Acid Rain Effects on Soils - Mobilize Aluminum
Increasing amounts of acids can "mobilize" aluminum ions which are normally present in an insoluble nontoxic form of aluminum hydroxide. It appears that when the soil pH dips to 5 or lower, aluminum ions are dissolved into the water and become toxic to plants.
Aluminum ions cause a stunting of the root growth and prevent the roots from taking up calcium. The result may be the overall slowing of the growth of the entire tree.
Lower soil pH and aluminum mobilization can reduce populations of soil microorganisms. Soil bacteria have the job of breaking down the dead and decaying leaves and other debris on the forest floor. The effect of this action is to release nutrients such as calcium, magnesium, phosphate, nitrate, and others. Low pH and high aluminum ion concentrations inhibit this process.
Higher amounts of acids can mobilize other toxic metals from the insoluble to the soluble ion forms in the same fashion as aluminum.
The toxic metals include lead, mercury, zinc, copper, cadmium, chromium, manganese, and vanadium.
These may all contribute to slow the growth of a tree. In addition, this combination of toxic metals may also adversely effect the growth of soil bacteria, mosses, algae, fungi, and earthworms.
