Acid rain.
Sulfur is taken up from the soil in the form of sulfate ions (S04=). Sulfur may also be absorbed as sulfur dioxide (S02) from the air through leaves in areas where the atmosphere has been enriched with sulfur compounds from industrial sources.
Western Plant Health Association. Western Fertilizer Handbook (Page 99). Waveland Pr Inc. Kindle Edition.
Dissolved oxygen.
Turned out to be a small wildfire. Twelve o'clock at night, it looked like the neighbor's house.