Soil pH and acidity

The degree of soil acidity or alkalinity is expressed in pH units. Soil pH affects many processes in soils, including nutrient availability, activity of microorganisms and root development.

The pH of natural soils typically ranges from 4.5 to 8.0. Soils with pH between 6.5 and 7.0 are considered to neutral, soils with pH above 7.5 are alkaline and soils with pH lower than 6.5 are acidic. Most plant nutrients become available at slightly acidic pH of 5.8 to 6.5.

 

The effect of soil pH on nutrient availability

At a soil pH lower than 5.2, nutrients such as calcium, magnesium, nitrogen, phosphorus and boron may become unavailable for plants, while the solubility and availability of trace elements, such as iron, aluminum, manganese, zinc and copper increases significantly and they may become toxic.

In highly acidic soils, the mineralization process of organic matter is slowed down and can even stop completely because microbial activity declines under low pH conditions. This results in a decreased availability of nitrogen and phosphorus.

In high pH soils, micronutrient deficiencies become common.

Phosphorous availability is reduced in both highly acidic soils, with pH lower than 5.5, as well as in alkaline soils with pH greater than 7.5. In acidic soils, phosphorus reacts with iron and aluminum and becomes unavailable, while in alkaline soils, phosphorus reacts with calcium.

Alkaline soils are characterized by presence of calcium, magnesium and sodium carbonates. At soil pH levels between 7.2 and 8.2 soil pH is dominated by calcium and magnesium carbonates and the soil is called a ‘calcareous soil’. At a soil pH above 8.2, the soil is dominated by sodium carbonates, as they become highly soluble. High sodium levels relative to calcium and magnesium may negatively affect soil structure.

 

Pools of acidity in soil

There are three pools of acidity in the soil: active acidity, exchangeable acidity and residual acidity.

Active acidity is the concentration of free hydrogen ions (H+) in the soil solution. Soil pH is a measure of the active acidity of the soil.

Exchangeable acidity refers to hydrogen and aluminum ions retained on the exchange complex of the soil, i.e. on the surfaces of soil colloids. It is the potential acidity of the soil, as adsorbed Aluminum (Al⁺³) and Hydrogen (H⁺) ions, along with Calcium (Ca⁺²), Magnesium, (Mg⁺²), Potassium (K⁺) and Sodium (Na⁺), are in equilibrium with the soil solution.

The higher the cation exchange capacity of the soil, the greater its buffering capacity is, as the soil can retain a larger amount of hydrogen and aluminum ions.

The buffering capacity of the soil refers to its ability to resist changes in pH. Since the active acidity is in equilibrium with the exchangeable acidity, adsorbed aluminum and hydrogen ions can replenish aluminum and hydrogen that were removed from the soil solution. Therefore, soils with high CEC and low pH will require more lime in order to increase the pH to the desired level.

While calcium, magnesium, potassium and sodium are considered base cations, , aluminum and hydrogen are considered acidic.

Aluminum is considered acidic due to the hydrolysis reactions it goes through in the soil solution. The hydrolysis of aluminum generates hydrogen ions, i.e. active acidity.

Al³⁺ + H₂O = Al(OH)²⁺ + H⁺

Al(OH)²⁺ + H₂O = Al(OH)₂⁺ + H⁺

Al(OH)₂⁺ + H₂O = Al(OH)₃ + H⁺

Al(OH)₃ + H₂O = Al(OH)₄^– + H⁺

Residual acidity is associated with aluminum and hydrogen ions that are bound to soil colloids, but not in an exchangeable form.

Determination of soil pH

There are several methods for determining soil pH. All methods measure the active acidity, i.e. hydrogen ions in the soil solution, but each method will provide different results for the same soil sample. Therefore, in order to properly interpret the results, it is important to understand the difference between the methods.

Methods based on water extraction:

• pH of the saturated paste extract
• 1:2 extract (1 part soil, 2 parts water)
• 1:5 extract

The more water is used for the extraction, the higher the measured pH would be, because the addition of water dilutes the hydrogen ions in the extract.

Methods that use chemical agent for the extraction:

In methods that use only water for the extraction, hydrogen ions that are bound to soil particles remain bound and are not released into the solution.

In order to obtain a result that better represents the field conditions, a diluted solution of potassium chloride (1.0M KCl) or calcium chloride (0.01M CaCl­₂) are commonly used. The salt concentration in the solutions intends to represent the salt concentration in the soil solution. Potassium or calcium in the extracting solutions replace some of hydrogen ions bound to soil particles and, therefore, pH measured using these methods is usually closer to the actual pH of the soil.

Readings are 0.5 to 1.5 units lower than pH measured in water, due to the higher concentration of hydrogen ions in the solution.