he organic-food business is booming: About 70 percent of Americans buy organic food occasionally, and nearly one quarter buy it every week, according to The Hartman Group, a market research firm. For most of us, the reason is simple: We want natural food that's better for us and for the environment, says Samuel Fromartz, author of Organic, Inc. But buying organic can cost you—as much as 50 percent more—so read on to know when it's worth it....