I have been using the square foot gardening and all is well so far. I don't know anything about Mel's mix so I did not respond to the post orginally.
I just used a mix of loam and compost that was delivered by a local company when I put my beds in.
I use the wooden raised planter beds for my vegetable gardens.
I use the companion planting protocol along with the square foot planting to get the most out of my space and to increase my chance of success with the square foot method.
I love this system. I know some people don't ( there are other threads where this is addressed and people have a different opinion than mine) but if you are limited in space and want to plant a decent amount of plants and still have a broad variety it can be the answer.
I believe the thinking is that you will have less production on your plants than if they were planted conventionally spaced out, but you will have alot more plants in your garden so it evens out.
I would suggest though that having very fertile, nutrient rich soil will make all the difference in the world. You will have so many plants drawing from the same soil and vying for nutrients that I believe a good portion of your success will depend on your soil.
I had a great quality soil to start with and I side dress with extra compost now that things are really taking off.
I have a total of 180 square feet planted this way with
several varieties of each of these vegetables:
lettuce
carrots
peppers
onions
tomatoes
cucumbers
potatoes
cabbage
watermelon
pumpkins
corn
beans
peas
winter squash
summer squash
strawberries
spinach
garlic
various herbs
I would like to add that( and this is just my opinion) doing the square foot gardening might work better in the raised bed planters.
My reasoning is that, you have better access for weeding, picking and such. I could be wrong, but if you do the square foot method in a regular garden bed it would be harder get down on the ground to weed without trampling something or to instead fit weeding tools in between the plants.
In the planters you really don't need to do any of that, I just lean over a little and reach in to do whatever I need.
Now for all the people that have alot of land, square foot gardening wouldn't appeal to them and they wouldn't need it since they have the space for a nice big walk through garden and plenty of room for all their crops.
But for those of us who have limited space it really increases our options.
I have the room to do a small walk through garden but it would only be approx. 18 X 22 feet and wouldn't allow for the amount of varieties I wanted to try.
I have 9 beds right now and would like to increase it more for next year.Its so hard to stop planting

I would only have been able to plant a third of what I wanted to this year if I hadn't used this method.
Honestly, I was holding my breath until I picked my first vegetable. There are no guarantees with any type of gardening though with so many variables.
I think the weather played a big role in my (still early) success, along with the new soil brought in.
I think it would be a good idea for anyone else that is doing the square foot method to keep this thread going to report your results.
Please keep us posted.