Soil type data types and sources are accessible via Internet from these sources (there may be others as well):
STATSGO
"State Soil Geographic Database", consolidated Soil Conservation Service soil maps at 1:250,000 scale. Not useful for small urban watersheds, but most appropriate for large projects. See STATSGO webpage for links to documentation and downloads:
U.S. General Soil Map (STATSGO) | NRCS NCGC
SSURGO
Detailed soil maps ("Soil Survey Geographic Database"). Usually electronic versions of county soil maps generated by SCS, 1160 datasets avaliable as of Spring 2002 out of 3066 counties in U.S. See SSURGO webpage at:
Soil Survey Geographic (SSURGO) Database | NRCS NCGC
EPA Surf Your Watershed
EPA has a watershed water quality research program, and data are meant to operate at the HUC level (unique Hydrologic Unit Code) at EPA's Surf Your Watershed:
Surf Your Watershed | Surf Your Watershed | US EPA
Hope that this helps.