Resources for soybean [Glycine max (L.) Merr] pods are supplied initially from newly assimilated chemicals. As reproduction progresses, dependence on stored reserves increases. Seed weight is sensitive to stored shoot reserves, but the role of stem resources in late seed filling is unclear. Pith and cortex tissues contain living cells in stems and play a key role in storage. We hypothesized that main stem tissue composition can predict concentrations of mobilizable resources, which can be used to calculate impacts of mobilizable chemicals on yield. A field experiment with determinate cultivars investigated stem tissue composition, mobile and immobile chemicals, and their relationship with fruit production. Pith was correlated with non-structural chemicals; xylem and phloem predicted structural chemicals. During late seed filling, mobile stem chemicals decreased 67.5%, and we found a negative relationship between pith and aborted pods. Results support our hypotheses and highlight the importance of pith in remobilized resources for seed production in soybean.