Preheat oven to 425 degrees. Whisk together flours, salt, and baking soda in a large bowl. Gather mixture into a mound; create a deep well in the center. Pour buttermilk into well. Gradually stir flour mixture into buttermilk with a wooden spoon, starting in center and working outward, until a dough forms.
Transfer dough to a lightly floured work surface and, with lightly floured hands, form a cohesive ball. (Do not overwork dough.) Pat ball into a 7-inch-wide domed round and transfer to a baking sheet. Across top of round, cut a 1-inch-deep X with a floured knife. Poke holes at 1-inch intervals to bottom of dough (about 28 holes total) with a floured wooden skewer.
Bake bread 30 minutes. Turn bread upside down; continue baking until cooked through and loaf sounds hollow when tapped on bottom, about 10 minutes more. Transfer loaf to a wire rack and let cool completely, about 2 hours, before slicing and serving. Bread can be made 2 days ahead and stored, whole, at room temperature, in a paper bag or wrapped in parchment.