Want to have the biggest pumpkin on the block , the neighborhood or the county  for Halloween? Here's advice from Bill Tall, owner of City Farmer's Nursery who sponsors an annual pumpkin growing contest for kids 16 and younger. Start by planting seeds this month. Tall likes organic 'Big Max' seeds, said to produce behemoths 100 pounds or more. “The trick is to grow them in good soil and keep them well watered and well fed,” he says. Tall recommends fertilizer high in phosphorous, rather than nitrogen, to encourage flowering. Allow only a few flowers to set fruit and then leave only one or two baby pumpkins to mature, placing them on a board or cardboard to protect them from pests. Encourage them to pack on pounds with plenty of water and by October, you could have a winner. Tall sells 'Big Max' seeds; kids who want to compete in City Farmer's contest get them for free. For contest details, visit the nursery at 4832 Home Ave., San Diego.