HELP! Clay Soil in concrete block raised bed on concrete slab

LouisePace(zone 6 Phila, PA)May 10, 2011

Newbie here with ZERO gardening experience. Please excuse me if this is in the wrong forum, has been asked/answered before, or is a stupid question!

I need soil help!!

So I'm renting an apartment with a back yard that is a concrete slab surrounded by a concrete block wall. There are existing planters made of concrete block along one corner which are filled with dirt. The planters are approximately 16 inches high and 2 feet wide. One side is 66 inches long and other is 86 inches.

There appear to be major drainage problems with the soil that is currently in there. Originally, I had intended on putting down rocks on the bottom to improve drainage, which I read somewhere. But after starting to dig, the top layer seemed to be a decent soil but after removing it the bottom half appears to be clay. It's reddish, sticks together, cracks form on top when dry and water pools when wet. It's also of unknown contamination so I don't want to grow any edibles in the soil.

My question is what to do about the soil. Do I dig it all out? Do I mix it with something? Do I leave it as is? What about added rocks to the bottom? (I just read on this site adding rocks to the bottom of a container is bad-- Do I treat the beds as one large container?) I'm only going to be here for two years and do not want to spend a lot of money on soil that I'm going to leave. I also don't want to waste my time trying to grow in impossible conditions.

I'm currently growing herbs in large pots filled with organic potting soil. My boyfriend wants to half bury the pots in the beds and grow non-edibles in the existing soil around the pots. We're really not picky about what we want to plant other than the herbs. Are there plants that would survive/thrive in the beds as is?

Any advice is greatly appreciated, as I am a newbie! Thanks!!

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"I just read on this site adding rocks to the bottom of a container is bad"

It is just a waste of time and root zone space but it is not "bad". One must understand how golf greens are built before saying it is bad to add rocks at the bottum for drainage.

Rocks at the bottum will work if you add a layer of fine rock on top of that then another layer of sand that way dirt will not fill in the rocks. You may be able to keep the dirt in there if it is the only choice. If you want to get rid of that clay/dirt and add some good soil that would be good.

    Bookmark   May 10, 2011 at 9:35PM
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calistoga_al ca 15 usda 9

If I am reading your post correctly your raised beds have a solid concrete bottom making raised beds into concrete containers without drain holes. The solution to your problem is to make holes through the bottom concrete or probably easier to make holes through the cinder blocks at the base where they contact the concrete. I am assuming the cinder blocks are mortared together. Without providing drainage it won't matter what you use for soil. Al

    Bookmark   May 11, 2011 at 9:33AM
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tapla (mid-MI z5b-6a)

Al is right. What differentiates between a raised bed and a container (hydrologically speaking) is whether of not you have the wicking action of the earth as an aid to drainage. In you're case, you have the impermeable concrete slab that essentially duplicates the effects of a container bottom.

With golf greens, the gravel beneath the sand the grass is rooted in essentially turns the green into a giant flower pot and ensures that when it rains the soil above can remain at near field capacity but doesn't puddle.

I don't think we need to use the words bad or good when we discuss "drainage layers" in containers. There are even degrees of effectiveness based on the size of the material in the "drainage layer. For example, a layer of peastone on the container bottom would be ineffective at increasing drainage, only serving to raise the position of the PWT in the container, while a single large rock that NEARLY fills the bottom of the container can be extremely effective at reducing the volume of soil that CAN be occupied by perched water, thus reducing the o/a volume of perched water a container CAN hold at container capacity. Those of you using heavy souls would be well-served to use an extra large container and simply overturn another pot that fits nicely into the bottom of the pot, which will also greatly reduce the volume of soil occupied by perched water.

In order for a "drainage layer" to be effective, the particles in the drainage layer should be no larger than 2.2X the size of the layer above. If it is larger, water will 'perch' in the soil above the intended drainage layer an d be reluctant to move downward.

It's very difficult to 'amend' clay so it drains w/o adding a SIGNIFICANT volume of larger particles (like coarse sand) and getting it well-homogenized. I think the best course would be to remove what you have and make your own soil from very coarse sand or a fine CALCINED clay and organic components.


    Bookmark   May 11, 2011 at 10:16AM
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LouisePace(zone 6 Phila, PA)

Thanks everyone for your help! It's clear to me now that I have two big concrete containers. I'm thinking that putting holes in the block is the only way to go. What is the minimum number that would be helpful? I'm thinking it's going to be very difficult since I don't have a hammer drill.

Al, I know you are very knowledgable and well respected. I'm so happy you have offered your advice.

I was wondering if you could recommend a specific soil recipe? Again, I really appreciate how helpful everyone is, since I'm a newbie. I really want to do things as best as I can!

    Bookmark   May 12, 2011 at 11:54AM
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tapla (mid-MI z5b-6a)

I got your email, Louise - Sorry, I've been away at a bonsai show & visiting friends/family.

This is a tough one. As I understand it, you have a concrete block planter constructed atop a concrete slab, with no way for the water to make its way into the sand below the slab. Drilling holes in the block walls won't improve drainage. The only 2 ways to effectively improve drainage would be to make sure there is a 'bridge' between the soil in the planter and the sand below the concrete so the water can percolate through the soil, or to make sure the soil you use is porous enough that it holds little or no perched water. This would mean that your soil particles would have to be large & stable, insofar as their structure is concerned. Even the soil you see me holding above would be inappropriate for your application because that soil depends on the wicking action of the earth (in RBs) to ensure it doesn't remain saturated after rain/watering. It looks great, but if you have high standards it would be a disaster in a container or in your app.

I'm thinking that about an 80% fraction of a mix of all purpose sand and all purpose gravel and 10% each of pine bark fines and Michigan (reed/sedge) peat or good compost would probably be the direction I would head. In subsequent years, I think all you'll need to do is mulch with pine bark to keep the soil healthy. If expense isn't a major consideration, you could use Turface MVP as the base ingredient for your soil. Topsoil and fine sand just isn't going to drain w/o help.

The alternative would be to remove the old soil and cut a hole in the slab so water can drain from the soil in the planter, but it would have to be a hole of considerable size - preferably at the lowest point on the planter's bottom.

If you have pictures, it's possible that something else might be devised that will allow the water to move out of the soil via some sort of passive wicking mechanism that we/you could devise, but I'd have to see the application in order to envision the possibilities.

There might be a way to employ a false bottom by placing FRP panel on top of cement blocks that are resting on the bottom & building it like a self-watering container in reverse - where you employ a wicking column of soil to REMOVE water instead of supplying it, but I don't know how far you want to chase the issue? ;-)


    Bookmark   May 15, 2011 at 3:41PM
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