Found the quote. It is the same old nonsense that claims since 3/4 of US exports were from the South in 1860, then the South was "really" paying the tariffs on imports.
Which is, of course, comprehensive nonsense.
The issue of exports vs. imports is one of balance of trade, not of whose pocket the tax payment comes out of.
In actual fact, the Pennsylvania merchant and the Iowa farmer paid exactly the same tariff on an imported item as the SC planter.
The south probably had disproportionately low consumption of tariffed items because many of these items were luxury goods, and the 40% of southerners who were enslaved didn't consume many of those.
On economic causes of civil war
Paul Collier and Anke Hoeffler
“the Yankees were for the most part fighting not to abolish slavery, but for their economic interests...the tariff became the single most important domestic economic issue prior to the Civil”
http://www.amazon.com/Understanding-Civil-War-Evidence-1-Africa/dp/0821360477