NEW YORK — Wal-Mart is using its massive size to drive down the price of organic food items from tomato paste to chicken broth to make them more affordable for its low-income customers. The world's largest retailer and nation's largest grocery seller said Thursday that it has teamed up with Wild Oats to sell a new line of organic foods, starting this month, that's at least 25 percent cheaper than the national organic brands it carries and in line with the prices of its branded non-organic alternatives. Wild Oats helped pioneer the organic food trend in the late 1980s but has largely disappeared from store shelves since 2007.