|MadSci Network: Earth Sciences|
Acid rain affects mainly the soil chemical properties rather than physical properties. Acid rain is caused by air pollutants (nitrous oxides and sulfur dioxide) from burning of fossil fuels that create nitric and sulfuric acids in precipitation and lower rainfall pH. The USGS defines as precipitation with a pH below 5. Even with no air pollution, rain will be slightly acid, about pH 5.5, because of carbon dioxide dissolving in it. The 5.5 pH causes soil in moderate to high rainfall areas to be acid because over time, the hydrogen ions displace basic cations, such as calcium, magnesium and potassium from the soil cation exchange sites into the soil solution and replaces them on the cation exchange sites with hydrogen or aluminum ions. At a pH above 5, most aluminum does not exist in ionic form but as insoluble aluminum hydroxides. Hydrogen ions in acid rain can combine with the hydroxyl ions in aluminum hydroxide and release the aluminum ions. The basic cations displaced from the cation exchange sites into the soil solution are then leached deep into the soil beneath the rootzone by rainfall. Acid rain speeds up this natural process. Therefore, acid rain is harmful because plants require calcium, magnesium and potassium, and aluminum ions are toxic to many plants. Low soil pH reduces the activity of soil microbes that decompose organic matter to release nitrogen in a plant available form. Low soil pH also reduces the availabilty of other essential plant mineral nutrients, such as phosphorus, boron and molybdenum. Low soil pH increases the availability of essential metal cations, such as iron, manganese, copper and zinc, which can result in plant toxicities. Soil color is not simply related to soil pH. Rather it is affected by many factors such as fertility, aeration, and organic matter content. I have noticed that raising the pH of sphagnum peat moss to pH 7 by adding limestone makes it darker. Sphagnum peat moss is a soil amendment sold in garden centers and has a natural pH of about 4 to 4.5. References Acid Rain FAQ Primer on Acid Rain Cation Exchange Capacity How soils become acid? Soil Acidity and Liming Soil pH and Nutrient Availability Soil Color
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