Water perches in soils when there is a layer of soil with particles more than 2.1x the size of the particles in the layer above it. An example would be water perching in silty loam situated above a layer of coarse sand mixed with gravel. You can actually approach this using a couple of different strategies, but I'm wondering if the planter will sit on the deck, and, if so, is the deck reinforced to handle the weight?
If it is, you might consider layering the soil like this:
6" of 2 parts of #2 granite and 1 part bark (granite: grower grit or #2 cherrystone if either are available)
6" of equal parts of unscreened Turface, granite, and bark
6" of equal parts of unscreened Turface, granite, bark, and sphagnum peat
6" of 2 parts of unscreened Turface and granite + 1 part each of bark and sphagnum peat
After you lay down the first layer, smooth it out and add 1" of the next layer and scuff up a bit and mix it into the top 1" of the lower layer. Repeat as you work through the layers.
If you wanted to use ballast, you could use a more water-retentive soil, but the ballast would reduce the volume of soil available for root colonization, a blessing or a scourge, depending on your perspective.
Al
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soil for container 3-3-3-5
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