Preheat the oven to 400°F, and line a sheet pan with parchment paper. In a large bowl, whisk together the flour, salt, and baking soda until well combined. Make a well in the center of the dry ingredients and pour in 1 1/2 cups buttermilk.
Using your hands or a wooden spoon, mix the ingredients until a loose dough forms. You’re looking for a dough that’s soft but not overly sticky or wet, and that holds together enough to make a loaf that can hold its shape on the sheet pan. If the dough is dry and crumbly, add up to 1/2 cup additional buttermilk, a tablespoon or so at a time, until it comes together.
When the dough is just mixed together—no streaks of flour or buttermilk—transfer it to the parchment-lined sheet pan. Using your hands, form the dough into a round that’s roughly 8 inches in diameter. Using a paring knife, cut a large “X” across the top of the loaf. Bake until soda bread is nicely browned, and sounds hollow when tapped on the bottom of the loaf, about 45 minutes. Let cool on the baking sheet until just warm, then slice and eat with good butter. To store, wrap in a slightly damp tea towel to prevent the crust from getting too hard, and keep on the counter.