SALINAS, Calif. — As Americans gather around Thanksgiving tables, chances are that the healthier parts of their menus — the tossed salads, broccoli casseroles or steaming bowls of roasted brussels sprouts — were grown here in the Salinas Valley.

A long strip of deep and fertile soil pinched by sharply rising mountains, the valley has more than doubled its output of produce in recent decades and now grows well over half of America’s leaf lettuce.

Yet one place the valley’s bounty of antioxidants does not often appear is on the tables of the migrant workers who harvest it.

Public health officials here describe a crisis of poverty and malnutrition among the tens of thousands of farmworkers and their families who tend to the fields of lettuce, broccoli, celery, cauliflower and spinach, among many other crops, in an area called the salad bowl of the nation.