When sulfur oxides and nitrogen oxides incorporate into rain, snow or mist, the result is acid precipitation: air pollutants that can damage people and property miles away from the industrial plants that caused them. Acid rain is particularly deadly to green growing things, impacting forest health, farmers' livelihood and the world's precious food supply.
Diminished Soil Nutrients
As acid rain falls onto soil, it may reduce in acidity because of contact with alkaline or basic substances such as limestone or calcium carbonate. The acid may also be simply immobilized if the sulfate or nitrate ions of the sulfuric and nitric acids are retained in soil or vegetation. This sounds like good news--but the interaction of acid rain with soil is bad news for any plants trying to grow there. Hydrogen ions from the acid deposits displace the calcium, magnesium and potassium, leaching them from the soil and making them unavailable to the plants that need them.
Acid can also mobilize the aluminum ions that were previously a harmless part of aluminum hydroxide. These freshly mobilized ions stunt root growth and prevent plants from taking up calcium.
Acidic or low pH conditions also can reduce the population of the soil microorganisms responsible for composting dead plant matter. The less composting going on, the less phosphate, nitrate and other nutrients available to local plant life.
Increased Soil Toxins
Increased acid levels mobilize (or make soluble) other metal ions besides aluminum. These metals, among them lead, mercury, zinc, copper, and cadmium, are toxic and will inhibit plant growth.
Reduced Seed Germination
In an experiment comparing the germination of seeds in different conditions, Aradhana Sinha simulated acid rain by treating one batch of seeds with 1 part per million sulfuric acid and another with 2 parts per million nitric acid. Seeds so treated demonstrated a 43 percent lower rate of germination and a 53 percent lower rate of sprouting than seeds in ordinary water. Nitric acid alone lowered the germination rate by more than half.
Effects on Plant Tissues and Photosynthesis
New growth is especially susceptible to acid rain; young rootlets and leaf shoots tend to be the most vulnerable to low pH conditions. Acid rain also damages the waxy outer coating that protects the leaves. This allow moisture to escape, dehydrating the plant. It also allows acid entrance to the plant system, where it suppresses photosynthesis. Acid-damaged leaves and needles also withstand the cold less effectively.