Can Rain Change the pH of Soil?

Chemical Interaction

• Water is made up of two hydrogen atoms and one oxygen atom, which is why it's called H2O. When rainwater falls to earth, its molecules break apart into positively charged hydrogen ions (H+) and negatively charged hydroxyl ions (OH-). Acid soil contains more free hydrogen than free hydroxyl ions, whereas the reverse is true for alkaline soil.

  The acidity of a soil is partly related to the kind of rock on which it rests. Granite is an example of an acid material while limestone is alkaline. Soil also gains acidity from nitrogen fertilizer and from penetration by large quantities of pure rainwater, which has a pH of about 5.6, or polluted rain that is more acidic. Crop absorption of nutrients can also affect pH.

Understanding pH

• Soil pH is based on a logarithmic scale, in which steps increase by mathematical powers of 10. The neutral level in pH, in which hydrogen balances hydroxyl, is represented by the number 7. It can be visualized as the center bottom point of an upward spreading parabola. The left side represents acidity and the right, alkalinity. A pH of 6 is 10 times more acidic and a pH of 5 is 100 times more acidic than a pH of 7. These powers of 10 are the same on the alkaline side, but are represented by 8 and 9.

  Too much or too little rainfall can create pH conditions that are not beneficial for crops. Agricultural extension agents say a range of 6 to 7 is best for most garden plants.

Leaching and Buffering

• Not only is rainwater naturally acidic, but it further aids soil acidification by speeding decomposition of organic matter into an acidic fluid. Certain characteristics of soil can buffer it against major pH change. When rainwater flushes nutrients from negatively charged clay particles, it replaces them with positively charged hydrogen ions, which become inactive when bound to the clay. Acidity measurements are based on the presence of "free" hydrogen ions that don't attach to soil particles. If soil is based on alkaline rock, such as limestone, this further buffers the affect of the rainwater.

Acid Rain

• Regular rainwater is acidic, because it dissolves carbon dioxide gas in the air into carbonic acid. But rainwater that is categorized as "acid rain" has a pH less than 5 and carries harsh pollutants, such as sulfuric and nitric acids from coal-burning industries and automobile exhaust. Rainwater does not need to be acid rain to have a pH-altering effect on soil. However, acid rain has a greater impact on soils that are based on acidic rock, such as those of the Northeast, because the underlying rock doesn't buffer the rain's acidity.