rogen oxide compounds. In the United States, 70 percent of sulfur dioxide pollution comes from power plants, especially those that burn coal. In Canada, oil refining and metal smelting account for 61 percent of sulfur dioxide pollution. Nitrogen oxides enter the atmosphere predominantly from the use of vehicles. When these chemicals are in the air, the sulfur dioxide and nitrogen oxides undergo a very complicated reaction with water vapor and other chemicals to yield sulfuric and nitric acid. These compounds can travel many kilometers in the air until the air becomes dense with moisture. At that point, clouds form in the acidic air so that when it precipitates, the acid then falls back to the earth.
             Acidity is measured on a pH scale that ranges from 0 to 14. Pure water has a neutral pH of 7 and is neither an acid nor a base. Acidic substances have a pH number from 1 to 6, where the lower the number, the higher the corrosiveness. Alkalis are bases that are not acidic substances and they have pH values ranging from 8 to 14. Precipitation with a pH value below 5.6 is considered acid rain. When rain falls with a pH level near 5.6, the alkaline chemical in the environment found in rocks, soil, etc. can absorb and neutralize the acid but if the pH level is lower than 5.6, the environment is unable to neutralize that acid rain. Acid rain has been linked to widespread environmental damage, including soil and plant degradation, depleted life in lakes and streams, and the erosion of many man made structures.
             The effect that acid rain has on soil is worrisome. Acid rain tends to wash away nutrients needed by plants. Sources of aluminum and mercury present in soil are also washed away due to acid rain. These alloys are then flushed into water systems or absorbed into plants causing those plants to die. In some places, the soil may be high in alkaline so it is protected by the effects of acid rain, but other thinner soils have ...