If the solo is written in a pedal part, of course, it will be
played by the feet. But this pedal solo can sound in any
range,
depending on the pitch on which the combination is built.
A 4' foundation in the Pedal division will sound in the soprano
or alto range, an 8' foundation in tenor or bass, and a 16' foundation
will sound in the bass range.
In addition, a skilled organist can play a solo line that appears in any part or range of a two-staff score with the feet. For example, a soprano-range solo written in the treble clef can be played by the feet on a 4' stop, freeing up the hands to play all of the other parts. Such versatility is a valuable skill in an organist.