I started up a new 55 and I have had dirt tanks in the past. my last set up was just flourite and it all grew fine for me. in this new set up though I put about 1/2 inch of organic soil capped by about 3 inches of flourite.
Anyone know how big of a difference it will make? My thinking was that the dirt would leak nutrients and that my plants roots would eventually reach the dirt as well.
You should be fine but expect some of your heavy root feeders to have an extended break in period until the root into the soil (that's my experience with crypts, sword, and some Val strands). Also I've had some problems with carpeting plants in substrates that have large caps (I.e. DHG will grow all the way down to the soil and THEN start growing upwards, resulting in a longer period to carpet).
I have found that I only like soil in small doses. The last tank I did soil in was a 5.5 gallon. It was no CO2, probably medium light. I put around 1/4 in of soil. It worked very well for me and I didn't have any of the negatives you get with a larger amount of soil. My heavy root feeders had incredibly large roots, even my various Rotala species had pretty crazy roots going.
I did notice after about 6 months, I had to supplement with a small amount of ferts and after about a year, probably more like a year and a 1/3rd, I was dosing the tank as if it didn't have soil, still getting better results than my non soil tanks.
I really think a small amount is the way to go. I have had success with an inch to two inches but I have had failures as well.
You also don't get crazy tannins which I personally enjoy not having.