Visible light occurs in the frequency range between 4x10^14Hz and 7x10^14 Hz with wavelengths between 400 and 700nm.
UV radiation has a frequency range between approximately 10^15-10^17 Hz and wavelengths between approximately 250-400nm. There is little literature on the use of just UV reflectance for PSS, however is often combined with VIS and NIR.
The visible band can be used to measure soil iron oxides and soil colour [1].
Commercial
1. Viscarra Rossel, R. A., Fouad, Y. & Walter, C. (2008) Using a digital camera to measure soil organic carbon and iron contents. Biosystems Engineering, 100(2), 149-159.