In a medium bowl stir together whole-wheat flour, 1/4 cup of the all-purpose flour, and the salt. Gradually stir in the water to make a soft dough, adding additional water, 1 teaspoon at a time, if necessary. Shape dough into a ball. Sprinkle some of the remaining 1/4 cup all-purpose flour on a work surface. Knead dough on floured surface until smooth, elastic, and slightly sticky. Cover and let stand 10 minutes.