MadSci Network: Earth Sciences

Re: How does acid rain affect soil?

Date: Fri Sep 14 22:44:09 2001
Posted By: David Hershey, Faculty, Botany, NA
Area of science: Earth Sciences
ID: 999616961.Es

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. 


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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