Ask Mr. Smarty Plants

"If planting this species in areas where it has not formerly grown it may be helpful to inoculate the soil with a rhizobium (soil-borne bacteria which form nitrogen-rich root nodules) for lupines."

It is in fact, not really necessary to inoculate the soil to successfully grow bluebonnets. Rhizobium does help bluebonnets and other, mostly leguminous, plants grow in poor, nitrogen-deficient soils. Given sufficient nitrogen fertilizer and grown in reasonably high pH soil, bluebonnets will grow and flower just fine. Also, bluebonnet seeds often come pre-inoculated from the seed seller.

Unfortunately, there are few sources for obtaining Rhizobium cultures. I did find one source, Becker Underwood in Australia with worldwide production and marketing, that lists a rhizobium inoculant for lupins. The USDA has a Rhizobium Germplasm Collection that provides free cultures for research but they do not have a database of strains available. You can see a list of other bacterial culture collections worldwide. Apparently rhizobium inoculant used to be available in powdered form in nurseries, but I have not been able to find any suppliers listed on the internet. You might check our National Suppliers Directory for nurseries in your area that specialize in native plants and telephone or e-mail them to see if they have it.