Function. Plants also have a pH rating. Plants whose pH matches the soil's pH can take up nutrients more efficiently than plants with a pH rating different than the soil. Pine trees and the needles that they produce prefer acidic soil.
Price of a Stamp. The current price of a first class stamp is 49 cents. A domestic first class letter (1 oz.) is 49 cents. The price of each additional ounce is 21 cents. It will cost you 34 cents to send a domestic postcard.
Short answer, currently 47 cents (US). Long answer, Forever stamps will always be worth whatever the current 1st class standard 1 ounce letter rate is. That rate may increase to 49 cents on Jan. 22nd. In which case, any Forever stamp would be worth 49 cents.