This recipe is a variation to Soba Tsuyu. Dashi is a soup and cooking stock considered fundamental to Japanese cooking. Dashi forms the base for MIso Soup, clear broth soups, Japanese noodle broths, and many Japanese simmering liquids. Most people use granulated or liquid instant substitutes found in Asian Markets or in most grocery stores (Asian food section) or in health stores. Usukuchi means thin flavor, not low sodium. Please do not confuse these two sauces, because their flavor is entirely different. Usukuchi is much lighter in color. All these items are available in Asian Markets. Serve sauces with grated daikon, momiji oroshi (red pepper spiced grated daikon), or grated fresh ginger root (né shoga) to be mixed at the table in each diner's kozara (saucer).