Rinse the stained white cotton in cold water to remove as much soy sauce as you can. Put a drop of dish detergent on the stain and gently scrub it into the fabric with a toothbrush.
Rinse it again in cold water. If the stain is still visible, put on rubber gloves and dampen a clean sponge with ammonia. Wipe this over the area.
Rinse the item in cold water until it no longer carries the ammonia odor. Spray any remaining stain on both sides with a laundry pretreating spray. Allow the spray to sit on the fabric for as long as the label indicates -- this is usually just a few minutes.
Wash the item with detergent and bleach. For white cotton, you may use either regular chlorine laundry bleach or color-safe bleach.