Soils have two pools of acidity – Active Acidity and Exchangeable Acidity.
The active acidity refers to the free hydrogen ions (H+) in the soil solution. Soil pH is measured in the soil solution and is an indicator of the Active Acidity. When lime is added to the soil, this acidity is easily neutralized.
However, there is another source of acidity in soils, which is the Exchangeable Acidity.
Most soil colloids (particles) carry a negative charge. As a result, positive ions (cations) are adsorbed on soil particles. Such ions include Calcium (Ca+2), Magnesium, (Mg+2), Potassium (K+), Sodium (Na+), Aluminum (Al+3) and Hydrogen (H+).
Aluminum and Hydrogen are considered to be acidic, while the others are considered to be basic.
Why is Aluminum considered to be acidic?
H+ directly reduces the pH of the soil solution, while aluminum participates in hydrolysis reactions, in which H+ ions are formed.
Hydrolyses by Aluminum
Al3+ + H20 = Al(OH)2+ + H+
Al(OH)2+ + H20 = Al(OH)2+ + H+
Al(OH)2+ + H20 = Al(OH)O3 + H+
The cations which are adsorbed on the soil colloids are in equilibrium with the soil solution and can be released into it (See article on CEC – soil Cation Exchange Capacity).
Therefore, the acidic cations, H+ and Al+3, can be released into the soil solution and decrease the pH of the soil solution.
Soils with high CEC, can hold more acidic cations and, therefore, have a greater buffering capacity, i.e. they can resist changes in pH. In such case, a greater amount of lime has to be applied in order to effectively increase soil pH.
Soil acidity affects plants in various ways: nutrient availability, toxicity of some metals and activity of beneficial microorganisms.
Effect nutrient availability
Plants absorb nutrients from the soil solution and in the right ionic form in which the plant can absorb them. Nutrients adsorbed on exchangeable sites on soil particles are in equilibrium with the soil solution and can also become available to plants.
Nutrients which are a part of insoluble minerals are not available to plants. These nutrients are also called “fixed nutrients”.
The acidity of the soil influences the solubility of minerals, and thus affect the availability of nutrients.
Most plant nutrients are available at slightly acidic pH of 5.8 to 6.5
Nutrient toxicity in acidic soils
In acidic soils, with low pH levels, metals such as Aluminum, Iron and Manganese might be released into the soil solution at high concentrations which may be toxic to many plants.
Effect on the activity of beneficial microorganisms:
Some soil bacteria are responsible for many reactions in the soil, such as decomposition of organic matter (contributes nitrogen and phosphorus) and the nitrification process. Those processes are significantly slowed down in acidic soils, and therefore limit nitrogen and other nutrients availability.
As mentioned above, measuring the pH of the soil represents the active soil acidity. There are various methods to measure soil pH. When comparing pH values of different soil samples, it is important to compare values that were obtained by using the same method.
Methods based on water extraction: For example, measuring the pH in the saturated paste, 1:2 extract (one-part soil to two - parts water) and 1:5 extract. Higher dilution results in higher pH reading, as the H+ concentration is more diluted.
In soils containing soluble carbonate minerals, the pH reading will be even higher as more water is added, because carbonate ions dissolve into the solution.
Methods that involve adding diluted chemical extractants: KCl or CaCl2 are commonly used. Using these methods, pH readings are 0.5 to 1.5 units lower than when using the water-dilution methods.
Using KCl or CaCl2 gives more consistent results, and represents better the field conditions.
The K and Ca in those extractants replace some hydrogen ions that are attracted to the soil particles. The hydrogen ions are then released into the test solution, lowering its pH more than using just water for testing the pH would.
Therefore, using 1.0M KCl or 0.01M CaCl2 result in lower pH reading than methods that are based on water extraction. It provides a better approximation of the soil pH under field conditions, as the soil solution also contains dissolved salts.