What's a gluten free alternative to farro?
Farro is essentially a variety of wheat, and I assume your recipe is using it in whole form, rather than milled in some way. Therefore, you'd want to substitute another grain that will provide at least a similar texture.

Farro



Rice is gluten-free, so you could use any kind of whole grain rice as a substitute.

Brown Rice



You could use wild rice, although its particular texture is quite different and may or may not compliment the soup.

Wild Rice





It's useful to know that there are actually four different kinds of gluten:


Gliadin in Wheat 
Hordein in Barley 
Secalin in Rye
Avenin in Oats


Because farro is a kind of wheat, it contains Gliadin, which is the form of gluten people are most allergic to. However only one in five people who are sensitive to wheat are allergic to the Avenin in oats.



If you're cooking for others and don't want to take a chance, then don't even consider using oats. However, if for yourself or someone who wants to try oats&ast;, they have a nice flavor that would be delicious in the soup.

Oats



Specifically, you'd want whole kernel oat groats — not cracked or rolled. Look for a package that says "gluten free", which really means they've taken precautions to keep the oats from being contaminated with gluten from wheat or other grains.

If you find you can tolerate oats with no problem, you may also be able to use rye and/or barley. 

&ast; Consult with your physician before trying any of these alternative grains.