How is it cheaper to ship goods to New York, land them, load them on a coastal packet, then ship them to New Orleans rather than ship them direct. Wouldn't that be the shortest route?
Additionally, New York had the deepest harbour, Charleston being only 12 feet deep large ships could never enter the harbour unless it was dredged - which was contemplated circa 1860 to attract trade.
Charleston harbor - which is located where two tidal rivers, the Ashley and the Cooper, meet - is actually quite deep. The channels that lead into the harbor are what needs care, but even in 1860 one of the four channels was 17 to 22 feet deep and the shallowest two were 11 feet deep.
And lastly, with the secession of the states, Southern exporters would see a rise in revenues and profits, and as a consequence, agricultural interests would seek more mechanical means of increasing production.
How so?
One of my minors is in Economics, but it's not a subject that one can understand in a few sentences. A trip to your local library should allow you to find a beginner's book on economics - the laws of supply and demand, Laffer curves, etc are not complicated. International economics & tariffs are only slightly more difficult, but a tax on imports IS an indirect tax on exports. Alternatively, you can learn online 'Mises Institute Home Study Course in Austrian Economics' for $350. Your study can also explain how economies of scale work to reduce costs by shipping > trans-shipment and the role of warehousing, and how tariffs are passed on, and the advantages of open markets which raise the standard of living in all participating countries. When higher profits are realized, they are almost invariably followed by investment in some means of mechanization - where possible - to futher reduce costs and increase profits.