Existing dirt & Mel's Mix?

The landscape design guy filled my bed with garden topsoil. I pulled my soil samples yesterday to dry so I can test. Can I incorporate Mel's mix into this? I've been growing in containers the past 3 years and for the first time used worm castings last year and had gorgeous plants. I have already planned to mix this in to the existing dirt in the bed. As I'm not going to have a traditional SFG space wise (but still use the principles) I can't pull the dirt out. It's too large of a space.

Well, strictly speaking Mel's mix has no dirt in it. If you want to proceed with what you have, you can boost the soil with adding a bit of sphagnum peat and a variety of composts. This is what I do with my inground flower beds and it works pretty well.

When making Mel's mix the recipe as described in the All New Square Foot Gardening book, mix together by volume 1/3 sphagnum peat, 1/3 vermiculite or perlite and 1/3 compost blend made from 5 different types of compost (chicken litter, cow, horse, rabbit,lobster, homemade with lots of organic matter, etc.). Do not buy anything that says soil on the bag label. Do not skimp on the variety of composts if you want good results in your garden. I often find sources of compost in the farm and garden section of Craigslist or Freecycle.

Maybe you have a good opportunity here to start out small with your SFG.  Could you divide a smaller, say four foot section,  out and take the top  6 inches of soil out and replace it with Mel's mix? I had a four by four box one year but didn't have enough Mel's next to fill it, so I stuck a piece of wood down the center and was able to fill half of it with Mel's mix and the other half I filled with compost. As time went by, little by slow, the whole box became Mel's mix. 

CC

It is expensive to convert all your beds to Mel's Mix.  Therefore, I have done this very slowly.  I did two boxes on 4" x 4" leg posts last year for strawberries and they look marvelous.  I still used my sandy loam bed for tomatoes, peppers, etc.  Last fall, I added another strawberry box with Mel's Mix.  This spring, I added (3) 4' x 4' boxes, 12 inches deep for potatoes.  I am adding another sandy loam bed as I type - 4' x 9' for tomatoes and peppers, since I need to rotate these.  I still have one more 4' X 4' box to fill with Mel's Mix that I am going to plant determinate tomatoes (small, bush type plants) into see how it goes.  I wish all my boxes were filled with Mel's Mix, but I'm having to slowly make my way there.

I don't think there should be any problem with you taking some of the dirt out of your box and replacing it with Mel's Mix.  I came up short on Mel's Mix for one of my 4' x 4' x 12" boxes for potatoes, so I filled the bottom with a green garbage bag full of leaves that I had collected from last fall.  They were wet inside the bag, as I was trying to create leaf mold.  These leaves filled about 4 inches of the 12 inch depth of the box.  I filled the rest of the box with Mel's Mix - on top of the leaves.  And what to you know - this particular box is FLOURISHING with 16 potato plants; while the other two boxes barely have a few potato plants sticking leaves through the top of the soil.  I have enough leaves left to try this in the 4' x 4' box I'm going to plant my determinate tomatoes in.  I think as long as you get anywhere near the Mel's Mix formula, you'll be okay.  And eventually, your box will be full of Mel's Mix and compost as you add each year.  I've decided that a big thing about the Mel's Mix is that it's pourous, which plants like.  Otherwise, I'd be stuck with gumbo or hauling in more sandy loam.

The easiest way for me has been to add the 4' x 4' boxes one or two at a time, due to cost.  I couldn't even begin to imagine filling my 9' x 20' sandy loam bed with Mel's Mix as it is just too cost prohibitive.  I just add LOTS of compost and chicken manure to the sandy loam and hand till it in.  It's all about your nutrients in the soil.  I was very pleased to see I have LOTS of earthworms in the sandy loam bed when I hand tilled it.  I want the worms to like it:-)  I need their help with aeration.