Exchangeable aluminium and total acidity

Changes in the composition of the soil solution have direct effects on the composition of the exchangeable ions. One example is acid deposition via precipitation, which leads to a decrease in pH (i.e. an increase in hydrogen ion concentration) in soil water and an increased release of aluminium ions (Al³⁺) into the soil solution. Some of the hydrogen and aluminium ions in the soil water then replace base cations (Ca²⁺, Mg²⁺, K⁺, Na⁺) that were previously electrostatically bound to soil particles. Consequently, the content of exchangeable hydrogen and aluminium ions also increases as the soil pH decreases. The sum of exchangeable hydrogen and aluminium ions is referred to as total acidity or titratable acidity. The proportion of exchangeable aluminium in the total acidity generally increases sharply at pH values below 4.5.

Swedish soils have a net negative charge

In Sweden, most soils have a net negative charge. One consequence of this is that cations are retained more effectively in the soil than anions. Examples of exchangeable cations include calcium (2⁺), magnesium (2⁺), potassium (1⁺), ammonium (1⁺), hydrogen (1⁺), and aluminium (3⁺). The numbers in parentheses indicate the charge of each ion. Both the hydrogen ion (H⁺) and the aluminium ion (Al³⁺) exhibit acidic properties and are often referred to as acid cations.

Al³⁺ can be considered an acid cation because it can release hydrogen ions when dissolved and hydrolysed in water, according to the following reactions:

Al³⁺ + H₂O ↔ Al(OH)²⁺ + H⁺

Al³⁺ + 2H₂O ↔ Al(OH)₂⁺ + 2H⁺

High concentrations of Al³⁺ in soil solution can have a toxic effect on plant roots. However, plant species vary in their sensitivity to Al³⁺. Common forest tree species are relatively tolerant and can withstand fairly high concentrations, whereas most agricultural crops are adapted to higher pH values and are therefore sensitive to low pH conditions accompanied by high concentrations of Al³⁺ in soil solution.

Determination of exchangeable aluminium and total acidity

Exchangeable aluminium

Approximately 10 g of air-dried humus or mineral soil is weighed into a 250 ml polyethylene shaking bottle. Then, 100 ml of 1 M KCl solution is added, and the sample is shaken for two hours. The solution is filtered through filter paper (ash-free filter paper, Munktell 00K, diameter: 18 cm). From the filtrate, 10 ml is taken and 40 ml of 0.125 M HCl is added. The aluminium concentration in this solution is analysed using ICP-AES (inductively coupled plasma atomic emission spectroscopy). The pH of the remaining 90 ml of extract is measured (pH-KCl).

Total acidity (titratable acidity)

To determine total acidity (TA), 50 ml of the same extraction solution (1 N NH₄OAc solution buffered to pH 7.0) is used as in the determination of exchangeable base cations. The solution is titrated to pH 7.00 using either 0.1 M NaOH (if the initial pH is below 7.00) or 0.1 M HNO₃ (if the initial pH is above 7.00). The titration is performed using the same automatic titration system as used for pH measurements.