Compelling evidence recently demonstrated that plants can take up nitrogen (N) as organic molecules. Yet, very little research addressed this issue in the context of organic horticulture, where N is provided as organic residues. Organic N and carbon (C) transported from roots to shoots could contribute significantly to the plant C and N accumulation. We posited that the type (organic or mineral) and amount of N fertilization affect the soluble organic N and C content of the growing medium, in turn influencing xylem sap N and C and fruit soluble solids content (SSC). To test this hypothesis, we collected growing medium, xylem sap, and fruit samples in a greenhouse cucumber crop grown in a peat-based growing medium and fertilized with organic (blood and feather meals) or mineral (ammonium nitrate) N fertilizers. The organic N source reduced growing medium concentrations of soluble mineral and organic N relative to the mineral source through microbial immobilization. Xylem sap C and N were positively linked to the soluble C and N contents of the growing medium, contributing to higher fruit SSC. A causal model is proposed, in which 62.6% of the variance observed in fruit SSC is explained by variation in mineral soluble N and soluble organic C in the growing medium and dissolved organic C and amino acids in xylem sap. Our results provide in situ indications that organic molecules in the growing medium are taken up by cucumber plants and contribute to fruit soluble solids in a context relevant to greenhouse horticulture.