Glad you found that info helpful!* Good question about acid rain though that is not to blame for the yellowing.* It's the excess rain washing the nutrients out of the soil.* That's why it's important to add organic matter to the soil, both as an amendment and a topdressing.*
Here's a site I googled about acid rain.
"The extra acidity in rain comes from the reaction of air pollutants, primarily sulfur oxides and nitrogen oxides, with water in the air to form strong acids (like sulfuric and nitric acid). The main sources of these pollutants are vehicles and industrial and power-generating plants."
When weeding, the best way to make sure you are removing a weed and not a valuable plant is to pull on it. If it comes out of the ground easily, it is a valuable plant.