Acidification is what happens when water or soil becomes too acidic. In the ocean, acidification is mainly caused by the burning of fossil fuels.
Scientists use the pH scale to measure the acidity of a substance. Drinking water, for example, is neutral, while sea water is normally a little basic, containing less acid. Ocean acidification, caused by pollution, shifts its place on that scale. More acid makes it harder for shellfish to grow shells and weakens coral reefs, among other effects. Soil acidification occurs when acidic chemicals are added to soil — this can happen as a result of acid rain and agricultural runoff.