Here the earliest textual sources read “upon the land”. The 1837 edition added all after upon, apparently accidentally. The 1908 RLDS edition restored the original reading (presumably by reference to 𝓟). Elsewhere in the text there are no examples of “upon all the land”, but there are 17 of “upon the land” without postmodification, of which six refer to a curse or cursing “upon the land”:
The last example is in the very same verse that we are considering here. Thus the text as a whole supports the original reading at the beginning of the verse, “a great curse upon the land”. The all was probably inserted because there are 15 examples in the original text of the related phrase “upon all the face of the land”. There is also one original example of “upon the face of all the land”, but this was changed in the 1837 edition to the expected phraseology, “upon all the face of the land” (see the discussion under 3 Nephi 8:20).
Summary: Remove the intrusive all in Ether 14:1, giving “a great curse upon the land” (the reading of the earliest textual sources).