about the drainage.

How to fix this situation now? Since the stuff doesn't reduce in volume through decomposition, I think the only thing to do would be to take out up to 6 inches of the stuff and then do lasagna composting?

My father is 70--if there is a way to fix it without digging out some of the clay, I would be happy, but am not sure any other way exists in a raised bed.

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What did yuou expect that "topsoil" to be? Many people think "topsoil" will be Loam, a specific soil type that is not all that common while "topsoil" is defined as the top 4 to 6 inches of soil from someplace. With some luck that "topsoil" might even have some organic matter in it.
What that clay needs is a lot of organic matter, a residual level (humus level) of between 5 and 8 percent. You could build a Lasagna style garden on top of what you have and that will, eventualy, get worked into the clay so that clay will then be workable.

    Bookmark   May 26, 2011 at 7:06AM
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ralleia(z5 Omaha, NE)

1) The clay hasn't reduced in volume such that sufficient organic matter to improve the soil composition can be added. The bed is essentially a CONTAINER next to a lawn--I can't add to an already-full container.

2) My brown-thumbed father bought the "topsoil." I'm not fool enough to buy unknown "soils" without touching the stuff. What he bought was probably clay dug out from some housing development--not topsoil at all.

Could someone who will read the OP well enough to understand the predicament comment on whether he is stuck removing hundreds of pounds of the stuff or not?

    Bookmark   May 26, 2011 at 9:03AM
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jean001a(Portland OR 7b)

As was said "You could build a Lasagna style garden on top of what you have and that will, eventualy, get worked into the clay so that clay will then be workable."

So, if the planter is full then, yes, you will need to remove some.

    Bookmark   May 26, 2011 at 12:33PM
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A quick fix to get you through this year's growing season might be to simply add organic matter to just the root ball areas.

That won't solve all the problems, but it's a lot easier to do it that way than to remove everything.

Over time, you can replace more of the clay with better soil.

That's not a perfect solution, but I shudder to think about having to remove/replace all that clay at one time (and even the lasagna composting method will take several months to improve the garden area).

    Bookmark   May 26, 2011 at 1:29PM
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rhizo_1 (North AL) zone 7

No, tn gardening, that is NOT a good temporary solution but would be very restrictive to any plants that might be grown there.

Removal of some of that soil volume will be necessary. The addition of organic matter of some sort will be the 'fix' for this situation. This can be in the form of compost, bark fines, even peat moss, etc. I'd try to incorporate as much as possible with a garden fork and/or spade.

Then, it could be planted into right away.

    Bookmark   May 26, 2011 at 2:02PM
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I beg to differ. If the root ball is growing in "good" organic matter, then the plants will be fine. Plants don't care too much about the row paths.

Perfect = no, but certainly well within the realm of good enough.

So, to more completely answer the original question: No, some digging of the clay will be necessary. However, complete removal of all of that topsoil is not necessary. In a round about way you can almost think of it as container gardening. The bigger the hole, the better.

    Bookmark   May 26, 2011 at 7:38PM
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Fix it in smaller steps.

You have a very large bed. Dig up a 5 ft section 6 to 12 inches deep and pile it on the adjacent 5 ft section. Repeat

Amend the dug up sections with compost/amendments, till in and plant stuff this year. One section could be a square foot garden.

Find a way to dispose of the piled up sections. Fill low spots in the yard. I have even heard of people putting 50 lbs of clay in the garbage can every week until it is all gone. ;-)

Repeat next year.


    Bookmark   May 26, 2011 at 9:55PM
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anubis_pa(z5 PA)

Maybe this was ruled out already but would it make more sense to build the raised bed up from 2.5 feet to 3 feet and add the compost on top? Rather than remove clay, give more room to add!

    Bookmark   May 26, 2011 at 11:29PM
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Hi anubis,
That is a great idea ;-)

    Bookmark   May 26, 2011 at 11:48PM
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And i was just gonna pet anubis's back for his lazy but brilliant solution. Good job.

    Bookmark   May 27, 2011 at 6:56AM
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If the root ball is growing in "good" organic matter, then the plants will be fine.
Of course, there is the fact that you're describing little "bathtubs" that will result in festering pockets of anaerobic soil as well as creating perfect little homes for root rot pathogens. Other than that...thumbs up, buddy!

    Bookmark   May 27, 2011 at 9:19AM
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ralleia(z5 Omaha, NE)

Mmmmmmmm--build it up another notch. I'll have to see if it can be done. That or the idea to do it in parts sounds workable.

I was rather frustrated when I saw how my dad had made the bed--instead of smaller beds that a person could work from all sides, he made one MONOLITHIC bed that against the chainlink fence, meaning that it could only be worked from two sides unless a person climbs up there (which is going to compress the soil, of course).

If we're going to have to dig some of the beds out, I might speak to him about subdividing the beds into five-foot sections such that they could be worked from three sides at least. The amount of work to do that is almost unimaginable though, so I might just encourage him to plant some well-behaved groundcover along the back 5-6 feet of it and only plant the front 2-3 feet for his garden. Or perhaps even dwarf fruit trees in the back, once we get the soil fixed.

    Bookmark   May 27, 2011 at 10:59AM
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tishtoshnm Zone 6/NM

To divide it, you can just use stepping stones to make the bed more manageable but also only compressing the soil in that designed spot. Gardening is an adventure!

