While I am unsure re: the situation re: rice, by the 1860 the market -- both nationwide and int'l -- for tobacco/tobacco products had been in a glut for nearly twenty years. Most slave owners in the central southern states, VA, etc, in fact made ends meet by selling "excess" slaves, tobacco being, as it were, sort money losing side-line to add a veneer of respectability.
However, unlike tobacco there was a huge, seemingly never-ending, ever increasing demand -- both nationwide and on int'l markets -- for cotton, esp. American cotton. New York, Philadelphia, Atlanta, etc. would loan the southern farmer money on the condition that he plant cotton -- for they knew there was a eager market for that product. So, more and more, the southern farmer (of any size but the very smallest) stopped a (healthy) diversity of crops and came to plant cotton...nothing but cotton. B/c that's where the money was, and b/c that is the one crop bankers would loan him money on and for.
It wasn't called "King Cotton" for no reason.
(Given the day and age and financial situtation, it's hard to believe that even with 40-acres-and-a-mule that the freed slave would, on average, have achieved much more than a subsistance level.)
I don't like comparisons like "subsistence level" when you are talking about historical comparisons. Compared to today's standards, middle class Americans of 1950 were "poor."
And compared to modern American standards Queen Victoria was middle class - an American secretary enjoys so many modern conveniences and so much better health care (for herself and her family) that an American secretary would have to think long and hard about trading circumstances with Queen Victoria.
2) No, they were not losing money. Both Time on the Cross and subsequent studies (Reconing with Slavery) concluded that slavery was not only profitable in almost all parts of the South, but viable because of the increasing value of property in slaves.
3) My doctoral dissertation was on Southern banking in the antebellum period. The bank loans were as much on slaves per se as they were cotton. (I found the same slaves used as collateral for several loans simultaneously!) Indeed, "King Cotton" proved a bust---Br. and France didn't need it at all.
4) The 40 acres and a mule argument was less about subsisetnce and more about independence. If we had done that, while there would always be a Jesse Jackson, it would be much tougher to ever argue that blacks were "economically discriminated against" after the Civil War, because they would have had basically what most whites in the South had---and, indeed, what most free farmers in the north had.