For yesterday's annual food edition, Mark Bittman, author of many cookbooks and the leading food writer for the New York Times Magazine, asked his readers what he should do. All they wanted, he found, was something on "big farming, small farming, sustainability, politics, poverty and, of course, truly delicious food." He found it all in the same place: California's Central Valley, running 450 miles, from Bakersfield up to Redding, between the Sierra Nevada Mountains and the coastal ranges. It's larger than nine states, where the climate allows for four growing seasons, and it's the largest patch of Class 1 soil in the world. It produces some 230 crops. It's a precious natural resource and, he found, it's in trouble.