Soil acidity is one of the most important soil factors affecting crop growth and ultimately, yield and profitability. It is determined by measuring the soil pH, which is a measure of the amount of hydrogen ions in the soil solution. As soil acidity increases, the soil pH decreases. Soils tend to be naturally acidic in areas where rainfall is sufficient to cause substantial leaching of basic ions (such as calcium and magnesium), which are replaced by hydrogen ions. Most soils in Kentucky are naturally acidic because of our abundant rainfall. Some nitrogen fertilizers are also a source of soil acidity, so fields with a history of N applications likely will be more acidic.
Schwab, Greg; Murdock, Lloyd W.; Ditsch, David C.; Rasnake, Monroe; Sikora, Frank J.; and Frye, Wilbur, "Agricultural Lime Recommendations Based on Lime Quality" (2007). Agriculture and Natural Resources Publications. 76.