    Bookmark   May 27, 2011 at 11:14AM
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Personally, I wouldn't plant anything along the back. Whatever you put there will be competing with veg crops for water, nutrients, rooting space, etc.
Maybe you could build small steps up either side of the planter and then put mulch or flag stone along the fence to make a 3 foot wide path along the back. Then you can step up and work the remaining 5 feet of the planter from both sides.

    Bookmark   May 27, 2011 at 11:18AM
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I'm not a gardener and I have little experience with clay as described but I would rent/borrow a decent tiller and try an experiment by tilling in lots of compost in a small area to see if it is doable.

I once got several wheelbarrows of clay-like material brought out from a small excavation uptown and I mixed in (by hand) a lot of compost as well as some peat. It seemed to work out well but the resulting stuff was used in a flowerbed. Flowers looked okay to me from what I could tell.


    Bookmark   May 27, 2011 at 12:12PM
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wayne_5 zone 6a Central Indiana

If there is room to start over, let the old area go.

    Bookmark   May 27, 2011 at 2:49PM
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haname(z9 AZ NE Phoenix)

8 feet by 40 feet? You can place wide boards across the width which would divide it into several smaller blocks that can be worked from 3 sides. Place them so that the divisions allow you to reach the middle of the section from all 3 sides The boards will disperse your weight so that walking on them will not cause as much compression.

Clay isn't a bad thing actually. It holds water and nutrients quite well, but just needs a little help. This could potentially be a really great garden. If you can incorporate a lot of peat moss or coconut coir, it will improve the tilth of the soil immediately and permanently. After that, all you would need to do is keep it well mulched and you'd have some great soil that would need a lot less supplemental water.

If I were in this situation I would set up some additional beds (if there's room) to take up the excess 'topsoil' so you can add plenty of peat moss. It will be hard to mix peat moss all the way down to the bottom in that main bed so I would mix in as much as I could in the top section, install some WORM TOWERS then add some composting worms as well as some nightcrawler type worms that will burrow deeply and help build up the lower levels and improve the drainage. Keep the worm tubes fed with organic matter. (Worm tube instructions at link below)

I would then plant a mix of deep-rooting cover crops (comfrey, alfalfa, clover, or whatever would work for this season and start growing now in your climate) which will root deeply and add organic material to those lower levels. Cut them down just before they begin to flower, and leave it all there on top as mulch. If necessary, spread it out a bit to let it dry some first, and chop it up a bit if you feel so inclined. Keep the bed mulched at all times. By next spring I think you'd have a great, very fertile bed ready for planting. Just open up holes in the thick mulch and put your seeds or plants in the soil. Leave the openings in the mulch to let the plants grow.

Along the back, plan to plant things that you can train up the fence (melons, cukes, beans, tomatoes, etc. If parts of the plant are allowed to grow out the other side, you can harvest from there. Large fruits will need support as they grow which can be done by making little hammocks from pantyhose tied to the fence.

Please keep us posted on how things go!

Here is a link that might be useful: Build a Worm Tower

    Bookmark   May 27, 2011 at 4:06PM
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wayne_5 zone 6a Central Indiana

I have raised beds.....7, 8, 12 feet wide. No kidding, I even walk on them. They are about 40percent good top soil; 30 percent sand; 30 percent Indiana peat moss [local]. It doesn't hurt to walk on this very loose structured soil.
Now hard clay is different. It would probably need walked on to bust the clods!

If there is good topsoil under all this overburden, I believe I would get rid of the "bad" stuff or plant red raspberries in it.

    Bookmark   May 27, 2011 at 6:20PM
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You can have a good garden there, for sure. I've had success growing roses and perennials in a region notorious for its clay hardpan by building lasagna-style beds right on top of the hardpan. But I didn't wait for the beds to decompose--I excavated holes down into the clay, chopping channels radiating from the sides of the holes, added worm-attracting amendments and a bit of bagged topsoil, and have kept everything mulched heavily. By continuing to add stuff that rots on top (barn litter is good)and keeping it heavily mulched, I'm continuing to create something the plants can grow root systems in, and also keeping the clay moist, allowing root penetration. So far so good, no dreaded bathtub effect, and it cuts down on watering.

    Bookmark   May 28, 2011 at 6:52AM
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Posted by gargwarb (My Page) on Fri, May 27, 11 at 9:19

thumbs up, buddy!


I'm not your buddy...and like SpiderLily7 works.

    Bookmark   May 31, 2011 at 12:44PM
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Excavating holes, digging out channels for drainage, filling with compost and mineral soil and then putting lasagna on top is a lot different from. "Dig a hole and fill it with organic matter", which results in the bathtub effect. (although without seeing Spiderlily's exact design, it could work or not. I would say give it some time before making the final decision.)

/Come on, be my buddy. I'll let you play my Wii and kiss my sister.

    Bookmark   May 31, 2011 at 1:40PM
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/Come on, be my buddy. I'll let you play my Wii and kiss my sister.

Why do I chuckle when I read a thread on the internet that mentions filling a hole with organic matter and your sister?

    Bookmark   June 1, 2011 at 5:19PM
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Because you've evidently met my sister.

    Bookmark   June 1, 2011 at 6:26PM
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