The amount or concentration of active hydrogen in the soil solution depends on the amount of acidity held by the clay and organic matter. Hydrogen ions on the clays or organic matter particles are known as reserve or exchangeable acidity. You must know how much exchangeable acidity a soil contains to determine the amount of lime needed. A buffer test called the Woodruff Buffer determines exchangeable acidity. Soil clay and organic matter particles carry a negative charge that is capable of holding positive-charged elements such as hydrogen, calcium, magnesium, ammonium, sodium, and aluminum.
Soils vary in the amount of positive elements they can hold.
The measure of a soil's capacity for holding positive-charged elements is called its cation exchange capacity. Much can be learned simply by knowing the pH of a soil. Most Midwestern agricultural soils have a pH range of 4.0 to 9.0. Soils with a pH of near 4.0 probably have free acids in the soil solution from the oxidation of sulfides or high exchangeable aluminum. Soils with a pH of 5.2 or less have high amounts of exchangeable aluminum and manganese.