How does acid rain release toxic aluminum from soil and rock?

Acid rain releases toxic aluminum from soil and rock through a process of chemical mobilization driven by increased acidity.

When acid rain falls, it introduces an excess of hydrogen ions ($H^{+}$) into the environment. In soils that lack sufficient buffering capacity—meaning they cannot effectively neutralize incoming acids—these hydrogen ions accumulate and increase the overall soil acidity.123

This acidic environment alters soil chemistry in the following ways:

• Increased Solubility: The presence of hydrogen ions increases the solubility of minerals within the soil and rock.2
• Mobilization: Aluminum, which is naturally bound to soil particles and rock, is released into the soil water in its toxic, dissolved form ($A{l}^{3+}$).42
• Nutrient Displacement: Once mobilized, these toxic aluminum ions can displace essential plant nutrients, such as calcium and magnesium, from soil particles.42

This process is particularly harmful to ecosystems because it simultaneously depletes the soil of nutrients necessary for plant growth and introduces a toxic substance that can damage tree roots and aquatic life.143