Preheat the oven to 200 degrees F.

Divide the popcorn and nuts between 2 ungreased 13 by 9-inch rectangular pans. Make sure the popcorn and nuts are evenly mixed.

In a medium saucepan, cook the brown sugar, butter, corn syrup, and salt over medium heat, stirring constantly until bubbly around the edges. Continue cooking for 5 more minutes, and then remove the pan from the heat. Stir in the baking soda until foamy. Pour the mixture over the popcorn and nuts, stirring until the corn is well coated. Bake, uncovered, for 1 hour, stirring every 15 minutes.