Agree with Ziggy953 totally... there seems to be a real division on deep sand beds as to what is science and what is myth. Even reading the stuff on wetwebmedia, it's kind of a toss up. I remember wracking my head on that "simple" question when I was setting up my tank: "How deep of sand do I want?"
But I can answer your question! 5 inches of sand establishes an anaerobic (oxygenless) environment where nitrates can be converted to nitrogen gas. 2 inches of sand does not establish an anaerobic environment because it's too shallow.
Now what happens if you have something in between those two magic numbers (although I think the normal DSB
depth is 6") is where the debating gets interesting!