I am aware that different plants thrive/need different pH levels to grow. What is it about the alkalinity/acidity of the soil that effects how a plant grows?
Note: I understand that this could have an in-depth answer about plant biology; but I am speaking a more basic level - not on the mechanics of molecular biology.
Soil pH affects the availability of nutrients to plants. At values outside the range of about 5-7, certain nutrients become unavailable to plants. (The Wikipedia article gets into more of the details of the chemistry, which are somewhat outside the scope of this site.)
For most plants in a home garden or landscape, you want to target a pH of about 6.0-6.5. There are some exceptions (blueberries, the azalea family, etc. like much more acidic soils).
The effect of soil pH is great on the solubility of minerals or nutrients. Fourteen of the seventeen essential plant nutrients are obtained from the soil. Before a nutrient can be used by plants it must be dissolved in the soil solution. Most minerals and nutrients are more soluble or available in acid soils than in neutral or slightly alkaline soils.
They have some good info here: http://www.esf.edu/pubprog/brochure/soilph/soilph.htm
