Listen to John Jeavons, and you’ll never sniff at composting again. Jeavons (rhymes with heavens) is a garden activist. He grows crops, but more important, from his perspective, he grows soil. Compost is to soil what good food is to the rest of us. Through Ecology Action, a nonprofit group he heads, Jeavons spreads the word: Crops that feed people also can nourish soil, and from that deceptively simple fact springs a method of cultivation he believes can save earth, if not the Earth.