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Thanks! What would be the cash crops on the north side of the Ohio river?
Same as Kentucky. Hemp, IIRC, and tobacco. Don't think there's any cotton country to speak of.
So what about further west? There seemed to be slavery all the way up the Missouri where it was legal. Political situation aside, would it have been feasible economically in Nebraska, Iowa and the Dakotas?
What are the circumstances of the continued legality? Do you have a POD worked out?
The anti-slavery enthusiasm of the north was not merely economic - it was also ideological. In Massachusetts, Vermont, and New Hampshire, emancipation mirrored the French Revolution's equivalent - it was explicitly part of a revolutionary agenda. It took riding a wave of war-borne radicalism to do so much so early in areas where slave ownership was nearly nonexistent.
In Pennsylvania, where the institution was much stronger, the same forces had to act in alliance with a longstanding and disproportionately powerful Quaker abolitionist lobby. Even then a compromise was necessary that extended the institution for generations.
Once that much had happened though, the Free North was nearly inevitable. The combination gave a model for compensated change, a threat of what the radical alternative might be, made the northwest ban easy to pass, almost made a southwest ban pass (one vote short!), and surrounded the northeast with free states. That and created a big population that self identified themselves as growing up in free states (with pride, obviously) and predominated in western migration.
In a very real way, the Revolution created the free states.
So what about further west? There seemed to be slavery all the way up the Missouri where it was legal. Political situation aside, would it have been feasible economically in Nebraska, Iowa and the Dakotas?
Mass cotton plantations somewhere else could reduce the price a bit if something very different is happening in Brazil, Egypt, or India. Eventually it'll come down on its own, as in OTL, of course. Or the slave trade could have continued a lot longer or been at a larger scale.
If we assume cotton demand stays the same, how much would the supply of slaves have to increase for King Cotton to have his thirst quenched? Are we talking more ~20% additional slaves or ~200% or ~2000%?