pH defines the relative acidity of alkalinity of a substance. The scale ranges from 0 being acid to 14 being alkaline. A pH value of 7.0 is neutral.

Acids are substances that release hydrogen ions (H+). The more H+ held in the soil, the greater it's acidity. Basic ions such as calcium (C++) make soils more alkaline.

Soil pH simply measures H+ activity and is expressed in logarithmic terms. This simply means that each unit change on the scale is a tenfold change in the acidity or alkalinity. A soil with a pH of 6 is ten-times more acid than a soil with a pH of 7. A soil that has a pH of 5 is 100 times more acidic than a soil with a pH of 7. A soil with a pH of 4 is 1000 times more acidic than a soil with a 7 pH.

How nitrogen fertilizers affect soil acidity

The nitrification process converts ammonium to nitrate. This process releases H+ ions. Nitrate furthers increases acidity by leaching calcium, magnesium (N never takes Mg), and potassium with it. As these alkaline ions are removed more hydrogen can be replaced in the soil.

How lime reduces soil acidity

One Ca++ ion from lime replaces two H+ ions on the soil exchange site. This process creates water (H₂0) and carbon dioxide (CO₂). As the H+ concentration is lowered soil acidity decreases.

There are other factors that affect soil acidity. Excessive rainfall can leach basic ions. Nitrogen fixation of legumes, crop removal, and organic matter decomposition also increase acidity. As the alkaline ions (Ca++, Mg++, K+) are removed they will need to be replaced or H+ will steadily increase, lowering pH.