Reading a collection of Karl Marx's journalistic writings in Dispatches for the New York Tribune: Selected Journalism of Karl Marx (Penguin Classics) I came across an interesting theme in his writings on the American Civil War, I shall here quote two articles:
"In 1859, on the occasion of John Brown's Harper's Ferry expedition, the very same Economist published a series of elaborate articles with a view to prove that, by dint of an economical law, American Slavery was doomed to gradual extinction from the moment it should be deprived of its power of expansion. That "economical law" was perfectly understood by the Slaveocracy. "In 15 years more," said Toombs, "without a great increase in Slave territory, either the slaves must be permitted to flee from the whites, or the whites must flee from the slaves.""
"The cultivation of the southern export articles, cotton, tobacco, sugar, etc., carried on by slaves, is only remunerative as long as it is conducted with large gangs of slaves, on a mass scale and on wide expanses of a naturally fertile soil, which requires only simple labor. Intensive cultivation, which depends less on fertility of the soil than on investment of capital, intelligence, and energy of labor, is contrary to the nature of slavery. Hence the rapid transformation of states like Maryland and Virginia, which formerly employed slaves in the production of export articles, into states which raise slaves to export them into the deep South. Even in South Carolina, where the slaves form four-sevenths of the population, the cultivation of cotton has been almost completely stationary for years due to the exhaustion of the soil. Indeed, by force of circumstances South Carolina has already been transformed in part into a slave-raising state, since it already sells slaves to the sum of four million dollars yearly to the states of the extreme South and Southwest. As soon as this point is reached, the acquisition of new Territories becomes necessary, so that one section of the slaveholders with their slaves may occupy new fertile lands and that a new market for slave-raising, therefore for the sale of slaves, may be created for the remaining section. It is, for example, indubitable that without the acquisition of Louisiana, Missouri, and Arkansas by the United States, slavery in Virginia and Maryland would have become extinct long ago. In the Secessionist Congress at Montgomery, Senator Toombs, one of the spokesmen of the South, strikingly formulated the economic law that commands the constant expansion of the territory of slavery. "In 15 years more," said he, "without a great increase in Slave territory, either the slaves must be permitted to flee from the whites, or the whites must flee from the slaves.""
My question is simply, how true are these claims? Was it a influencing factor in the Republican containment strategy? How real was the exhaustion of the soil, how urgent was the situation, and what role did this play in the outbreak of the Civil War?
While I would be glad to hear from experts in agricultural history who might have the specialization to dispel this, if it is a myth, I can assure you that soil exhaustion is still treated as a major factor in the expansion of slavery in the South.
Harold Underwood Faulkner emphasized it in this multi-volume Economic History of the United States, written in the early 20th century. More recently Stanley Engerman included it in his article on the Antebellum South in the second volume of the Cambridge Economic History of the United States (c. 2000).
From what I can see, some individuals have taken this very real phenomenon and exaggerated it (especially how poor the soil had become) as a way to somehow explain away the expansion of slavery in the South. However their abuse of the thesis shouldn't mean we should throw the baby out with the bathwater.
Taking a broader view of slavery, plantations, and soil exhaustion due to intensive agriculture, I can tell you that this was a very real phenomenon. Reinaldo Funes wrote an environmental history of Cuba (c. 2004) telling how the area around Havana was turned from forests into savannas, with extremely exhausted soil. Plantation owners would get a decade or two out of the soil without investing too much into letting the land recover the nutrients required for agriculture, then expand into new lands, repeating the cycle.
It wasn't that the lands once sold stopped producing cotton, sugar cane, etc. It was that they now produced less and were therefore far less valuable. What drew people further West was the promise of easily realized wealth in the form of almost virgin soils with access to the Mississippi river basin, and thus an easy, natural, transportation network.
If you read an author who says slavery's expansion was NECESSARY due to soil depletion, they are oversimplifying and possibly being disingenuous. If you read an author who just speaks of it as a major factor spurring on expansion, due to monetary interest, they're right.
Hope it helps.