Here in Jacob 5:24, the 1830 typesetter added it since he interpreted the that as a subordinate conjunction rather than as a direct object. We do not have the original manuscript here, but it is quite reasonable to assume that Oliver Cowdery accidentally omitted the small word it as he took down Joseph Smith’s dictation or while copying from 𝓞 into 𝓟.
When comparing the two possible readings from a usage point of view, we discover evidence both for and against each reading. The original Book of Mormon text has a number of examples of “behold that” where behold is in the imperative and that is the subordinate conjunction (although some cases have a short intervening phrase, each marked below with an asterisk):
On the other hand, in the verse immediately following Jacob 5:24, we have a direct object, this, coming directly after an imperative behold:
However, note here that inversion has taken place: the helping verb have comes before the subject pronoun I, whereas the example in verse 24 has the noninverted order “behold that I have nourished”. Still, the Book of Mormon allows either inverted or noninverted order after a clauseinitial direct object pronoun such as this:
inverted examples
noninverted examples
But in all these examples the direct object pronoun is this rather than that; besides the possible example here in Jacob 5:24, there are no examples of the pronoun that serving as the direct object in initial position.
Finally, when we consider the verb nourish in the olive tree allegory, we discover that (whenever there is a choice) the direct object always occurs after the verb. In fact, there are ten instances where the verb nourish is followed by the direct object pronoun it (besides the potential one here in Jacob 5:24):
All of this internal evidence suggests that interpreting that in Jacob 5:24 as a direct object pronoun is considerably more unusual than interpreting it as a subordinate conjunction following the imperative behold, although the latter construction is relatively rare in the text. Moreover, there is scribal evidence for the accidental omission of the object pronoun it, as in the following clear example where Oliver Cowdery momentarily omitted the word while copying from 𝓞 into 𝓟:
In this instance, 𝓞 is extant and has the it.
Summary: Accept the 1830 typesetter’s inserted it in Jacob 5:24 (“behold that I have nourished it also”); the object pronoun it could have readily been omitted while copying from 𝓞 into 𝓟 or as Oliver Cowdery was taking down Joseph Smith’s dictation.