According to PBS' "Slavery and the Making of America", slaves would usually be given weekly rations, usually using corn meal, lard, molasses, peas, some meat, and flour. Occasionally, the slaveholder would give fresh produce along with these, but it's unclear how common this was. These rations, because they certainly weren't nutritionally sound, contributed to slaves developing conditions like rickets or scurvy from vitamin D and C deficiencies. These rations were also not much for a slave to go by, so sometimes slaves would try to steal food from their owners. This was usually punished very harshly.

An interesting aspect of this topic is geophagy, which was eating soil to get nutrients missing from a person's diet, According to this 1984 NYT article on the practice. The article also mentions that geophagy is practiced to an extent in Western Africa, where a majority of American slaves were from, and so, the geophagy among slaves in the American South may have cultural links to their origin.