In a large bowl combine all the dry ingredients: both flours, brown sugar, salt, and baking soda.
Add in the buttermilk and gently mix to combine. Do not knead or over mix.
Grease two 9x5 loaf pans, and divide the batter evenly between them. Bake at 350 degrees for one hour. (I made mine round in two cake pans, and reduced the cooking time slightly because they were more shallow. Bake until the tip of a knife comes out clean.)