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Overview
The measurement of a solution's acidity or alkalinity is called its pH. When you measure a solution's pH, or its ̶0;power of hydrogen,̶1; you are measuring its concentration of hydrogen ions. Pure water possesses a pH of 7 and lies in the middle of the scale, which ranges from zero to 14. A value of less than 7 means that the solution is an acid, while a value of more than 7 means it is a base. One unit of change reflects a tenfold change in a solution's acidity.
Testing
Paper test strips ̵2; litmus paper ̵2; provide one method of testing the pH of soil or other substances. Liquid test kits and digital meters are also available. Most plants prefer slightly acidic soil ranging from 5.5 to 6.0. Over time, the pH of garden soil or an aquarium can change, so test the pH periodically. Rainfall can affect a garden's pH, while driftwood and other aquarium ornaments can influence the acidity of your tank water.
Lowering pH
Phosphoric acid is the most common acid used to lower the pH of garden soil. Other acids that lower pH include nitric and sulfuric acids. However, these acids are used less frequently because they are more hazardous. Organic gardeners occasionally use food-grade citric acid to lower the pH of their soil. A heavy rain can also lower the pH of your soil. Regardless of how much acid you add to soil or water, it will not register a pH of zero.
Hydrochloric Acid
Concentrated hydrochloric acid possesses a pH of zero. It is a highly corrosive liquid used in the human digestive system. Strong acids break water apart into hydrogen ions and hydroxide ions. The stronger the acid, the more it increases the concentration of hydrogen ions. Concentrated hydrochloric acid breaks water down very effectively and creates the highest concentration of hydrogen ions, making it the lowest value on the pH scale.