Aluminum

• Acid rain "activates" or "mobilizes" aluminum ions, which are dormant and nontoxic in most soils. However, when the pH balance of the soil changes due to acid rain and the pH drops to 5 or lower, those once-harmless aluminum ions change their chemical properties and become harmful to plants. The change occurs because the acid rain mixes with the aluminuml activating the dormant toxins in the aluminum through a chemical reaction. Chemically, sulfuric acid in the rain reacts with the aluminum hydroxide causing a "neutralization reaction." Aluminum ions can contribute to slow growth in roots and prevent roots from taking in the nutrients they need.
  
  

Limestone Neutralization

• When acid rain mixes with soil, it changes both the chemical and physical makeup of the soil. Soil acts as a natural filter that can sometimes remove acidity from rain and change the chemical makeup of the water. This process is sometimes known as Limestone Neutralization and occurs with alkaline or basic soils; ones that have a higher amount of limestone or calcium carbonate can remove the acid almost immediately. This change takes a lot of the nutrients out of the soil because they were needed to neutralize the acid, thus changing the chemical and physical makeup of the soil. These soils are typically found in the middle and western United States.
  
  

Effects on Plants and Trees

• Acid rain, while harmful to plants and trees, does not kill them instantly. The acid in the rain harms the leaves and petals, stripping away nutrients and weakening their defense structures. After the acid rain soaks into soil, that soil becomes contaminated with toxins, which are then consumed by the plant. The plant grows weaker and, depending on the circumstances, either dies or is significantly weakened.
  
  

Cation Exchange

• During an acid rainfall, a cation exchange may occur when the ion particles of magnesium, potassium and calcium attach themselves to the clay particles of the soil. The positively charged metal ions attract and stick to the negatively charged clay particles, binding them together. This attraction of these two different ions is powerful enough to withstand water filtering through the soil. The water is unable to break the bind the acid rain particles have on the soil, so the soil now becomes permanently contaminated. The ion particles of the acid rain are retained by the top soil, and the metal ones (which are vital to plant growth) are washed down into the subsoil where plant roots do not reach. This process occurs because the acid rain particles are lighter and the now heavier metal particles sink down.