Soil pH affects the growth of plants primarily through influence on nutrient availability, and plants have different abilities to tolerate either acid or alkaline pH levels. Reducing acidity requires adding lime to the soil, but the effectiveness and the quantities needed depend on the balance between active and reserve or potential acidity. A professional soil test will usually give a recommendation based on this balance.
A Definition Of Acidity
When a molecule of water breaks up or ionizes, it splits into a positively charged hydrogen ion (H+) and a negatively charged hydroxyl ion (OH-). When a solution of water and soil has more hydrogen ions than hydroxyl ions, it is said to be acid. If the two are balanced, it is neutral. When more hydroxyl ions are present, it is alkaline. The scale used to express the relative numbers of hydrogen and hydroxyl ions is called the pH scale. Neutral is 7, numbers below that are acid and higher numbers are alkaline. The lower the number, the greater the imbalance between hydrogen and hydroxyl ions and the stronger the acidity.
The pH of the solution used in a soil test, the soil sample plus water, is called the active acidity. It is a measure of the number of hydrogen ions currently present. The active acidity is the pH of the soil that a plant would experience at that time.
Hydrogen ions, however, can be held by soil particles, nutrients, or bits of organic matter. These are not in the solution, so they are not measured. If you add ground limestone, carbonates in the lime would react with some of the hydrogen ions, neutralizing them. However, other hydrogen ions held in the soil would be released to replace those neutralized. These ions held in the soil are called the "reserve acidity." To raise the pH of the soil effectively, both active acidity and reserve acidity must be neutralized.
The Buffer Index
The estimate of reserve acidity is often called the "buffer index," the capacity of the soil to resist a change in pH. It is used to estimate the amount of lime that will be needed to raise the pH. A low buffer index reading means that the reserve acidity is high and substantial amounts of lime need to be applied to raise the active acidity, the pH that a plant experiences.
Cation Exchange Capacity
The substances that can hold hydrogen ions, the can increase the buffer index, are those with a high cation exchange capacity, or CEC. Cations are positively charged ions, such as the hydrogen ion. Many nutrient ions are also positively charged and are attracted to the negatively charged clay and humus particles.
Different clay minerals can hold different amounts of hydrogen ions, with the clay found in cool temperate and arid areas having a higher CEC than that commonly found in warmer areas. Some organic substances also have a higher CEC than others, with peat moss and coir having a great capacity to hold ions than sawdust or redwood bark.