Take my advice for what it is (a newbie as well), but it is my understanding that without a barrier of some sort, the grass will be able to come up and through (or if not the grass, then some larger weeds). The theory is that you in fact do not want your roots growing into the existing soil, especially if there is grass on the surface. The underlying soil is presumably less nutritious than the mix above, and potentially has soil diseases present. Further, I doubt the tender root system is going to be able to effectively penetrate an existing mat of grass. Much like everything else in the universe, roots follow the path of least resistance and tend to move towards better soil. In this case, it would be the Mel's Mix.

That being said, if you lay down four or five layers of newspaper first, it should kill most of the grass and weeds underneath and still allow some downward growth. Just not sure that is really what you want though.

Working backward, as it were, in your questions, I would have to agree with m lorne, the grass, with its mat of roots will for an impenetrable barrier to the roots of your garden plants. It will also need, at the very least, to be "smothered" somehow (newspaper, cardboard, etc.) or it will grow up through your garden and take it over in short order. They will probably still try to sneak up from below around the edges if you don't use a solution like landscape fabric.

My advice would be to remove the sod, leveling the area inside the boxes to get the maximum depth of soil mix, lay down the landscape fabric, which will allow moisture to move through, but not roots. I feel that the roots invading from outside are a much worse problem than the roots inside, in their nice, custom made, soil, being unable to get out. I had amazing results with my plants confined to about 7" of "Mel's Mix" last year. If you are concerned about extra dept for crops like carrots and leeks, one great solution is to make, out of much thinner boards (say inch or half inch, quarter-inch would probably work) small 1x1 or 1 x 2 sq. ft. boxes between 4 and 6 inches high and put them on top of individual squares in your garden to add depth. I grew some very nice leaks last year that way, I planted them, put the "collar" onto the square, and added soil as they grew. For carrots you would add and fill the "collar" and then plant the seeds, although, as Mel points out, if your carrots do outgrow the depth of the beds and reach the landscape fabric they will just grow in an L shape, slightly funny looking but perfectly good to eat.

As for the vermiculite, my advice, mirroring the advice of many others here is search, search, search, and pick up the phone, as Mel advises in the book. I finally found mine at a local nursery for around $27 for an extremely bulky, compressed, 8cf. bag, after long and disappointing searches of "the usual suspects" and finding prices on various size bags that would have been well above the $25 per 4cf.

If I had to sacrifice any of the Mel's Mix ingredients for economic expedience the peat moss would be my first choice. It tends to be close to the cost of the vermiculite, at least in my area, and seems to break down and loose its beneficial properties faster than the vermiculite. Using a bed with Mel's Mix in the long haul you will end up with a mixture that is progressively more compost every year, as the peat moss, which was originally plants, breaks down, dissolves, and looses its water holding properties, the vermiculite will slowly break down into smaller particles but will continue to "work" for a long time, and the compost will be replaced as you add more each time you replant a square. I could only find medium vermiculite, and plan to add a little new vermiculite each year to keep the levels up, as I have found that it does wonders both for the Mel's Mix, and in the old days, when I followed Mel's pre-raised-bed directions, that involved supplementing my native soil, the vermiculite was by far the best addition then (not counting the compost into this equation at all).

Thank you for your posts. The more I think about it, the more I'm convincing myself not to skimp. I'm already in over my head from a cost perspective, so I might as well do everything possible to aid in my success.

I do think that I would rather use newspaper rather than weed cloth just from a personal preference.

If anyone has any further suggestions, I would be very appreciative. I've been reading through older posts trying to learn as much as I can.

Depending on what kind of grass it is, the newspaper may not do much.

I put my beds on an area that still has some remnants of the bermuda that I've been trying to kill for 2 years. If I didn't put landscape fabric down, the bermuda would have come up in the box and caused a mess, guaranteed. Even WITH the fabric, there's not guarantee that won't happen.

I'd certainly dig out the sod, even if you don't put down landscape fabric and use newspaper. Your plants are gonna have a hard time growing through the thick grass thatch and roots....

I should mention, for whatever its worth, that I'm located in Iowa. The only issue with my soil is the slope which is why we had to go with raised beds. The grass is patchy (thanks to my dog) and is just the traditional type of grass we have in this area (don't know the name). Reading the replies I think I'll remove the sod.

I was just a little confused as I've seen other posts on here that indicate they put their boxes on existing grass without issue. Anyone have experience with that?

Bermuda and a particularly noxious woody weed are the only things I've had penetrate cardboard or newspaper on the bottom, and both of those are noxious enough where it didn't really matter whether or not I removed the sod first, so I quit bothering with all the extra initial and just dealt with what punched through.

Early in the season last year I determined I need a couple more 4 cu.ft. bags of vermiculite. I have a relatively cheap source, but it's about an hour drive.
I was able to get a significant price break from a local Agway by telling them I knew I could get it for half the price by driving an hour. It went like this:
"How much for the vermiculite?"
"$28"
"Eeesh! That's almost worth a drive back to *cheap source*"
"You know where you could get it cheaper?"
"Yeah, it's $16.50 for the exact some bag at *cheap source*"
"Well, *pause* I could probably go down to say $19."
"Sold!"

Now if I needed a few rather than two, I'd still make the drive. I'm a cheapskate after all. ;)

I used weedcloth to suppress my grass and it worked well. I did have one interesting result though. My four kids had a 4x4 box to themselves. Three of them planted eggplant. My oldest son's did the best by far, for no apparent reason. When I pulled the plants at the end of the season, his eggplant had a long vertical root that had wriggled through a hole in the weed cloth. My regular soil is pretty good, and it definitely made a big difference with eggplant.

I'm sure you've seen this thread, but I thought I'd link to it just in case.

Here is a link that might be useful: Loose attic insulation is cheap source of vermiculite

I wouldn't put down weed cloth but I would use cardboard. It's great for a weed and grass barrier and the worms love it which will help you down the line. Just lay in a good layer, wet very well then proceed with what you plan.

Well for what it's worth I live in Alabama where the grass grows thick and the soil isn't soil, it's bright red clay! Call me lazy or what have you, but I built my two 4x4 beds with 4ft long, 1 inch thick, 1ft tall boards that just sat on top of the grass... I didn't even trench so that the boards would be partially underground. I filled them up with Mel's Mix and they did great all year last year... I had so many tomatoes and okras I was giving them away to people at work! Every now and then I get a grass stem sprouting up out of the bed (I'm talking one sprout with maximum 3 blades) but that's maybe twice a month and it's never been a problem to just pull that sucker out. Maybe my laziness will catch up to me, but since my beds are a foot deep I kinda don't think it will. Not saying you shouldn't clear the grass first... just saying if you're pinched for time like me I don't think it will matter as long as the bed is deep enough. :)