Stir together 1/2 cup soy sauce, onion, garlic, oil and ginger root. Place chicken in plastic bag and place in a bowl. Pour soy sauce over chicken and close bag. Chill for 3 hours, turning chicken occasionally to distribute marinade. Drain. Skewer chicken neck skin to back. Tie legs to tail; twist wing tips under back. Place chicken breast side up on rack in shallow roasting pan. Roast in 375°F oven for 1-1/2 hours or until drumstick moves easily. Mix 1/4 cup agave with 2 tablespoons soy sauce. Baste chicken with agave-soy mixture during last 15 minutes roasting time.