Is it because of cost?

I have a 12 inch deep that i filled the first 6 inches with topsoil and then filled it the rest of the way with Mel's Mix.

I haven't had a problem with it.

HTH!
V.

Definitely not the cost, I calculated the cost of the vermiculite (5x3.5 ft3 bags) at about $120, which is cheap at 1/3 the 6 y3 we need for my beds. And less than $100 for each of the peet and compost. That's about $300 for all the growing material I need. Cheap!

The top soil would be to appease my family. Thank you Veganmom for letting me know it would work if I cave.

Here is a link that might be useful: Sinfonian's pics

I don't see a problem with it. The only potential problem with top soil is that it is an incredibly generic term and can mean anything. It might be soil very suitable for growing things and it might be garbage.

The top soil will also almost certainly have a lot of weed seeds in it, just the nature of the beast.

However, in a 15" bed it really doesn't matter. Do as you were planning and place it on the bottom as filler. Even if it were the worst soil on earth, 6" of it would still give you 9" of great growing area which is enough for just about anything. If it does have a lot of weed seeds they would be buried deep enough they would be unlikely to ever emerge.

If the top soil appears to have some clay in it it will be a water and nutrient sponge. If this is the case you may want to mix it right into the mix for this added benefit. Then again you live in Seattle so I don't know that water retention is all that important to you.

Anyway, my point is you aren't going to hurt yourself if you use it to appease your family.

I suppose I do wonder why your family would be so controlling as to not let you just 'try something new', but that's really none of my business.

Good point about being controlling. They're not. They just have 150 years of combined garden experience to my 0, so they are trying to help. They just "know" better is all. As for weeds, they don't believe any garden is weed free because they always use or amend top soil.

Three way mix will work fine as long as you ammend it with some good compost and make sure it's not too acidic. Steve Solomen's organic fertilizer recipe (should find with Google) will also help if your 3 way mix isn't a particularly good batch. Regular use of a weeding sickle to slice down the weed seedlings just below soil level quickly solves the weed seed problem and is fast (5 mins for a 4 x 4 bed).

Have fun with your new garden and let us know how it goes.

I don't think it would be a problem for me in western Oklahoma, but I remember that drainage was my #1 concern in Federal Way. But with 6" of mix on top, shouldn't be a problem.

Sinfonian, what did you end up doing? Since this is essentially what I'm struggling with now... how deep to go and how/whether to tinker with the "mel's mix" and still have good results.

I'm concerned about the cost a little, too. But I like those deeper beds.

I'd just state that I was trying a new method that is different than the way they've always done it and go with the peat/vermiculite/compost anyways. See how it goes for a year.

Maybe you can teach THEM something!

Kristimama, the short of it is I stuck to my guns and did straight Mel's Mix. Though the second batch (did 6.5 cubic yards in two batches) included an extra half yard of compost because of the griping that it wasn't going to work with all that vermiculite and peat moss.

The long of it is check out my blog. I chronicled the whole affair and then some...

Here is a link that might be useful: Sinfonian's garden adventure!