From what I know about the debate over slavery in the pre-Civil War US, it seems like everyone pretty much agreed that if Southern slavery wasn't allowed to expand westward (or southward), it would collapse. I understand why people believed this, but I haven't come across any modern writing about whether they were right.
Is my impression about 19th century opinion correct? And what do contemporary historians have to say about the idea that Southern slavery had to expand? I'm familiar with the debate about the profitability of slavery, but I'm not aware of anything that addresses this specific question.
It wasn't that slavery was unsustainable unless it kept expanding, it's that the southern states were worried that if the western territories were admitted as free states, they would eventually side with the northern states to ban slavery nationwide.
Yep. I think one of the best overviews comes by way of Carolyn Merchant in her American Environmental History. Her third chapter (in the edition I have anyway) details the environmental impacts of cotton and tobacco production. Both of these agricultural products take vast amounts of nutrients to remain viable and in an era without chemical fertilizers- the South could not sustain the agricultural production that had made it wealthy.
Planters had largely moved from the initial coastal plantation areas further west- and ultimately they would have had to continue moving over the course of generations to find suitable crop land. Their heavy labour intensive agriculture simply depleted soil nutrients until cash crops were no longer viable.
There were a couple of solutions to this. First, there were phosphate deposits in the Carolinas, but there use as a fertilizer was not wide spread until after the Civil War. The Pacific Guano Company (whose operations were mostly in the Atlantic oddly enough) followed the lead of South American phosphate mining operations in Chili and Peru and hunted for offshore islands rich in guano, which could be processed into a powerful fertilizer which could restore areas that had experienced intensive tobacco cultivation. Secondly, they could use floods- especially common along the Gulf states, to restore soil nutrients. Basically flooding their fields with river water which carried with it silt and soil runoff, whose nutrients could help revitalize the fields. This system, while effective, was expensive. Dams and dykes had to be repaired and broken to flood fields, slave mortality was higher because working in mud and standing water is just asking for mosquitos and disease outbreaks.
However, the main solution was simply to move- to find new, still fertile land on which you could produce the standard cash crops and then divide up and sell your old land to poor farmers. So yeah- ultimately- if we look at slavery through an environmental lens- we can argue that the cash crop driven, slave powered economy of the South was environmentally unsustainable and required expansion to continue.
A really good discussion of slavery is Robert Fogel's "Time on the Cross". A quick summary is slavery was great for planters. Slaves worked much harder than free labor. It wasnt going anywhere. Cotton farming was extremely profitable. The South was one of the richest places in the world. Turns out whipping people is good motivation. Slavery was a rational economic institution.