Soil acidity and liming

Soil pH is a useful indicator of the relative acidity or alkalinity of a soil. The pH scale ranges from 0 to 14, and the soil is assigned a value from the pH scale to describe the acidity or alkalinity. Since pH 7 falls midway along the scale, pH values that are equal to 7 are said to be neutral. However, pH values that fall below 7 are acidic, while pH values above 7 are alkaline.

By definition, the pH of a soil is the measurement of the concentration of hydrogen ions in soil water. Recall that the hydrogen ion is an acid cation. The greater the concentration of hydrogen ions in the soil water solution, the lower the pH. In return, the lower the pH value, the greater the acidity of the soil will be. The concentration of hydrogen ions in the soil solution is directly proportionate to and in equilibrium with the hydrogen ions retained on the soil’s cation exchange complex. Thus, the hydrogen ions retained by clay particles replenish, or buffer, the hydrogen ions in soil water.

Table 1. pH of some common items.

Item	pH	Item	pH
Most acid soils	4.0 - 6.0	Lemon juice	2.2 - 2.4
Orange juice	3.4 - 4.0	Vinegar	4.0 - 4.5
Acid rain	3.0 - 5.0	Clean rain water	5.5 - 5.7
Fresh milk	6.3 - 6.6	Blood plasma	7.2 - 7.4
Mild soap solution	8.5 - 10.0

Source: Hue, N.V. and Ikawa, I. Acid Soils in Hawaii: Problems and Management. CTAHR.
http://www.ctahr.hawaii.edu/huen/hue_soilacidity.htm

Soil pH is an important soil property, because it affects the chemical, biological, and physical processes of the soil. Thus, pH is often considered the “master variable” of soil. Its importance in nutrient management cannot be understated. To understand the significance of pH, its effects are listed below: