The color and pH drop by air oxidation sounds like sulfidic soil, usually seen along the coasts but if the soil is trucked in from somewhere I would guess Fe2S contamination from a mining operation.
Fe2S is jet black. Take a small sample of the soil and drip hydrochloric acid on it. If it has Fe2S in it, you'll immediately smell rotten eggs as hydrogen sulfide (H2S) is liberated. Aeration of sulfidic soils causes formation of sulfuric acid, which is exactly as you describe: battery acid (in lead-acid batteries).
If it does indeed come from a mine, I would be extremely concerned about acid liberation of the other heavy metal contaminants that might be present. The low pH will liberate metal ions that would otherwise be trapped as insoluble salts.
Last edited by Bocomo; 06/12/12 02:39 PM.
