Hopefully the two soil types are similar ENOUGH that they "connect" and form some capillary channels that allow enough drainage that the top 3" don't get so waterlogged that they go too anaerobic ("Hypoxic"). I like the words "hypoxic" and "hypoxia" because they remind me that lack of oxygen is "toxic".
Also, I doubt that many garden containers get totally anaerobic - a pot would have to be pretty deep and clayey, or totally waterlogged, to allow NO oxygen to penetrate. "Anaerobic" and "anoxic" mean "NO oxygen".
Maybe, if you do add any more water, even misting the surface, do it with dilute hydrogen peroxide. That releases oxygen into the soil. Dilute 3% H2O2 1:32, like 1 Oz per Quart or 1.5 tsp per cup.
Also, consider bottom-watering the bottom 3" layer, and only misting the top 3" layer.
Seed flats are most at risk because they don't have big active root balls able to suck water out of waterlogged soil and transpire it out their leaves.
Also, shallow flats are more vulnerable to perched water because they are so shallow that a perched layer 1-2" tall may be the entire root zone.
>> I hope that I remember Rick's information and water them carefully
I'm not sure how to remedy it if the top layer is already water-logged. Keep it on a heat mat and run a fan, so it evaporates faster? Carefully poke little slips of cotton flannel or cotton butcher's twine down the side of each cell 4" to 6" deep to wick water from top layer to cotton, then from cotton to bottom layer.
Al (Tapla) pointed out that you can squeeze out SOME perched water by tapping the pot or tray on a hard surface. The inertia of the water causes it to try to keep moving down when the tray goes 'thump'. I imagine that the damp soil also compresses slightly and squeezes a tiny amount of water out. I can also imagine flinging muddy soil all over the kitchen table! I've tried this and gotten some water out the bottom, and some soil out of the top.
P.S. If you had one big flat instead of many small cells, you could remove the perched layer by tilting the flat so that one corner was several inches lower than the other corner. All the perched water would flow into the lower corner and "overflow" into the lower layer, since any given soil mix will only support a certain small depth of perched water layer. Or stir the soil in that corner enough to mix the layers. That establishes a capillary channel that will drain the entire top l;ayer into the bottom layer and eventually out of the tray through bottom holes.
You can dry out the BOTTOM layer of all perched water by placing the tray or pot on top of a terrycloth towel or cotton flannel. If the terrycloth touches the soil mix through the holes, it will establish a capillary connection and wick water out of the soil until the towel is soaked.
You can keep the cotton towel cleaner if you lay absorbent paper towels over the fabric. But the paper towel has to touch soil through the holes in the tray.
If you need to wick a LOT of water out, or, like me, want continuous protection against obsessive overwatering or excess rain, set the tray on top of a cotton flannel pad, and lay a long wick of cotton flannel under the pad and let it dangle DOWN a foot or so lower than the tray. The pad will wick water out of the tray through capillary action, then the dangling wick will drain the water out of the pad, through capillary action PLUS gravity.
Next year, you could have the advantages of two soil layers without that capillary-gap / two-perched-layer problem by letting the two soil types intermingle a litlte. Drop in the bottom layer but don't smooth or tamp it. Add around 1" of the top soil type, and stir each cell just a little, SHALLOWLY, so the top 2" intermingle somewhat. Then add the last 2". Now the transition should be gradual enough that the two layers are connected by enough capillary channels.that the top layer won't have its own perc hed water table.
I wish I could master the art of not over-watering! Up to now, I've treated seed flats (cells) the same way I treat big containers: top-water until water comes out the bottom, "so that salts don't build up".
DUHH! The two situations are totally different. Big containers are watered and fertilized for months or years, but I guess that if you "prick out" small seedlings with just a few leaves and one thread-like root, you never need to water it again after the initial dampening.
