In the spring and fall, many lucky kids get a break from their school desk routines for a Zenger Farm field trip. This urban farm is an oasis of green in outer southeastern Portland, Oregon: 6 acres of farm set alongside 10 acres of wetlands, with pear, plum and apple trees growing behind fields of vegetables. A tire swing hangs from the old distinguished cedar tree. And a big yellow school bus hauls in groups of kids throughout the growing season, with the help of Friends of Zenger....