Add the rye berries to the sponge. Using a wooden spoon, stir in the rye flour, whole wheat flour, caraway seeds and 1 tablespoon plus 1 teaspoon of salt. When the dough becomes too stiff to mix, scrape it out onto a floured work surface. Using a dough scraper, lightly knead the dough, adding any leftover flour, until the dough forms a cohesive but sticky mass. Transfer the dough to a clean bowl. Cover and let stand in a cool place until the dough increases in volume by two-thirds and holds an impression when lightly pressed, about 2 hours.