What can I use as a substitute for 1/2 cup of barley in a 2 quart soup?
Barley's there to bulk out the soup, and add a bit of flavor and texture. It doesn't thicken or have any other special function, so you don't need to add anything to replace it.
If you want to add something with a roughly similar size with some texture then short grain rice like risotto or paella rice will do, however I prefer Orzo, which is a type of very small pasta. You can also smash up some short pasta like penne, farfalle, or shells and put them in. Just remember that rice and pasta cooks much faster than barley, so add it at the end of cooking and get it off the heat as soon as it is al dente so it doesn't overcook and get mushy. Add a handful of frozen peas to it to get the temperature down quick.
Becky Epstein, author of Substituting Ingredients, The A to Z Kitchen Reference, 4th Edition, suggests farro, spelt, or wheat berries.