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Container drainage hypotheticals

Posted by DHLCAL none (My Page) on
Tue, Apr 29, 14 at 4:19

I understand that:

A) In a normal container situation with a commercial potting mix, there will be a perched water table (PWT).

B) However, there isn't a PWT issue if we are talking about a raised bed without a bottom or a Smart Pot-type fabric pot sitting on soil (as opposed to, say, sitting on concrete). In these cases, the water will be drained into the ground.

So, I am wondering what happens if I have:

1)A plastic container with one or two large hole (maybe like 4 inches diameter) or numerous smaller holes (say, like 40 0.5 inch holes) cut or drilled into the bottom and the container sits on soil.

If the potting mix in the container is in contact with the soil below through the holes, would the water drain into the ground below more like the raised bed or Smart Pot situation? Would we have to make sure that the soil sitting under the pot and in the immediate surrounding area has more or less the same texture as that of the potting mix (or maybe exactly the same texture as the potting mix, achieved by spreading a layer of the potting mix under the pot) so there is no clear break in between the potting mix and the ground)?

2) A Smart Pot fabric container sitting on a deep pan with a diameter significantly larger than that of the Smart Pot filled with 3 or 4 inches of potting mix (or just soil?), with the pan itself then placed on concrete.

Would the fabric container wick the water into the potting mix or soil in the pan, thus basically pull all or a part of the PWT into the pan below the Smart Pot?

I am thinking about this because a good portion of our small yard, particularly the parts of it that has the best sunlight, is covered by concrete. I am wondering if the above can help moving the PWT outside of the Smart Pot itself and into the pan below.

Follow-Up Postings:

RE: Container drainage hypotheticals

Perhaps it is already known in these forums, but I liked this article from Oregon State University.

I think your observations make sense, but perhaps rather than trying to expand the bottom reservoir, just adjust your mix with something to enhance water retention? More peat perhaps.

This line was interesting:

"After containers are saturated and drained, 45 to 65% of the container should be filled with water."

Perhaps with a high permeability smart pot shoot for the high end, and 65%. That could be roughly gauged with a bathroom scale. A pint's a pound the world around.

Here is a link that might be useful: Physical properties of container media

RE: Container drainage hypotheticals

  • Posted by tapla z5b-6a mid-MI (My Page) on
    Tue, Apr 29, 14 at 18:08

We talk about pot-in-pot or pot-in-trench type growing all the time. Having a "bridge" between the soil in your container and the earth (or a soil 'reservoir below the main pot), employs the earth (or the second pot) as a (giant) wick, which from a hydrologic perspective turns your container into a mini raised bed.


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