There’s probably a Master’s thesis on it ;-).
Very interesting information in the next excerpt about the economic conditions for the slaves.
The monthly allowance appears to be on top of subsistence gardening, fishing, etc (recall Grandma Betsy from Chapter 1).
While the living conditions seem harsh by today's standards, they don't strike me as much different from those of poor whites in that era.
Note the monthly corn allowance, a southern dietary staple unwittingly responsible for the scourge of Pellagria http://www.eufic.org/article/en/artid/origins-maize-pellagra/