The possibility is investigated that soil CO₂ and soil carbonate, deficient in ¹⁴C, enter a tree via the root system and are assimilated together with atmospheric CO₂ during photosynthesis. Such a situation would produce regional variations in the ¹⁴C content of organic materials, causing difficulties in the application of a universal ¹⁴C time scale. Carbon-14 activities of samples from beech trees grown in a highly calcareous soil were compared to ¹⁴C activities of samples from beech trees of the same age grown in a normal soil to see if part of the carbon assimilated during photosynthesis might originate from ¹⁴C-deficient carbonates in the soil. No significant assimilation of soil CO₂ or soil carbonate during photosynthesis was detected. Thus, only in very special cases could the assimilation of the amount measured in this study constitute a source of error in ¹⁴C dating. -- AATA