Join Date: Feb 2012
Location: South east asia
Using "fresh" soil isn't an instant disaster. Its just more troublesome to manage as the soil slowly changes to take on submerged qualities. Ammonia may leech out, bacteria activity on organic materials produce CO2 and other substances; depletes the substrate of oxygen. Water quality maybe unstable till it settles.
However, with good lighting, CO2 and ferts, many plants don't seem to mind. I've done a few tanks with "fresh" top soil (planting HC immediately submerged also), and they've turned out okay.