What to put below Mel's mix in a tall raised garden bed?

I am building a set of 30" tall raised garden beds (equivalent of 44' x 4' in space, which will require about 16 cubic yards of fill-in).

It would seem excessive (and expensive) to fill it all the way with Mel's mix. So I just want to put 12" of Mel's mix at the top, with some sort of a filler soil below it that will be good for draining.

What is the best filler to use below? So far I'm contemplating something like do the bottom 12" in sand, then 8" of a topsoil over that (loam soil + compost processed through a 1/2" screen). I don't want to do it all the way in sand though, because then I'm likely to have leftover sand which is a pain to get rid of. And don't want it all the way topsoil since it compresses like crazy, and doesn't drain as well after a few years.

However, preferably I don't want two different mixes, so maybe I just mix something together. Or alternatively mix in vermiculite.

What says the collective wisdom on this one?

Deonb, you definitely do not want the expense of 30" of Mel's Mix - the cost of that would be prohibitive, and it would be unnecessary. The top 6" to 8" of MM would be all you will need.

I have beds with the bottom filled with "topsoil" I purchased from a local Topsoil & Mulch company, and you are correct about it compacting. The topsoil I got had a lot of clay in it, and it has heavily compacted over the years. The top of the beds with Coir and Vermiculite added to mix remains loose and friable, but below that is very compacted and tends to hold water too long.

You will not want a high sand content fill as that would dry way too fast for good moisture retention. Since you are looking at 2' of fill before adding the 6" of MM, you might want to look at either the Hugelkultur method, or a topsoil mixed with loose organic material such as peat moss.

Thanks! Yeah I saw this a while ago, then forgot all about it when I discovered SFG. But it's a great idea to combine the two in this case.

Deon, Clean washed sand is recommended by the SFG Foundation because it does drain well. Hugalkulture, being organic, will eventually break down and the height within the bed will drop over the years.

I kind of like what OhioGardener mentioned about adding peat moss, but I would suggest adding it to sand or a sand/top soil mix.

My raised beds are 24" deep so that I do not need to squat to tend them. I filled the bottoms slightly more that one foot with composted steer manure then topped with MM. As the steer settled I added more MM. Every second year I scoop out the MM, turn the bottom material then refill the beds.