What is soil acidity?
Soils can have two types of acidity: • Active or soil solution acidity • Reserve or exchangeable acidity Hydrogen ions in the soil solution are referred to as active acidity. Active acidity can be measured by a salt pH soil test. Note The salt pH of soil indicates whether limestone is needed to reduce soil acidity. A soil salt pH of 6.0 to 6.5 is generally best for most crops. As salt pH drops by one unit (for example, 6.0 to 5.0), acidity increases by 10 times. A soil with a salt pH of 4.0 would be 100 times more acidic than a salt pH of 6.0 (Figure 3). Figure 3 pH scale. 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 cl
