As famous as the tobacco companies are for their accurate and comprehensive studies, the only thing this study from 1977 says is that Magnesium and K increased yields and market value. K much more so than Mag, and that K may help remove certain "health hazards" from the smoke while adding others.
It ends with:
"Consequently, K fertilizer would appear to have both positive and negative effects on smoke quality, and bioassays of the experimental samples will be necessary to evaluate the potential benefits or hazards to health associated with varying K levels in tobacco leaf"
it was an example on how to search, if you want more pertinent studies refine the search or do your own trials