Here the 1840 edition replaced the preposition on with in, probably accidentally. The RLDS textual tradition continued with the in until the 1908 RLDS edition, which restored the original on (most likely by reference to 𝓟). 𝓞 is not extant here but probably read on (like 𝓟). Elsewhere the text prefers the preposition on in this construction that deals with a cardinal direction. For discussion of that evidence (and one possible exception), see nearby under Alma 50:15.
Summary: Maintain in Alma 50:13 the preposition on in the phrase “on the south”, the reading of the earliest textual sources.