Literary: While the two branches planted in poor soil refer to specific populations that remain unknown to us, even without a historical identification, the function of these two branches in the allegory is to set up a contrast between the fruitful branches in the poor soil and the next branch that will be planted in good ground, but with ambivalent results.
On the level of allegory, the tension is set between the expectation that a poor circumstance yields a poor result and that a good circumstance will yield a good result. The examples become diametric opposites of the expectation. The historical connection will be apparent in the next example, but the moral lesson of the allegory comes through without any connection at all. The Lord is telling us that we may not use our circumstances as an excuse for our spiritual progress. We may not say to him that we could not believe because we were poor, or that our lives were difficult. We cannot say to him that we would have been faithful had we been comfortable. Indeed, there is a highlighting of the scriptural theme of the difficulty of producing spiritual fruits amidst the distractions of prosperity.
Botanical: "It might also seem odd that one of the trees planted in poor soil should produce good fruit. One of the branches was planted in "a poor spot of ground . . . poorer than the first" (Jacob 5:22-23). Nevertheless, this plant thrived. Although olives sometimes do well in poor soils because of their long maturing period and ability to tolerate considerable salinity, boron, etc., it is only with much attention to cultural practices that productive trees will grow on poor soil. When all of the important cultural factors are carefully optimized, olive trees will grow and produce a crop on poor soil. Accordingly, the unusual poorness of the soil in this part of the allegory draws attention to the extraordinary care and power of the Lord of the vineyard. The production of good fruit by the plant under these circumstances is attributable exclusively to the fact that the Lord had "nourished it this long time" (Jacob 5:23)." (Hess, Wilford M., Daniel J. Fairbanks, John W. Welch, and Jonathan K. Driggs "Botanical Aspects of Olive Culture Relevant to Jacob 5" In: The Allegory of the Olive Tree. FARMS, Provo Page 507)