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. Nitrogen fertilization can also contribute to soil acidity as the nitrification of ammonium increases the hydrogen ion concentration in the soil through the following reaction:
NH4+ + 2O2- → NO3- = H2O + 2H+
Ritchey, Edwin L.; Murdock, Lloyd W.; Ditsch, David C.; McGrath, Joshua M.; and Sikora, Frank J., "Agricultural Lime Recommendations Based on Lime Quality " (2016). Agriculture and Natural Resources Publications. 102.